Work of Breathing Under Anesthesia

Understanding the relevant aspects of pediatric respiratory physiology provides a rationale for the recommendations presented later in this chapter. Several factors may increase the work of breathing under anesthesia (Table 17-1). These factors can be broadly divided into two groups: first is the increased work of breathing that may be imposed by the anesthesia breathing system, including inspiratory and expiratory resistance of breathing circuits, circuit dead space, and rebreathing of carbon dioxide. Second are changes in the mechanics and in the control of breathing induced by general anesthesia. These include decreased total respiratory compliance, decreased lung volume, airway closure, and upper airway obstruction. Thus, during general anesthesia, many factors conspire to increase the demands placed on the respiratory system while simultaneously decreasing the patient’s ability to cope with these demands.

Table 17-2 compares some selected infant and adult respiratory physiologic values. In general, neonates and infants have less respiratory reserve than adults and may demonstrate an unpredictable response to any additional work of breathing through a circuit under general anesthesia. Patients breathing spontaneously with the Mapleson D, E, and F circuits will rebreathe alveolar gas if the fresh gas flow is less than 2.5 to 3 times the predicted minute ventilation.³ Under normal circumstances, most of these patients will increase their minute ventilation in response and thereby prevent significant hypercarbia. However, even some anesthetized adults are unable to respond to increases in inspired carbon dioxide by hyperventilation and thus become significantly hypercarbic.⁴

Olsson and Lindhal examined the response of spontaneously breathing anesthetized infants⁵ (nitrous oxide and 1% to 1.5% halothane) to an increase in inspired carbon dioxide. Patients younger than 6 months did not significantly increase their minute ventilation in response to the added carbon dioxide, whereas patients older than 6 months showed a markedly attenuated response to increased inspired carbon dioxide. When the halothane concentration was reduced to 0.8%, the ventilatory response to carbon dioxide returned in patients younger than 6 months; thus, inhalational anesthesia attenuates the ventilatory response to carbon dioxide in small infants.

General anesthesia also adversely affects infants’ ventilatory response to increased respiratory work in the forms of tubular resistance, dead space, and valvular resistance. For a given increase in apparatus dead space, the infant must increase minute ventilation to a proportionately greater degree to maintain normocarbia when compared with the older child.⁶ The neonate is prone to respiratory compromise in the face of prolonged increases in respiratory work. The newborn’s diaphragmatic muscle is immature, having a smaller proportion of fatigue-resistant high-oxidative muscle fibers.⁷ In addition, the infant diaphragm is less able to generate a maximal inspiratory force compared with the adult diaphragm.⁸

Face Masks

The face mask must be able to provide a good seal to the child’s face to facilitate effective positive-pressure ventilation, avoid entrainment of room air, and reduce operating room (OR) pollution. Other desirable face mask features include low dead space, increased patient comfort, and construction with transparent material that allows observation of patient color and the presence of secretions or vomitus (Fig. 17-1); in addition, transparent masks are less threatening to the patient.⁹ The ability to ventilate the patient by mask is of the utmost importance, but low dead space is also desirable to avoid retention of carbon dioxide and increased respiratory work. However, in practice, the functional dead space of most face masks is significantly less than the measured dead space because streaming of gas eliminates significant rebreathing.^6,9

Maintaining an airway with a face mask occasionally can be challenging in children because the anesthesiologist’s fingers can cause external compression of the compliant upper airway. Care should be taken to ensure that fingers do not stray from the mandible onto the soft tissues surrounding the airway. Mask anesthesia with spontaneous ventilation should probably not be used for prolonged procedures in small infants because infants are unable to tolerate the increased work of breathing and may respond with apnea or hypoventilation. Inhalational induction of anesthesia may be facilitated by the application of an odorant (e.g., lip gloss) to the inside of the face mask to disguise the smell of the potent volatile agents and the unpleasant plastic odor of the mask and breathing circuit.

Tracheal Tubes

Selection of the correct tracheal tube type and size, along with its accurate and secure positioning, is vitally important.

Education and the Supraglottic Airway

Airway management during the administration of anesthesia requires precise positioning of the supraglottic airway to maximize ventilatory exchange and minimize the risk of hypoventilation, laryngospasm, and mechanical distortion of the upper airway. Instruction of supraglottic airway use to anesthesiology trainees must be methodical and controlled and must permit adequate experience for the trainee to understand the indications and contraindications for use.²⁵ Fiberoptic examination of the supraglottic airway after insertion is invaluable for anyone learning to see position variation and how malposition may adversely affect airway function. Experience with different types of supraglottic airways is important for trainees because different institutions use different types.

Insertion Technique

The original insertion technique for LMA insertion has been modified by many users during the past 30 years. Ideal insertion of a supraglottic airway places the airway channel immediately above the glottic inlet, with the distal end resting in the upper esophageal sphincter and the upper border of the device bowl below the hyoid bone. The airway should slide along the hard and soft palates, between the tonsillar pillars, and into the hypopharynx. Insertion techniques for supraglottic airways with cuffs have been described with the cuff completely deflated, partially inflated, and completely inflated (Fig. 17-4). The potential problem with insertion with the cuff inflated is that the epiglottis will fold downward, and the device will be high in the hypopharynx, thereby increasing pressure on the base of the tongue and the lateral oropharyngeal wall. Insertion of a supraglottic airway upside-down and partially inflated, with rotation of the device after it enters the pharynx, has been advocated by many authors.^26,27 Large inflation volumes may cause a greater incidence of sore throat and increase the risk of excessive pressure on the pharyngeal mucosa.

An adequate depth of anesthesia must be attained prior to supraglottic airway insertion. Insertion of the device before attenuation of upper airway reflexes may provoke emesis or regurgitation. Satisfactory anesthesia can be achieved with both intravenous (IV) and inhaled agents.^28,29

Laryngeal Mask Airway

Since its introduction into clinical practice in the late 1980s, several models of the LMA have been developed (Box 17-2). In addition to the LMA Classic, single-use LMAs (LMA Unique), flexible LMAs, intubating LMAs (Fastrach), the ProSeal LMA, and Supreme LMA are currently available. The Fastrach LMA is noted for its ease of insertion and for the fact that it can serve as both a conduit for ventilation and tracheal intubation. The intubating LMA is available only in sizes 3, 4, and 5. All other models are available in smaller sizes. The LMA C Trach is an advanced model of the LMA Fastrach intubating LMA that has imaging capability of the larynx after insertion of the LMA. Although the C Trach has been most frequently used in adults, successful pediatric experience has been reported for children aged 9 to 17 years.³¹ The ProSeal LMA and the LMA Supreme are perhaps the most advanced of the LMAs and consist of a more anatomically designed bowl that results in a higher leak pressure without excessive pressure on the hypopharynx. A gastric conduit is built into the LMA Supreme and the ProSeal LMA and permits gastric decompression.

The medical literature regarding the LMA is extensive and documents its utility in pediatric patients with normal and difficult airways.

air-Q

The air-Q intubating laryngeal airway (Mercury Medical, Clearwater, FL) is a supraglottic airway that can be used by itself or as a conduit for tracheal intubation. It is available in sizes to fit a wide range of patients from neonates to large adults. One recent version, the ILA-SP, is self-pressurized. A study of 352 patients, aged newborn to 18 years, reported a mean leak pressure of 20.4 cm H₂O, complications in 14 patients, and no cases of regurgitation or aspiration. Successful placement on the first attempt was performed in 95% of the patients.³² Tracheal intubation via the air-Q can be performed efficiently and effectively with guidance from a flexible fiberscope.³³

Ambu Airway

There are five models of the Ambu laryngeal airway: the AuraFlex, AuraStraight, AuraOnce, Aura-I, and Aura40 (Ambu Inc., Ballerup, Denmark). The AuraFlex is available in sizes 2 through 6 with a small half size (2.5). All other models are available in sizes 1 through 6 with two small half sizes (1.5 and 2.5). The Aura40 is a curved reusable supraglottic airway, and the AuraOnce is a curved single-use supraglottic airway. Performance of the Ambu laryngeal airway compares favorably with other supraglottic airways, although few published studies have been done in pediatric patients.³⁴

I-Gel

The I-Gel (Intersurgical, Liverpool, NY) is a supraglottic airway with a laryngeal cuff composed of a thermoplastic elastomer that produces a perilaryngal seal without an inflatable cuff. The purpose of this design is to provide an adequate laryngeal seal while minimizing the risk of pressure trauma to the hypoharyngeal tissue. A bite block and gastric drain port are also built into the device to permit decompression of the stomach. Reported experience with the I-Gel in children is limited but indicates a high insertion success rate and a mean leak pressure of 25 cm H₂O.³⁵ There are 7 sizes of the I-Gel that cover patients weighing 2 kg to more than 90 kg.

Fundamental Requirements of the Pediatric Breathing System

The pediatric breathing system should have a low resistance and low dead space, be efficient for use with both spontaneous and controlled ventilation, and be easy to humidify and scavenge (Box 17-3). If the circuit contains valves, they should offer minimal resistance to gas flow and must be reliable. In addition, the circuit should have a low compressible volume and be lightweight and compact. The circle system with carbon dioxide absorbent is commonly used for pediatric anesthesia, as is the Mapleson D and F systems for manual ventilation and in PACU settings.

Mapleson D, E, and F System Development

The Mapleson D, E, and F systems are direct descendants of the original T-piece system introduced in 1937 by Philip Ayre of Newcastle, England.³⁸ The original Ayre T-piece has been greatly modified over the years and is rarely used today. All have the common feature of containing no valves that direct gas flow toward or away from the patient. The most commonly used configurations are the Mapleson D and F systems (Figs. 17-5 and 17-6). The Mapleson F system is a modification that is generally attributed to Jackson Rees, who added a reservoir bag with an adjustable valve to the expiratory limb.³⁹ When used with the valve largely open, the movement of the reservoir bag allows monitoring of spontaneous ventilation. With the valve partially closed, manual compression of the bag allows controlled ventilation. The Mapleson D system has an expiratory valve located at the end of the expiratory limb and functions in a manner similar to the Mapleson F circuit. The volume of the expiratory limb should exceed the patient’s tidal volume. The Mapleson D and F circuits can be used with mechanical ventilation by removing the reservoir bag and connecting a ventilator.

The absence of valves with the Mapleson systems can result in rebreathing of exhaled gases in a setting of decreased fresh gas flow (FGF). The use of capnometry to determine if there is rebreathing (PiCO₂ >0) allows titration of FGF to eliminate rebreathing if desired. In general, an FGF of not less than 2.5 and up to 3 times the minute ventilation will usually eliminate rebreathing (Box 17-4).⁴⁰ Neonates generally require an FGF of 3 L/min. FGF may need to be increased in the presence of a large tracheal tube leak. Significant FGF will be required in larger children and adults to prevent rebreathing, where peak inspiratory flows may exceed 20 L/min. FGF of this magnitude is both difficult to humidify and expensive. In general, a circle system is more efficient in terms of FGF requirements during anesthesia, particularly in older children and adults. A valved self-inflating breathing bag is appropriate for manual ventilation of larger children and adults when inhaled anesthetic delivery is not being used, particularly during patient transport.

The Bain circuit, described by Bain and Spoerel in 1972, is functionally identical to the Mapleson D system from which it is derived.⁴¹ In the Bain circuit, the FGF is carried in a smaller inner hose, contained within the expiratory limb, attached to the machine using a special adapter (Fig. 17-7). The outer expiratory limb ID is 22 mm and is 7 mm for the inner fresh gas tubing. FGF requirements for the Bain system are similar to those recommended for the Mapleson D and F systems.

Before using the Bain circuit, it is important to verify that the fresh gas inner tube is not fractured or disconnected. In this situation, fresh gas will enter the expiratory limb near the reservoir bag, and a huge dead space will result.⁴² Pethick’s maneuver has been recommended to test the integrity of the circuit.⁴³ First, the patient end of the circuit is occluded and the reservoir bag is filled. The patient end is then opened, and a high flow of oxygen is passed through the circuit using the oxygen flush valve. The high gas flow exiting from the patient end of an intact fresh gas hose will cause a reduction in pressure in the expiratory limb because of the Venturi effect, collapsing the reservoir bag. If the inner tube is disrupted, oxygen under pressure will enter the expiratory limb and distend the reservoir bag. This technique has been criticized because small leaks may go undetected, and the test itself may cause circuit disruption. Similarly, if the inner tube is intact but does not extend completely to the end of the expiratory limb, significant dead space will exist that may not be detected. An adapter has been described that tests the integrity of the inner and outer tubing separately (Fig. 17-8).⁴⁴ A similar malfunction of a breathing system caused by disconnection of the central coaxial tubing can occur with a coaxial circuit used with a circle system.⁴⁵

There has been substantial investigation of the functional characteristics of Mapleson circuits.⁴⁶ The near uniform use of capnography during anesthesia has greatly changed the clinical use of these systems and has largely supplanted previous work. Based on the capnogram, FGF can be adjusted as needed to ensure appropriate levels of rebreathing of exhaled gases.

Mapleson A (Magill) Circuit

The Mapleson A (Magill) circuit has the advantage of economy of FGF when used for the spontaneously breathing patient (Fig. 17-9). However, when used for controlled ventilation, this advantage is lost, and a Mapleson D or circle system is a better choice. For anything but the shortest duration of controlled ventilation, there are more appropriate alternatives to this system.

Circle Systems

The circle system is the most commonly used breathing circuit in the United States. Its main advantages are those of economy of anesthetic gas use, decreased environmental pollution, and conservation of heat and moisture. However, early experiences with the use of adult circle systems to anesthetize small children resulted in respiratory compromise in spontaneously breathing patients.⁴⁸ High resistances of the valves, tubing, and soda lime and a relatively large apparatus dead space were the factors that led to early recommendations that the circle system not be used for children younger than 6 years.⁴⁹ Since that time, developments in circuit design have led to the introduction of circle systems with smaller canister and tubing volumes, decreased compliance, and lower resistance valves and connectors. In contrast to the earlier experiences, recent clinical experience in pediatric patients has been extremely favorable. Compared with the Mapleson D or F circuit, the circle is more economical, offers better heat and moisture conservation, and is easier to scavenge.³ Although the resistance of circle systems has been found to be slightly higher than or similar to that of a Mapleson D or F system, it does not appear to significantly affect the minute ventilation or blood gas homeostasis of spontaneously breathing infants.⁵⁰ To add some clinical perspective, the resistance to breathing of a circle system is significantly less than that of a size 3 or 3.5 mm tracheal tube. Hence, circle circuits appear to be acceptable for short-term spontaneous ventilation for healthy older infants.

Factors that Affect Delivered Tidal Volume

Several factors can make the patient’s actual delivered tidal volume differ significantly from that delivered into the circuit by the ventilator. These include compressible volume, tracheal tube leak, and augmentation of tidal volume by FGF. The volume delivered to the patient may be less than or greater than that expected by the anesthesiologist, but the importance of these factors can be reduced or eliminated using available technology. The use of end-tidal capnography provides a continuous monitor of ventilation with or without technology to address compressible volume and augmentation of tidal volume.

Miscellaneous Aspects of Mechanical Ventilation

Practical Hints for Ventilating Neonates and Infants

Although the healthy newborn has a normal spontaneous respiratory rate of approximately 40 breaths/min, in practice, most healthy newborns who undergo anesthesia are adequately ventilated at somewhat lower respiratory rates, such as 20 to 30 breaths/min.⁶³

Some of the factors that determine the adequacy of mechanical ventilation of the neonate have been modeled mathematically.⁴⁸ Although these data were derived several decades ago and modern anesthesia systems have since evolved significantly, the authors’ conclusions remain pertinent. The anesthesia circuit, ventilator bellows (if present), and humidifiers all have significant compliance. During inspiration a portion of the total flow delivered by the ventilator (Qt) is expended in distending the circuit (Qc), thus the inspiratory flow to the patient (Qp) will be reduced accordingly (Qt = Qc + Qp; Fig. 17-10). This explains in part why very small infants can be ventilated safely with ventilators that have minimum inspiratory flows that greatly exceed those of a spontaneously breathing infant.

The disparity between Qp and Qt is greatest at the start of inspiration, when the circuit pressure is low, and the circuit is most compliant. During the latter portion of the inspiration (after 360 ms), as the circuit becomes pressurized and less compliant, Qp approximates Qt (Qp/Qt = 0.9). Thus, as inspiratory time is prolonged, there may be a disproportionate increase in tidal volume. Conversely, if the inspiratory time is short, Qp will be small in relation to Qt for most of the inspiratory period. To complicate matters, at rapid respiratory rates, shorter inspiratory times (500 ms) may be necessary to allow sufficient time for exhalation. Therefore, when rapid ventilatory rates are used with short inspiratory times, significantly higher inspiratory flows and pressures may be required to ventilate the patient effectively. At lower respiratory rates with longer inspiratory times, a proportionately larger increase in the duration of the later, more effective period of inspiration occurs, and lower inspiratory flows may be sufficient. This may explain the apparently paradoxic deterioration in gas exchange occasionally observed in small infants when they are ventilated at rapid respiratory rates and the inspiratory time is shortened.

High-Frequency Oscillatory Ventilation

Although high-frequency oscillatory ventilation (HFOV) is an established therapy in the neonatal and pediatric intensive care units (ICUs), the perioperative use of HFOV is less well described. As the complexity and number of procedures performed in sicker and smaller neonates increase, anesthesiologists will be even more exposed to patients who require this mode of ventilation. Perhaps the most common scenario is the neonate who is already receiving HFOV prior to patent ductus arteriosus ligation or exploratory laparotomy for necrotizing enterocolitis. However, anesthesiologists may occasionally encounter critically ill neonates who are receiving conventional ventilation, in whom HFOV represents a viable elective option for perioperative ventilation. Importantly, HFOV also represents an invaluable intraoperative rescue strategy that might be considered in certain circumstances.

The most important issues relating to the perioperative use of HFOV include the anesthesiologist’s relative unfamiliarity with this technology, the inability to monitor end-tidal carbon dioxide or to deliver volatile agents, and the inability to transport patients while delivering HFOV. Furthermore, it is possible that the neonate who requires HFOV is too sick to transport to the OR. Under these circumstances, the anesthesiologist will be working in the relatively unfamiliar environment of the neonatal intensive care unit (NICU).

On the other hand, although formal data are lacking, it is often the anesthesiologist’s experience when HFOV is used during thoracotomy for patent ductus arteriosus ligation that lung retraction appears to be relatively well tolerated, with less interference from ventilation with surgical exposure. Indeed, some anesthesiologists advocate the elective conversion to HFOV prior to this procedure.⁶⁹ The use of HFOV has been reported during congenital diaphragmatic hernia repair, necrotizing enterocolitis, excision of pulmonary bullae, and surgery for congenital cystic adenomatoid malformations.^70-72 The assistance of the neonatologist and the respiratory therapist can be helpful in adjusting HFOV settings and troubleshooting. A total IV anesthetic must be used, typically with an opioid-based technique. Because accurate end-tidal carbon dioxide measurement is not possible, transcutaneous or blood gas monitoring of carbon dioxide is needed. If the neonate requires transport prior to surgery and is already on HFOV, it is useful to test his or her ability to tolerate short periods of manual ventilation prior to leaving the NICU.

An in-depth discussion of HFOV weaning and adjustment strategies is beyond the scope of this chapter. However, the basic approach includes adjustment of the fraction of inspired oxygen (FiO₂) and mean airway pressure to optimize oxygenation and manipulation of amplitude, usually referred to as ΔP, and optimizing inspiratory time to achieve the desired carbon dioxide elimination. A commonly monitored parameter of ventilation is the presence of abdominothoracic oscillation. Optimum alveolar recruitment is achieved when lung expansion to 8.5 to 9 ribs is observed on routine chest radiograph.

Beneficial Effects of Humidification

Beneficial effects of humidification include the reduction of heat and water loss from the respiratory tract, the prevention of airway damage, and the prevention of inspissation of secretions that could cause tracheal tube obstruction.

Methods of Providing Humidification in Pediatric Systems

Several methods are available to reduce heat and water loss from the airway during anesthesia. Even the tracheal tube itself may act as a low-efficiency passive HME, as the moisture in exhaled gases condenses within the tube and is then added to the inspired gas mixture. In the absence of either active or passive humidification, measurements of humidity at the distal end of the tube demonstrate that the tracheal tube alone will produce an inspired relative humidity of 30% at its distal end.⁷⁸ However, this is less than the 50% relative humidity reported to be necessary to allow normal mucus flow.

Humidifiers in common use are either active, adding exogenous heat and water to the breathing system, or passive, conserving and allowing the rebreathing of the patient’s endogenous heat and moisture. Alternatively, the use of a circle system with low FGF will generate heat and humidity that can be inhaled by the patient. Each method of humidification has its specific advantages and disadvantages.

Gas Sampling Site

The site of gas sampling from the breathing circuit appears to be important in patients weighing less than 12 kg.⁸⁵ When gas is sampled from the distal end of the tracheal tube with a sampling catheter, the accuracy of measurement was considerably improved over measurements made at the tracheal tube circuit connector (proximal sampling). On the other hand, distal versus proximal sampling may not affect the accuracy of measurement when a nonrebreathing circuit and ventilator with no fresh gas blowby is used, such as a Servo 900C system (Siemens, Munich, Germany).⁸⁶ Gas need not be sampled from the distal tip of the tracheal tube but may be sampled with similar accuracy from the point of narrowing of the tracheal tube adapter (Fig. 17-13).⁸⁷

Breathing System

When the Mapleson D circuit is used, end-tidal values are often decreased compared with arterial values. The relatively high FGF required in these circuits dilutes the end-expired carbon dioxide, thereby artifactually lowering the end-tidal CO₂ reading. However, this “dilutional” effect is attenuated when certain ventilators are used, such as the Sechrist Infant Ventilator (Sechrist, Anaheim, CA).⁸⁸

Mainstream Versus Sidestream Analysis

When mainstream capnometers have been compared with sidestream analyzers in children, they were found to be as accurate as distally sampling sidestream analyzers and considerably more accurate than proximally sampling sidestream analyzers.⁸⁹ The sampling rate of sidestream capnometers affects their accuracy. High gas sampling rates (>250 mL/min) exaggerate the dilution effect by entraining fresh gas and diluting expired carbon dioxide. On the other hand, low sampling rates (<100 mL/min) can prolong the response time of the capnometer such that the ability to display a valid waveform at rapid respiratory rates is lost.⁹⁰ Badgwell and colleagues⁹¹ have examined the effect of ventilation at rapid respiratory rates on the capnographic waveform and demonstrated significant artifactual elevation of the sidestream baseline attributable in part to longitudinal mixing of gas within the sampling line. No artifactual elevation was observed in flow-through capnograph waveform baselines. This study utilized multiplexed mass-spectrometer gas analysis with a sampling line that was approximately 50 m long. The authors predict that artifactual elevation of the baseline waveform would be less when using stand-alone sidestream monitors with shorter sampling lines.

Patient Factors

In general, the accuracy of sidestream measurements is reduced when used with small infants and in the presence of lung disease. With many neonatal lung diseases, measurements of end-tidal carbon dioxide do not accurately reflect arterial carbon dioxide.⁹² Also, patients with cyanotic heart disease who have reduced pulmonary blood flow or mixing lesions also tend to have reduced end-tidal carbon dioxide values compared with arterial measurements.⁹³ Furthermore, for capnometry to be useful even as a monitor of trend in those patients, the arterial/end-tidal carbon dioxide difference must remain constant. Unfortunately, patients with cyanotic heart disease may have variations in shunt fraction, pulmonary blood flow, and dead space/tidal volume ratios during anesthesia and surgery. These variations will produce a variable arterial/end-tidal carbon dioxide difference, thereby reducing the utility of capnometry, even as a monitor of trend.

Although expired carbon dioxide monitoring may not always accurately reflect arterial carbon dioxide tension in children, it does appear to be very useful as a trend monitor in patients without cyanotic heart disease. Capnometry will continue to have a vital role in maintaining the safety of pediatric anesthesia.

Blood Pressure Determination

Periodic blood pressure determination is an essential part of monitoring during anesthesia,⁹⁴ and a variety of techniques and equipment are available to achieve this (see also Chapter 12). ^95,96 All noninvasive forms of blood pressure determination depend in part on a snugly fitting cuff that contains an air-filled bladder that can be inflated to controlled pressures. The American Heart Association has developed recommendations for indirect blood pressure determination, including those for cuff sizes.⁹⁵ The width of the inflatable bladder of the cuff should be 0.4 times the circumference of the extremity, and its length should be at least twice its width to cover at least 80% of the extremity circumference. Cuffs that are too narrow will result in an overestimation of the blood pressure, and cuffs that are too wide will underestimate the blood pressure.^97,98 Given the same degree of mismatch, cuffs that are somewhat too wide will result in less error than cuffs that are too narrow.⁹⁹ As a result, a series of blood pressure cuffs of various sizes must be available for the accurate, noninvasive determination of blood pressure in pediatric patients.

Electrocardiographic Monitoring

The electrocardiogram (ECG) is an excellent noninvasive monitor of heart rate and rhythm (see also Chapter 13). Apart from sinus bradycardia and tachycardia, rhythm changes are not common in pediatric patients. Premature atrial and ventricular contractions, junctional rhythms, and respiratory variations are seen on occasion. Rhythm changes in pediatric patients generally relate to some aspect of the ventilatory, metabolic, anesthetic, or surgical management rather than to a primary cardiac event. They can generally be identified using standard limb lead electrode positions, although special leads may be helpful in some instances.¹¹²

Although the ECG is a very useful monitor of heart rate and rhythm, it can continue to be normal even in the presence of moderate to severe circulatory or ventilatory abnormalities. Indeed, the ECG can appear normal, at least briefly, without systemic perfusion. As a result, additional monitors must be used to supplement the ECG to assess the adequacy of systemic perfusion.

Pediatric ECG monitoring requires equipment capable of processing higher frequency signals.¹¹³ At a minimum, the equipment must be able to identify individual heartbeats and provide accurate rate analysis. Normal values for QRS amplitudes can vary depending on the frequency of sampling in digitized systems because slower sampling frequencies may miss the peak QRS amplitudes.¹¹⁴

Although myocardial ischemia is a rare event during uncomplicated pediatric anesthesia, it can occur in some circumstances.¹¹⁵ Kawasaki disease is an acute exanthematous condition usually seen in infants and children younger than 5 years.¹¹⁶ The illness is associated with coronary artery aneurysms in 20% to 25% of patients. These aneurysms generally resolve over time, but the lesions may become stenotic. Cardiac ischemia is well recognized in these patients, and appropriate ECG monitoring is indicated perioperatively.¹¹⁷ Other pediatric clinical situations in which cardiac ischemia may occur include surgery involving the coronary arteries, congenital coronary artery abnormalities, and systemic air embolism.

Pulse Oximetry

Hypoxemia is a major concern during any anesthesia procedure. Its incidence is increased during anesthesia administered to children aged 2 years and younger.¹¹⁸ Pulse oximetry provides a reliable method to detect hypoxemia prior to the time it is detected clinically (see also Chapter 11).¹¹⁹ Although pulse oximetry values may change several seconds after the arterial saturation changes,¹²⁰ it is sufficiently rapid to detect changes in oxygen saturation as needed in clinical practice.

Pulse oximetry is applicable to all pediatric patients, including neonates.^121,122 The probes must allow the emitter and detector portions of the probe to face each other across some pulsatile bed, and careful attention to probe placement is essential. Oximetry during anesthesia induction and emergence can also be difficult, because the probes may displace or dislodge with the patient’s movements. Fetal hemoglobin (HbF) does not affect pulse oximetry but does have an oxyhemoglobin dissociation curve to the left of normal adult hemoglobin.¹²³ Common sources of inaccurate pulse oximetry values (SpO₂) are summarized in Table 17-5.

Unlike adult patients, hyperoxia is of concern in neonates with immature retinas because elevations in arterial oxygen tension are associated with the development of retinopathy of prematurity. The upper limit of desirable oxygenation varies from infant to infant depending on a variety of factors that include postconceptual age and the clinical stability of the infant. In infants with a patent ductus arteriosus, the pulse oximetry probe should be located at a site that indicates preductal saturation; this value will reflect the oxygenation state of blood supplying the brain and retina.¹²⁴ The range of oxygen saturation of concern in hyperoxia occupies the flat portion of the oxyhemoglobin dissociation curve, where large changes in oxygen tension occur with small changes in hemoglobin saturation. Typical safe limits for SpO₂ for the stable preterm infant are 85% to 93%.¹²³ Preterm infants born at 24 to 27 weeks’ gestation have improved survival when managed with SpO₂ of 91% to 95% compared with 85% to 89%.¹²⁵ A single best range has not been established.¹²³ Nonetheless, pulse oximetry is a valuable aid to the anesthesiologist caring for any patient, particularly the neonate, infant, and young child.

Tissue Oximetry

Tissue oximetry offers a noninvasive method to monitor areas of body perfusion.^126,127 It uses near-infrared spectroscopy (NIRS) to assess tissue oxygenation.¹²⁸ Both cerebral and somatic monitoring can provide the clinician with important information regarding patient condition,^126,129 and multiple products are available from different manufacturers for this purpose. Technologic approaches to better address the infant patient have been introduced or are in process from multiple manufacturers.¹²⁶ NIRS has been used as a routine monitor perioperative status of infants following stage 1 palliation of univentricular congenital heart disease.¹³⁰ The role of NIRS in optimizing patient management continues to evolve.^131-132

Temperature Monitoring and Maintenance

The normal human tightly regulates body temperature within a narrow range using defense mechanisms that prevent both hypothermia and hyperthermia.¹³³ These defense mechanisms are more effective in adults compared with children, children compared with infants, and less premature infants compared with more premature infants. General anesthesia broadens the range of body temperature that must be reached before homeostatic mechanisms become active.¹³⁴ Mild to moderate degrees of intraoperative hypothermia are common because of factors that include altered temperature homeostasis, cool environments, exposed skin surface, administration of cool fluids, increased evaporative losses, and redistribution of heat.¹³⁵ Mild to moderate degrees of hypothermia can adversely affect outcome measures such as shivering, infection, platelet function and bleeding, cardiac events, drug metabolism, and PACU discharge time.^136-138

The American Society of Anesthesiologists Monitoring Standards state that intraoperative temperature monitoring is indicated whenever a clinically significant change in body temperature is intended, anticipated, or suspected.⁹⁴ In combination with the limited ability of the infant and young child to maintain normothermia during anesthesia, this strongly suggests that body temperature should be measured for the majority of young infants and in most pediatric patients undergoing anesthesia.

The approaches generally used to maintain the body temperature of nonanesthetized young infants—clothing, blankets, and heated and humidified enclosures—often cannot be applied in the OR setting. Warmed ORs can be used, but the degree of warmth necessary to maintain body temperature generally makes working conditions uncomfortable, particularly when wearing surgical gowns. Radiant warmers and the use of heated, humidified anesthetic gases can reduce heat loss to a limited degree.

Fluid warmers are useful in maintaining body temperature, particularly during procedures in which large volumes of either room temperature or 4° C blood or blood products are administered.¹³⁹ However, at low fluid administration rates, warmed IV fluid tends to cool before reaching the patient. The Hotline fluid warming system (Smiths Medical, St. Paul, MN) uses a water-jacketed administration set able to deliver warm fluid to the end of the administration set at low and high flow rates more effectively than conventional dry warmers. This is advantageous with infants and young children, in whom the absolute rate of fluid administration is low, but the proportionate fluid administration rate to the patient is high.

In contrast to approaches of reducing heat loss, forced-air warmers can effectively add heat to patients. These warmers are manufactured with a variety of air dispersal products, allowing their adaptation for a variety of surgical procedures and patient sizes (Fig. 17-14).^140-142

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