Instrumentation

Clinical Applications

A high correlation between PaO₂ and P_tcO₂ has been verified by many studies,^{34,48,101,172,175,185} particularly when the range of PaO₂ is 30 to 100   torr. In fact, brief periods of hypoxemia that are reflected by a fall in P_tcO₂ may not be detected by intermittent PaO₂ sampling. These episodes are frequently associated with nursing interventions, such as turning of the patient, vital sign measurement, dressing changes, suctioning, and chest physiotherapy.

Thick skin reduces the accuracy of the P_tcO₂ because fewer deep capillaries are present beneath the sensor site. In addition, thicker skin offers more resistance to oxygen diffusion than thinner skin, and it has higher oxygen consumption; thus the P_tcO₂ over thick skin will be lower than the PaO₂. For this reason, the transcutaneous electrodes should not be applied over areas of thickened skin, such as calluses or the soles of the feet.

The P_tcO₂ electrode also should be placed on the trunk rather than over extremities because extremity perfusion will be influenced more readily by temperature and cardiac output. The patient should never be positioned on top of a sensor because this may decrease local blood flow.

The relationship among P_tcO₂, PaO₂, and cardiac output have been documented in a somewhat predictive pattern: the P_tcO₂ correlates linearly with the PaO₂ when the cardiac output is greater than 65% of normal.^{172,173,174,175} Tremper¹⁷⁵ reports a high correlation between the PaO₂ and P_tcO₂ levels when the cardiac index is greater than 1.54   L/minute per m² BSA. If cardiac output is compromised significantly (<  65% of normal), the P_tcO₂ will be less than 80% of the PaO₂. This poor correlation reflects a compromise in tissue perfusion and often is observed during episodes of low cardiac output or shock even before the PaO₂ falls.^174,175

Nursing Considerations

Erythematous marks may develop at the electrode site, resulting from heat produced by the electrodes. Although these marks may disturb the family and staff, actual blisters (second-degree burns) seldom develop if the electrodes are changed as recommended by the manufacturer.

The schedule for rotation of electrode sites on the skin surface should be strictly maintained and documented. The erythematous marks caused by an electrode may last for hours or days following electrode removal, but rarely leave scars. Many studies recommend a maximum of a 3- to 4-hour interval for each electrode location,¹⁸³ but the nurse should check the manufacturer's recommendation for each electrode used.

Accurate transcutaneous monitoring requires meticulous electrode and machine calibration. The nurse should be especially aware of the following:

In some hospital units, nurses are required to obtain an arterial sample for blood gas analysis after every electrode change, to compare the child's arterial blood gas values with transcutaneous values. As noted, the nurse must be knowledgeable about the procedure for machine calibration and maintenance (consult operator's manual and unit protocols) and must also be able to recognize electrical drift and/or mechanical error.

Maintenance should include frequent observation of the fluid space in the sensor. Gain or loss of fluid results in an erroneous P_tcCO₂ measurement. Each nurse must be familiar with interpretation of measurements, procedures for troubleshooting, and setting of alarm systems.

Mode of Operation

An instrument probe, housed in a clip or on an adhesive strip, may be placed on the finger, toe, foot, hand, or ear lobe. In the probe are two light-emitting diodes that emit red and infrared light through the tissue to a photodetector (Fig. 21-31). The red light absorption will be inversely related to the amount of saturated hemoglobin (i.e., hemoglobin that is bound to oxygen) passing through the tissue. Well saturated hemoglobin absorbs little red light and desaturated hemoglobin (i.e., hemoglobin not bound to oxygen) absorbs a large amount of red light.

Fig. 21-31 Pulse oximeter. Two light-emitting diodes (light sources) transmit a red and an infrared light through the pulsatile tissue bed. The photodetector must be placed directly across the pulsatile tissue bed from the light source. Oxygenated (saturated) and deoxygenated (desaturated) hemoglobin absorb red and infrared light differently, and the hemoglobin saturation (percent of total hemoglobin that is oxygenated) is related inversely to the amount of red light absorbed.

Because the probe is placed over a pulsatile tissue bed, the pulse oximeter unit computes a pulse rate, and can signal the strength of the pulse. However, patient movement can produce artifact that significantly affects the accuracy of the pulse measurement. A difference between the pulse rate displayed by the pulse oximeter and the heart rate detected by the cardiac monitor should prompt an immediate evaluation of the patient and the patient's systemic perfusion.

Pulse oximeters require pulsatile blood flow to operate properly. These monitors generally provide accurate results over a wide range of clinical conditions (including hypotension, low cardiac output, and hypothermia).^{29,42,72,100,157,173,177,183} However, the response time in the presence of hypoxemia varies widely from instrument to instrument. Some units may fail to reflect acute, severe hypoxemia and may overestimate the hemoglobin saturation. This tendency may be observed more frequently with finger probes than with ear probes.^100,158

Most oximeters provide a low signal alert, which may indicate diminished pulse intensity under the probe. Accuracy of the measured O₂ saturation is not necessarily altered by low pulse intensity,¹⁰⁰ but the low signal alert should prompt evaluation of the patient and the patient's systemic perfusion.

The pulse oximeter may be a useful adjunct to cardiovascular monitoring. It enables monitoring of the pulse in an ischemic limb and reflects changes in pulse rate in unstable patients and in patients during pacemaker support.

Pulse oximeters are not reliable in the presence of methemoglobinemia and carbon monoxide poisoning, because the effect of these hemoglobins on light absorption is not factored into the calculation of hemoglobin saturation. As a result, when these conditions are present, the pulse oximeter will display a falsely high hemoglobin saturation, reflecting only the percent saturation of the child's normal hemoglobin, rather than the saturation of the child's total hemoglobin.

Troubleshooting During Pulse Oximetry

Several operator-controlled variables may cause a poor signal (and resulting alarm), including the presence of ambient light and movement artifact. To eliminate ambient light, the monitored area can be wrapped loosely with an opaque material such as gauze. The nail bed should be clean before sensor application.^36,77,189

Movement artifact is difficult to control in pediatric patients. Ear clip sensors are often the least likely to be disturbed with movement. However, if movement artifact is a significant problem the oximeter may be placed on a restrained extremity. In addition, application of a disposable sensor on the hand or foot may result in less movement artifact than that occurring with placement over a finger or toe. Occasionally, the patient may require sedation (with physician input and order).

Neonatal studies suggest that attachment of the probe to the patient before it is attached to the device may shorten the time required to obtain an initial signal.¹³² The nurse must be familiar with the rapidity of signal response required for the pulse oximetry devices used in the critical care unit.

Instrumentation

The exhaled carbon dioxide is the tension (in mm Hg) or partial pressure of carbon dioxide in expired (exhaled) gas. The highest exhaled carbon dioxide tension is present at the end of the expiration; this end-tidal CO₂ or P_ETCO₂ is also known as the end-expiratory CO₂. The P_ETCO₂ normally trends with the patient's arterial carbon dioxide tension.

The continuous measurement of the P_ETCO₂ by infrared spectroscopy is called capnometry. Capnography includes a display (graph) of the waveform of the carbon dioxide tension throughout inspiration and exhalation and the monitor typically provides a digital display of the P_ETCO₂. Analysis of the capnogram is discussed in the section that follows (see Clinical Applications of Capnography).

End-tidal CO₂ monitors can now be purchased as part of a mechanical ventilation system or as separate monitors. Capnometry can also be performed via nasal cannula during spontaneous breathing.^170a

The infrared CO₂ analyzer consists of three components: an inferred radiation source, a gas sampling chamber, and a detector. Carbon dioxide absorbs infrared radiation of specific wavelengths, so as the infrared rays are passed through expiratory gas, a detector then registers the intensity of the radiation in the gas (and conversely, the absorption of infrared radiation) to determine the tension or partial pressure of CO₂.¹⁷¹ This analyzer therefore enables evaluation of alveolar ventilation and CO₂ elimination.

Two types of P_ETCO₂ monitoring methods are used clinically—mainstream and sidestream analyzers. In mainstream analyzers the detector module is placed in the ventilator circuit at the proximal end of the endotracheal tube, in line with the expired gas flow; this placement allows for rapid response to changes in P_ETCO₂. However, the mainstream module can be affected easily by condensation and mucus, and requires frequent cleaning of the sensor module.

In sidestream modules a low flow (50 to 150   mL per minute) vacuum aspirates a small sample of expired gas to the analysis module located in the monitor. This sampling method is also affected by condensation and mucus, which can occlude the sampling line. Sidestream technology has also been found to be relatively inaccurate at small tidal volumes, increased I:E ratios, and high resistance states.^95,106

The presence (but not the tension) of CO₂ in exhaled gas can be detected through use of a colorimetric device attached to the proximal end of an endotracheal tube (or between an endotracheal tube and a resuscitation bag or mechanical ventilation system). Colorimetric CO₂ detection devices change color (typically from purple to yellow, but the color change may vary from device to device) when CO₂ is present in the gas flowing through the device. Typically the color change develops within about 6 breaths, and the color will change permanently after several minutes or hours of use. These devices are considered qualitative (i.e., indicating presence or absence of CO₂) rather than quantitative devices, and are not discussed further in this chapter (see Chapter 9 for additional information).

Clinical Applications of Capnography

The correct use of a P_ETCO₂ device requires an understanding of the patient's alveolar-arterial (A-a) CO₂ gradient. In addition, the P_ETCO₂ must be evaluated in conjunction with the clinical examination.

Analysis of Capnograph

The waveform that is displayed by variations in exhaled CO₂ throughout the respiratory cycle is known as a capnogram. To use the capnogram, the clinician must first be familiar with a normal P_ETCO₂ waveform (Fig. 21-32).¹⁸³

Fig. 21-32 Capnogram phases. A, Normal capnogram illustrating each phase of exhalation (see text, Analysis of Capnograph). B, Rebreathing capnogram: Inspiratory level does not return to zero. C, Curare cleft: Cleft in the alveolar plateau indicates a return of diaphragmatic activity in a patient receiving neuromuscular blockade. D, Increased alveolar plateau slope is representative of small airway obstruction (e.g., asthma, bronchiolitis).

A capnogram is divided into four phases. Phase A-B is the inspiratory cycle, when no carbon dioxide is detected. Phase B-C is the beginning of exhalation (emptying of dead space and alveolar gas), at which point carbon dioxide tension rapidly increases.¹⁸³ Phase C-D reflects exhalation of predominantly alveolar gas, and is also known as the “alveolar plateau.” Point D is the “end-tidal” point at which carbon dioxide exhalation is at its maximum level (i.e., the P_ETCO₂). Phase D-E is the beginning of inspiration (inhalation of CO₂ free gas) where the waveform returns to zero.

Nursing Considerations

When capnometry is in use, the nurse must be able to calibrate the instrument and must be aware of the relationship between the patient's alveolar (or end-tidal) and arterial CO₂ tensions. The nurse must be able to correlate P_ETCO₂ values with the clinical status of the patient and must be aware of sources of instrument error.

The absolute P_ETCO₂ at any one time is usually not as important as the trends documented by this equipment, although a sudden fall in P_ETCO₂ in an intubated patient should prompt immediate suspicion of spontaneous extubation (see Fig. 9-21). The P_ETCO₂ should be compared with either a venous or arterial PCO₂ to ensure that the two correlate. In addition, it is critical to verify effectiveness of oxygenation and ventilation through careful clinical assesment.¹⁸³

Clinical Applications

Arterial oximetry allows direct, continuous measurement of oxygen saturation, which is preferable to intermittent measurement. The indwelling electrode does not require frequent repositioning (e.g., as is necessary with transcutaneous oxygen monitoring).

The disadvantages of arterial oximetry are related to vascular effects of a foreign body and the potential for inaccurate results. The catheter may lodge against the arterial wall, producing arterial spasm. Fibrin clots may form at the catheter tip, resulting in inaccurate readings and risk of embolism. Hemodilution (particularly a hematocrit <  30%) may result in erroneous oxygen saturation calculations. Finally, electrode failure has been reported.

Use of the arterial oximeter may be limited in pediatric patients because they have small vessel size. The risks of infection, thromboembolic events, and other potential complications of the indwelling arterial oximeter have not been documented in pediatric patients. Currently, arterial oximetry is not performed frequently in children because further research is required to verify its effectiveness.

Description

The SvO₂ can be monitored with a fiberoptic 5-French, 5-lumen balloon-tipped pulmonary artery catheter that also enables measurement of pulmonary artery pressure, central venous pressure, and pulmonary wedge pressure, and for thermodilution cardiac output calculations.

Continuous monitoring of S_CVO₂ is performed using a fiberoptic central venous catheter. Pediatric central venous oximetry catheters are available in 4.5 and 5.5-Fr sizes and three different lengths (Edwards PediaSat, Edwards Lifesciences, Irvine, CA).^110a

The hemoglobin saturation is determined by spectrophotometric reflection. The catheter port used for SvO₂ monitoring is connected to an optical module, which transmits a narrow band width (wavelength) of light to the tip of the fiberoptic catheter. This light will be reflected by saturated (oxygenated) hemoglobin differently than by desaturated hemoglobin (i.e., hemoglobin not bound to oxygen).

The light emitted by the catheter is reflected by the hemoglobin and transmitted back via a separate fiberoptic filament to the module (Fig. 21-33). The microprocessor analyzes the amount of light transmitted and reflected and averages the signal over 2 to 3 seconds. A tandem recorder trends the SvO₂ on a graph recording. A light intensity indicator (at the tip of the catheter) is also recorded and monitored; changes in the light intensity may indicate a change in catheter position, inadequate blood flow, or damage to the fiber optics.^19,62,81

Fig. 21-33 A, Continuous central venous oxygen saturation (S_CVO₂) microprocessor/monitor and B, multilumen central venous catheter. The fiberoptic catheter contains filaments that transmit red light to and from the blood. The light is reflected by circulating hemoglobin and transmitted via a second optical fiber to the microprocessor, where the hemoglobin saturation is determined. Catheters are available in 3 lengths. (Vigileo monitor and PediaSat catheter photograph courtesy of Edwards Lifesciences, Irvine, California.)

Two different reflection spectrophotometry processor units are available: a three-wavelength and a two-wavelength device. The three reference wavelength processor is thought to be more accurate over a wide range of physiologic conditions, particularly changes in hemoglobin concentration. When a device with two reference wavelengths is used, the hemoglobin concentration must be entered whenever it changes⁵⁵ (typically this calibration is performed at regular intervals, based on institutional protocol and manufacturer's recommendations). The bedside team must familiarize themselves with published experience of the device in use to be aware of accuracy of the device and potential drift.^55,110a,187

Operation

Unit calibration is recommended before insertion of the catheter. This calibration is performed using the catheter, the processor, and a standard optical reference provided with the catheter. If the SvO₂ monitor is part of a pulmonary artery catheter, the appropriate pressure transducers also must be calibrated and connected to their respective monitoring systems and catheter ports before the catheter is placed.

Insertion into the central vein is accomplished using a standard Seldinger technique. Pressure waveforms and SvO₂ readings are monitored during insertion to guide catheter placement.

If the fiberoptic is contained in a pulmonary artery catheter, placement is correct when the pulmonary catheter is correctly positioned. If the tip of the pulmonary artery catheter is advanced too far into a pulmonary artery, a falsely high SvO₂ will be calculated, reflecting the proximity of the fiberoptic to blood that is oxygenated by surrounding alveoli.¹⁵⁴

Recalibration after insertion should be performed according to manufacturer's recommendation: a mixed venous blood sample is sent for laboratory measurement of the oxygen saturation using a co-oximeter (i.e., not a calculated saturation). Simultaneous with the blood sampling, the nurse should document the mixed venous oxygen saturation displayed by the device. Once the laboratory analysis of the blood sample is complete, the device should be recalibrated based on the laboratory measurement.

Clinical Applications

The normal mixed venous oxygen saturation is usually between 65% and 75%. A rise in SvO₂ reflects one of four conditions: (1) increased oxygen delivery, caused by a rise in cardiac output, or an increase in arterial oxygen content; (2) reduced oxygen consumption, as observed with hypothermia, neuromuscular blockade, and anesthesia; (3) the presence of a left-to-right intracardiac shunt; or (4) mechanical interference from the measuring unit (e.g., a wedged catheter).

The SvO₂ may fall for the following reasons: (1) decreased oxygen delivery, resulting from decreased cardiac output, hypoxemia associated with pulmonary dysfunction, or anemia, or (2) increased oxygen consumption caused by shivering, seizures, hyperthermia, sepsis, or agitation. The SvO₂ will track with changes in cardiac output, but there are other factors, such as oxygen demand and oxygenation of blood in the lungs, that influence the SvO_2.

The SvO₂-monitoring system has been used in adult patients since 1981, and its efficacy has been documented in a variety of physiologic conditions.^{19,55,62,81,93,122,146,154,187} The enthusiasm for its use in adult patients is based on its accuracy and reliability as well as its ability to reflect changes in cardiac output and arterial oxygenation instantly and continuously.

The use of venous fiberoptic oximetric catheters has been successful in the management of pediatric patients following surgery for congenital heart disease and for pediatric patients with septic shock. Muller and co-workers described the use of a 2-French fiberoptic probe that is introduced through a single-lumen central venous catheter in a series of three infants who underwent a stage 1 Norwood procedure.¹²⁶ The fiberoptic probe was inserted approximately 2.5   cm beyond the central venous catheter tip, allowing for continuous central venous oximetry (S_CVO₂). The group found good correlation between the fiberoptic S_CVO₂ and laboratory SvO₂ evaluated by co-oximeter (r = 0.912. 95% CI: 0.716-0.975).¹²⁶ However, they reported that the fiberoptic probe was less accurate in conditions of very low saturation (<  40%).

More recently, de Oliveira reported that goal-directed resuscitation using the end point of a superior vena caval oxygen saturation (ScvO₂) ≥  70% significantly improved the outcome of pediatric patients with septic shock (28-day mortality 11.8% vs. 39.2%).⁴³ Since 2006, pediatric S_CVO₂ catheters have been approved by the Food and Drug Administration for use in children in the United States.

Disadvantages

Changes in the calculated SvO₂ (or S_CVO₂—in this section they are used interchangeably) are not always indicative of changes in patient condition. The high incidence of artifact during continuous SvO₂ monitoring seems to be linked to the fact that SvO₂ determination is dependent on reflected light. As rapid blood flow passes the catheter tip, consistent light reflection may not occur. Faulty connections, fiberoptic fracture, occlusion of the catheter tip by emboli, and wedging of the tip against the vessel wall also may produce inaccurate SvO₂ readings.^122,154

Whenever a fall in SvO₂ is noted, the patient's oxygenation and systemic perfusion must be assessed. Clinical evaluation always should be used to confirm any changes associated with deterioration in the SvO₂. The function of the monitor should be assessed only after the patient's condition is evaluated.

Factors reducing the accuracy of fiberoptic SvO₂ monitoring include lack of in-vitro and in vivo calibration, lack of intensity calibration during insertion, bent or broken optics, catheter tip close to or facing the vessel wall, increased carboxyhemoglobin or methemoglobin, and extreme hypoxemia (<  40%).¹⁷¹

The limited variety of available sizes of the fiberoptic catheters prevents its use in very small children. Although the 2- to 4-French SvO₂ fiberoptic catheter can be used in infants and for central vein oximetry, these small fiberoptic catheters do not enable other hemodynamic measurements and calculations (e.g., CO, PAWP, vascular resistances) that are possible with a pulmonary artery catheter with fiberoptic and thermistor. With the availability of the pediatric fiberoptic central venous catheter more experience is being gained with continuous monitoring of central venous oxygen saturation as a surrogate for the mixed venous oxygen saturation in critically ill children.^110a More data about its reliability and effectiveness is anticipated.

Instrumentation

Esophageal pressure is measured using an air-containing balloon that is sealed in a catheter connected to a pressure transducer. The catheter is inserted into the thoracic esophagus. A change in pressure imposed on the balloon is conveyed via the catheter to the pressure transducer, and the pressure is displayed on the monitor or ventilator.

Catheter dimensions range from an internal diameter of 1 to 1.2   mm (for use in newborns and smaller children) to an internal diameter of 1.4 to 1.7   mm (for use in large children and adults). Adequate balloon volume is essential for accurate detection of the P_es.^20,171 Typically, a balloon volume of 0.5   mL is sufficient; however, the range of gas volume should be determined from the catheter packaging; the packaging indicates the clinical situations in which more or less volume may be indicated.¹⁷¹

The balloon is positioned by passing an empty balloon catheter into the stomach via the nares or mouth. A volume of 0.5   mL of air is then injected into the system and the catheter is attached to the pressure transducer.¹⁷¹ A positive pressure swing indicates that the balloon is in the stomach. The catheter is then withdrawn until a negative pressure deflection is identified on the monitor, indicating the balloon is in the thoracic esophagus. The catheter is then pulled back an additional 5 to 10   cm, which positions the entire balloon in the esophagus.

The final position of the balloon in the mid- and lower esophagus can be verified by chest radiograph.²⁰ Correct position of the balloon can be validated by using the “occlusion test,” which is accomplished by having the patient take a spontaneous breath against a closed airway while observing the ΔP_es and change of pressure at the airway opening (ΔP_AO).¹⁷¹ If the balloon is in correct position, the clinician will find near unity between the ΔP_es and ΔP_AO throughout the inspiratory cycle.¹⁷¹

Clinical Applications

Esophageal pressure monitoring has a wide range of clinical applications. As noted, P_es is beneficial in determining the patient's work of breathing imposed by underlying lung disease and the addition of the ventilator circuit. Evaluation of the work of breathing allows adjustment of mechanical support to decrease the patient's metabolic demands and/or identify if the patient is ready for extubation.

In addition to work of breathing assessment, P_es monitoring provides information regarding respiratory muscle function. More specifically, monitoring of the P_es in conjunction with gastric pressure monitoring enables calculation of diaphragm force-generation, relatively isolated from intercostal and other accessory muscles, and from elastic recoil of the chest wall. Some authors have suggested that isolated diaphragm function should be assessed using P_es measurement in patients with suspected diaphragm weakness or paralysis.²⁰

Esophageal pressure measurement may be beneficial in any clinical situation characterized by decreased lung compliance.^20,166,171 If poor respiratory system compliance is determined to be caused by chest wall edema, low lung compliance, or abdominal compartment syndrome, the clinician may be better able to titrate PEEP without causing lung trauma.

Nursing Considerations

The bedside nurse must be able to assess the catheter position and document depth and position at the insertion location, as well as P_es measurements. The catheter must be secured at the nares without causing tissue breakdown around the nares or mouth.

Description

A tank or shell surrounds the thorax; negative pressure is created around the thorax by a vacuum. The negative pressure “pulls” the thoracic cage outward, thereby increasing intrathoracic volume and reducing intrathoracic pressure. A pressure gradient is then present between the mouth and the intrathoracic space (where the pressure is now approximately −  10 to −  15 cm H₂O), so air flows into the alveoli. Exhalation occurs passively when the vacuum cycles off.¹⁸³

In order to generate the subatmospheric pressure around the thorax, the shell must seal around the arms, neck and lower abdomen. This required proper fit. If diapers are used, they should be secured outside (below) the shell.

Clinical Applications

Negative pressure ventilation may be used for support of children with chronic respiratory failure secondary to neuromuscular disease, such as the child with phrenic nerve injury or muscular dystrophy. The child's lung tissue must allow normal gas diffusion (i.e., pulmonary interstitial disease cannot be present), and the child must be able to maintain a patent airway (with an effective cough reflex).

An advantage of this form of ventilation is that endotracheal intubation is not required. Supplementary oxygen may be administered by nasal prongs or face mask.

Advantages and Disadvantages

There are some distinct disadvantages to negative pressure ventilation, which limits its use. The tanks are cumbersome for the patient and caretakers and they render the child virtually immobile. “Shell” devices must be fitted precisely to the child's thorax to obtain a good seal. Even if the tank fits properly, it is frequently difficult to achieve a good seal around the arms, neck, and abdomen. Air leaks diminish the effectiveness of the machine, thus reducing chest expansion and alveolar ventilation.

Exaggerated dilation of the thoracic great vessels and diminished cardiac output have been reported during negative pressure ventilation. In addition, venous pooling may develop in the legs.

In spite of the challenges during use, negative pressure ventilators may be extremely useful in the care of chronically ill ventilator-dependent patients. This form of ventilation support may be particularly useful for home ventilator therapy because the devices are relatively easy to operate and do not require an advanced (invasive) airway. With practice, the child is able to talk during negative pressure ventilation.²⁸

Nursing Considerations

As with all other devices, the nurse must be knowledgeable about the operation of a negative pressure ventilator. At all times a resuscitation bag and mask with an O₂ source should be available to provide manual ventilation in case of machine malfunction.

The child's heart rate, blood pressure and systemic perfusion must be closely monitored when instituting negative pressure ventilation and whenever the ventilator is adjusted. It is also important to monitor the patient closely for signs of venous dilation and signs of hypovolemia (potentially caused by venous pooling outside of the thorax).

If the child will be discharged home with this ventilator, a teaching program must be implemented for the family in the hospital. The parents must be able to provide bag-mask ventilation and troubleshoot common problems with the ventilator.

Description

Ventilators may be classified according to the mechanism that terminates inspiration. However, most new ventilators use a combination of cycling mechanisms and a variety of ventilation characteristics.

Volume-cycled ventilators are preset to deliver a specific tidal volume during inspiration. Once this volume is delivered, inspiration stops, allowing for passive exhalation. Pressure-cycled ventilators use a preset peak inspiratory pressure (PIP) at which inspiration is terminated; gas is delivered until the peak pressure is achieved without regard to the volume delivered. Time-cycled ventilators, most commonly used in the neonatal/infant population, are preset to allow a specific inspiratory time; gas will be delivered until that time is reached, without regard for the volume delivered or the peak pressure produced.^88,183

Changes in the child's pulmonary resistance and compliance will affect the tidal volume that is actually delivered to the lung. For this reason, close attention to exhaled volume is required. Some centers measure the patient's effective tidal volume (Vt_eff) to ensure that an acceptable tidal volume is delivered. Most ventilators available today are capable of real-time measurements of Vt_eff. Effective tidal volume is defined as the actual volume (mL) of gas being delivered to the lung per kilogram (kg) of ideal body weight; it is measured in mL/kg. The most accurate measurements are obtained at the patient wye (the connector that joins the inspiratory and expiratory limbs of a two-limb circuit to the patient's airway); measurement at this point alleviates the need to calculate the compliance factor of the ventilator circuit.³² Depending on the level of PEEP and underlying pathology, acceptable values of Vt_eff range from 5 to 8   mL/kg.¹⁸³

The Vt_eff may be calculated using pressure measurements obtained by the ventilator, with correction for tubing compliance (i.e., the circuit compliance factor). Calculation is performed as follows:

Calculation of Effective Tidal Volume Using Ventilator Values (Values Measured at the Ventilator, not from the patient wye)

* The greater the difference between the P_Plateau and level of PEEP or change in pressure (ΔP), the greater the volume lost in the ventilator circuit.¹⁸³

^† The circuit compliance factor is available from the ventilator circuit manufacturer and varies according to the circuit diameter.

Most newer-generation pediatric ventilators have incorporated flow-sensing devices or pneumotachographs at the patient's wye to more accurately determine inhaled and exhaled volumes.³² Monitoring respiratory volumes at the patient's airway removes the inaccuracy resulting from the aforementioned volume loss related to the circuit tubing compliance.³²

Most pediatric patients requiring mechanical ventilation in the critical care unit are placed on positive pressure ventilators. Selection of the appropriate ventilator should be based on the following factors:^46,88,183

Characteristics of an Ideal Pediatric Ventilator

The categories in Box 21-2 represent three groups of criteria for selection of the ideal ventilator for pediatric use. See Table   21-11 for more detail regarding each alarm.

Frequently, ventilators with the widest possible clinical application are more practical to purchase than a large number of ventilators with very specific applications. The information in Box 21-2 and Table   21-11 should be considered when selecting ventilators for use with critically ill children, as well as selection of a specific mechanical ventilator for a particular patient.

Clinical Applications

Many of the newer-generation ventilators are capable of providing mechanical support across all or most patient populations. The incorporation of microprocessors into newer-generation ventilators has enabled this wide range of use.

Manufacturer's specifications and recommendations and hospital clinical trials should facilitate the selection of the ventilator(s) that can best serve each patient (e.g., the patient size limit for use of neonatal ventilators is generally 8-15   kg body weight). The clinical condition of the patient will determine the ventilator functions needed to provide optimal ventilation.

Nursing Considerations

Throughout mechanical ventilatory support, the bedside nurse is responsible for assessing effectiveness of ventilation. The use of mechanical ventilation does not ensure that the child is ventilated effectively. The ventilator settings must be evaluated constantly in light of the child's clinical appearance.

When ventilator function is in doubt, the child should be ventilated manually with a hand-resuscitator bag. Table   21-11 offers a troubleshooting guide for use when problems arise during mechanical ventilation. It is intended to address equipment (rather than patient) problems. For more information, see Chapter 9, Positive Pressure Mechanical Ventilation and the section, Nursing Care of the Child During Mechanical Ventilation).

High-Frequency Oscillatory Ventilation (HFOV)

High-frequency oscillatory ventilation (HFOV) employs a piston moving at extremely high frequencies (about 180 to 1500 cycles/minute) to create positive and negative pressure swings. Oscillatory ventilation does not produce bulk gas delivery. It uses a continuous gas flow to eliminate CO₂ and deliver oxygen to the lung's ventilatory units.

In a controlled randomized multicenter NIH trial,⁷⁵ HFOV offered no advantages over conventional ventilators in the treatment of neonatal respiratory failure. In this study the incidence of bronchopulmonary dysplasia was similar to conventional ventilation, and mortality rates were equal in both groups.

A randomized, crossover trial of HFOV versus conventional mechanical ventilation in 70 critically ill children with ARDS was published in 1994 by Arnold et al^12a Children treated with HFOV demonstrated improved oxygenation and decreased use of supplementary oxygen 30   days later.

High-Frequency Jet Ventilation (HFJV)

The typical HFJV uses an oxygen source and a high-pressure source to deliver gas through a small-bore injector cannula that extends into the endotracheal (ET) tube. This system allows delivery of relatively large tidal volumes at relatively low peak airway pressures. A flow interrupter adjusts the frequency and relative inspiratory time. Valve devices applied to the expiratory limb of the circuit allow the application of PEEP. A continuous infusion of saline into the path of the jet humidifies inspired air. Often a conventional ventilator is used in tandem as the gas and oxygen source for the HFJV unit.

HFJV is used most often used as a rescue therapy for patients with respiratory failure unresponsive to conventional ventilation, as evidenced by rising inspiratory pressures, persistent hypoxemia, and hypercarbia despite maximal conventional ventilatory support. In recent years high frequency jet ventilation (HFJV) has become less widely used than HFOV.

Mechanisms of Gas Exchange with High Frequency Ventilation

The mechanisms of gas exchange during high-frequency ventilation are not well understood. As previously mentioned, bulk gas flow is not a major mechanism of gas exchange during HFV. Some gas exchange probably occurs simply because of nonhomogeneous alveolar filling and pressures.¹⁸³ Other explanations include gas exchange resulting from turbulent mixing of gas molecules (“augmented dispersion”), gas convection, and diffusion.

Multiple mechanisms are probably involved in gas exchange during high-frequency ventilation.¹⁸³ Ultimately, the effectiveness of these mechanisms must be determined by the evaluation of the patient's response to support.

Nursing Considerations During High Frequency Ventilation

High-frequency respiratory support is very different from conventional mechanical ventilation. The nurse must be familiar with the principles of operation, assessment of effectiveness of ventilation, and the potential complications of the technique.

The assessment of the infant or child on HFV differs from conventional ventilation in the following ways:

Complications of High-Frequency Ventilation

Many of the potential complications of HFV are identical to the complications of conventional mechanical ventilation, but the development of the complication may be more difficult to detect during HFV than during positive pressure ventilation.

Shape of the Tube

Some ET tubes are curved sharply to enable rapid intubation to the point of curvature. These tubes should not be used for more than a few hours, because it is difficult to pass a suction catheter beyond the curvature of the tube. Tubes with sharp curvature are designed for orotracheal use, so they can be very difficult to place nasotracheally.

Position Markings

A radiopaque line should be present along the length of the ET tube to allow radiographic verification of the tube's position. In addition, markings should be present at 1-cm intervals on the tube. Such markings allow the nurse to verify appropriate depth of insertion regularly so that tube displacement is detected immediately.

The depth of the tube insertion at the patient lips or nares should be recorded in the patient record and nursing care plan. The nurse should check the depth of insertion whenever the tube is retaped, when vital signs are obtained or if the patient deteriorates suddenly.

Cuffed Versus Uncuffed Tubes

In the past, uncuffed ET tubes were generally used for infants and children less than 8   years of age, because the cricoid diameter of a child is quite narrow and was thought to provide a natural seal around the tube. However, evidence published in recent years has demonstrated that the use of cuffed tubes during in-hospital care of young children produces no higher incidence of complications than the use of uncuffed tubes.⁹²

Cuffed tubes may reduce the incidence of aspiration, and cuffed tubes may be preferable to uncuffed tubes in some patients (i.e., those with poor lung compliance, high airway resistance, or a large glottic air leak), provided the endotracheal tube size, position and cuff inflation pressure is monitored.⁹² The cuff inflation pressure is maintained according to manufacturer specifications, typically at 20 to 25   cm H₂O.⁹²

Endotracheal Tube Size Selection

The diameter of the child's trachea is smallest at the level of the cricoid cartilage; therefore an ET tube may pass easily through the vocal cords yet be too large at the level of the cricoid cartilage. The ET tube size is appropriate if a small, audible air leak is present when inspiratory pressure of approximately 20- to 30-cm H₂O is provided. This small leak indicates that the tube is probably small enough to avoid excessive pressure on the trachea below the level of the vocal cords.

If the tube is too large, an air leak is not detected even at positive pressure of more than 25 to 30   cm H₂O. A tube that is too large can cause injury to the trachea.

If the tube is too small, an air leak is detectable at even low (<  10 to 15   cm H₂O) inspiratory pressures. If the tube is too small and allows a substantial air leak, it may be impossible to provide adequate oxygenation or ventilation through the ET tube.⁹²

The child's body length provides the best parameter for estimation of appropriate ET tube size.¹⁴² The relationship between body length and proper ET tube size has been used in the development of the Broselow resuscitation tape, which enables determination of appropriate endotracheal tube sizes, resuscitation equipment sizes, and drug dosages using the child's body length (see Figure 1-1).^3,108

Several formulas enable estimation of ET tube size from age in children. The most popular formulas for estimation of uncuffed and cuffed tube sizes for children 1 to 10   years of age are:⁹²

These formulas provide a relatively accurate estimate of ET tube size (within 0.5   mm) in most children 1 to 10   years of age. Additional guidelines for estimation of proper ET tube size include the approximation of the size of the patient's little finger or the equivalent of the size of the child's nares.

Essential equipment for endotracheal intubation trays is listed in Table   21-12. Suggestions for intubation equipment size according to the child's age in years are listed in the table.

Table 21-12 Essential Equipment for Endotracheal Intubation

Note: Intubation should be attempted by an experienced or closely-supervised clinician with resuscitation equipment immediately available.

* An ET tube size 0.5 mm larger and one 0.5   mm size smaller than predicted size should be immediately available to accommodate unexpected anatomical deviations.

From Czervinske MP, Barnhart SL: Perinatal and pediatric respiratory care, St Louis, 2003, Saunders.

Clinical Use

Self-inflating resuscitation bags are particularly useful for resuscitation carts because they can be used without oxygen flow, if needed. If used during resuscitation, delivery of a high concentration of oxygen is typically needed, so the bag should include a reservoir and should be attached to oxygen flow as soon as possible. Self-inflating bags require less skill to use than non-self inflating bags.

These bags are also useful for patient transport, when it is frequently impossible to predict how much air/oxygen to carry. If the oxygen source is exhausted, the self-inflating bags still enable effective ventilation of the patient with room air until additional oxygen is obtained.

When the bag is used for ventilation through an ET tube, a pressure gauge should be used, joined to the bag (with a Y connector if necessary). This enables monitoring of peak inspiratory pressures delivered.

Most bags have a pop-off valve to prevent delivery of high pressures. These valves are not precise and it may be necessary to inactivate the pop-off valve to deliver adequate bag-mask ventilation during emergencies.

The bags typically contain a valve near the adaptor where the bag can be joined to the ET tube or a mask. Unless a low-resistance valve is present, the valve opens only when the bag is squeezed (to deliver breaths to the patient) and it closes while the bag recoils and refills with air. An adjustable PEEP valve can be added to maintain positive end-expiratory pressure.

Advantages

The advantages of self-inflating bags include ease of operation (operation of valves is not needed to deliver gas flow) and ability to operate without an oxygen or gas source.

Disadvantages

There are several disadvantages to the use of self-inflating bags. A reservoir must be added to the bag to enable provision of a FiO₂ greater than 0.60.

If an inspiratory pop-off valve is present in older bags (set at 40   mm Hg to prevent the delivery of high inflation pressures), the pop-off valve may prevent delivery of adequate tidal volume during bag-mask ventilation, particularly if there is low airway or lung compliance.

The operator must be familiar with appropriate manual ventilation technique. Because these bags have thicker walls than nonself-inflating bags, it is more difficult for the operator to assess the compliance of the patient's lungs during bag-mask or bag-tube ventilation. A quick, snapping motion on inspiration should be avoided because this technique creates excessive inspiratory pressure that will open the pop-off valve (if one is present) and result in loss of tidal volume (i.e., air flow delivered to the patient).

Gas flow does not occur during spontaneous patient inspiration unless a low-resistance valve is present in the bag to allow the patient to draw in gas from the bag between manually delivered breaths. In many models a valve between the mask-adapter and the bag is opened only by the force of bag compression, so gas flow (and delivery of supplementary oxygen through the bag) during spontaneous ventilation is impossible.

The volume delivered during manual ventilation may be variable and depends on the speed and force of bag compression (and recoil) and patient lung compliance. The oxygen concentration delivered to the patient can also vary widely; if a reservoir is attached to the bag an FiO₂ of 1.00 (100% oxygen) may be reliably delivered.

Clinical Use

The non-self-inflating bag (“anesthesia bag”) is used in patients with advanced airways; it is not used for mask ventilation. This bag is very useful for assisting respiratory efforts of spontaneously breathing patients, but can also be used for those with no spontaneous respiratory effort.

The operator can deliver a relatively consistent tidal volume to patients who have decreased lung compliance. If the patient is breathing spontaneously, the patient can receive gas flow from the bag between delivered breaths.

Advantages

There are many advantages of this type of resuscitation bag. A FiO₂ of 1.0 can be provided without the addition of a reservoir bag. The bag and adaptor itself contain no internal valves that might dysfunction. During ventilation the provider can easily assess the patient's lung compliance because the walls of the bag are not thick and there are no valves between the bag and the patient's endotracheal tube/airway.

As noted previously, patients can breathe spontaneously and receive a continuous flow of oxygen even when the bag is not squeezed. A CPAP/PEEP valve (that provides resistance to exit of gas from the bag) and a pressure gauge easily can be added to the system.

Disadvantages

The use of this bag requires much greater skill than the use of the self-inflating bag. The use of the bag is best learned in a controlled environment. The operator must be able to monitor the patient and the patient's chest rise while ensuring that gas flow into and out of the bag is adequate.

The provider must be able to adjust gas flow into the bag so the bag will quickly reinflate between delivered breaths yet avoid delivery of high inspiratory pressure to the patient. If gas flow to the bag is insufficient, it will be impossible to deliver an adequate tidal volume and respiratory rate.

The risk of pneumothorax is significant when this bag is used by an inexperienced operator. If the gas flow to the bag is too high in light of the patient's minute ventilation, or if the PEEP valve is adjusted incorrectly, the bag may quickly become distended and ventilation may be delivered at a high pressure. Finally the bag requires a continuous flow of gas, which may be a problem during field resuscitations, such as transports.

Description

A ventriculostomy catheter is introduced into one of the lateral ventricles, usually on the patient's nondominant side. A twist drill is used to create a burr hole through which the catheter is advanced. Once the ventricle is reached the catheter may be tunneled through the scalp via a separate incision or sutured to the scalp at the insertion site. The catheter is then joined to an external fluid-filled monitoring system (with transducer) and to a stopcock and a closed, sterile drip chamber and receptacle for CSF collection.

External ventricular drainage systems include tubing between the catheter and the drip chamber, a stopcock, a drip chamber with a one-way valve, and a collection bag (Fig. 21-35). Many systems contain a built-in level indicator to facilitate leveling of the transducer with the patient's lateral ventricles (typically the external auditory canal is used as a landmark); the lateral ventricles are considered the zero reference point of the system.

Fig. 21-35 Intraventricular intracranial pressure (ICP) monitoring system with an external ventricular drainage system for controlled drainage of cerebrospinal fluid. A, The system consists of an intraventricular catheter joined by tubing to a drainage system with adjustable height and a drainage bag. The system also typically has a stopcock, an injection sampling port, and a clamp. The zero reference point for the system is typically between the outer canthus of the eye and the external auditory canal (see leveling device placed at that level). B, The drip chamber pressure level (horizontal arrow) is placed a prescribed height (in cm) above this zero reference point. The drainage stopcock can allow continuous drainage or be turned to allow only intermittent drainage when the child's ICP exceeds a prescribed threshold. If the system is functioning and the stopcock is turned to drainage, CSF will drain from the child's ventricle into the collection chamber and ultimately into the drainage bag once the patient's ICP is sufficiently high. If the drip chamber is placed 27.2   cm (rounded to 27   cm) above the child's ventricles, drainage should occur if the child's ICP equals 27.2   cm H₂O or 20   mm Hg (1.36   cm H₂O pressure = 1   mm Hg pressure; 27.2   cm H₂O pressure = 20   mm Hg pressure).

(Redrawn and modified from Owen A: Clinical guideline: external ventricular drainage. Great Ormond Street Hospital for Sick Children, Institute of Children's Health and University College of London, Revised, September, 2009.)

The flow of CSF to the collection unit is regulated by the height of the drip chamber above the ventricles; this height (in centimeters) creates resistance to CSF drainage equivalent to centimeters of water pressure. For every 1.36   cm the drip chamber is elevated above the ventricle, 1   mm Hg intraventricular/intracranial pressure is required to produce drainage of CSF into the chamber.

Typically the system and stopcocks are arranged to allow automatic drainage once the ICP reaches a pressure specified by the physician (or on-call provider). For CSF drainage to occur, the pressure in the ventricles must exceed the pressure created by the column of water between the ventricle and the drip chamber (1.36   cm H₂O pressure = 1   mm Hg) in the collection system. If the drip chamber is placed 27.2   cm above the ventricle, resistance to CSF drainage is equal to 27.2   cm H₂O pressure, or 20   mm Hg pressure. When intracranial pressure exceeds 27.2   cm H₂O (or 20   mm Hg), if the stopcock is set to allow drainage between the ventricle and the drip chamber, CSF will flow into the collection system.

If the system is set to allow continuous CSF drainage, the measured ICP will never exceed the pressure equivalent to the centimeters (of water) that the drip chamber is elevated above the ventricles. Some providers may request that some or all ICP measurements be obtained with the drainage briefly interrupted (i.e., the stopcock is temporarily open only between the transducer and the ventricle). Such measurements may be requested if there is any deterioration in the patient's clinical status. Once the isolated ICP measurement is obtained, the stopcock is again opened to the drip chamber as well as between the transducer and the catheter.

If intermittent drainage is preferred, the stopcock is typically closed between the ventricle and the drip chamber (i.e., “off” to drainage) for most periods, and the stopcock is only opened to allow drainage at times specified by the physician or on-call provider. If such intermittent drainage is provided, the ICP may continue to rise and CSF drainage won't occur unless the stopcock is turned to allow drainage from the ventricle into the drip chamber.

The external transducer is secured to a fixed reference point, as ordered by the physician or on-call provider. Usually the transducer is leveled at the lateral ventricle; the landmark used is typically the external auditory meatus.¹⁰² Some centers use the outer canthus of the eye and others use the midpoint of the imaginary line between outer canthus of the eye and the external auditory meatus. While any of these points is acceptable, it is essential that a single leveling point is established, and every practitioner must use the identical zero reference point. Use of the same zero reference point enables identification of changes over time.

Risks and Complications of Intraventricular Monitoring and Drainage

The need for a ventriculostomy is weighed against the potential complications or difficulties. Use of an intraventricular catheter creates the risk of infection.¹¹³ The system should always be closed and sterile technique must be used if it is necessary to enter the system. Opening the system to sample CSF or rezero and flush the system (not the ventricular catheter) can increase the risk of infection.¹¹²

Care of the insertion site varies with institution policy, but observation of the site, maintenance of the integrity of the dressing, and immediate reporting of redness or drainage are routine nursing responsibilities. If a catheter infection is suspected, the catheter is removed and appropriate antibiotic therapy initiated. If ICP monitoring is still required, another catheter may be placed in the opposite lateral ventricle. There are commercially available EVD catheters with antibiotic-coated tips that may reduce infection rates, but their routine use in clinical practice requires more research.²⁴

When the catheter is passed through brain tissue, there is a risk of interrupting blood vessels. Although the reported incidence is low, resulting hemorrhage may be considered clinically insignificant, and may not be reported in the literature.¹¹³ In one pediatric study, hemorrhagic complications were detected more often with the use of external ventricular drainage systems as compared with fiberoptic monitoring.⁸ In an effort to prevent hemorrhage, coagulopathies should also be corrected before the placement of any device.¹¹³

Blood or brain tissue can obstruct the tip of the catheter any time during catheter use. Obstruction also may occur from compression of the ventricles by cerebral swelling. Additionally, the catheter can migrate or be misplaced or become displaced during use. The external tubing can also become kinked or can remain inadvertently clamped. If CSF drainage decreases or is lower than expected, obstruction or misplacement should be suspected. The pressure tracing may also become dampened or eliminated. The cause of obstruction and location of the catheter tip can be determined by means of a computed tomography (CT) scan.

The flow of CSF is caused by a pressure gradient between the ventricles and the drip chamber. The flow chamber should not be placed below the zero reference point, because that could cause excessive drainage with resulting collapse of the ventricles. Ventricular collapse can result in lateral or upward brain herniation or tearing of bridging veins as the brain tissue is pulled from the dura. Subdural or subarachnoid hemorrhage can result.

During periods of high volume CSF drainage, the child may develop hyponatremia because CSF sodium is unavailable for reabsorption and is lost in the external drain. It may be necessary to replace the CSF drainage with an equivalent volume of intravenous saline solution to maintain euvolemia and adequate serum sodium concentration.

If the system drip chamber is placed too high in relation to the zero reference point (e.g., as a result of patient movement or inadvertent movement of the drainage point), it will create too much resistance to drainage. As a result, CSF drainage will be inhibited until the ICP is high enough to overcome the resistance created by the fluid column between the ventricle and the drip chamber. This increase in ICP could be detrimental.

Maintenance and Nursing Considerations

The nurse must monitor the patient and the system closely to detect clinical changes, ensure accuracy of ICP measurements, and prevent complications. Whenever the patient deteriorates clinically, the nurse should assess the patient, check the monitoring and drainage system, and notify a physician or on-call provider immediately. If the ICP measurement or drainage changes acutely, the nurse should assess the patient first and then check the system.

Description

Both the fiberoptic and the microchip ICP transducer can be used to monitor ICP. Either device may be placed in the lateral ventricles, the brain parenchyma, or the subdural, subarachnoid, or epidural space.

The fiberoptic monitoring device transmits light along a fiber (the optical fiber, or fiberoptic, contained in the catheter) to the tip of the catheter and transmits reflected light back to the base unit. A pressure transducer is located near the catheter tip. The mechanical diaphragm of the transducer moves with changes in pressure, and the movement will change the intensity of the light reflected off the diaphragm. The light refraction is then analyzed to derive a pressure reading that is displayed digitally on the ICP monitoring unit display (Fig. 21-36). The fiberoptic catheters must be zeroed before insertion; they are calibrated at the factory and cannot be recalibrated after insertion.

Fig. 21-36 Camino fiberoptic intracranial pressure-monitoring system. The fiberoptic catheter is joined to an amplifier-connector, and it transmits signals to the Camino module. The module displays digital ICP pressure measurement, but must be joined to a bedside monitor to display a waveform and provide audible high-pressure alarms. A, Camino monitoring system with intraparenchymal or intradural monitoring. Inset shows connection of catheter and monitor cable—at this connection, zeroing of the catheter is performed before insertion. B, Camino monitoring system for epidural monitoring. (Redrawn from illustrations of fiberoptic intracranial pressure-monitoring system by Integra Camino, San Diego, California.)

The microchip transducer has a pressure sensor at the tip. A change in pressure moves the position of this sensor and causes changes in electrical resistance; these changes are transformed and displayed as ICP measurements. The microchip transducers must be zeroed before insertion; they are calibrated at the factory and cannot be recalibrated after insertion.

Both fiberoptic and microchip ICP monitoring catheters can be coupled with a conventional bedside monitor for waveform display, paper recording, and generation of audible pressure alarms. The portable pressure monitoring unit must be calibrated at regular intervals with the bedside monitor digital display; note that this calibration is not the same as zeroing and calibration of the ICP transducer system (as discussed in the next paragraphs).

The advantages of the fiberoptic and microchip transducers over other systems are accuracy, versatility, small size, and minimal maintenance requirements. Because the transducers are located very near the tip of the catheter, leveling of the transducer to an anatomic reference point is unnecessary. As a result, valid pressure measurements are obtained even with frequent change in patient position.

This system is not dependent on transmission of pressure through a fluid-filled segment of tubing; artifact from air bubbles or particles does not occur. Zeroing is necessary only once before insertion and catheter calibration is performed at the factory. Catheters are also available to enable simultaneous ICP monitoring and CSF drainage (Fig. 21-37), as well as brain temperature monitoring.

Fig. 21-37 Camino intraventricular monitoring catheter with cerebrospinal fluid drainage system. The sheath of the intraventricular catheter provides a Y connection to a CSF drainage system. Drainage may be accomplished intermittently or continuously, with simultaneous pressure monitoring. The height of the drainage chamber above the ventricles will determine the ease of CSF drainage. (Redrawn from illustrations of intraventricular monitoring catheter with cerebrospinal fluid drainage system by Integra Camino, San Diego, California.)

Risks and Complications

The fiberoptic and microchip catheters require careful handling to avoid damage; the catheter must be replaced if it is damaged. Drifting of the zero reference point may be observed and can increase daily for as long as the catheter is in use.¹¹³ Manufacturer's recommendations should be followed regarding catheter replacement when monitoring is required for longer than 5   days.

Infection remains a potential problem with any ICP monitoring device. However, because there is no fluid incorporated in these devices, the incidence of infection may be lower than with fluid-filled monitoring systems. As with any invasive monitoring system, however, attention should be given to ensuring sterile technique during insertion, and aseptic technique during catheter and tubing care.

Complications associated with fiberoptic or microchip ICP monitoring are similar to those described previously for ICP monitoring and ventricular drainage. These include hemorrhage and obstruction, migration, and misplacement.

Maintenance and Nursing Considerations

These catheters are fragile. Care must be taken to avoid any tension on or compression of the catheter. All providers who care for patients with a fiberoptic or microchip ICP catheter must be extremely careful during handling of the patient and catheter. Damage to the light fibers of a fiberoptic catheter can occur readily. At regular intervals, the nurse should assess the security of the catheter at the insertion site.

The catheter must be inserted under sterile conditions and maintained with aseptic technique. The dressing around the insertion site must remain clean and dry. The risk of infection increases the longer a catheter remains in place.¹¹³

These catheters are zeroed before insertion; once the catheter is inserted, the zero is never adjusted. However, the calibration of the bedside monitor should be checked according to the manufacturer's recommendations, to ensure correlation between the ICP monitor and bedside monitor.

If the patient requires transfer to another location, ICP monitoring may be continued during transfer because the portable monitoring unit contains a rechargeable battery. If ICP monitoring is not performed during patient transfer and the catheter is disconnected, the ICP monitor will not lose calibration and the catheter transducer tip does not lose its zero reference point. However, once the ICP monitoring unit is reconnected to a bedside monitor, the bedside monitor must be recalibrated with the ICP monitor.

If a ventriculostomy drainage system is used, nursing documentation should include the characteristics and amount of drainage. The condition of the drainage system (open vs. clamped), and the appearance of the insertion site and dressing are also documented.

The ICP and CPP are documented at frequent, regular intervals along with changes in ICP associated with patient care activities, such as suctioning. The patient's tolerance of maintenance activities and any unexpected responses or deterioration are reported to a physician or other on-call provider.

Description

Another alternative method of ICP measurement is the use of a conventional fluid-filled transducer tubing system connected to the subarachnoid space with either a catheter or subarachnoid bolt or screw, respectively (Fig. 21-38). The transducer and tubing preparation is similar to that used for other fluid-filled monitoring systems; in these neurologic monitoring systems, however, a flush solution and continuous flow device are not used.

Fig. 21-38 Subarachnoid bolt. The bolt may be placed in the frontal area through a burr hole. To monitor pressure using a fluid-filled monitoring system, a short piece of (flushed) noncompliant tubing is used to join the bolt to a transducer. A fiberoptic catheter also can be placed for subarachnoid pressure monitoring.

A short, direct segment of tubing from the bolt (or catheter) is joined to a transducer. Care must be taken to prime the transducer system with preservative-free saline to eliminate all bubbles before joining the system to the bolt or catheter.

Advantages and Disadvantages

The advantages of using a subarachnoid bolt or screw include ease of insertion, minimal risk of brain tissue injury, and its improved accuracy in the presence of central edema, when the lateral ventricles may be compressed and difficult to catheterize.^10,114

A disadvantage to the bolt is that it can become occluded with blood clots, tissue, or debris, which can dampen the ICP waveform and cause erroneous results.^10,114 This system may not be accurate when the ICP is elevated (the very condition it is designed to monitor).¹⁸² As with other devices that enter the skull, infection, leak of CSF, and hemorrhage can occur.^10,102 Subarachnoid bolts or screws are difficult to place in infants because of the pliability of the infant skull.

Maintenance and Nursing Considerations

Nursing care and maintenance of the system are similar to other ICP monitoring devices and include evaluating and documenting the integrity of the system, and taking precautions to prevent infection.

Description

Brain tissue oxygen monitoring catheters continuously measure the partial pressure of oxygen in local brain tissue (PbtO₂); the PbtO₂, in turn, reflects the net balance of cerebral oxygen supply and cellular oxygen consumption.¹¹¹ This advanced technology has led to a better understanding of the impact that low oxygen states have on the injured brain.¹⁰⁵

The brain tissue oxygen monitoring system consists of a triple lumen probe that is inserted via a burr hole into the parenchyma of the brain. When detailed information on cerebral oxygenation in an injured area is desired, the device is placed on the affected side of the brain, in the penumbra area (ischemic but viable tissue surrounding an infarcted or injured area). The probe may also be placed in the nonaffected hemisphere for a reflection of more global cerebral oxygenation.

The tip of the flexible microcatheter contains a polarographic Clark-type electrode. Oxygen diffuses from the cerebral tissue through the polyethylene-coated catheter wall and into an inner electrolyte chamber where the partial pressure of oxygen is converted to a current between a cathode and an anode.^82,118 The amount of current generated is proportional to the PbtO₂ and is displayed in mm Hg.⁸²

After insertion, the probe is connected by an electrical cable to the monitoring system supplied by the manufacturer. Depending on the monitor selected, the screen displays PbtO₂, brain tissue temperature, and ICP.

Advantages, Disadvantages, and Contraindications

The advantage of using this system over conventional ICP monitoring is that it enables the practitioner to evaluate cerebral oxygen delivery and demand in the brain. This can allow the practitioner to identify levels of brain tissue oxygen that may be associated with secondary brain injury, identify and correct conditions associated with reduced cerebral tissue oxygenation, and plan treatment interventions more specifically aimed at improving the oxygenation of the brain.^99,188

As with any intracranial monitoring system, there is a risk of infection, hematoma, and displacement of the probe. After insertion of the monitor probe, it can take up to 2   hours for the PbtO₂ to equilibrate after the microtrauma of the insertion procedure.¹⁸¹ The practitioner must be aware of this delay in measurements when making clinical decisions based on these parameters. Technical care should be taken in handling these catheters because they can be inadvertently dislodged, misplaced, or broken.

Contraindications to using this system include uncorrected coagulopathy and an infection at the insertion site.¹⁸¹

Maintenance and Nursing Considerations

The insertion supplies for the brain oxygen monitor are similar to those needed for inserting a fiberoptic ICP monitor (see Box 21-3). Follow the manufacturer's directions for assisting with inserting the probe, and calibrating and setting up the monitoring system.

After setup is complete and the probe's external ends are attached to the brain tissue oxygen monitor, a cable can be used to connect the brain oxygen monitor with the patient's bedside monitor so that values are displayed and alarm parameters can be set.

After insertion, the nurse should evaluate the insertion site frequently and maintain the sterile dressing. The PbtO₂, brain temperature, and ICP values should be assessed and documented routinely as part of the patient's overall assessment, and when there is a change in clinical status or a clinical intervention. When the child must be transported, the probes are gently disconnected from the bedside monitor at the patient end using gentle pulling; the probe device should not be twisted.¹⁸¹

Description

Most of the cerebral venous circulation drains into the external jugular veins, where oxygen saturation can be monitored by spectrophotometry. Because the oxygen saturation in the jugular vein reflects the oxygen remaining after cerebral perfusion and oxygen extraction has occurred, trends in jugular venous oxygen saturation (SjO₂) reflect the balance of global cerebral oxygen delivery to consumption and can be used to evaluate the effectiveness of therapies.

The SjO₂ is used in conjunction with other monitored parameters such as ICP and CPP to evaluate and manage patients with cerebral ischemia. Continuous measurements are obtained via a fiberoptic catheter that is placed retrograde into the internal jugular vein. The catheter is threaded over an introducer and advanced until it sits in the jugular venous bulb.⁸⁹ Although either the right or left jugular vein can be used, the right internal jugular vein is usually cannulated because it drains a greater proportion of blood from the sagittal sinus.¹⁸⁰

The monitoring system uses wavelengths of light (two or three wavelengths, depending on the manufacturer). The light is emitted from the tip of the catheter and then reflected light is detected by a photosensor and transmitted to the base unit. A microprocessor determines the reflection of the different wavelengths of light and because oxygenated hemoglobin (hemoglobin saturated with oxygen) reflects light differently than desaturated hemoglobin, the relative reflection of light allows estimation of the relative percent of hemoglobin in the jugular vein that is saturated with oxygen. The percentage of the oxygenated hemoglobin in the total hemoglobin is expressed as SjO₂.²⁶

Advantages, Disadvantages, and Contraindications

A fall in the SjO₂ will identify cerebral ischemia more specifically than ICP monitoring or evaluation of the CPP. However, a decreased SjO₂ represents more global ischemia rather than a particular area of local injury.

One disadvantage of this monitoring system for pediatric patients is catheter size. Because the catheter may impede venous drainage or cause obstruction of the internal jugular vein, continuous SjO₂ monitoring is usually not attempted for children younger than 8   years of age or those who weigh less than 30   kg.¹⁸⁰

Potential complications include hematoma formation, venous thrombosis, infection, and the potential for catheter-associated bloodstream infection. The catheter can also become clogged with debris, resulting in false readings.

Continuous SjO₂ monitoring is contraindicated in the patient with an infection at the insertion site (in the neck), neck trauma, impaired venous drainage, or jugular venous occlusion or malformation. This monitoring may not be appropriate for the patient with coagulopathy, brain stem or cervical spine injury, or tracheostomy.¹⁶⁴

Maintenance and Nursing Considerations

Insertion supplies include sterile garb for those involved in the insertion procedure, skin antiseptic and local anesthetic, pressure transducer tubing, the catheter, and the oximetric monitor. Prepare the oximetric monitor according to the manufacturer's instructions.

After the catheter is placed, it is flushed, attached to a continuous flush solution, and attached to the monitoring system. Only flush solution should infuse into the catheter. An occlusive, sterile dressing is applied over the insertion site.

Follow the manufacturer's instructions for obtaining a jugular venous blood gas for calibration of and correlation with the oximetric monitor.⁸⁹ The nurse should aspirate the blood calibration specimens from the catheter at a rate of 1 to 2   mL/minute (i.e., very slowly).^114,164 This technique ensures accuracy of the measurements by avoiding contamination of the sample from extracranial blood that also drains into the jugular vein; such contamination could result in falsely high oxygen saturation in the blood calibration specimen.

The nurse should record or store the SjO₂ readings hourly and the insertion site, catheter integrity, and stability should be assessed hourly. Inaccurate measurements can result when the child's head or neck is rotated because the catheter can become improperly positioned within the jugular vein. Slightly repositioning the patient's head or the catheter may help the nurse determine if a desaturation reading is accurate or is a result of catheter misplacement.

Follow the manufacturer's recommendations for calibration of the device and recalibrate the device whenever there is concern about the accuracy of the readings. Accuracy can be affected by the hemoglobin concentration, systemic arterial oxygen saturation, core body temperature, and serum CO₂ levels.¹⁶⁴

Complications of continuous SjO₂ monitoring include potential infection, pneumothorax, nerve injury, thrombosis, and carotid artery puncture.^114,178 To reduce the risk of infection, strict sterile technique must be used during insertion and aseptic technique used when manipulating the catheter.

Description

Transcranial cerebral oximetry, also known as near-infrared spectroscopy (NIRS), is a technique used for observing changes in regional cerebral oxygenation (rSO₂). Sensors contained in adhesive electrodes are placed (noninvasively) on the patient's forehead and light waves of near-infrared wavelength penetrate the scalp, skull, and brain tissue to a depth of a few centimeters. The light waves are then absorbed differentially by oxygenated and deoxygenated hemoglobin, so analysis of light absorption can enable estimate of the cerebral tissue oxygen saturation.

The system measures light waves that are absorbed or scattered back to two electrode detectors. The device is able to identify the difference between light reflected from shallow tissue (i.e., the skull) and that reflected from tissue under the skull to identify the deeper tissue light reflection. The percent of the total hemoglobin that is oxygenated is calculated and displayed on the monitor as a numerical value.^26,169

Advantages, Disadvantages, and Contraindications

The NIRS monitoring system provides real-time continuous monitoring of tissue oxygenation. This oxygenation can be used to evaluate the effect of interventions on tissue oxygenation. The technology is not dependent on the patient's pulse, blood pressure, or temperature, so it can be used in a variety of clinical situations.

A disadvantage to this technology is that brain swelling can affect the accuracy and reliability of NIRS.²⁵ Additionally, the light waves only penetrate a few centimeters and cannot be used to determine rSO₂ measurements beyond that depth.

The accuracy of the measurements is affected by cerebral infarction, extraaxial hematomas, and extracranial blood.²⁶ This monitoring technology may not be used when bandages cover the forehead where the sensors must be placed.

Maintenance and Nursing Considerations

A separate transcranial cerebral oxygen monitor must be supplied by the manufacturer. It displays both the rSO₂ value and a graphic trend of the value over time. Small pediatric sensors are available and should be used for children less than 40   kg body weight. Because the skull of a child is much thinner than the skull of an adult, the algorithm for detecting the pediatric rSO₂ is different than the algorithm used in adults.^26,169

The nurse should follow the manufacturer's instructions for applying the disposable sensors to the frontal temporal area of the forehead (to the left and right of midline) and attaching them to the NIRS monitor. The sensors are not placed on the scalp.

The nurse should note the baseline rSO₂ value. Rather than comparing a given result with an absolute range of normal values, the continuous rSO₂ readings are most useful for evaluating trends and changes from the patient's baseline.²⁶ The rSO₂ is not a stand-alone value, and the nurse should incorporate the reading with other data collected from the patient's neurologic assessment and other monitoring devices.¹⁶⁹

Description

The measurement of cerebral blood flow is important whenever the patient at risk for cerebral ischemia and infarction, particularly risk of cerebral vasospasm and vessel occlusion. Transcranial Doppler ultrasonography (TCD) is a noninvasive method for evaluating cerebral blood flow velocity through the cranium.

A 2-MHz ultrasound probe is held to a thin area of the skull, usually above the zygomatic arch, and velocity signals are displayed as pulsatile waveforms that can be recorded.¹⁸⁰ The measurement is performed by a trained technologist and then interpreted by a radiologist.

Advantages and Disadvantages

TCD is portable and does not require patient transport. The measurement technique is usually performed intermittently, because extended monitoring is hampered by difficulty in maintaining the ultrasound probe in position over a long period of time.⁹¹ Patient movement can affect the accuracy of the study results.

Maintenance and Nursing Considerations

The nurse should anticipate that the measurement of cerebral blood flow via TCD will be performed intermittently, usually daily. It will be combined with other technologic monitoring for determining the patient's overall cerebral oxygenation and neurologic status.¹⁸⁴

Description

The electroencephalogram (EEG) is used most frequently to monitor critically ill patients with intracranial hypertension or refractory status epilepticus, or those at risk for secondary ischemia after brain injury. Electrodes are placed on the scalp and head to detect neuronal electrical impulses; these impulses are recorded and the graphic recording is the EEG. The EEG waveform reflects the physiologic state of the cerebral tissue and is an indication of the cerebral metabolic state.¹¹⁴

Advantages and Disadvantages

Continuous cerebral EEG monitoring (cEEG) is useful for unconscious patients, those in status epilepticus, and those in a coma, including barbiturate-induced coma; it is also useful for evaluation of the patient receiving neuromuscular blockade. Many critically ill patients may require neuromuscular blockade for several reasons, such as ventilator management; therefore the cEEG can offer more sensitive detection of localized seizure activity in these patients and allows prompt treatment of status epilepticus.⁹⁶ Continuous EEG can be combined with other multimodality brain monitoring, allowing the clinician to identify when the brain is at risk for injury and intervene before there is permanent damage.⁵⁷

The EEG electrodes must be placed by a trained technologist, who must be available to reapply any electrodes that loosen during patient transport, movement, or care. Electrical interference from equipment in the critical care environment and patient movement can interfere with the EEG recording.

Maintenance and Nursing Considerations

The nurse will prepare the patient and assist with the placement of EEG electrodes, as needed. Electrodes should not be placed over areas of skin breakdown. Before electrode placement, the skin should be prepared with soap and water or alcohol. Because a loose or misplaced electrode will impact the EEG waveform, the nurse should be vigilant in assessing electrode placement and wires while performing patient care.

Description

The bispectral index (BIS) is a processed EEG parameter that is used to monitor the patient's response to sedatives and anesthetic agents.¹³⁵ Data are obtained from a single electrode strip that is placed across the patient's forehead. Brain activity in this localized area is reflected in the BIS value, a derived parameter reported on a 0 to 100 linear scale.¹¹ A value of zero indicates no brain activity, whereas an awake and interactive patient will have a BIS value near 100.

Advantages and Disadvantages

When it is difficult to assess the child's neurologic status or level of sedation, the BIS, in conjunction with other neurologic monitoring technologies, can provide useful information. This technology is beneficial for the patient who is placed in a drug-induced (e.g., pentobarbital) coma for ICP management.

BIS data can be tracked over time and can provide real-time feedback about the patient's response to interventions. Muscle activity from the patient's face and forehead can cause signal artifact and interfere with the BIS data reliability. The BIS should not be considered a stand-alone monitoring technology, but should be used in combination with other monitoring strategies to augment the clinical assessment.

Maintenance and Nursing Considerations

A separate BIS monitor must be supplied by the manufacturer. It displays both the BIS value and a graphic trend of the value over time. Follow the manufacturer's instructions for applying the disposable sensor to the patient's forehead. The nurse should monitor clinical signs of neurologic function (e.g., the Glasgow Coma Scale Score, Table 11-6 or inside back cover) and record the BIS value on a regular basis and in response to therapeutic interventions, including titration of sedatives and administration of neuromuscular blocking agents.

Procedure

The speculum probe is positioned in the ear canal by pulling the ear lobe down and back for children less than 3   years of age, and up and back for older children.¹⁷⁹ The probe should fit snugly but not be painful. To initiate measurement, a scan button is depressed.

The sensor receives emitted infrared energy that is fed through an analog to a digital converter. The resultant temperature is displayed within seconds and can be displayed in either Celsius or Fahrenheit degrees. After use the disposable speculum probe cover is removed.

Advantages and Disadvantages

This method of temperature assessment is convenient and rapid. Additionally, there is relatively low potential for cross-contamination because the probe covers are disposable. Young children may be extremely frightened of an instrument that enters any orifice, including the ear.

The presence of otitis media may result in inaccurate readings. Tympanic temperature measurements can also be affected by the user's positioning of the device during measurement and the size of the child's ear canal.⁴⁰

Description

Using infrared technology, the temporal artery thermometer captures the heat energy from the skin overlying the temporal artery, just below the skin on the forehead. A hand-held infrared scanner is used and provides an estimate of core body temperature using two sensors and an algorithm that incorporates a factor compensating for measured ambient temperature.⁹⁰

Procedure for Use

To obtain a temporal artery temperature reading, the scanner is placed on the skin at the center of the patient's forehead. Begin the measurement by pressing and holding the scan button. Sweep the sensor located at the head of the thermometer horizontally across the forehead to the hairline. While still holding down the scan button, remove the sensor briefly from the skin and place it behind the earlobe to touch the skin overlying the mastoid process. Release the scan button for the digital temperature measurement to display. The skin tap is used to control for evaporative cooling of the forehead that occurs if the patient is sweating. If the temperature at the mastoid is greater than the measurement over the temporal artery, the temperature at the mastoid is displayed.

Advantages and Disadvantages

Temporal artery temperature measurement is rapid and noninvasive. It can be performed quickly and may not disturb the child. The instrument is easy to clean and relatively inexpensive. However, the accuracy of this method of temperature measurement has been debated and the clinician should select the method most appropriate for each patient care situation.^90,133

Description

Warming blankets use either forced air or convection to warm the patient. Forced air efficiently transfers heat to the patient by circulating hot air through a “blanket” that covers the patient.

Circulating warm water mattresses raise the patient's temperature and reduce conductive heat losses. The mattress is placed under the patient and warmed water circulates through the mattress.

A water mattress can also be used as cooling device: cool water is circulated through the mattress and conductive cooling lowers the patient's body temperature. Various manufacturers supply the heating or cooling unit and disposable blankets or mattresses.

Advantages and Disadvantages

Devices vary by manufacturer and they heat or cool based on a preset target body temperature. Many include a patient temperature probe and a servo-controlled mechanism to maintain a selected patient temperature.

There is an increased risk of burns when using a circulating water mattress with the child who is cold and peripherally vasoconstricted. The vasoconstricted child has decreased surface capillaries to dissipate heat.

Cooling blankets are not always well tolerated by patients and the blankets can cause thermal injuries to the skin. External cooling can result in reflex shivering and vasoconstriction as the body attempts to generate heat and counteract the cooling process. Shivering increases the patient's temperature, causes discomfort, and increases metabolic demands.

Maintenance and Nursing Considerations

The manufacturer's instructions should be followed for the setup, application, and safety management of any patient heating or cooling device. The nurse should monitor the patient's temperature and response to therapy, and assess the effectiveness of rewarming or cooling interventions. If substantial heat output of a device is necessary to maintain the child's skin temperature, the child's cardiac output may be inadequate.

Description

The closed infant warmer is a useful bed for infants who require maintenance of a controlled thermal environment. Air in the closed incubator is warmed and recirculated.

Temperature in the closed infant warmer can be set manually or by a servo control that regulates the thermal environment in the incubator in response to the infant's skin temperature. The servo control mechanism operates by presetting a desired skin temperature; the heating element within the incubator adjusts the environmental temperature automatically. The use of a servo control device should prevent wide fluctuations in environmental temperature and will conserve the infant's energy.

Advantages and Disadvantages

In addition to providing heat, the closed infant warmer minimizes convective heat loss. Ambient humidity of about 40% is maintained in the incubator without the use of additional water. This is an advantage because the water can be a site for bacterial growth.

Access to the infant for general care occurs through portholes, which limit access to the infant in a critical situation. Heat loss can occur when the incubator is opened and can result from radiant heat transfer if the walls of the incubator are subjected to a cool environmental temperature (e.g., near an air conditioning vent or in an area with drafts, near windows, or near outside walls).

Nursing Considerations

When operating the closed warmer in servo control, place the skin temperature probe on the infant's abdomen away from bony areas.¹¹⁹ The infant should not lie on top of the probe.

The portholes of the isolette should be used to gain access to the patient; the door of the incubator should not be opened for routine care. If the infant must be exposed for procedures, a portable radiant warmer should be placed over the infant. Because the incubator impedes access to the infant, critically ill patients with numerous invasive monitoring lines may benefit from being placed in an open bed with a radiant warmer.

Description

When infants and other critically ill patients require close monitoring, quick access, and temperature control, the use of open radiant warmers with servo control provide the most effective method of temperature regulation. The radiant warmer consists of electrically heated elements placed over the patient's bed to emit radiant heat above the patient. The heating elements can be obtained as part of a system with an infant bed or as a separate unit that can be placed over a bed.

Skin probes can be used to enable servo control heat regulation. Manual mode can be used to control the heat output, but the patient's temperature must be monitored closely to avoid wide temperature swings.

Advantages and Disadvantages

Quick access to the patient is facilitated by the use of over bed warmers. Unobstructed visibility allows continuous observation of the patient and of all equipment surrounding and attached to the patient. The servo control device usually includes alarms for high and low temperature as well as for indication of continuous or prolonged heating.

Disadvantages to the open warmer include heat loss by convection if the room is drafty and increased insensible water loss that may be as high as twice the normal rate. Assessment of hydration must be performed with even greater vigilance than usual when an overbed warmer is used, and fluid intake adjusted as appropriate.

Maintenance and Nursing Considerations

Whenever radiant warmers are used, the nurse must monitor the patient's core and skin temperature. The servo mode is safer for continuous thermal support and the manual mode should only be used for short-term warming.⁴⁹ The skin temperature probe should be placed on the infant's abdomen away from bony areas¹¹⁹ and the infant should not lie on top of the probe.⁴⁹ A rectal temperature probe should not be used for warmer control because the skin can be burned before normal core temperature is reached.