Pediatric Anesthesia and Critical Care

The management of congenital heart disease (CHD) has progressed significantly over the past three decades. Most congenital heart lesions are now amenable to anatomic or physiologic repair early in infancy. Advances in diagnostic and interventional cardiology, the evolution of surgical techniques and conduct of cardiopulmonary bypass, and refinements in postoperative management have all contributed to a substantial decrease in morbidity and mortality associated with CHD. The approach to repairing CHD as early as possible, preferably in the neonatal period, has had significant implications for the anesthetic care of these critically ill infants during cardiac surgery. To meet this challenge, a clear understanding of neonatal respiratory and cardiac physiology, neonatal responses to anesthesia and surgery, and the pathophysiology of complex congenital heart defects is necessary.

Pathophysiology

Care of the critically ill neonate requires an appreciation of the special structural and functional features of immature organs. The neonate appears to respond more quickly and extremely to physiologically stressful circumstances; this can be expressed in terms of rapid changes in, for example, pH, lactic acid, glucose, and temperature.

The physiology of the preterm and full-term neonate is characterized by a high metabolic rate and O ₂ demand (a twofold to threefold increase compared with adults), which may be compromised at times of stress because of limited cardiac and respiratory reserve. The myocardium in the neonate is immature, with only 30% of the myocardial mass being composed of contractile tissue, compared with 60% in mature myocardium. In addition, neonates have a lower velocity of shortening, a diminished length–tension relationship, and a reduced ability to respond to afterload stress. Because the compliance of the myocardium is reduced, the stroke volume is relatively fixed and cardiac output is heart rate dependent; therefore, the Frank-Starling relationship is functional only within a narrow range of left ventricular end-diastolic pressure. The cytoplasmic reticulum and T-tubular system are underdeveloped, and the neonatal heart is dependent on the trans-sarcolemmal flux of extracellular calcium both to initiate and sustain contraction.

Cardiorespiratory interactions are important in neonates and infants. In simple terms, ventricular interdependence refers to a relative increase in ventricular end-diastolic volume and pressure, causing a shift of the ventricular septum and diminished diastolic compliance of the opposing ventricle. This effect is particularly prominent in the immature myocardium. Therefore, a volume load from an intracardiac shunt or valve regurgitation, and a pressure load from ventricular outflow obstruction or increased vascular resistance, could lead to biventricular dysfunction. For example, in neonates with tetralogy of Fallot and severe outflow obstruction, hypertrophy of the ventricular septum can contribute to diastolic dysfunction of the left ventricle and an increase in end-diastolic pressure. This does not improve immediately after repair in the neonate, as it takes some weeks or months for the myocardium to remodel; therefore, an elevated left atrial pressure is not an unexpected finding after neonatal tetralogy repair. This circumstance can be exacerbated further if there is a persistent volume load to the left ventricle after surgery, such as from residual ventricle septal defects (VSDs).

The mechanical disadvantage of an increased chest wall compliance and reliance on the diaphragm as the main muscle of respiration limits ventilatory capacity in the neonate. The diaphragm and intercostal muscles have fewer type I muscle fibers (i.e., slow-contracting, high oxidative fibers for sustained activity), and this contributes to early fatigue when the work of breathing is increased. In the newborn, only 25% of fibers in the diaphragm are type I, reaching a mature proportion of 55% by 8 to 9 months of age. Diaphragmatic function can be compromised significantly by raised intra-abdominal pressure, such as from gastric distention, hepatic congestion, or ascites.

The tidal volume of full-term neonates is 6 to 8 mL/kg and, because of the mechanical limitations just mentioned, minute ventilation is respiratory-rate dependent. The resting respiratory rate of the newborn infant is between 30 and 40 breaths per minute, which provides the optimal alveolar ventilation to overcome the work of breathing and match the compliance and resistance of the respiratory system. When the work of breathing increases, such as with parenchymal lung disease, airway obstruction, cardiac failure, or increased pulmonary blood flow, a larger proportion of total energy expenditure is required to maintain adequate ventilation. Infants therefore fatigue readily and fail to thrive.

The neonate has a reduced functional residual capacity (FRC) secondary to an increased chest wall compliance (FRC being determined by the balance between chest wall and lung compliance). Closing capacity is also increased in newborns, with airway closure occurring during normal tidal ventilation. Oxygen reserve is therefore reduced, and in conjunction with an increased basal metabolic rate and oxygen consumption two to three times adult levels, neonates and infants are at risk for hypoxemia. However, atelectasis and hypoxemia do not occur in the normal neonate because FRC is maintained by dynamic factors, including tachypnea, breath stacking (early inspiration), expiratory breaking (expiratory flow interrupted before zero flow occurs), and laryngeal breaking (auto–positive end-expiratory pressure [PEEP]).

Organ immaturity of the liver and kidney may be associated with reduced protein synthesis and glomerular filtration, such that drug metabolism is altered and synthetic function is reduced. These problems can be compounded by the normally increased total body water of the neonate compared with the older patient, along with the propensity of the neonatal capillary system to leak fluid out of the intravascular space. This is especially pronounced in the neonatal lung, in which the pulmonary vascular bed is almost fully recruited at rest and the lymphatic recruitment required to handle increased mean capillary pressures associated with increases in pulmonary blood flow may be unavailable.

The caloric requirement for neonates, especially preterm neonates, is high (100 to 150 kcal/kg per 24 hr) because of metabolic demand. The task of supplying nutrition for growth becomes even more difficult when necessary limits are placed on the total amount of fluid that may be administrated parentally or by the enteral route. Hyperosmolar feedings have been associated with an increased risk for necrotizing enterocolitis in the preterm neonate, or to the neonate born at term who has decreased splanchnic blood flow of any cause (e.g., left-sided obstructive lesions).

Physiologic Approach to Congenital Heart Disease

Specific classification of congenital heart defects is difficult because of the complex nature of many lesions. Basing identification and classification on physiology brings an organized framework to the intraoperative anesthetic management and postoperative care of children with complex CHD.

Single Ventricle Physiology

Single ventricle physiology is used to describe the situation wherein complete mixing of pulmonary venous and systemic venous blood occurs at the atrial or ventricular level and the ventricles then distribute output to both the systemic and pulmonary beds. Because of this physiology:

Ventricular output is the sum of pulmonary blood flow (Qp) and systemic blood flow (Qs).
Distribution of systemic and pulmonary blood flow is dependent on the relative resistances to flow (both intra- and extra-cardiac) into the two parallel circuits.
Oxygen saturations are the same in the aorta and the pulmonary artery.

This physiology can exist in patients with one well-developed ventricle and one hypoplastic ventricle and in patients with two well-formed ventricles.

In the case of a single anatomic ventricle, there is always obstruction to either pulmonary or systemic blood flow because of complete or near complete obstruction to inflow and/or outflow from the hypoplastic ventricle. In this circumstance, there must be a source of both systemic and pulmonary blood flow to assure postnatal survival. In some instances of a single anatomic ventricle, a direct connection between the aorta and the pulmonary artery via a patent ductus arteriosus (PDA) is the sole source of systemic blood flow (hypoplastic left heart syndrome) or of pulmonary blood flow (pulmonary atresia with intact ventricular septum); this is known as ductal dependent circulation . In other instances of a single anatomic ventricle, intracardiac pathways provide both systemic and pulmonary blood flow without the necessity of a PDA. This is the case in tricuspid atresia with normally related great vessels, a nonrestrictive VSD, and minimal or absent pulmonary stenosis.

Single ventricle physiology can exist in the presence of two well-formed anatomic ventricles when there is complete or near complete obstruction to outflow from one on the ventricles:

Tetralogy of Fallot (TOF) with pulmonary atresia (PA) where pulmonary blood flow is supplied via a PDA or multiple aortopulmonary collateral arteries (MAPCA)
Truncus arteriosus
Severe neonatal aortic stenosis and interrupted aortic arch (in both lesions a substantial portion of systemic blood flow is supplied via a PDA)
Heterotaxy syndrome, in which there are components of systemic venous (superior vena cava, inferior vena cava, hepatic veins, azygous veins) and pulmonary venous return to both right and left sided atria, and in which atrial morphology is ambiguous

With single ventricle physiology, the arterial saturation (SaO ₂ ) will be determined by the relative volumes and saturations of pulmonary venous and systemic venous blood flows that have mixed and reach the aorta. This is summarized in the following equation:

\begin{matrix} Aortic saturation = [(Systemic venous saturation) \\ \times (Total systemic venous blood flow) \\ + (Pulmonary venous saturation) \\ \times (Total pulmonary venous blood flow)] / \\ (Total systemic venous blood flow \\ + Total pulmonary venous blood flow) \end{matrix}

$\begin{matrix} Aortic saturation = [(Systemic venous saturation) \\ \times (Total systemic venous blood flow) \\ + (Pulmonary venous saturation) \\ \times (Total pulmonary venous blood flow)] / \\ (Total systemic venous blood flow \\ + Total pulmonary venous blood flow) \end{matrix}$

A typical example is as follows:

{SaO}_{2} = [(65) (3.3) + (98) (2.8)] / (3.3 + 2.8) = 80 %

${SaO}_{2} = [(65) (3.3) + (98) (2.8)] / (3.3 + 2.8) = 80 %$

Intercirculatory Mixing

Intercirculatory mixing is the unique situation that exists in transposition of the great vessels, in which two parallel circulations exist because of the existence of atrioventricular concordance (RA-RV, LA-LV) and ventriculoarterial discordance (RV-Ao, LV-PA). This produces a parallel rather than a normal series circulation. In this arrangement, blood flow will consist of parallel recirculation of pulmonary venous blood in the pulmonary circuit and systemic venous blood in the systemic circuit. Therefore, the physiologic shunt or the percentage of venous blood from one system that recirculates in the arterial outflow of the same system is 100% for both circuits. Unless there are one or more communications (atrial septal defect [ASD], PFO, VSD, PDA) between the parallel circuits to allow intercirculatory mixing, this lesion is incompatible with life.

An anatomic right-to-left (R-L) shunt is necessary to provide effective pulmonary blood flow, whereas an anatomic left-to-right (L-R) shunt is necessary to provide effective systemic blood flow. Effective pulmonary blood flow, effective systemic blood flow, and the volume of intercirculatory mixing must always be equal. Total systemic blood flow is the sum of recirculated systemic venous blood plus effective systemic blood flow. Likewise, total pulmonary blood flow is the sum of recirculated pulmonary venous blood plus effective pulmonary blood flow. Recirculated blood composes the largest portion of total pulmonary and total systemic blood flow, with effective blood flows contributing only a small portion of the total flows. This is particularly true in the pulmonary circuit in which the total pulmonary blood flow (Q _P ) and the volume of the pulmonary circuit (LA-LV-PA) is twofold to threefold greater than the total systemic blood flow (Q _S ) and the volume of the systemic circuit (RA-RV-Ao). The net result is transposition physiology, wherein the pulmonary artery oxygen saturation is greater than the aortic oxygen saturation.

Arterial saturation (SaO ₂ ) will be determined by the relative volumes and saturations of the recirculated systemic and effective systemic venous blood flows reaching the aorta. This is summarized in the following equation:

\begin{matrix} Aortic saturation = [(Systemic venous saturation) \\ \times (Recirculated systemic venous blood flow) \\ + (Pulmonary venous saturation) \\ \times (Effective systemic venous blood flow)] / \\ (Total systemic venous blood flow) \end{matrix}

$\begin{matrix} Aortic saturation = [(Systemic venous saturation) \\ \times (Recirculated systemic venous blood flow) \\ + (Pulmonary venous saturation) \\ \times (Effective systemic venous blood flow)] / \\ (Total systemic venous blood flow) \end{matrix}$

A typical example is as follows:

{SaO}_{2} = [(50) (1.2) + (99) (1.1)] / 2.3 = 73 %

${SaO}_{2} = [(50) (1.2) + (99) (1.1)] / 2.3 = 73 %$

Simple Shunts

Shunting is the process whereby venous return into one circulatory system is recirculated through the arterial outflow of the same circulatory system. Flow of blood from the systemic venous atrium or right atrium to the aorta produces recirculation of systemic venous blood. Flow of blood from the pulmonary venous atrium or left atrium to the pulmonary artery produces recirculation of pulmonary venous blood. Recirculation of blood produces a physiologic shunt. Recirculation of pulmonary venous blood produces a physiologic L-R, whereas recirculation of systemic venous blood produces a physiologic R-L shunt. A physiologic R-L or L-R shunt commonly is the result of an anatomic R-L or L-R shunt. In an anatomic shunt, blood moves from one circulatory system to the other via a communication at the level of the cardiac chambers or great vessels. Physiologic shunts can exist in the absence of an anatomic shunt. Transposition physiology is the primary example of this process.

Effective blood flow is the quantity of venous blood from one circulatory system reaching the arterial system of the other circulatory system. Effective pulmonary blood flow is the volume of systemic venous blood reaching the pulmonary circulation, whereas effective systemic blood flow is the volume of pulmonary venous blood reaching the systemic circulation. Effective pulmonary blood flow and effective systemic blood flows are the flows necessary to maintain life. Effective pulmonary blood flow and effective systemic blood flow are always equal, no matter how complex the lesions. Effective blood flow usually is the result of a normal pathway through the heart, but it can occur as the result of an anatomic R-L or L-R shunt.

Total pulmonary blood flow (Qp) is the sum of effective pulmonary blood flow and recirculated pulmonary blood flow. Total systemic blood flow (Qs) is the sum of effective systemic blood flow and recirculated systemic blood flow. Total pulmonary blood flow and total systemic blood flow do not have to be equal. Therefore, it is best to think of recirculated flow (physiologic shunt flow) as the extra, non-effective flow superimposed on the nutritive effective blood flow.

Shunts causing an increase in pulmonary blood flow may be simple or complex, occurring among the ventricles, atria, or great arteries, and they are described by the ratio of pulmonary blood flow (Qp) to systemic blood flow (Qs), or Qp/Qs. Patients may be acyanotic or cyanotic, have one or two ventricles, or have a single outflow trunk, yet have a significant increase in Qp/Qs and be at risk for congestive heart failure (CHF) and pulmonary hypertension ( Table 110-1 ).

TABLE 110-1

Simple Shunts: Defects and Surgical Procedures Contributing to an Increased Ratio of Pulmonary Blood Flow (Qp) to Systemic Blood Flow (Qs)

Type of Shunt	Acyanotic	Cyanotic
Two ventricles	ASD VSD CAVC DORV	D-TGA/VSD PA/VSD
Single ventricle	—	TA ± TGA HLHS DORV/MA Norwood/Sano procedure BT shunt
Aortopulmonary (AP) connection	PDA Truncus arteriosus AP window	PA/MAPCA

AP, Aortopulmonary; ASD, atrial septal defect; BT, Blalock-Taussig; CAVC, complete atrioventricular canal; DORV, double-outlet right ventricle; D-TGA, dextro-transposition of the great arteries; HLHS, hypoplastic left heart syndrome; MA, mitral atresia; MAPCA, multiple aortopulmonary collateral arteries; PA, pulmonary atresia; PDA, patent ductus arteriosus; TA, tricuspid atresia; TGA, transposition of the great arteries; VSD, ventral septal defect.

In patients with large L-R shunts and low pulmonary vascular resistance, a substantial increase in pulmonary blood flow can occur. If the increase in pulmonary blood flow and pressure continues, structural changes occur in the pulmonary vasculature until eventually pulmonary vascular resistance (PVR) becomes persistently elevated. The time course for developing pulmonary vascular obstructive disease depends on the amount of shunting, but changes with some lesions may be evident by 4 to 6 months of age. The progression is more rapid when both the volume and pressure load to the pulmonary circulation is increased, such as with a large VSD. As PVR decreases in the first few months after birth and the hematocrit falls to its lowest physiologic value, the increased L-R shunt, and therefore volume load on the systemic ventricle, can lead to congestive cardiac failure and failure to thrive.

The end-diastolic volume is increased in patients with an increased Qp/Qs ratio, but the time course over which irreversible ventricular dysfunction develops is variable. Generally, if surgical intervention to correct the volume overload is undertaken within the first 2 years of life, residual dysfunction is uncommon.

The volume load on the systemic ventricle and increased end-diastolic pressure contribute to increased lung water and pulmonary edema by increasing pulmonary venous and lymphatic pressures. Compliance of the lung is therefore decreased, and airway resistance increased secondary to small airway compression by distended vessels. Lungs may feel stiff on hand ventilation and deflate slowly. Besides cardiomegaly on the chest radiograph, the lung fields are usually hyperinflated. Ventilation-perfusion mismatch contributes to an increased alveolar–arterial oxygen gradient, and dead-space ventilation. Minute ventilation is therefore increased, primarily by an increase in respiratory rate. Pulmonary artery and left atrial enlargement may compress main-stem bronchi, causing lobar collapse. Symptoms and signs of CHF to note in neonates and infants are shown in Box 110-1 .

Box 110-1

Symptoms and Signs of Cardiac Failure in a Neonate or Infant

Poor Growth

Poor feeding
Diaphoresis

Increased Work of Breathing

Tachypnea
Grunting
Flaring of ala nasi
Chest wall retraction

Decreased Cardiac Output

Tachycardia
Gallop rhythm
Cardiomegaly
Poor extremity perfusion
Hepatomegaly

Manipulating PVR is an important means of limiting pulmonary blood flow and pressure. During anesthesia, PVR can be maintained or increased by using a low fraction of inspired oxygen (Fi o ₂ ) and altering ventilation to achieve a normal pH and Pa co ₂ . Care must be taken at induction of anesthesia, because patients may have a diminished contractile reserve. Preload, contractility, and heart rate must be maintained; afterload reduction is often well tolerated and will reduce pulmonary flow and myocardial work.

Complex Shunts

In complex shunts, there is additional pulmonary or systemic outflow obstruction, and the Qp/Qs ratio is determined by the size of the orifice, the outflow gradient, and the resistance across the pulmonary or systemic vascular bed. The obstruction may be fixed as with valvular stenosis, or dynamic as in forms of TOF.

Outflow Obstruction

Severe outflow obstruction in the newborn may be associated with ventricular hypertrophy and vessel hypoplasia distal to the level of obstruction. The increased pressure load can cause ventricular failure, with mixing or shunting at the atrial or ventricular level (or both) necessary to maintain cardiac output if there is complete outflow obstruction. Maintenance of preload, afterload, and normal sinus rhythm is important to prevent a fall in cardiac output or coronary hypoperfusion. As the time to develop significant ventricular dysfunction is longer in patients with a chronic pressure load than in those with a chronic volume load, symptoms of CHF are uncommon unless the obstruction is severe and prolonged.

Pulmonary Hypertension

Pulmonary hypertension may be idiopathic. Patients with CHD typically have pulmonary hypertension secondary to increased pulmonary artery flow and pressure, pulmonary venous obstruction, or left atrial hypertension from systemic ventricular atrioventricular valve dysfunction or ventricular systolic or diastolic dysfunction. Factors that increase PVR and pulmonary pressures include light anesthesia with a poorly attenuated stress response, hypoxemia, hypoventilation with a fall in FRC and respiratory acidosis, metabolic acidosis, hypothermia, prolonged bypass with associated inflammatory response and capillary leak, and administration of protamine or blood products (e.g., platelets; Box 110-2 ).

Box 110-2

Causes of Abnormally Elevated Pulmonary Artery Pressure

Left-to-right shunt lesion (e.g., large ventricular septal defect or patent ductus arteriosus)
Pulmonary arteriolar smooth muscle hypertrophy (e.g., pulmonary vascular obstructive disease)
Increased pulmonary venous pressure
Mechanical obstruction of the pulmonary circulation
Anatomic defects (e.g., pulmonary vein or branch pulmonary artery stenosis)
Pulmonary embolus
Raised intrathoracic pressure
Lung hyperinflation
Lung hypoinflation and hypoplasia
Decreased alveolar oxygen tension
Acidemia (respiratory or metabolic)
Inflammatory response to cardiopulmonary bypass
Drugs: protamine
Hyperviscosity (from polycythemia)
Blood product administration (platelets)
Artifactual (e.g., monitoring problems, catheter malposition)

After repair of defects with large L-R shunts, pulmonary artery pressures may remain elevated immediately after bypass, as the pulmonary arteries initially remain reactive to factors that increase PVR. While the patient is on bypass, factors contributing to this elevated pressure include compression and atelectasis of the lung, and pulmonary edema from inadequate venting of the left atrium or from the humoral and cellular response to bypass. Attenuation of the stress response with deep anesthesia using high-dose narcotics will prevent increases in PVR. A high Fi o ₂ and hyperventilation to induce a respiratory alkalosis will reduce PVR, and boluses of bicarbonate may be necessary to maintain metabolic alkalosis. Ideally, the pH should be approximately 7.45 to 7.50 and the arterial CO ₂ should be 30 to 35 mm Hg. A strategy of hyperventilation to induce a respiratory alkalosis and lower PVR may have an adverse effect on central nervous system recovery by lowering cerebral blood flow. The pattern of ventilation and maintenance of lung volumes is important: atelectasis and decreases in lung compliance can cause a significant rise in PVR and pulmonary pressures. Changes in ventilation must be made cautiously and reassessed frequently.

Several intravenous vasodilators, including the nitric oxide (NO) donors nitroprusside and glycerol trinitrate, the phosphodiesterase inhibitor milrinone, the eicosanoids prostaglandin E ₁ and prostaglandin I ₂ , tolazoline, and isoproterenol have been used to treat postoperative patients with elevated PVR. The chief limitation of these pharmacologic agents is that their vasodilatory effects are not specific to the pulmonary vasculature; therefore, vasodilation of the systemic vasculature and systemic hypotension may accompany reduction of pulmonary hypertension.

Inhaled NO selectively dilates smooth muscle cells in small pulmonary vessels and lowers PVR. The selective effect of inhaled NO on the pulmonary vasculature is a result of the rapid uptake and inactivation by hemoglobin as NO diffuses from alveoli to the lumen of lung capillaries. The usefulness of inhaled NO for patients with congenital heart disease and pulmonary hypertension has been documented in several populations. After surgery, NO has been shown to reduce pulmonary artery pressure and PVR in patients with pulmonary venous obstruction, such as total anomalous pulmonary venous connection and mitral stenosis, to a lesser extent in patients with a large preexisting L-R shunt, and in those with cavopulmonary connections (Fontan physiology) or pulmonary hypertensive crises related to cardiopulmonary bypass (CPB). NO has also improved both pulmonary hypertension and impaired gas exchange in patients who have undergone lung transplantation. Patients with a variety of other pulmonary vascular or parenchymal diseases, including persistent pulmonary hypertension of the newborn, primary pulmonary hypertension, acute respiratory distress syndrome, and acute chest syndrome in sickle cell disease have also shown significant improvements in oxygenation from treatment with inhaled NO.

Recent therapeutic advances have significantly improved the prognosis for patients with pulmonary arterial hypertension. The role of newer pulmonary vasodilating drugs such as the phosphodiesterase type V inhibitor sildenafil, and endothelin I blocking drugs, such as bosentan, have shown encouraging results. The value of these drugs in children with CHD is yet to be established.

Preoperative Evaluation

Patients with complex defects require frequent evaluation and often repeated cardiac operations as a staged approach to surgical repair. Previous anesthetic, bypass, or surgical problems should be noted. In general, providing continuity of care in these patients, such as by a dedicated cardiac anesthesia service, is useful to ensure consistent management practices, and it enhances the long-term relationship with patients and families.

Failure to thrive is an important indicator of cardiopulmonary compromise. Symptoms as described in Box 110-1 should be noted. Murmurs and extra heart sounds may be difficult to interpret if tachycardic, but a palpable thrill usually indicates a significant murmur. Failure to thrive, lethargy, and poor exercise tolerance are significant symptoms in older children. Orthopnea, syncope, and palpitations may also be described. Recurrent respiratory infections and wheezing are common in patients with L-R shunts. Four-limb blood pressures should be compared, and room air baseline peripheral arterial saturations should be noted along with potential airway problems. The chest radiograph should be analyzed for cardiomegaly, pulmonary congestion, airway compression, and atelectasis. Echocardiographic assessment and cardiac catheterization results provide valuable information about anatomic structure, myocardial function, intracardiac pressures, shunting, and gradients across obstructions. They should be interpreted in conjunction with the cardiologist and surgeon. Patients with cardiac failure are often stabilized on digoxin, diuretics, and oral vasodilators such as captopril. Preoperative digoxin levels and hypokalemia must be checked.

The consequences of chronic hypoxemia also need special consideration. Polycythemia increases oxygen-carrying capacity, but when the hematocrit rises to greater than 65%, the increased blood viscosity causes stasis and potential thrombosis, and it exacerbates tissue hypoxia. Dehydration must be avoided, and intravenous (IV) maintenance fluids should be begun while the patient is fasting preoperatively. Bleeding disturbances, common in cyanotic patients, can result from thrombocytopenia, defective platelet aggregation, or clotting factor abnormalities.

Monitoring

The monitoring technique used for a patient should depend on the child's condition and the magnitude of the planned procedure. For elective patients, noninvasive monitoring (electrocardiography, pulse oximetry, capnography, and a noninvasive blood pressure cuff) is placed before induction of anesthesia.

Monitoring by electrocardiogram (ECG) is essential, because significant rhythm disturbances can occur before and after bypass, particularly with VSD and outflow tract surgery. Myocardial ischemia occurs in pediatric patients mostly because of anatomic and shunt-related problems rather than coronary occlusive disease. Anomalous coronary arteries are associated with a number of complex defects, such as transposition of the great vessels and pulmonary atresia. Ischemia also occurs when coronary perfusion pressure falls, such as in hypoplastic left heart syndrome, truncus arteriosus, and critical aortic stenosis. Ventricular fibrillation can occur in these settings, particularly on induction of anesthesia. Ischemia after bypass can result from air embolism or complications related to surgery, such as coronary reimplantation or coronary compression from conduits.

Pulse oximetry is an important monitor before and after bypass, as peripheral arterial saturation levels provide an indicator of pulmonary blood flow. The anesthesiologist needs to know the patient's baseline, prebypass peripheral O ₂ saturation (Sp o ₂ ), and the anticipated level after surgery. Causes for lower than expected Sp o ₂ , in patients with single-ventricle physiology, include pulmonary venous desaturation and intrapulmonary shunt, reduced pulmonary blood flow, and low cardiac output. For patients who have undergone a two-ventricle repair, a lower than expected Sp o ₂ is usually secondary to intrapulmonary shunting, because of parenchymal lung disease (e.g., atelectasis, edema) or restrictive pulmonary defects (e.g., pleural effusion, pneumothorax). After repair of a neonatal right ventricular outflow tract, such as TOF or truncus arteriosus, a small atrial communication is an advantage as it provides a R-L atrial shunt. Although these patients may be cyanotic immediately after surgery, the R-L shunt will decrease and the Sp o ₂ will rise as the compliance of the right ventricle improves.

Once the patient is anesthetized, a direct arterial line is placed percutaneously or through a cutdown. The site of the arterial line placement needs careful consideration. For example, patients undergoing placement of a modified Blalock-Taussig shunt from the subclavian or innominate artery should have the radial arterial line placed in the opposite extremity. Similarly, a right radial arterial line is necessary when repair of coarctation of the aorta is planned. The arch anatomy and possible aberrant arterial vessels are additional considerations when planning arterial access. Aortic root pressure monitoring may be necessary immediately after bypass if the peripheral arterial pressure is damped from hypothermia or low output state. Alternatively, a femoral artery catheter may provide a more reliable arterial waveform after CPB, particularly in newborns and infants, and it is often preferable to a peripheral arterial catheter. Care must be taken to prevent thrombus and distal limb ischemia, and femoral lines are best removed early once the patient is in stable condition. Caution is necessary when flushing arterial catheters in neonates and infants, because retrograde flow into the carotid arteries is possible.

Some centers routinely use central venous pressure monitoring for all cardiovascular surgery. Percutaneous central venous access enables titration of volume replacement and administration of vasoactive infusions before CPB, and during CPB it may provide a measure of the adequacy of cerebral venous drainage. Insertion of central venous catheters can be particularly difficult in pediatric patients, and central venous lines should be used with caution in neonates and infants because of the risk for infection and superior vena cava thrombosis, which can have significant sequelae if collateral veins are poorly developed. Transthoracic right and left atrial lines can be inserted by the surgeon for hemodynamic pressure monitoring and drug infusions after bypass. They have a low complication rate. In addition, they can be left in situ for longer during postoperative recovery and then easily removed in the intensive care unit (ICU). Swan-Ganz catheters are rarely used in pediatric cardiac surgery because of anatomic limitations. Direct pulmonary artery catheters can be inserted by the surgeon to measure pulmonary saturations, to detect residual outflow tract gradients, and for thermodilution measurement of cardiac output. Oximetric catheters can be placed percutaneously for continuous measurement of mixed venous oxygen saturation (SvO ₂ ) in the superior vena cava.

Ultrasound-guided technique has been shown to increase the overall success rate and reduce the incidence of traumatic complications associated with central venous cannulation. The anatomy of the central venous drainage should be known before attempting percutaneous cannulation. Heterotaxy syndrome and possible vein occlusions after previous catheterization are considerations, and if in doubt, ultrasound evaluation of the position and size of a central vein before cannulation is useful.

Neurologic Monitoring

Long-term neurodevelopmental impairment is common in newborns and infants undergoing repair for complex CHD. The etiologies of adverse neurologic sequelae in these patients are multifactorial and include prenatal, preoperative, intraoperative, and postoperative factors. Cerebral protection is a concern during bypass for congenital heart surgery, particularly if deep hypothermic arrest or low-flow bypass is used, and the importance of routine perioperative monitoring of the brain is increasingly recognized. Tympanic or nasopharyngeal temperature monitoring is used to assess the adequacy of cerebral cooling and rewarming. Continuous electroencephalographic monitoring, transcranial Doppler, and frontal lobe near-infrared spectroscopy or cerebral oximetry can be used to evaluate cerebral blood flow velocity and perfusion, and O ₂ delivery and extraction.

Intraoperative Echocardiography

Intraoperative transesophageal echocardiography (TEE) has achieved a role in intraoperative monitoring of patients undergoing repair of CHD. The development of smaller probes has allowed transesophageal monitoring to replace epicardial echocardiographic imaging in many cases, and it is performed routinely. Placement of a transesophageal probe after the induction of anesthesia in the operating room enables reevaluation of the anatomy before surgical intervention, but, more importantly, the adequacy of surgical repair can be evaluated as soon as the patient is weaned from CPB. Interference of the probe with the airway and the effect on unstable hemodynamics before and after CPB must be evaluated carefully to avoid the complications of this monitoring.

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