Circuit

The VA ECMO circuit is composed of polyvinyl chloride tubing attached to venous and arterial cannulas (Fig. 7-1). The venous cannula is inserted into the internal jugular vein and advanced through the superior vena cava and right atrium to the tricuspid valve. The arterial cannula is placed into the right common carotid artery with the tip of the cannula advanced to the innominate artery. Postoperative cannulation immediately after cardiac surgery is often accomplished through the median sternotomy incision with direct cannulation of the right atrial appendage and the aorta. In adolescent or adult patients, the femoral artery and vein are often used for VA ECMO cannulation.

Fig. 7-1 Venoarterial extracorporeal membrane oxygenation circuit.

(From Biddle M, Gulanick M, Berra K: Interdisciplinary team in cardiac rehabilitation. In Moser DK, Riegel B, editors: Cardiac nursing: a companion to Braunwald’s heart disease. Philadelphia, 2008, Saunders.)

The bridge connects the arterial and venous lines and is located near the cannula. The bridge is routinely clamped during ECMO support with brief periods of unclamping to prevent clot formation caused by stagnation of blood in the bridge. If the patient must be separated from ECMO for mechanical complications or for trial periods off ECMO support, the open bridge isolates the patient from the circuit while allowing blood flow to continue through the circuit to prevent stagnation and circuit thrombosis.

Blood from the venous cannula drains passively into a small reservoir called the bladder. The ECMO pump draws blood from the bladder, which works like the right atrium. The function of this bladder is to prevent negative pressure from pulling the vessel wall into the cannula, so it reduces risk of damage to the vena cava. The bladder is connected to a servo regulator mechanism that reduces or stops the pump flow if venous return decreases below a minimum threshold.

The tubing from the bladder joins to the ECMO pump, which is either a roller pump or a centrifugal pump. For the past few decades, the roller pump has been used most commonly for ECMO. However, centrifugal pumps are becoming more popular, especially in cardiac ECMO programs and for extracorporeal cardiopulmonary resuscitation. The roller pump functions on the principles of compression and displacement; blood is displaced as rollers travel the length of the raceway (the segment of tubing contained within the pump head). A strong polymer tubing, called Tygon, is used in the raceway because it is resistant to creasing and erosion.

Blood leaving the pump enters the membrane oxygenator. The membrane consists of a thin silicon rubber sheath with a plastic screen spacer inside to create a semipermeable membrane separating the gas compartment from the compartment containing the patient’s blood. As the patient’s blood flows past one side of the membrane, oxygen diffuses from the gas side across the membrane into the blood, and carbon dioxide from the blood diffuses through the membrane into the gas compartment. The carbon dioxide is then removed, or “swept out” of the system, by the ventilating gas. This ventilating gas is also referred to as the sweep gas, and it is regulated by a flow meter from an oxygen blender. The amount of oxygen in the sweep gas and the available surface area for diffusion determines the amount of oxygen delivered to the patient’s blood as it flows through the oxygenator. The rate of carbon dioxide removal is also dependent on the amount of sweep gas flow and the surface area. The movement of both oxygen and carbon dioxide are dependent on the pressure gradients for the gases across the membrane.

As the blood moves through the ECMO circuit, heat is lost, so ECMO systems use a heat exchanger to keep the blood warm. The heat exchanger is located either between the oxygenator and the patient or integrated into the oxygenator. When the heat exchanger is placed between the oxygenator and the patient, it also serves as a bubble trap.

The blood returns to the patient from the heat exchanger through an arterial cannula inserted into the right common carotid artery. The tip of the cannula is just proximal to the junction of the brachiocephalic artery and the aorta. With this type of cannulation, ECMO becomes essentially a cardiopulmonary bypass system. The greater the ECMO pump flow, the greater the oxygen delivery.

Venoarterial ECMO Flow and Function

In VA ECMO, blood is drained from the right side of the heart and is returned to the arterial side of the circulation, so that it bypasses the heart. Highly oxygenated blood is returned to the arterial circuit, where it mixes with the blood that the native heart is ejecting into the arterial circulation. Ventilator support is minimal, so blood returning from the lungs is relatively desaturated. As ECMO flow increases, the relative percentage of highly oxygenated blood in the arterial circulation is increased. Therefore, the patient’s partial pressure of arterial oxygen (PaO₂) increases with increased ECMO flow. With inadequate ECMO flow, the relative percentage of desaturated blood is increased, so the PaO₂ is low.

Flow from the ECMO circuit delivered into the aorta is continuous and nonpulsatile. As a result, as ECMO flow increases the patient’s native (intrinsic) cardiac output decreases and the systemic arterial blood pressure waveform is dampened by the nonpulsatile blood flow into the aorta. At 100% ECMO flow, the arterial pressure waveform is flat with only an occasional pulse that may result when slow ventricular filling triggers occasional ventricular systole. Such complete bypass is not typically provided if the patient has some cardiac function.

Typically VA ECMO is run at 80% bypass, so approximately 80% of the patient’s total cardiac output is diverted to the ECMO circuit and then returned to the arterial circulation. At this level of flow, the blood pressure waveform is dampened but preserved, with a pulse pressure of approximately 10 to 15 mmHg. At 80% bypass, the resulting admixture of saturated blood (80% of normal cardiac output) and desaturated blood (from intrinsic cardiac activity) will create sufficient arterial oxygen tension (PaO₂), hemoglobin saturation and oxygen delivery. The PaO₂ can be expected to be normal or even high.

One method to assess adequacy of oxygen delivery is monitoring of the oxygen saturation of blood in the venous side of the circuit; this oxygen saturation is the effective mixed venous oxygen saturation (SvO₂). An SvO₂ of approximately 70% to 75% indicates that the ECMO flow is providing sufficient oxygen delivery. Because the normal range of cardiac output varies widely between and within pediatric age ranges, the typical ECMO flow rate varies widely. Approximations of flow are 100 for infants, 80 for children, and 50 mL/kg per minute for adults.

Hemodynamic Changes During Venoarterial ECMO

During VA ECMO, the patient’s PaO₂, SvO₂, and blood pressure will be affected by changes in the ECMO flow and the patient’s intrinsic (native) cardiac output and differences between the two.

ECMO Support with Single Ventricle Physiology

In patients with single ventricle physiology or shunt-dependent pulmonary blood flow, controversy exists about management of the shunt during MCS. There is debate regarding whether the shunt should be completely closed, partially closed, or left open during MCS. The challenge is to maintain balance between systemic and pulmonary circulation. If the shunt is completely closed during support, there is a risk of pulmonary infarction. If the shunt is open during ECMO, increased ECMO flow may be needed to maintain adequate pulmonary and systemic blood flow and systemic oxygen delivery.⁵³

There are limited data regarding the role of ECMO after stage 1 palliation (and its variants) for hypoplastic left heart syndrome. ECMO is a useful tool for treatment of potentially reversible conditions, such as acute shunt thrombosis and transient depressed ventricular function⁵³; ECMO may be used electively in the immediate postoperative period. However, the use of ECMO after the Norwood procedure remains controversial because the infant’s anatomy is complicated, the therapy is expensive, and survival rate is low.^18,51,53,57

Circuit

VV ECMO is an alternative to VA ECMO that may be preferred for treatment of respiratory failure. In VV ECMO, venous blood is drained, sent through the circuit for oxygenation and carbon dioxide removal (ventilation), and returned to the venous side of the circulation.

The VV ECMO circuit has the same major components and flow pattern as the VA ECMO circuit (described previously), but blood is infused back into the venous circulation by a second venous cannula or a double-lumen single venous cannula. In infants, a double-lumen cannula is placed in the right internal jugular vein and blood is withdrawn through this catheter and returned to the right atrium. In older patients, two or more venous cannulas are placed, often in the right internal jugular vein, the femoral vein, or both.

Venovenous ECMO Flow and Function

Because VV ECMO delivers highly oxygenated/saturated blood to the venous side of the heart, the right heart cannula must be carefully positioned and rotated, and native cardiac function must be adequate. The range of PaO₂ typical during VV ECMO is 50 to 70 mmHg, lower than during VA ECMO.

Because both the venous drainage and reinfusion cannula are in the venous system, some blood will be cycled through the ECMO circuit over and over again. This phenomenon is known as recirculation. As ECMO flow increases, so does the percentage of blood that will be recirculated. The greater the volume of blood that is recirculated, the lower the amount of blood flow that is sent forward through the tricuspid valve, into the lungs, left heart, and to the body, and the lower the efficiency of gas transfer between the oxygenator and the patient. The degree of recirculation is monitored by comparing the oxygen saturation of the venous drainage (SvO₂) with the arterial saturation (SaO₂). If the SvO₂ is higher than normal and the SaO₂ is low, the volume of recirculation is excessive and either the blood flow rate or cannula placement requires adjustment.

Hemodynamic Changes during Venovenous ECMO

Venous cannula position can impede forward flow, as can right or left atrial volume, and poor contractility. If forward flow is impeded, PaO₂, cardiac output and oxygen delivery fall.

If the PaO₂ is low or falls during VV ECMO, the cannula position should be checked and the physician should be notified to make any adjustments. In addition, therapy to improve cardiac output may include volume administration with packed red blood cells (to maximize oxygen-carrying capacity) and initiation of inotropic support. Occasionally, conversion from VV ECMO to VA ECMO is needed if hypoxia persists, particularly in the presence of hypotension and poor cardiac output despite maximal ventilator and inotropic support. VA ECMO is also considered if the child develops nonperfusing arrhythmias or cardiac arrest.

Although there is no difference in the rate of survival, or rates of intracranial hemorrhage and infarction, seizures and brain death in pediatric respiratory patients for VV versus VA ECMO, VV ECMO is associated with significantly more cannula problems, higher incidence of CPR on ECMO, and use of inotropic support. In addition, there is also a trend toward more renal failure and use of hemofiltration on VV ECMO.⁶²

Neonatal Respiratory Failure

Neonatal ECMO has generally been considered an invasive rescue therapy with an identified set of risks; it is reserved for patients with a high predicted mortality who fail to respond to optimal conventional therapies. In newborns, a predicted mortality rate of 80% or greater was historically the main indication for ECMO. Much work was done in the early years of ECMO therapy to establish criteria for 80% predicted mortality. The most commonly used variable is the OI. An OI greater than 35 on three or more postductal arterial blood gases, 30 to 60 minutes apart, is consistent with an 80% predicted mortality. In addition, a single OI of 60 is a significant predictor of mortality.

Some centers used the A-a oxygen difference (A-a DO₂), also known as the A-a O₂ gradient. An A-a difference or gradient of more than 600 for a period between 4 and 12 hours is an accepted indication for ECMO support. However, some centers simply report the use of ECMO in patients with severe hypoxemia (PaO₂ <50 mm Hg), severe acidosis (pH <7.25), and acute deterioration resulting in a PaO₂ of approximately 30 to 40 mm Hg; these are all considered indications for ECMO.

ECMO may be initiated for patients with 50% to 80% predicted mortality if ECMO offers a higher potential for survival than conventional therapy. Examples of such patients include those with meconium aspiration and congenital diaphragmatic hernia with an OI of 25 to 40. The inclusion criteria of a minimum gestational age greater than 34 weeks’ gestation and weight greater than 2 kg is somewhat arbitrary, but is based on the increased risk of intraventricular hemorrhage (IVH) and difficult intravenous access in premature neonates.

Contraindications for neonatal ECMO include the presence of irreversible and uncontrolled bleeding or coagulopathy, IVH greater than grade II, birth weight <2 kg, or gestational age <34 weeks, lethal chromosomal abnormality or congenital malformation incompatible with life, and duration of mechanical ventilation >14 days.⁴³

With more extensive use of therapies such as high-frequency ventilation, surfactant, and nitric oxide, there has been a marked decline in the need for neonatal ECMO.³⁸ If patients do not respond to these therapies, ECMO should be offered in a timely manner.

Pediatric Respiratory Failure

There is less consensus regarding indications for ECMO support for pediatric patients than for neonates with respiratory failure. Severe respiratory failure in children has many etiologies. A child is generally considered to be a candidate for ECMO if death is believed to be nearly certain despite maximal conventional therapy, and the lung disease is believed to be reversible and other organ systems are intact.²⁹ In children, the OI has not been shown to be as predictive of mortality as it is in neonates. Many centers use the criteria of OI >40 and rising, hypercarbia and pH <7.1, a PaO₂:FiO₂ ratio less than 100 and falling, ventilator support for less than 14 days, and no contraindications such as significant neurologic or hemorrhagic conditions, or failure of more than three organ systems.⁴³ With the use of lung protective strategies and adjunctive therapies such as high-frequency ventilation, surfactant, and nitric oxide, it has become harder to establish criteria for initiation of ECMO support. Contraindications for pediatric ECMO are similar to those in the neonatal period; these include diagnoses incompatible with life, intractable hemorrhage or coagulopathy, or severe central nervous system abnormality.

Cardiac Failure

Cardiac ECMO support has steadily increased over the past decade.²⁴ Although isolated left ventricular failure is relatively rare in children, right ventricular failure, pulmonary hypertension, and hypoxemia are often associated with circulatory collapse in children with congenital heart disease. The most common causes of circulatory failure in infants and children are cardiovascular surgery (postcardiotomy), end-stage cardiomyopathy, and acute myocarditis. The most common indications for ECMO in cardiac patients are severe hypoxia, severe pulmonary artery hypertension, cardiogenic shock, cardiac arrest, and failure to wean from cardiopulmonary bypass after surgical repair.

With ECMO support, postcardiotomy myocardial recovery should occur in approximately 72 hours to 7 days. If there are no signs of myocardial recovery during this time, a cardiac catheterization is needed to assess for residual structural defects. If none exist, the child is listed as a candidate for cardiac transplant. If residual structural defects are identified, surgical reintervention is scheduled.

Contraindications for the use of cardiac ECMO include incurable malignancy, advanced and presumed irreversible multisystem organ failure, extreme prematurity, and severe central nervous system abnormality or hemorrhage.⁵⁹ In addition, if a patient will not be a transplant candidate, the patient should be carefully evaluated before ECMO. During the past 10 years, many contraindications have been removed from the list or labeled as relative rather than absolute contraindications.

ECMO Circuit Emergencies

There are many types of ECMO circuit emergencies, so ECMO staff and the bedside nurse must be constantly vigilant to prevent a circuit problem and rapidly respond to and correct any circuit emergency. The following circuit components need to be rapidly assessed in an ECMO emergency:

The circuit check can direct the ECMO specialist’s immediate response to the emergency. If there is blood ejecting from the circuit and air being pumped, then immediate removal from ECMO is required before the problem can be repaired. The ECMO specialist must identify and fix the pump problem while the patient’s nurse and additional critical care unit (PCCU) staff support the patient off ECMO. Immediately clamp the venous line, open the bridge, and clamp the arterial line to remove the patient from the ECMO circuit. Because the patient is dependent on the ventilator, provide ventilation with 100% oxygen or shift the patient back to pre-ECMO ventilator settings. In addition, increased volume and inotropic support may be required, and full cardiopulmonary resuscitation may be needed.

If there is no squirting blood or air being pumped, the patient can be maintained on ECMO while the problem is corrected. ECMO circuit emergencies and problem troubleshooting are summarized in Table 7-2.

Table 7-2 Troubleshooting the ECMO Circuit

AC, Alternating current; ECMO, extracorporeal membrane oxygenation; UPS, universal power source.

Ventricular Assist Device Flow and Function

Most VAD pumps used in children are either displacement pumps (pulsatile or pneumatic devices) or rotary pumps (continuous flow devices). The pulsatile or pneumatic pumps mimic the natural contraction (pumping action) of the heart. Flow rates depend on preload and the size of the external pump. Average flows for an infant-sized pump (12 or 15 mL) are 0.5 to 1.3 L/min and for a child-sized pump (25 or 30 mL) are 1.3 to 3.3 L/min.⁴ The external pump size can be changed to accommodate the child’s growth and increased stroke volume.

The most common continuous flow devices are axial or centrifugal pumps. Both types have a central rotor containing permanent magnets. Controlled electric currents that run through coils in the pump housing apply forces to the magnets causing the rotors to turn. Axial flow rates vary depending on the size of the device implanted. The child-sized device has been reported to provide an average flow of 0.3 to 2.5 L/min.⁴ The pediatric centrifugal Bio-Medicus Bio-pump (Medtronic, Inc. Minneapolis, MN) has both a 50- and 80-mL pump head size to accommodate infants <10 kg and children >10 kg, respectively. The 50-mL pump can provide flow up to 1.5 L/min and the 80-mL pump can provide >2 L/min.²⁷

The VAD console and energy source varies depending on the VAD type. Most pneumatic consoles display both left- and right-sided heart support, the pump rate in beats per minute, systolic and diastolic or fill pressures, and vacuum drive pressures. The consoles also have backup units in case of malfunction. These backup units can be automatically or manually converted, depending on the device. In addition, the consoles can be operated by external electrical power as well as internal batteries. Battery life varies by device with an average of 1 to 2 hours. External backup pumping devices should be attached to the console in case of an emergency.

The console of the centrifugal pump displays the speed and blood flow rate, which are manually adjusted by the operator. Inlet and outlet pressure monitors can be used to guide pump speed and prevent tubing collapse. External electrical power and internal batteries operate the console if transport is needed.

All VADs are preload dependent—the amount of blood returning to the heart is the amount of blood pumped to the body. The VAD is sensitive to impedance to flow, so hypertension and mechanical obstruction must be corrected. Both the LVAD and RVAD allow blood to bypass the failing ventricle. This decompresses that ventricle, decreases myocardial work, and reduces oxygen demand while maintaining adequate systemic perfusion to sustain end-organ function. VAD support has been shown to improve myocardial contractility. It also reverses beta receptor downregulation (documented to occur with heart failure), restoring myocardial response to the inotropic effects of adrenergic stimulation.⁴⁸ VAD support can also normalize chamber geometry and reduce myocardial fibrosis, hypertrophy, and disruption in cytoskeletal proteins.^10,34

Continuous Flow Pumps

The continuous flow (nonpulsatile) VADs use either centrifugal or axial flow pumps. Each has advantages and disadvantages.

Centrifugal Pump

The centrifugal pump is external, requires cannulation via a thoracotomy or sternotomy, and can be used for single or biventricular support. The most common centrifugal pump is the Bio-Medicus Bio-Pump (Medtronic, Inc. Minneapolis, Minn). The Bio-Pump uses two magnetically coupled, polycarbonate rotator cones that spin to create centrifugal force along a vertical axis.¹¹ The rotors are shaped to accelerate the blood circumferentially and thus cause it to move toward the outer rim of the pump. The constrained vortex pump design creates subatmospheric pressure at the tip of the cone, establishing suction in the venous cannula.⁴⁰ Blood enters at the apex of the cone and is ejected tangentially at the base of the cone (Fig. 7-3). The cone design retains any small air bubbles.

Fig. 7-3 Bio-Medicus centrifugal pump.

(From Karl TR, Horton SB, and Brizard C: Postoperative support with the centrifugal pump ventricular assist device (VAD). Seminars in Thoracic and Cardiovascular Surgery: Pediatric Cardiac Surgery Annual 9:83–91, 2006.)

The pump output is proportional to revolutions per minute and is adjusted according to the venous return. Spins averaging 10,000 to 20,000 rpm will create a blood flow of 5 to 6 L/min in larger pumpheads.³⁶ This type of pump can support neonates and older children with postoperative cardiac failure but competent lung function.^36,58

The advantages of the centrifugal pump include: no need for an oxygenator, low priming volume (pediatrics, 50 mL), low requirements for heparin and little hemolysis, adequate decompression of the left ventricle, easy transport, and low cost. However, adequate pulmonary function is required and the chest must remain open, as with ECMO.

The centrifugal VADs include Bio-Medicus Bio-pump, Levitronix CentriMag (Levotronix, Waltham, Mass.), RotaFlow (Jostra, Hirrlingen, Germany), and the Capiox Terumo (Terumo Cardiovascular Systems, Ann Arbor, Mich.). The Levitronix CentriMag is marketed in Europe, but available in the United Stated only as an investigational device.¹¹ The advantage to this device is that it can be attached to cardiopulmonary bypass cannula already in place. However, cannula adaptors may be needed for smaller patients.

Axial Flow Pump

Axial flow pumps reflect recent industry efforts to develop smaller, lighter, quieter, implantable pediatric VADs. Axial flow pumps are designed to assist the left ventricle as a bridge to transplant or destination therapy. The pump has a magnetically levitated impeller that rotates in a cylindrical chamber so that blood accelerates toward the rotor’s axis, usually 7 to 11.5 cm in length, with spins of 7500 to 12,000 rpm (Fig. 7-4). A flexible motor cable supplies power to the implanted pump. The axial flow pumps have been used successfully in 5 to 16 year olds with BSA ≥0.7 m².

Fig. 7-4 Axial continuous flow pump

(Redrawn from an illustration of the Axial Flow Pump, Micromed DeBakey VAD, by Micromed Technology, Houston, Texas.)

Advantages of the axial pumps are small size, ease of implantation and explantation, low rates of infection, and minimal formation of thrombus.¹⁶ The disadvantages are nonpulsatile flow, limited sizes for children, and need for ventricular apical cannulation.¹⁶

The MicroMed DeBakey VAD/MicroMed DeBakey VAD Child (MicroMed Cardiovascular, Houston, Texas), HeartMate II LVAS (Thoratec, Pleasanton, Calif.), and Jarvik 2000 (Jarvik Heart, New York, N.Y.) are currently used axial flow VADs for adults and selected children with a BSA >0.7 m². An infant (3-15 kg) and child (15-25 kg) Jarvik 2000 are being developed through the National Heart, Lung and Blood Institute (NHLBI) Circulatory Assist Program.⁴

Pulsatile Pumps

Pulsatile VADs contain a reservoir. Blood is ejected by the pump either electronically through the movement of pusher plates or with compressed air movement of the bladder. These devices propel blood in synchrony with the patient’s ventricular ejection, producing pulsatile arterial blood flow. These devices are paracorporeal systems and consist of a pneumatic compressor-operated diaphragm pump with inflow and outflow valves. A transparent polyurethane pump housing allows inspection for potential thrombus development. The external pump position enables fast and safe pump changes if required.⁵⁴

The Berlin Heart Excor Pediatric VAD (Berlin Heart, Berlin, Germany), Medos HIA-VAD (Medos Medizintechnik, Aachen, Germany) and the Thoratec VAD (Thoratec) provide both single and biventricular support. The Berlin Heart and Medos HIA-VAD are available in the United States under compassionate use appeal to the FDA. The Thoratec device can only be used in patients with BSA >0.8 m².¹¹

Adult pulsatile devices used in older children include the HeartMate XVE LVAS (Thoratec), Abiomed BVS 5000 (Abiomed, Danvers, Mass.), and Novacor LVAS (WorldHeart Corp. Oakland, CA). These devices are suitable for patients with BSA >1.2 m² and who require pump flows of more than 2 L/min.

The advantages of pulsatile devices are feasibility for long-term support, potential use without the need for mechanical ventilation, mobility, ability to transition out of the ICU, and need for only low-dose anticoagulation. Disadvantages include thromboembolic complications, infection, and cost.

Neurologic Assessment

The pediatric patient receiving MCS is at risk for cerebral embolism and bleeding, so frequent neurologic assessment is required. Routine cranial ultrasound examinations are performed to assess for the presence of IVH in neonates and infants with an open fontanelle. Clinical neurologic assessment is particularly challenging in patients with centrifugal VAD or ECMO support with open chest cannulation; such cannulation typically requires that patients be sedated with possible neuromuscular blockade. In patients receiving neuromuscular blockade, the only clinical signs of seizures may be abrupt changes in heart rate, blood pressure, skin perfusion, and pupil size. If seizures are suspected, a cranial ultrasound examination or head computed tomography scan will be obtained to look for evidence of bleeding. An electroencephalogram will be required to assess for the presence of seizures and cerebral cortical activity.

When ECMO cannulation uses the internal jugular vein and right common carotid artery, the head is maintained in the midline position to avoid obstruction of the vein or artery. Such compression could compromise cerebral venous drainage or perfusion.

Near-Infrared Spectroscopy (NIRS)

NIRS provides continuous, real-time data regarding tissue oxygenation that can be used in conjunction with clinical and laboratory assessment to evaluate systemic oxygenation and perfusion. The NIRS system uses adhesive sensors placed on the forehead and over the abdomen. Each adhesive sensor contains a light source and two optodes (fiberoptic bundles), often placed at angles from one another to evaluate light absorption and reflection of different tissue depths. As light passes through the skin—and in the case of the brain, through the skull—and into tissue, oxygenated and deoxygenated hemoglobin and oxygenated cell mitochondria affect light absorption and reflection. The NIRS devices calculate the average oxygen saturation of underlying arteries, capillaries, and veins, to reflect a tissue or regional tissue oxygen saturation (rSO₂) that is similar to a mixed venous oxygen saturation. This tissue oxygen saturation is digitally and continuously displayed.

Evaluation of both cerebral and somatic (typically evaluated over the abdomen) oxygenation can help identify changes in regional oxygenation and blood flow. A change in NIRS saturation can indicate declining or inadequate cerebral or somatic oxygenation and allow corrective action to prevent hypoxic injury.

The cerebral rSO₂ value will vary depending on the level of sedation or agitation and the location of the sensor. It will also be affected by factors influencing cerebral blood flow such as increased intracranial pressure (can decrease blood flow), changes in PaCO₂ (hypocarbia produces cerebral vasoconstriction and decreased cerebral blood flow, and hypercarbia produces cerebral vasodilation and increased blood flow) and severe hypoxemia (produces cerebral artery dilation and attempts to increase cerebral blood flow). The target cerebral rSO₂ is greater than 50%.²⁵

The somatic tissue oxygenation should be higher than cerebral tissue oxygenation, because the kidneys receive 25% of cardiac output at any time. The target difference between somatic and cerebral rSO₂ should be greater than 10%, with somatic readings approximately 10% to 15% points higher than cerebral readings.²⁵

NIRS monitoring can be a valuable tool during ECMO support to identify a compromise in tissue oxygenation that may result from reversible problems with cannula placement, and it can assist in the evaluation of systemic oxygen delivery and readiness for decannulation. If a change in cannula position or a circuit malfunction occurs, the flow deficit will produce a fall in rSO₂. Furthermore, if the rSO₂ falls significantly during weaning and clamping of the ECMO circuit, then additional time on ECMO will be needed. If the rSO₂ remains stable in a patient with other signs of adequate systemic perfusion, the patient is likely to be sufficiently stable for decannulation.²⁵

Sedation and Analgesia

Sedation and analgesia administration after ECMO or VAD placement will vary depending whether the patient is cannulated through an open or closed chest, and must be titrated to patient response. Continuous sedation and analgesia is maintained with fentanyl (0.5-2 mcg/kg per hour) or morphine sulfate (0.02 mg/kg per hour) and midazolam (0.05-0.1 mg/kg per hour) infusions. Intermittent doses can be provided for agitation, painful procedures, or unexplained hypertension and tachycardia.

Neuromuscular blockade may be needed for the hemodynamically unstable patient receiving MCS. Pancuronium (0.1 mg/kg per dose) or a Cis-atracurium infusion (0.2 mg/kg per hour) can be used. Minimal use of neuromuscular blockade is advantageous to allow spontaneous movement and assess for seizure activity. For further information, see Chapter 5.

Respiratory Assessment

All patients receiving MCS are initially intubated and their lungs are mechanically ventilated. Lung sounds should be auscultated and chest expansion assessed during hand ventilation (with bag). The depth of endotracheal tube insertion should be adjusted if breath sounds are decreased on the left side or if placement is too low or too high on chest radiograph. Patients receiving only left ventricular support via centrifugal devices require full ventilator support. Patients requiring more long-term pneumatic assistance with a VAD should be weaned to extubation once stable.

During VA ECMO, mechanical ventilation settings are generally reduced to an inspired oxygen of 21% to 40%, peak inspiratory pressure of 20 to 25 cm H₂O, positive end-expiratory pressure of 4 to 10 cm H₂O, and respiratory rate of 5 to 10 breaths per minute. The goal of the resting ventilator settings is to maintain functional residual capacity and prevent barotrauma. In patients with minimal lung disease and native cardiac output, gas exchange will then occur within the lungs and through the membrane oxygenator.

Daily chest radiographs and frequent arterial and venous blood gas samples are used to monitor respiratory function. Bilateral opacification of lung fields is generally apparent on chest radiograph within the first 24 hours of ECMO, resulting from a reduction of positive airway pressure and complement activation leading to capillary leak and pulmonary edema. Patients with persistent pulmonary air leaks while receiving ECMO may require very low positive airway pressure to allow the lungs to heal. Pulmonary care should include chest vibration and suctioning of the endotracheal tube as needed to clear secretions. Limitation of suctioning may be needed if bleeding is exacerbated by the procedure.

Potential respiratory complications include pneumothorax or hemothorax, pleural effusions, and pulmonary hemorrhage. Needle aspiration or chest tube insertion are options for emergent decompression of a pneumothorax or hemothorax or drainage of a pleural effusion. If pulmonary hemorrhage develops, management consists of minimizing the target ACT, increasing the platelet count, and minimizing suctioning. If bleeding becomes more severe, epinephrine (0.1 mL/kg of 10,000 dilution) may be instilled into the trachea and positive end expiratory pressure instituted. See Chapter 9 for information regarding mechanical ventilation.

Cardiac Assessment and Support

The patient’s cardiac output, hemodynamics, and device flow must be monitored continuously to ensure adequate systemic perfusion. The critical care nurse must understand the device operation and components to interpret hemodynamic changes and ensure effective MCS. Inadequate systemic perfusion can result from inadequate intravascular volume, inappropriate vascular resistance, (native) myocardial dysfunction, arrhythmias, or tamponade.

Troubleshooting VAD Pump and Console

As more VADs are placed in children as a bridge to transplant, longer support durations are inevitable. Consequently, VAD dysfunction may become more prevalent. The VAD-trained critical care nurse should examine all external device components such as cannulas, drive lines, battery support, and all mechanical and electrical connections. The nurse should be familiar with the VAD alarms and troubleshooting techniques (see Table 7-4).

The Bio-Medicus centrifugal console has low- and high-flow alarms. Flow can frequently change in response to blood pressure or vascular resistance. Constant visual, auditory, and tactile monitoring is required. Visual and tactile checks of the circuit can detect line “chattering” that would indicate inadequate preload or cannula obstruction. Other visual checks include examination of lines, connector sites, and the centrifugal head for changes in color that could indicate fibrin clot formation. In addition, auditory and tactile checks are needed to detect vibration of the centrifugal head.

The pneumatic console delivers air pressure to compress and empty the blood pump sac delivering pulsatile flow. Each blood pump must fill and eject completely to give the patient an adequate cardiac output. Cardiac output is optimized by adjusting the pressure and vacuum values on the console. Most pneumatic consoles have low-fill alarms, indicating inadequate pump preload before ejection. The nurse should assess the patient’s volume status for any changes or signs of right-sided heart failure during single LVAD support.

Use a flashlight daily to assess external pumps for thrombi formation and complete VAD emptying. Illuminate the pump at an angle to verify transmission of a white light through to the opposite side. If a pink light is noted, then the VAD is not completely emptying. The physician should be notified immediately whenever clot formation is noted or suspected in the circuit or pump. The use of heparin-coated pumps and circuits will hopefully reduce the incidence of thromboembolic complications in the future.

Hematologic Assessment

Thrombosis and bleeding are the most common complications of MCS.⁶⁰ Patients receiving MCS are susceptible to bleeding from all surgical sites and should be monitored closely (e.g., cannula sites, incisions, chest tubes). All saturated dressings should be measured and included in the total output calculation. All bodily fluids such as stool, urine, gastric drainage, and sputum should be checked for blood. The nurse should monitor the child’s abdominal girth and assess for increased abdominal distension and firmness. In addition, the nurse should try to avoid any procedure that may produce bleeding such as heel sticks, venipunctures or injections, insertion of a nasogastric tube, or urinary catheter or rectal probe insertion.

The patient’s hematocrit, fibrinogen, and platelet counts are routinely monitored, and appropriate blood products are administered as needed. Blood product replacement protocols may vary among institutions. In general, the target platelet count is >100,000/mm³, fibrinogen 100 mg/dL, and hematocrit >35%. If possible, blood transfusions should be limited for patients awaiting transplant, because frequent exposure can increase the panel reactive antibody, which can complicate matching for a future transplantation.¹¹ Blood products should be irradiated, depleted of leukocytes, and negative for cytomegalovirus.¹¹ If excessive nonsurgical bleeding is encountered, aminocaproic acid, aprotinin, and recombinant factor VII may be administered.

Anticoagulation

Anticoagulation management can be an extremely challenging part of management in patients receiving MCS. The goal of anticoagulation therapy is a balance of preventing pump thrombosis formation and bleeding. Blood exposure to an artificial circuit triggers the coagulation cascade and causes clots to form in the circuit. Anticoagulation is therefore necessary to prevent clot formation and the resultant consumptive coagulopathy that would occur as procoagulants are depleted by the clotting process.

Most anticoagulation regimens begin with intravascular heparin sodium infusion generally titrated in the range of 20 to 60 units/kg per hour to achieve a desired aPTT of 60 to 80 sec or an ACT of 140 to 180 sec (VAD) or 200 to 240 sec (ECMO). These ranges can vary depending on the type of support, the technology used to measure them (e.g., Hemochrons, I Stats), the patient’s risk for bleeding, and whether the device circuit is coated with heparin or another bioactive surface. The heparin infusion is usually started once chest tube output has decreased. Patients receiving long-term VAD support can eventually transition to oral anticoagulation, such as warfarin or antiplatelet therapy. Protocols to guide anticoagulation therapy are typically available for specific devices, with recommended INR ranges.

The control of the coagulation system on ECMO is complex. The ACT is followed every hour with titration of the heparin dose. However, AT III levels are low in many patients receiving ECMO, and the combination of AT III and a heparin cofactor is needed for anticoagulant effects. The combination of AT III and a heparin cofactor stops the conversion of fibrinogen to fibrin and ultimately prevents clot formation. A deficiency in AT III can cause heparin to be ineffective, leading to excessive heparin use and the potential for clotting of the circuit. Replacement of AT III or fresh frozen plasma is necessary to restore adequate AT III levels and the anticoagulant effects of heparin.

Patients receiving ECMO also have thrombocytopenia, because platelets are deposited onto artificial surfaces and lost to clot formation in the circuit. In addition, after exposure to the circuit, many remaining platelets are nonfunctional. Thrombocytopenia will ultimately lead to bleeding. Platelet transfusions are given to reverse thrombocytopenia and reduce the risk of hemorrhage.

Fluid and Electrolyte Replacement and Balance

Patients typically are quite edematous during MCS because they often require fluid resuscitation before device insertion, and often develop capillary leak before and after device insertion. Aggressive diuresis can lead to intravascular volume depletion and cardiac compromise. Diuretic therapy is initiated early to promote the removal of edema fluid. A furosemide infusion (0.2 to 0.3 mg/kg per hour) is commonly used to promote diuresis. Chlorothiazide and bumetanide can be added to promote diuresis, or renal dose dopamine infusion can enhance renal perfusion and glomerular filtration. When the child receives diuretics and requires frequent blood replacement, the child’s serum potassium, calcium, and other electrolytes should be closely monitored and replaced as needed.

Renal and Hepatic Function

During the first 24 to 48 hours of MCS therapy, patients often develop acute tubular necrosis with oliguria and rising blood urea nitrogen (BUN) and creatinine. The kidneys and liver may be compromised by low cardiac output, prolonged period of hypoxia or hypotension before initiation of support, prolonged cardiopulmonary bypass (if postoperative), multiple blood transfusions, thromboembolism, antibiotics or other toxic medications, and severe hemolysis.

Monitoring of urine output, BUN and creatinine levels, and liver function studies for worsening or improvement in end-organ perfusion is imperative. Once adequate cardiac output has been restored through the use of MCS, renal and hepatic function should improve. Interventions to minimize renal and hepatic dysfunction include minimizing blood transfusions, providing early nutritional support, monitoring drug levels, and discontinuing of renal or hepatic toxic medications if possible.

If urine output does not improve, the patient may have an open patent ductus arteriosus (PDA) or a combination of inadequate flow, low cardiac output, and low mean arterial pressure compromising renal blood flow. Surgical ligation of the PDA may be warranted if a PDA is documented by echocardiogram.

In the case of rising creatinine levels, hyperkalemia, persistent oliguria, and maximal diuretic support, it may be necessary to perform dialysis via peritoneal dialysis, perform ultrafiltration via the ECMO, or centrifuge VAD circuit. Ultrafiltration is accomplished through the use of a hemofilter connected to the ECMO or centrifuge VAD circuit; water passes through the filter from the plasma, reducing hypervolemia. Ultrafiltration may need to be stopped in the case of low atrial pressure, hypotension, or circuit shutdown from low volume detected by the bladder box.

Nutrition

Infants and children receiving MCS typically have compromised nutritional status secondary to presupport clinical decompensation. Parenteral nutrition and intralipid therapy are usually started within 48 to 72 hours after initiation of MCS with gradual increase to achieve a caloric goal of 80 to 100 kcal/kg per day. Lipid intake should not exceed 1 g/kg per day, or it should be infused through the bladder port to prevent accumulation and embolism within the ECMO circuit. In addition, ranitidine is added in the parenteral nutrition or as a supplementary intravenous medication to inhibit gastric secretions secondary to stress.

Once inotropic support is minimal and bowel sounds have resumed, enteral feedings can be started slowly through a nasogastric tube. Oral feedings can be attempted if the child is extubated. In patients with poor systemic perfusion before MCS, mesenteric artery blood flow is compromised, so feedings should be started slowly with careful monitoring for signs of feeding intolerance such as abdominal distension, high gastric residuals, no stools, or bloody stools. These signs may also be associated with necrotizing enterocolitis in the infant and may not be present until feeding is initiated. The nutritional support service should be involved in the child’s care to determine the current nutritional status, recommend age-appropriate diet or formula and caloric goals, and monitor progress through laboratory values such as total protein and albumin.

Infection

Patients receiving MCS support are at high risk for nosocomial infection secondary to surgical incisions or open chest, cannulae sites, invasive monitoring catheters, drainage tubes, mechanical ventilation, and compromised nutrition and immune status. Early indicators of infection may absent or difficult to detect during MCS. Thrombocytopenia results from platelet destruction by the ECMO circuit or VAD pump. In addition, core body temperature can be affected by the ECMO heat exchanger.⁴⁷ The white blood cell count with differential should be obtained daily, and routine blood, urine, and endotracheal tube cultures should be obtained to monitor for infection. Patients with unexplained hemodynamic instability, coagulopathy, elevated white blood cell count, or fever should be promptly treated for suspected sepsis pending culture results (see Chapter 6).⁶⁰

Prophylactic broad-spectrum intravenous antibiotics are administered before mechanical support and are continued after insertion. Cannulation through an open sternum poses a greater risk for mediastinitis. Therefore, in patients with an open sternum, antibiotic coverage is broadened to include a second- or third-generation cephalosporin in combination with a course of vancomycin or oxacillin.^9,60

The nurse should monitor the patient closely for signs and symptoms of infection during MCS. In addition, the nurse should use meticulous aseptic and sterile technique when required for procedures or direct patient care.

Skin Assessment

Children receiving MCS are at high risk for developing skin breakdown related to suboptimal nutritional status and immobility. Furthermore, the patient with a pneumatic VAD will have external pumps on the surface of the abdomen, and cannulas, and drive line connections may be a potential source for the development of skin breakdown. VAD site dressing care (either the pump or percutaneous components) is performed according to manufacturer’s recommendations. Strict sterile technique is preferred until the skin has adhered to the Dacron on the cannulae. The nurse should carefully remove encrustations that result from wound secretions to prevent skin necrosis at the cannulae exit sites.

Exit sites should be covered at all times, and the use of age-appropriate binders or pouches is recommended to stabilize pumps or components. Several VADs use proprietary materials with guidelines regarding wound care. Many companies do not recommend oil-based products (ointments), alcohol, or acetone that might degrade the pump case or cannula. (e.g., the Thoratec VAD outer pump casing is sensitive to acetone).

Frequent skin assessment is critical to early detection and prevention of skin breakdown. Early initiation of enteral nutrition and mobility (if requiring long-term use of a pneumatic VAD) can prevent the development of skin breakdown. However, infants or children receiving ECMO might not tolerate position changes. In such patients, low air-flow beds can be used to maintain skin integrity. A gel pillow under the occiput is often used to prevent skin breakdown. Pressure points should be assessed routinely, and passive range-of-motion exercises should be performed to maintain skin integrity and prevent muscle contractures.

Activities of Daily Living and Rehabilitation

For children receiving ECMO or centrifugal VAD support, activities of daily living are limited to passive range-of-motion exercises throughout the duration of support. For the child requiring long-term use of a pneumatic VAD, the goal is extubation and rehabilitation. The primary nurse and multidisciplinary care team facilitate early extubation, discontinuation of invasive catheters, and early ambulation. Patient, parent, and physical therapist involvement are crucial to successful rehabilitation. Allowing the patient to gain independence in activities of daily living and device management (if age appropriate) encourages both functional and psychological well-being. The patient’s family should participate in the rehabilitation phase of recovery.

The multidisciplinary team should develop and follow a daily schedule of activity or exercise plan, rest periods, and sleep during the night. If the child is of school age, daily class work is incorporated into the schedule with input from the hospital or local school district teacher.

Psychological Support

One of the most stressful experiences for any parent is a child’s critical illness. Psychological support in addition to patient and family education are essential components of postoperative MCS care. If emergent VAD placement occurs, patients may wake from anesthesia unaware of the device, and the family often has no time to prepare.

Parents often face overwhelming feelings of anxiety, fear, and helplessness, yet can remain hopeful for successful therapy.¹⁷ Age-appropriate explanations of components and functions can gradually introduce the device to the patient and family. The nurse also provides crucial emotional support during the recovery period.

When the child’s condition is stable, visits from classmates and friends are encouraged, provided that visitors are screened for infection. Visits from other VAD recipients can be a positive experience for the patients requiring support as a bridge to transplant and for their families.