Anesthesia for Pediatric Cardiac Surgery

Shunt Lesions

Shunts are intracardiac connections between chambers (e.g., ASD or VSD) or extracardiac connections between a systemic and pulmonary artery (e.g., PDA). The direction of blood flow through the shunt depends on the relative resistances on either side of the shunt and on the size of the shunt orifice. The direction and magnitude of shunt at the atrial level are additionally governed by the relative differences in ventricular compliance and respective AV valve function. With a nonrestrictive VSD or PDA that does not impede blood in either direction, the main determinant of the direction of blood flow is relative resistance between the pulmonary and systemic vascular beds. The effect that a shunt lesion has on the cardiovascular system depends on both the size of the shunt and its direction.

Left-to-right shunts occur when the PVR is lower than the SVR, so that blood flow is preferentially directed toward the lungs, resulting in increased PBF. In patients with large left-to-right shunts and low PVR, this increase in PBF can be substantial and result in three pathophysiologic problems: (1) congestion of the pulmonary circulation; (2) intravascular volume overload with resulting increased cardiac work for the LV; and (3) excessive PBF, resulting in progressive elevation in PVR. Volume overload causes ventricular dilation that places the heart at a mechanical and physiologic disadvantage, resulting in reduced diastolic compliance. Diastolic changes lead to engorgement of the respective venous beds, which produces the signs and symptoms of clinical congestive heart failure (CHF) early in the natural history of volume-overload condition. The demand for increased cardiac output placed on the LV is limited in the infant by virtue of its immature structure, so that large left-to-right shunts may outstrip the capacity of the left side of the heart to maintain adequate systemic perfusion.

Surgical closure of a hemodynamically significant VSD usually provides immediate benefit by dramatically lowering left ventricular volume output demands. Occasionally, the sudden increase in wall stress imposed on a dilated ventricle that must now pump solely against SVR can produce worsening ventricular failure during the early postoperative period after eliminating the low-resistance “pop off” into the pulmonary circulation. If the left-to-right shunt is not repaired, prolonged exposure to increased PBF results in progressive elevations in PVR. Fixed changes in pulmonary arterioles may occur, leading to pulmonary vascular obstructive disease, which may become irreversible. Table 78.1 lists common left-to-right shunt lesions.

Right-to-left shunts occur when pulmonary vascular or right ventricular outflow tract resistance exceeds SVR, thereby reducing PBF. The systemic circulation receives an admixture of deoxygenated blood via the shunt. This manifests clinically as cyanosis and hypoxemia. Pure right-to-left shunting resulting from increased PVR is seen in the Eisenmenger complex and persistent pulmonary hypertension of the newborn with shunt at both the atrial and ductal levels. More commonly, PVR is low, and the right-to-left shunt is produced by a more complex lesion with obstruction of pulmonary outflow proximal to the pulmonary vasculature. A classic example of right-to-left shunt is TOF, where shunting occurs through the VSD because of pulmonary outflow obstruction. Systemic perfusion is generally normal with right-to-left shunting lesions unless hypoxemia becomes severe enough to impair O ₂ delivery to tissues. Right-to-left shunting produces two pathophysiologic problems: (1) reduced PBF resulting in systemic hypoxemia and cyanosis, and (2) increased impedance to right ventricular ejection, which may ultimately lead to ventricular dysfunction and RV failure. However, the physiologic mechanisms designed to compensate for pressure overload rarely create abnormalities in systolic or diastolic function early in the natural history of the disease process. In contrast to lesions that produce excessive ventricular volume, lesions that produce isolated pressure overload typically require years to cause ventricular dysfunction and failure.

Mixing Lesions

Mixing lesions constitute the largest group of cyanotic congenital heart defects (see Table 78.1 ). In these defects, the mixing between the pulmonary and the systemic circulation is so large that the systemic and pulmonary artery O ₂ saturations approach each other. The pulmonary-to-systemic flow ratio $\dot{\text{Q}}\text{p}/\dot{\text{Q}}\text{s}$ is independent of shunt size, and is completely dependent on vascular resistance or outflow obstruction. The pulmonary and systemic circulations tend to be in parallel with one another rather than in series (see Table 78.1 ). In patients with no outflow obstruction, flow to the systemic or pulmonary circulation depends on the relative vascular resistances of both circuits, such as with univentricular hearts or double-outlet RV. If SVR exceeds PVR, as in the typical circumstance, the tendency is toward excessive PBF, and the predominant pathophysiologic process is left-to-right shunting. These patients have increased PBF, ventricular volume overload, and a gradual elevation of PVR over time. If PVR exceeds SVR, as may occur episodically in ductal-dependent lesions such as hypoplastic left heart syndrome (HLHS), systemic blood flow predominates and PBF dramatically decreases, causing hypoxemia ( Table 78.2 ).

In patients with a mixing lesion and left ventricular outflow obstruction, PBF may be sufficiently excessive to impair systemic perfusion. In patients with mixing lesions and a right ventricular outflow obstruction, such as a single ventricle with subpulmonic stenosis, systemic-to-pulmonary flow can vary from balanced flow to significantly decreased PBF in which the severity of hypoxemia depends on the degree of obstruction. Typical mixing lesions include truncus arteriosus, univentricular heart, total anomalous pulmonary venous return, pulmonary atresia with large VSD, and single atrium.

Obstructive Lesions

Obstructive lesions range from mild to severe. Severe lesions manifest in the newborn period with a pressure-overloaded, diminutive, or profoundly dysfunctional ventricle proximal to the obstruction. Such lesions include critical aortic stenosis, critical pulmonic stenosis, coarctation of the aorta, and interrupted aortic arch. Although aortic and pulmonary atresia represent the most extreme variants of outflow tract obstruction, they are associated with such significant hypoplasia of the ventricle (HLHS and pulmonary atresia with intact ventricular septum, respectively) that the ventricle’s function does not contribute to the circulatory physiology. As with other critical obstructive lesions, these extreme variants have ductal-dependent circulations, but beyond that similarity they are perhaps better understood as univentricular hearts for which the management characteristics of a mixing lesion dominate in importance.

In critical neonatal left-sided obstructive defects, systemic perfusion depends on desaturated blood flow from the RV via the PDA; the coronary perfusion is supplied by retrograde flow from the descending aorta (see Table 78.2 ). In right-sided lesions, PBF is supplied from the aorta via the PDA and right ventricular function is impaired.

Pathophysiologic problems in critical neonatal left-sided heart obstructive lesions include (1) profound left ventricular failure, (2) impaired coronary perfusion with an increased incidence of ventricular ectopy, (3) systemic hypotension, (4) PDA-dependent systemic circulation, and (5) systemic hypoxemia. The pathophysiologic problems in critical neonatal right-sided heart obstructive lesions include (1) right ventricular dysfunction, (2) decreased PBF, (3) systemic hypoxemia, and (4) PDA-dependent PBF. Apart from the most extreme variants that become evident in the neonatal period, infants and children with outflow obstruction (e.g., mild-to-moderate aortic or pulmonary stenosis, coarctation of the aorta) manifest compensatory mechanisms for pressure overload, and often remain clinically asymptomatic for many years.

Regurgitant Valves

Regurgitant valves are uncommon as primary congenital defects. Ebstein malformation of the tricuspid valve is the only pure regurgitant defect manifesting in the newborn period. However, regurgitant lesions are frequently associated with an abnormality of valve structure, such as incomplete or partial AV canal defect, truncus arteriosus, and TOF with an absent pulmonary valve. The pathophysiology of regurgitant lesions includes (1) volume-overloaded circulation and therefore (2) progression toward ventricular dilation and failure.

When considering the incidence of all the congenital heart defects, three uncomplicated left-to-right shunts (VSD, ASD, PDA) and two obstructive lesions (pulmonic stenosis, aortic coarctation) constitute 60% of all congenital cardiac defects. Mixing lesions, complicated obstructive defects, and right-to-left shunting lesions account for the vast majority of the remaining 40%. The latter group of defects, which are more difficult to manage, have a significantly higher morbidity and mortality rate.

Chronic Consequences of Congenital Heart Disease

The chronic effects of CHD are a consequence of the imposed hemodynamic stress of the defect or the residua and sequelae after cardiac surgery. These effects continue to alter normal growth and development of the cardiovascular system and other organ systems throughout life. Complete surgical cures are rarely achieved, and some repairs are palliative rather than corrective; therefore, abnormalities before and after repair produce long-term effects in patients with CHD. Many of the abnormalities are trivial and have no major import. Others affect major organ system processes, such as ventricular function, the conduction system of the heart, central nervous system (CNS) growth, or PBF. Whether anesthetizing these patients for their primary or subsequent cardiac repair or for noncardiac surgery, these chronic changes should be ascertained and reflected in the anesthetic plan.

The myocardium is continually remodeled by specific hemodynamic stresses in utero and throughout life. Abnormal hemodynamic loading conditions associated with CHD interrupt the normal ventricular modeling process ( Fig. 78.2 ). Abnormal ventricular remodeling typically begins in utero and stimulates an increase in ventricular mass. Increased ventricular mass is due to both hyperplasia and hypertrophy of myocytes in response to altered wall stress on the developing ventricle. The resultant biomechanical deformation of the ventricle alters its geometry, affecting normal systolic and diastolic function.

Abnormalities of ventricular performance at rest and with exercise can be detected in patients with chronic hemodynamic overload and complex cyanotic lesions. These abnormalities in ventricular function are the consequences of chronic ventricular overload, repeated episodes of myocardial ischemia, and residua or sequelae of surgical treatment (ventriculotomy, altered coronary artery supply, inadequate myocardial protection). Although chronic volume or pressure overload of the LV results in CHF, the compensatory mechanisms for pressure overload create less physiologic disturbance than volume overload, particularly in diastolic function. Consequently, CHF occurs later in the natural history of isolated obstructive lesions that do not require treatment in the neonatal period. Similarly, chronic right ventricular volume overload as seen in pulmonic insufficiency after TOF repair is more likely to be associated with chronic ventricular dysfunction and failure than a pressure-loaded RV that manifests with residual pulmonic stenosis. In fact, the most potent combination for inducing ventricular dysfunction and failure occurs when a pressure overload is superimposed on a dilated, volume-overloaded ventricle (e.g., postoperative TOF with pulmonary insufficiency and branch pulmonary artery stenosis).

Initial manifestations of CHF reflect alterations in ventricular compliance that result from a variety of biophysical responses to abnormal loading conditions. The ventricular dilation and compensatory hypertrophy that accompany excessive intravascular volume provide effective compensation to preserve normal systolic wall stress, but alterations in diastolic wall stress become evident ( Fig. 78.3 ). Ultimately, chronic or severe pressure overload causes similar changes as the resultant myocardial hypertrophy outgrows vascular supply and results in ischemia and fibroblast proliferation. Permanent changes in myocardial structure and function are the end result.

In patients with cyanotic heart defects, the long-term compensation for chronic hypoxemia is major redistribution of organ perfusion with selected blood flow to the heart, brain, and kidney and decreased flow to the splanchnic circulation, skin, muscle, and bone. Chronic hypoxemia is associated with increased work of breathing in an attempt to increase O ₂ uptake and delivery. The most dramatic complications are decreased rate of somatic growth, increased metabolic rate, and an increase in hemoglobin concentrations.

Congenital syndromes may have associated CHD that will influence long-term outcome ( Table 78.3 ).

Surgical Procedures and Special Techniques

The ultimate objectives for congenital heart surgery are (1) physiologic separation of the circulation, (2) relief of outflow obstruction, (3) preservation or restoration of ventricular mass and function, (4) normalization of life expectancy, and (5) maintenance of quality of life. The available surgical procedures to accomplish these objectives are diverse and complex ( Table 78.4 ). In general, operations performed for congenital heart defects can be divided into corrective and palliative procedures. The type and timing of repair depend on the age of the patient, the specific anatomic defect, and the experience of the surgeon and the team (see Table 78.4 ).

Palliation in infancy is usually performed when anatomic parts are missing, as in pulmonary atresia (absent RV and pulmonary artery), tricuspid atresia (absent RV and tricuspid valve), HLHS (aortic atresia and hypoplastic LV), univentricular heart (absent RV or LV), and mitral atresia (absent LV). These palliative procedures can be further subdivided into those that increase PBF, those that decrease PBF, and those that increase mixing (see Table 78.4 ). Palliative procedures that increase PBF include shunts (Blalock-Taussig, central, and Glenn), outflow patch, and enlargement of the VSD. Those that decrease PBF include pulmonary artery banding and ligation of a PDA. Those that improve intracardiac mixing include atrial septostomy (balloon, blade, and Blalock-Hanlon).

The improvements in surgical technique, coupled with advancements in anesthetic and technologic support, make repair in early infancy not only feasible but in many cases preferable. Currently, repair in infancy can be offered for a number of congenital heart defects, as shown in Table 78.4 . The timing of surgical intervention reflects medical necessity, physiologic and technical feasibility, and optimal outcome. Cardiac defects that require a PDA to sustain sufficient systemic blood flow or PBF (e.g., pulmonary atresia, HLHS, interrupted aortic arch, critical aortic stenosis, and critical pulmonic stenosis) require an intervention in the neonatal period. A variety of defects are optimally repaired in early infancy. Lesions such as transposition of the great arteries (TGA) may exhibit better left ventricular function if the arterial switch operation is performed in the first few weeks of life when the PVR has recently been high enough to increase left ventricular systolic pressure, whereas other repairs may manifest less volatile postoperative physiology if deferred a few weeks or months until PVR has consistently fallen (e.g., TOF, AV canal defect). Each defect may have mitigating factors for which deferred definitive repair will enable an optimal surgical result (e.g., TOF with aberrant coronary branching pattern or multiple VSDs; TGA with VSD and severe left ventricular outflow tract obstruction).

Pediatric cardiovascular surgery aims to preferentially repair defects in infancy rather than palliate. This trend reflects improved technical capabilities coupled with a desire to limit the morbidity and mortality associated with long-term medical management and the sequelae of multiple palliative operations. Early correction should decrease the incidence of the chronic complications of CHD, such as the problems associated with ventricular overload, cyanosis, and pulmonary vascular obstructive disease. Early repair also may have the selective advantage of enhancing organ system protection during repair because of poorly understood factors promoting resistance to injury and enhanced recovery potential (i.e., enhanced plasticity). With the continued improvement in surgical techniques and the early treatment of CHD, specific organ systems such as the brain, heart, and lungs will be spared the detrimental effects of chronic derangements of hemodynamics and O ₂ delivery.

Procedures for the treatment of CHD continue to evolve to decrease long-term morbidity and enhance survival. For example, the long-term problems with right ventricular dysfunction and failure associated with the Mustard procedure for repair of TGA encouraged many surgical groups to develop the neonatal arterial switch operation, which probably provides an anatomic correction with better long-term results. A second example of the continuing evolution of technique is surgery for TOF. Long-standing pulmonary insufficiency after right ventricular outflow repair for TOF is associated with right ventricular dysfunction and failure. Preservation of the pulmonary valve at initial repair using a combined transatrial and transpulmonary approach during correction and the early insertion of a pulmonary homograft in the setting of pulmonary insufficiency are techniques employed to avoid the long-term problems of right ventricular dysfunction and failure.

Surgery for HLHS, once considered a fatal disease, has achieved significant long-term survival after a series of staged reconstructive procedures. The use of RV-PA conduits as an alternative to traditional systemic-to-pulmonary shunts confers some advantage in survival after stage 1 palliation owing to elimination of diastolic runoff into the pulmonary circulation with concurrent unloading of the systemic RV. Myocardial perfusion improves with higher diastolic pressures, no run off to the pulmonary circulation, and decreased myocardial work. The long-term impact of a right ventriculotomy in a univentricular heart is unknown. In 2008, the Pediatric Heart Network, an organization sponsored by the National Institutes of Health, completed enrollment in a randomized control trial comparing the effects of modified Blalock-Taussig shunt (mBTS) to RV-PA conduits as part of the stage 1 Norwood procedure. Recent results revealed infants treated with an RV-PA shunt had improved survival over those with mBTS, although the long-term outcomes were not different.

Neurologic outcome after surgical repair is an ongoing concern. Preoperative cerebral blood flow (CBF) was shown to be diminished in patients with a variety of congenital heart defects, and low CBF values were associated with periventricular leukomalacia. Some centers routinely advocate regional low-flow cerebral perfusion and measurement of regional cerebral O ₂ saturation index and CBF velocity using transcranial Doppler imaging during arch reconstruction in this population. Reductions in the regional cerebral O ₂ saturation index or CBF greater than 20% of baseline are treated aggressively in an attempt to increase cerebral O ₂ delivery by increasing the mean perfusion pressure, red blood cell (RBC) transfusion, and maintenance of high normal levels of Pa CO ₂ to achieve cerebral vasodilation.

Techniques have evolved to a “three-region” perfusion strategy for aortic arch reconstruction in the Norwood procedure. This technique involves direct perfusion of the coronaries and distal thoracic aorta as well as continuous cerebral perfusion via innominate cannulation. The arch repair occurs from distal to proximal at warmer patient temperatures, theoretically allowing decreased coronary and splanchnic ischemic times, decreasing the risk of cardiac dysfunction and abdominal organ damage, and mitigating the negative hypothermic effects on the hematologic system.

Surgical management has evolved in a broader application of certain surgical procedures initially designed for a specific defect. For example, modifications of the Fontan operation, which was originally devised for patients with tricuspid atresia, are now being used to repair a variety of univentricular hearts, including HLHS. Initially, the wider application of the Fontan operation to include complex defects once considered inoperable was associated with a rise in morbidity and mortality. However, this trend has been reversed in recent years by several groups who have demonstrated improved outcome with the staging of the operation (superior cavopulmonary anastomosis, subsequent completion of the Fontan operation), the creation of a fenestration between the RA and LA at the time of the Fontan operation, and the use of modified ultrafiltration (MUF) at least in the early years.

However, as patients who undergo the Fontan procedure grow older, they present with the unique pathophysiologic challenges of refractory arrhythmias, a failing single ventricle, protein-losing enteropathy, and plastic bronchitis. Most of these adults are cared for in a combined pediatric and adult cardiac program and require intensive multidisciplinary care to optimize cardiorespiratory status. Ingenuity and innovation such as demonstrated with the Fontan procedure have permitted continued improvements in survival for all patients with CHD. As incisions in the myocardium become smaller and sutures more precisely placed, and as improvements in surgical techniques continue to evolve, the complications of ventricular dysfunction, arrhythmias, and residual obstruction should decline, contributing to improved patient quality of life.

One final difference unique to congenital heart surgery that has a major impact on anesthetic management relates to the type of cardiopulmonary support. Because of the complexity of repair in small patients, surgery often requires significant alterations in the bypass techniques, such as the use of deep hypothermic CPB at temperatures of 18°C and total circulatory arrest. Despite widespread use of these techniques during CPB, their physiologic effects on major organ system function are just beginning to be understood. These effects are discussed in subsequent sections.

Intraoperative Echocardiography

The use of echo-Doppler imaging is now regarded as standard of care for almost all pediatric cardiac surgeries. Two-dimensional echocardiography combined with pulsed-wave Doppler ultrasonography and color flow mapping provide detailed morphologic and physiologic information in the majority of operative cases. Using echo-Doppler in the operating room, anatomic and physiologic data can be obtained before CPB, thus refining the operative plans. Prebypass echo-Doppler precisely defines anesthetic and surgical management. Because of the unrestricted epicardial and TEE approaches in anesthetized patients, new findings are frequently discovered and management plans changed accordingly ( Fig. 78.4 ).

Postbypass echo-Doppler evaluation is able to immediately assess the quality of the surgical repair and cardiac function by examining ventricular wall motion and systolic thickening. This technique can show residual structural defects after bypass, which can be immediately repaired, thus avoiding the patient leaving the operating room with significant residual structural defects that will require reoperation at a later time ( Fig. 78.5 ). By identifying patients with right or left ventricular contraction abnormalities after bypass, as determined by a change in wall motion or systolic thickening, echo-Doppler provides guidance for immediate pharmacologic interventions. Importantly, postbypass ventricular dysfunction and residual structural defects identified by echo-Doppler imaging are associated with an increased incidence of reoperation and higher morbidity and mortality rates. Thus, this monitoring tool is helpful in assessing surgical repair and identifying operative risk factors, which will hopefully improve outcomes.

Two techniques for intraoperative echo-Doppler imaging have been described: epicardial and TEE. Using TEE, the probe is placed after induction of anesthesia and intubation and is then available for monitoring of the patient. The advantage of this technique is its utility as a continuous monitor of cardiac structure and function, without interrupting surgery. Because of its ideal imaging location, TEE has been especially helpful in evaluating pulmonary venous return and the integrity of the left AV valve after mitral valvuloplasty, complete AV valve repair, and correction of complex CHD. Early limitation in views has been virtually eliminated as a result of clinical experience and improved biplane images. Pediatric biplane TEE probes have now extended the patient weight limits to neonates between 2.5 and 3 kg. Potential hazards of TEE that merit particular vigilance include descending aorta and airway compression because of probe size or during probe flexion. There have been reports of esophageal damage during hypothermia and low or no flow states because of the heat energy the probe produces while connected to the TEE machine, leading most institutions to pause the imaging ability of the probe, disconnect the probe from the machine, or even remove it during the CPB period of the procedure.

A second technique for intraoperative echocardiographic analysis in children is the epicardial approach. This approach requires passing a clean, short-focused 5.0- or 7.0-MHz transducer over the anesthesia screen into a sterile sheath, where it then can be placed on the epicardial surface of the heart. This technique best facilitates the probe manipulations necessary for thorough interrogation of the major structures and dynamic function of the heart. The advantage of this approach is that all views can be obtained in patients of any size. Among the disadvantages are the need for sufficient operator skill and experience to perform the manipulations, the need to interrupt surgery to manipulate the probe, and the possible deleterious impact of direct myocardial mechanical manipulation. Given current TEE capabilities, epicardial imaging is rarely employed.

Specialized Central Nervous System Monitoring

The primary goal of brain monitoring is to improve our understanding of cerebral function during cardiac surgery so that effective brain protection strategies can be developed. Because many of the determinants of normal brain perfusion become externally controlled by the cardiac team during CPB, such as flow rate (cardiac output), perfusion pressure, temperature, hematocrit, and Pa CO ₂ , a knowledge of the effect of these factors on the brain in neonates, infants, and children is essential. Numerous intraoperative techniques have been used for monitoring the brain to prevent secondary brain injury from hypoxia, ischemia, emboli, and electrophysiologic derangements. These have primarily included the following three modalities in isolation or combination: (1) electroencephalography (EEG) to assess perfusion-related changes in cortical activity; (2) transcranial Doppler imaging to measure arterial flow and resistance; and (3) near-infrared spectroscopy (NIRS) to provide a measure of venous-weighted, tissue oxyhemoglobin saturation. Additionally, the measurement of CBF and metabolism with specialized clinical research tools has been very important in furthering our understanding of brain function during and after surgery. Multimodal neurologic monitoring is also used to guide CPB, deep hypothermic circulatory arrest (DHCA), and regional low-flow cerebral perfusion techniques in neonatal arch reconstruction.

Electroencephalographic monitoring allows detection of ischemia or recognition of an adequate decrease in cerebral metabolic activity during hypothermia before DHCA. EEG is helpful in monitoring physiologic functions of the CNS during deep hypothermic bypass and total circulatory arrest. For example, during deep hypothermia and before total circulatory arrest, the processed EEG can identify residual cerebral electrical activity. Isoelectric silence can then be induced by further cooling and any further brain activity detected by EEG. Because this residual electrical activity during arrest is associated with ongoing cerebral metabolism, an isoelectric state may prevent ischemic injury to the brain during circulatory arrest. EEG also may be useful in detecting the level and depth of anesthesia. Postoperative electroencephalographic analysis has demonstrated subclinical seizure activity in a number of high-risk patients, potentially linking these abnormalities to poorer neuropsychologic outcome. The value of intraoperative electroencephalographic monitoring after CPB and the significance of the findings remain to be determined.

Transcranial Doppler imaging has been used primarily for research purposes in infants and allows detection of venous or arterial flow abnormalities and the detection of microemboli. This technology uses the Doppler principle to detect shifts in the frequency of reflected signals from blood in the middle cerebral artery to calculate blood flow velocity. Because the diameter of this large cerebral artery is relatively constant, flow velocity can be used to approximate CBF. Transcranial Doppler imaging has several advantages: (1) it is noninvasive, (2) it does not require radiation exposure, (3) it is a continuous monitor, and (4) it captures rapid alterations in blood flow velocity caused by temperature or perfusion changes. The limitations of transcranial Doppler monitoring include (1) reproducibility, especially at low flow rates, when small movements of the patient’s head can dramatically alter the signal, and (2) the lack of validating studies during hypothermic CPB, when hypothermia, reduced flow rates, and the laminar flow characteristics of non-pulsatile perfusion may limit the accuracy of CBF velocity measurements.

Transcranial Doppler imaging has been used to investigate the effect of CPB and DHCA on cerebral hemodynamics in children, as well as to assess the incidence of cerebral emboli. Recent studies examining the brain using transcranial Doppler have enabled several investigative groups to provide important information regarding questions of normal and abnormal brain perfusion during cardiac surgery in children. Questions regarding cerebral perfusion pressure, autoregulation, effect of Pa CO ₂ , and temperature have been addressed using transcranial Doppler imaging in children. This technique also has provided qualitative information regarding the presence of gaseous emboli in the middle cerebral artery during cardiac surgery.

NIRS is a noninvasive monitor of cerebral tissue oxygenation, reflecting the balance of O ₂ delivery and consumption. Cerebral NIRS reflects O ₂ saturation of the venous compartment, and values correlate with jugular venous bulb saturations. There is significant interest in studying the ability of NIRS to predict outcomes, particularly on the neurodevelopmental spectrum. A study of infants undergoing the stage 1 Norwood procedure for HLHS demonstrated an association between low NIRS (particularly when less than 50%-60%) and poor neurodevelopmental outcomes. Although there was not a linear association, it provides evidence that NIRS can be used to detect clinically consequential hypoxia. Somatic and cerebral saturations are further clinically relevant in the immediate postoperative period by predicting overall morbidity and mortality in patients with HLHS stage 1 palliation. The number of minutes of cerebral desaturation below 50% predicts morbidity and serves as an early warning sign for hypoxia, bleeding, and/or low cardiac output state. A decrease of greater than 20% from baseline in renal saturation as detected by NIRS for a period of 20 minutes has been associated with a longer duration of mechanical ventilation and ICU convalescence.

A 2016 review of NIRS use in pediatric cardiac surgery identified several prospective trials, mostly observational, that found inconsistent results overall regarding improved clinical outcomes with NIRS use. Importantly, the current evidence does not provide clear thresholds for cerebral or somatic NIRS, below which the chance of morbidity would increase. Differences in longer-term cardiac, renal, or neurodevelopmental outcomes have not been demonstrated based on differences in NIRS values. Despite these reservations, NIRS remains an important clinical tool to interpret alongside other monitoring technologies.

CBF studies using xenon clearance technology have improved the understanding of cerebrovascular dynamics in young children during CPB and especially during deep hypothermia and after periods of circulatory arrest. In general, this investigational tool has described the effects of CPB, temperature, and various perfusion techniques on CBF and, indirectly, on brain metabolism ( Fig. 78.6 ). Studies using this methodology have shown that some of the mechanisms of CBF autoregulation, such as pressure-flow regulation, are lost with deep hypothermia and that cerebral reperfusion is impaired after a period of total circulatory arrest.