Peads Cardiac Yao Flashcards
What is TOF?
In 1888, French physician Étienne-Louis Arthur Fallot described a congenital heart defect with four characteristics: (1) large VSD, (2) right ventricular (RV) outflow obstruction, (3) overriding aorta, and (4) right ventricle hypertrophy (RVH). Broadly defined, TOF is a complex of anatomic malformations consisting of a large malaligned conoventricular VSD; rightward and anterior displacement of the aorta such that it overrides the VSD; and a variable degree of subvalvar RVOT obstruction due to anterior, superior, and leftward deviation of the conal(infundibular) ventricular septum. In addition, RV abnormalities in the septal and parietal bands of the crista supraventricularis exacerbate the infundibular RVOT obstruction. RVH is the result of chronic RVOT obstruction. The most common associated lesion is a right aortic arch with mirror image arch vessel branching that occurs in 25% of patients with TOF. In this lesion, the innominate artery gives rise to the left carotid and left subclavian arteries and the right carotid and subclavian arteries arise separately. TOF/PS and TOF with pulmonary atresia are two broad subsets of TOF. A third rare variant of TOF occurs when the pulmonary valve is absent.
Tetralogy of Fallot with Pulmonary Stenosis
TOF/PS has the four features of TOF in conjunction with varying degrees of valvar PS. At one end of the spectrum, the pulmonary valve annulus is mildly hypoplastic with minimal fusion of the pulmonary valve leaflets (Fig. 38.1A). At the other end of the spectrum, the pulmonary annulus is very small and the pulmonary valve leaflets are nearly fused. RVOT from valvar PS is fixed, whereas the subvalvar component of RVOT obstruction is dynamic. If left uncorrected, compensatory RVH from valvar or subvalvar components of the RVOT increases the mass of the RV and the infundibulum and worsens subvalvar obstruction. The anatomy of TOF/PS can usually be definitively delineated by two-dimensional transthoracic echocardiography, so further imaging with cardiac catheterization is rarely necessary or indicated.
Tetralogy of Fallot with Pulmonary Atresia
TOF with pulmonary atresia has the features of TOF with infundibular and pulmonary valvar atresia in conjunction with varying degrees of pulmonary artery (PA) atresia. TOF with pulmonary atresia patients are divided into four groups based on the degree of PA atresia. Group 1 patients have isolated infundibular and pulmonary valve atresia with main PA and branch PAs of near-normal size and architecture. In some of these patients, the main PA may extend to the atretic infundibulum. In others, atresia involves a short segment of the main PA (Fig. 38.2). Patients in this group have pulmonary blood flow supplied from a patent ductus arteriosus (PDA). In group 2 patients, the main PA is absent, but the left and right PAs are in continuity and supplied by a PDA. Group 3 patients have severely hypoplastic native PAs, and the left and right PAsmay not be in continuity. Pulmonary blood flow is from collateral vessels from the aorta to the PAs, known as major aortopulmonary collateral arteries (MAPCAs). A PDA may be present as well. Some segments of lung may be perfused from MAPCAs, some only by the native PAs, and others by both sources (Fig. 38.3). Group 4 patients have no native PAs, and all pulmonary blood flow is derived entirely from MAPCAs.The anatomy of MAPCAs in TOF with pulmonary atresia can almost never be clearly delineated by two-dimensional transthoracic echocardiography alone. Cardiac catheterization and/or magnetic resonance imaging or magnetic resonance angiography are usually necessary to delineate collateral anatomy and determine QP:QS.
Tetralogy of Fallot with Absent Pulmonary Valve
TOF with absent pulmonary valve is the least common variant of TOF and accounts for only 3% to 6% of cases of TOF. Thesepatients have a displaced infundibulum, VSD, and varying degrees of RVOT obstruction as well as aneurysmal dilation of the main and branch PAs from in utero pulmonary regurgitation. The pathophysiology of TOF is complicated by symptoms of tracheobronchial compression that are similar to those caused by an anterior mediastinal mass. These patients usually must be positioned on their sides even when mechanically ventilated.
What is the pathophysiology of TOF/PS?
The pathophysiology of TOF/PS is a right-to-left (R-L) shunt characterized by the presence of an interventricular communication (VSD) and partial obstruction to RV outflow caused by RV infundibular and valvar stenosis. This is a complex shunt because resistance to RV outflow is a combination of the resistance from the obstructive lesions and the PVR. If the resistance from the RV obstructive lesions is high, changes in PVR will have little effect on shunt magnitude and direction. In most patients with TOF/PS, there is a fixed (valvar) and dynamic (variations in caliber of the RV infundibulum) component to RVOT obstruction. In addition, because the aorta overrides the VSD and the RV, desaturated systemic venous blood tends to stream out of the RV directly to the aorta, even in the presence of mild RVOT obstruction resulting in further systemic desaturation. The degree of cyanosis is related to the percentage of desaturated hemoglobin. A markedly hypoxemic child with baseline arterial saturations in the 70% range who is anemic may appear less cyanotic than another child with similar oxygen saturation and a hematocrit of 65%. The SaO2 is determined by the relative volumes and saturations of recirculated systemic venous blood and effective systemic blood flows that have mixed and reached the aorta. This is summarized in the following equation:This is demonstrated in Figure 38.4. Notice that the patient has a QP:QS = 0.5:1.
What is shunting, and how is QP:QS calculated?
Shunting is the process whereby venous return from one circulatory system is recirculated into the arterial outflow of the same circulatory system. Flow of blood from the systemic venous atrium (right atrium) to the aorta produces a physiologic R-L shunt with recirculation of systemic venous blood. Flow of blood from the pulmonary venous atrium (left atrium) to the PA produces a physiologic left-to-right (L-R) shunt withrecirculation of pulmonary venous blood. A physiologic R-L or L-R shunt is usually caused by a communication (orifice) at the level of the cardiac chambers or great vessels; however, physiologic shunts can exist in the absence of an anatomic shunt; transposition physiology is the best example. 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 and effective systemic blood flows 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 may 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 because the amount of recirculated systemic or pulmonary venous blood do not have to equal. QS (systemic cardiac output) tends to remain constant to maintain end-organ blood supply, even in the presence of a physiologic R-L shunt (systemic venous recirculation), although the SaO2 will be lower. Conversely, a physiologic L-R shunt (pulmonary recirculation) causes pulmonary volume overload. The calculation of QP:QS can be made by the Fick equation, where the pulmonary and systemic blood flows (QP and QS) are calculated individually by dividing the rate of oxygen extraction by the change in oxygen content in the arterial and venous sideof each circulation. The rate of oxygen extraction is the same as effective systemic or pulmonary blood flow and must be equal, so these numbers cancel out in the equation. The calculation of oxygen content is simplified when the determination is made using low inspired concentrations of oxygen (fraction of inspired oxygen [FIO2]), the contribution of oxygen carried in solution (PO2 × 0.003) can be ignored, so calculation of QP:QS can be simplified using oxygen saturations. Failure to account for the contribution of oxygen carried in solution, when calculation oxygen content of blood, will introduce substantial error. (SAO2 − SSVCO2) / (SPVO2 − SPAO2) where A = arterial, SVC = superior vena cava, PV = pulmonary vein that can be assumed to be 98% in the absence of significant pulmonary disease, and PA = pulmonary artery.
What is the pathophysiology of TOF with pulmonary atresia?
The pathophysiology of TOF with pulmonary atresia is characterized as single ventricle physiology because there is a VSD and little or no antegrade flow from the RV. In single ventricle physiology, there is complete mixing of pulmonary venous and systemic venous blood at the atrial or ventricular level and the ventricle(s) distributes output to both the systemic and pulmonary vascular beds. As a result of this physiology, the following are observed: Ventricular output is the sum of pulmonary blood flow (QP) and systemic blood flow (QS). Distribution of systemic and pulmonary blood flow isdependent on the relative resistances to flow (both intracardiac and extracardiac) into the two parallel circuits. Oxygen saturations are the same in the aorta and the PA. Single ventricle physiology can exist with one hypoplastic ventricle and one well-developed ventricle or with two wellformed anatomic ventricles when there is atresia or near atresia of outflow from one of the ventricles. Examples include the following: TOF with pulmonary atresia where pulmonary blood flow is supplied through a large PDA or MAPCAs Truncus arteriosus Severe neonatal aortic stenosis and interrupted aortic arch; in both lesions, a substantial portion of systemic blood flow is supplied through a PDA. Heterotaxy syndrome In cases where there is a single anatomic ventricle, there is complete or near-complete obstruction to inflow and/or outflow from the hypoplastic ventricle, so there must be an alternate source of either systemic or pulmonary blood flow to ensure postnatal survival. An example of alternate source of blood flow is in ductal-dependent circulation where a PDA forms a direct connection between the aorta and the PDA is the alternate (sole) source of systemic blood flow in hypoplastic left heart syndrome or of pulmonary blood flow in PA with intact ventricular septum. In other instances of a single anatomic ventricle, intracardiac pathways provide both systemic and pulmonary blood flow as in tricuspid atresia with normally related great vessels, a nonrestrictive VSD and minimal or absent PS.
. How is SaO2 determined in single ventriclephysiology?
The SaO2 is determined by the relative volumes and saturations of pulmonary venous and systemic venous blood flows that have mixed and reached the aorta. This is summarized in the following equation:The primary goal in the management of patients with single ventricle physiology is the optimization of systemic oxygen delivery and perfusion pressure to prevent end-organ (myocardial, renal, hepatic, splanchnic) dysfunction, so the systemic and pulmonary circulations must be balanced. The term balanced circulation is used because both laboratory and clinical evaluations have demonstrated that maximal systemic oxygen delivery (the product of systemic oxygen content and systemic blood flow) is achieved for single ventricle lesions when QP:QS is at or just below 1:1. Increases in QP:QS in excess of 1:1 are associated with a progressive decrease in systemic oxygen delivery because the subsequent increase in systemic oxygen content is more than offset by the progressive decrease in systemic blood flow and by diastolic hypotension due to runoff into the pulmonary circulation. Decreases in QP:QS below 0.7 to 0.8:1 are associated with a precipitous decrease in systemic oxygen delivery because the subsequent increase in systemic blood flow is more than offset by the dramatic decrease in systemic oxygen content.
What is a “pink Tet”?
The term pink Tet refers to any patient with TOF/PS or TOF with pulmonary atresia who is not cyanotic. In these patients, QP:QS is sufficiently high (usually greater than 0.8:1 in the presence of a normal mixed venous saturation and pulmonary vein saturations) to maintain a deoxyhemoglobin concentration less than 5 g per dL with SaO2 generally >80%. Examples are TOF/PS patients with minimal valvar and subvalvar PS and all patients with TOF with pulmonary atresia where pulmonary blood flow is supplied from a large PDA and/or MAPCAs.
What are hypercyanotic spells? How are they treated?
Hypercyanotic or “Tet” spells are sudden episodes of profound cyanosis in TOF patients that may be life-threatening. These spells occur more frequently in patients who are severely cyanotic at baseline but may occur in any patient with TOF. The peak frequency of spells is between 2 and 3 months of age. The onset of spells usually prompts urgent surgical intervention, so the anesthesiologist may care for an infant who is at great risk for spells during the perioperative period. The etiology of spells is not completely understood, but acute infundibular spasm that acutely decreases flow through the RV outflow tract and increased the magnitude of the R-L shunt through the VSD is thought to be causal. Crying, defecation, feeding, fever, and awakening from sleep all can be precipitating events. Paroxysmal hyperpnea is the initial finding with an increase in the rate and depth of respirations followed by increasing cyanosis and potential syncope, convulsions, or death. During a spell, the infant will appear pale and limp secondary to poor cardiac output. Hyperpnea maintains and worsens hypoxic spellsby increasing oxygen consumption and lowering the mean intrathoracic pressure, which increases systemic venous return. RV volume and pressure increase because there is an obstruction to RV outflow increasing the R-L shunt and the degree of cyanosis. During a spell, the infant will appear pale and limp secondary to poor cardiac output. Hyperpnea has several deleterious effects that can maintain and worsen a hypoxic spell. Hyperpnea increases oxygen consumption through the increased work of breathing. Hypoxia induces a decrease in SVR, which further increases the magnitude of the R-L shunt. Treatment of a “Tet spell” is focused on increasing pulmonary blood flow and decreasing the magnitude of the R-L shunt. It includes the following: Administration of 100% oxygen Compression of the femoral arteries or placing the patient in a knee-chest position to transiently increase SVR and reduce the magnitude of the R-L shunt Manual compression of the abdominal aorta to increase SVR and reduce the R-L shunt. This maneuver is particularly effective in the anesthetized patient. A cardiac surgeon can manually compress the ascending aorta to increase impedance to left ventricle (LV) ejection and reduce the R-L shunt. Administration of IV morphine sulfate (0.05 to 0.1 mg per kg) for sedation reduces oxygen demand and reduces hyperpnea by depressing the respiratory drive. Administration of 15 to 30 mL per kg of a crystalloid solution IV. Increasing preload to the heart increases the diameter of the RVOT and reduces dynamic RVOT obstruction. Administration of sodium bicarbonate to treat the severe metabolic acidosis that can be seen during a spell. Correction of the metabolic acidosis will help normalize SVR and reduce hyperpnea. Empiric administration of sodium bicarbonato 2 mEq per kg IV) in the absence of a blood gas determination should be considered during a spell. Phenylephrine (5 to 10 μg per kg IV or 2 to 5 μg/kg/min as an infusion) can be used to increase SVR and reduce R-L shunting. In the presence of severe RV outflow obstruction, phenylephrine-induced increases of PVR will have little or no effect on RV outflow resistance. Treatment with phenylephrine increases SVR and may decrease R-L shunting across the VSD and augment preload by decreasing unstressed venous volume, but the underlying cause of the spell is not treated. β-Adrenergic agonists are absolutely contraindicated. By increasing contractility, they will cause further narrowing of the stenotic infundibulum. Administration of β-adrenergic antagonists such as propranolol (0.1 mg per kg IV) or esmolol (0.5 mg per kg IV followed by an infusion of 50 to 300 μg/kg/min) may reduce infundibular spasm by depressing contractility. In addition, slowing of heart rate may increase preload by improving diastolic filling, increasing heart size, and the diameter RV outflow tract. Caution is warranted when administering βadrenergic agonists after administering α-adrenergic agonists because the LV may not tolerate the increased SVR after βmediated reduction in contractility. Extracorporeal membrane oxygenation resuscitation is another alternative in refractory episodes when immediate operative intervention is not possible.
What palliative surgical procedures are available for treating patients with TOF/PS?
Palliative shunt procedures to increase pulmonary blood flowand relieve cyanosis are used when complicated surgical anatomy or institutional practice precludes definitive repair at the time of presentation. The palliative shunts are systemic-topulmonary artery (L-R) shunts that increase pulmonary blood flow. The volume load imposed on the LV by these shunts parallels the increases in pulmonary flow that they produce. The progressive hypertrophy of the body and infundibulum of the RV that is characteristic of TOF is unchanged, so RV outflow tract obstruction increases in the interval from shunt placement to definitive repair. The shunts can be summarized as follows: Waterston shunt: a side-to-side anastomosis between the ascending aorta and the right PA. This procedure is performed through a right thoracotomy without CPB. Potts shunt: a side-to-side anastomosis between the descending aorta and the left PA. This procedure is performed through a left thoracotomy without CPB. Waterston and Potts shunts are of historic interest only. These shunts were difficult to size—too small of an orifice overly restricted pulmonary blood flow, whereas too large an orifice resulted in pulmonary overperfusion and congestion and predisposed the patient to development of unilateral pulmonary vascular obstructive disease. Furthermore, these shunts were difficult to take down at the time of the procedure and frequently distorted PA anatomy making definitive repair more difficult. Central shunt: a synthetic tube graft between the ascending aorta and the main or branch PA. This shunt can be performed with or without CPB through a thoracotomy or median sternotomy. It is usually only used when prior shunt procedures have failed. Blalock-Taussig shunt (BTS): an end-to-side anastomosis of the right or left subclavian artery to the ipsilateral branch PA. Currently, a modification of this procedure known as the MBTS is used. A Gore-Tex tube graft (usually 3.5 to 4.0 mm diameterin infants) is interposed between the subclavian or innominate artery and the branch PA. These shunts usually are performed on the side opposite the aortic arch through a thoracotomy without CPB (Fig. 38.5).
What palliative catheter-based procedures are available for patients with TOF?
A catheter-based palliative RVOT stent is an alternative to a systemic-to-pulmonary artery shunt in patients who are not candidates for a complete repair. These patients are usually symptomatic low birth weight infants with multiple comorbidities or infants with unfavorable anatomy (small PAs).The procedure is performed in the cardiac catheterization lab or an operating room with biplane fluoroscopy. The stent is placed through a femoral vein or a hybrid perventricular approach where the apex of the RV is approached through a subxiphoid incision. The indications for the procedure are not clearly defined and vary from institution to institution. Results including early morbidity, time to complete repair, and rate of PA growth have been encouraging when compared to an MBTS.
What definitive surgical procedures are available for treating patients with TOF/PS?
Most patients with TOF/PS have an elective full correction between the ages of 2 and 10 months. Definitive repair for TOF/PS in the neonatal period is done in some centers if favorable anatomy is present, whereas other centers delay surgery as long as possible until cyanotic episodes require urgent repair. Surgery is aimed at relieving the RV outflow obstruction by resection of the hypertrophied muscles that obstruct the RV outflow tract and enlargement of the RV outflow tract with a pericardial patch. A transannular pericardial patch is placed across the pulmonary valve annulus and into the main PA unless the pulmonic annulus is near-normal size and the pulmonary valve is only mildly stenotic. A transannular patch is avoided if possible because the pulmonary valve is rendered insufficient and free pulmonary regurgitation is present until the valve is repaired or replaced at an older age. If stenosis of the PA extends to the bifurcation, the pericardial patch can be extended beyond the bifurcation of the PAs. In neonates, the VSD is closed through the right ventriculotomy that was created for resection of the RVOT obstruction and placement of thetransannular patch. In infants and older children, the VSD can be closed through the right atrium and the tricuspid valve. An important surgical consideration for patients with TOF/PS is the presence of coronary artery abnormalities. Approximately 8% of patients have either the left main coronary artery or the left anterior descending artery as a branch of the right coronary artery. In these cases, a right ventriculotomy to enlarge the RVOT could endanger the left coronary artery. In such cases, an extracardiac conduit (RV to main PA) may be necessary to bypass the outflow tract obstruction and avoid injury to the coronary artery.
What palliative and definitive surgical procedures are available for patients with TOF with pulmonary atresia?
Patients with TOF with pulmonary atresia in groups 1 and 2 are dependent on prostaglandin E1 (PGE1) to maintain pulmonary blood flow through a PDA so surgery to establish a reliable source of pulmonary blood flow must be performed in the neonatal period. These patients may undergo a palliative shunt procedure or a definitive procedure. A definitive procedure includes placement of an RV to PA and closure of VSD, usually through the ventriculotomy used for the proximal attachment of the RV to PA conduit. Patients in groups 3 and 4 are more challenging to manage. These patients have single ventricle physiology with a tendencytoward excessive pulmonary blood flow (QP:QS >2 to 3:1) as the PVR drops in the neonatal period. In group 3 patients, neonatal repair with placement of an RV to PA conduit is undertaken to place the PAs in continuity with the RV in an effort to promote native PA growth. In this circumstance, the VSD is left open as a source of R-L shunting and delivery of desaturated blood to the systemic circulation because the RV cannot deliver an adequate cardiac output to the left atrium across the hypoplastic pulmonary vascular bed. These infants then undergo multiple cardiac catheterization procedures in order to dilate and stent the hypoplastic native PAs and to embolize MAPCAs, which provide pulmonary blood flow that is competitive to the blood flow supplied by native PAs. MAPCAs that provide pulmonary blood flow to segments of lung not supplied by native PAs must be unifocalized to the proximal pulmonary circulation. Unifocalization involves removal of the collateral vessel from the aorta with anastomosis to the RV to PA conduit or a proximal PA branch. This establishes antegrade blood flow from the RV to the pulmonary vascular bed. Usually, 80% to 90% of the pulmonary vascular bed (10 to 12 bronchopulmonary segments) must be in direct continuity with the RV to ensure that adequate cardiac output can cross the pulmonary bed and the VSD can be closed. In group 4 patients, the initial intervention may be to create an RV to PA conduit and unifocalize several large collaterals to the distal end of conduit from the RV. Alternatively, several large collaterals can be unifocalized to an MBTS or central shunt. These procedures serve to promote pulmonary vascular growth, prevent the development of pulmonary vascular obstructive disease, and control the QP:QS.
What are the long-term outcomes of patients with TOF?
Complete surgical repair of TOF has been performed for over 40 years with actuarial survival of 85% at 30 to 35 years. Most patients are largely asymptomatic and RV dysfunction may only be evident on echocardiography and exercise stress testing. Factors that adversely affect long-term survival include older age at initial repair, initial palliative procedures, and presence of residual PS and/or pulmonary regurgitation. RV dysfunction from chronic pressure and volume overload as well as the development of ventricular dysrhythmias may prompt pulmonary valve repair or replacement.
What preoperative history and physical examination information do you want?
The medical history and review of systems should include an assessment of ability to perform age-appropriate activities that will aid in the evaluation of cardiac function and reserve. The observation by a parent that the patient cannot keep the same pace as siblings is often a reliable clinical sign that cyanosis or congestive heart failure is worsening. Symptoms of low cardiac reserve include sweating, dyspnea, and circumoral cyanosis that may first occur during feeding, which is a strenuous activity for a newborn. Interpretation of vital signs must be age specific.Growth curves also are useful because congestive heart failure will inhibit age-appropriate gains in weight, height, and head circumference. It is not unusual for patients with severe congestive heart failure to weigh less at 3 or 4 months of age than at birth. Interestingly, cyanotic children often do not manifest this failure to thrive. It is often difficult to differentiate signs and symptoms of congestive heart failure from a mild upper respiratory tract infection in infants and children, so surgery may need to proceed even when the diagnosis is not clear. Children and infants with increased pulmonary blood flow are predisposed to multiple respiratory tract infections so surgery may need to proceed in these patients to reduce pulmonary blood flow and the propensity for chronic lung infections. Physical examination might reveal cyanosis, clubbing, or signs of congestive heart failure similar to those seen in adults, such as hepatomegaly, ascites, edema, or tachypnea. Some manifestations of congestive heart failure such as rales may not be heard in infants and children, so the severity of heart failure may be more reliably assessed by the signs and symptoms outlined previously. Physical examination should include an evaluation of the limitations to vascular access and monitoring sites imposed by previous surgeries. A child who has undergone a BT or MBT palliative shunt procedure may have a diminished pulse or unobtainable blood pressure in the ipsilateral arm, preventing accurate blood pressure and pulse oximetry measurement in that extremity. Venous access should also be assessed preoperatively because this may affect the mode of induction of anesthesia.
Which other abnormalities need to be considered inthis patient?
Approximately 8% of children with congenital heart disease have other congenital abnormalities. Patients with TOF/PS or TOF with pulmonary atresia have a higher incidence of 22q11.2 deletion syndrome. Different phenotypes of this syndrome include DiGeorge syndrome, velocardiofacial syndrome, and conotruncal anomaly face syndrome. These syndromes are characterized by hypocalcemia; immunodeficiency; facial dysmorphia; palate anomalies; velopharyngeal dysfunction; renal anomalies; possible tracheoesophageal fistula; as well as speech, feeding, neurocognitive, behavioral, and psychiatric disorders. Many of these defects may complicate airway management.
What premedication will you give to a child with congenital heart disease?
Premedication alleviates anxiety and eases separation of children from parents. In infants, premedication facilitates placement of an IV catheter for induction of anesthesia, alone or in combination with inhaled anesthetics. Midazolam (1 mg per kg PO) may be sufficient in infants and young children, but the addition of ketamine (3 to 5 mg per kg PO) is sometimes necessary in older children with heightened anxiety or in patients who are tolerant to benzodiazepines from previous procedures. Intramuscular (IM) ketamine (2 to 3 mg per kg) and glycopyrrolate (10 μg per kg) alone or in combination with midazolam (0.1 mg per kg) works well in children who will not take oral medications.
How will you induce anesthesia in this patient if intravenous (IV) access cannot be obtained?
Induction of anesthesia with IM ketamine (3 to 5 mg per kg IM), succinylcholine (5 mg per kg IM), and glycopyrrolate (10 μg per kg IM) is an alternative to IV induction in infants and neonates with difficult peripheral IV access. Glycopyrrolate is an important adjunct to reduce airway secretions associated with ketamine administration and to prevent the bradycardia, which may be associated with succinylcholine. This technique provides prompt induction of anesthesia, allowing for immediate control of the airway with endotracheal intubation. This is useful in circumstances where it is anticipated that initial IV access will have to be obtained through the external jugular vein, femoral vein, or internal jugular vein. IM rocuronium (1 mg per kg IM) can be substituted for the succinylcholine, but the time to adequate intubating conditions and duration of action is variable.
Why would end-tidal carbon dioxide (EtCO2) monitoring be of particular use in a patient with TOF/PS?
EtCO2 monitoring is particularly helpful in patients with congenital heart disease because the difference between arterial CO2 and alveolar CO2 reflects physiologic dead space ventilation and is indicative of changes in lung perfusion. Anacute reduction in pulmonary blood flow (decreased cardiac output, pulmonary embolus, or increased intracardiac R-L shunting) will increase this gradient. In a patient with TOF/PS, a gradual reduction in EtCO2 often precedes a decrease in SaO2 as the first manifestation of the increased R-L intracardiac shunting during a “Tet spell.”
What is near-infrared spectroscopy (NIRS) and what does it measure?
NIRS is a real-time, online monitor of cerebral tissue oxygenation. This technology is based on the physical principle that light of an appropriate wavelength passing through a solution of a colored compound (chromophore) will be absorbed by the compound. As a result of this absorption, the intensity of the light emerging from the solution will be lower than the intensity of the light projected into the solution. This principle through application of the Beer-Lambert equation “log [IO / I] = c α d” allows quantification of the concentration (c) of a chromophore if the emergent light intensity (I) is measured and the following are known: Extinction coefficient (α), a constant that describes the absorption characteristics of a particular chromophore at a given wavelength of light Thickness of the solution (d) Incident light intensity (IO) NIRS technology is particularly suited to use in neonates and infants because the thin skull and small head allow light to be transmitted through one side of the head and detected on the other side, a technique known as transmission spectrometry. Cerebral oxygen saturation (ScO2) as measured by all NIRStechnology is the combined oxygen saturation of an uncertain mix of arterioles, capillaries, and venules. Traditional pulse oximetry differs in this respect from NIRS because it is capable of isolating and measuring the arteriole component by gating measurements to arterial pulsatility. It has been previously assumed that ScO2 represented contributions of cerebral arterial and venous blood in a ratio of 25:75, with the contribution of capillary blood felt to be negligible. More recent data suggest that in children, the average ratio is 15:85. The issue is further complicated by the fact that there is significant variability in the ratio (from 0:100 to 40:60) between patients.
What are the important management issues during creation of a palliative shunt?
Unilateral lung retraction will be required for surgical exposure if a thoracotomy approach is used. The resulting atelectasis may severely compromise ventilation and worsen oxygenation and CO2 removal. The main or branch PAs will have to be partially occluded by a clamp to allow creation of the distal anastomosis of all palliative systemic to PA shunts. The resulting increase in physiologic dead space and PaCO2 and EtCO2 gradients may compromise oxygenation and CO2 removal. Efforts to optimize ventilation, reduce PVR, and increase pulmonary blood flow should be initiated before PA occlusion. Partial occlusion of the aorta with a clamp will be necessaryduring creation of Waterston, Potts, and central shunts. The resulting increase in LV afterload may compromise LV systolic function, and inotropic support may need to be initiated. All of the palliative shunts impose a volume load on the LV. Inotropic support may be necessary to ensure systemic and shunt perfusion after shunt creation. Palliative shunts are mildly restrictive simple shunts. It is important to maintain SVR and reduce PVR to maintain pulmonary blood flow in these patients. Be prepared to treat an episode of hypercyanosis.
What is the effect of inhalation anesthetics on airway reflexes, myocardial contractility, systemic vascular resistance (SVR), and pulmonary vascular resistance (PVR) in children?
Sevoflurane, isoflurane, and desflurane induce a dose-dependent reduction in cardiac output and SVR with mild depression of contractility at 1 and 1.5 minimal alveolar concentration (MAC). At these concentrations, sevoflurane and isoflurane do not alter the ratio of PVR to SVR enough to induce any change in QP:QS. Isoflurane and desflurane are not good choices for inhalational induction because their pungency is responsible for a high incidence of airway complications in children. Sevoflurane has cardiovascular effects similar to isoflurane and is a good agent for inhalational induction.
What are the pre-cardiopulmonary bypass (preCPB) anesthetic goals for a patient undergoing definitive surgical correction of TOF/PS?
Maintain heart rate, contractility, and preload to maintain cardiac output. Maintaining euvolemia is important to prevent exacerbation of dynamic RVOT obstruction from hypovolemia and reflex increases in heart rate and contractility. Avoid increases in the PVR:SVR ratio. Increases in PVR relative to SVR and decreases in SVR relative to PVR will increase R-L shunting, reduce pulmonary blood flow, and produce or worsen cyanosis. This effect is more pronounced when the RV outflow obstruction is not severe. Maintain or increase SVR. This is particularly important when RV outflow obstruction is severe, and changes in PVR will have little or no effect on shunt magnitude and direction. Treat episodes of hypercyanosis. Maintain contractility. Depression of contractility, particularly in the face of severe RV outflow obstruction, may produce RV afterload mismatch and drastically reduce pulmonary blood flow. The exception to this is a patient in whom the dynamic component of infundibular obstruction is active. Reducing contractility in these patients may reduce RV outflow obstruction through relaxation of the infundibulum.pulmonary vasoconstriction. An arterial O2 tension lower than 50 mmHg increases PVR over a wide range of arterial pH values. This effect is most pronounced when the pH is lower than 7.40. Reduce PCO2. Consistent reductions in PVR and increases in pulmonary blood flow and PO2 are seen in children with R-L shunts who are hyperventilated to a PCO2 near 30 mmHg and a pH near 7.50. Hypocarbia reduces PVR through production of an alkalosis, but hypercarbia increases PVR independent of changes in arterial pH. Patient with preoperative pulmonary hypertension who are hyperventilated post-CPB to a PCO2 of 30 to 35 mmHg and a pH of 7.50 to 7.56 have a reduction in PVR when compared with ventilation that produces normocarbia or hypercarbia. Reduce pH. Both respiratory and metabolic alkalosis reduce PVR, whereas respiratory and metabolic acidosis increase PVR. Variation in lung volumes. At small lung volumes, atelectasis compresses extra-alveolar vessels, whereas at high lung volumes, hyperinflation of alveoli compresses intra-alveolar vessels. Therefore, PVR is normally lowest at lung volumes at/or near the functional residual capacity. Positive endexpiratory pressure (PEEP) may cause an increase in PVR by increasing alveolar pressure through hyperinflation. However, a decrease in PVR generally is seen in situations in which PEEP works to recruit atelectatic alveoli and increase arterial PO2. Vasodilator agents. PGE1, prostaglandin I2 (PGI2), nitroglycerin, sodium nitroprusside, and tolazoline are IV drugs that induce pulmonary vasodilation as well as systemic vasodilation. There is no IV drug that selectively acts as a pulmonary vasodilator. Inhaled nitric oxide, PGE1, and PGI2 are the only specific pulmonary vasodilators available.
How does heparin administration and activated coagulation time (ACT) monitoring for CPB differ in children as compared with adults?
Adequate anticoagulation, usually defined as an ACT greater than 400 seconds, is necessary before cannulation and the commencement of CPB. Most of the evidence for assessing the adequacy of anticoagulation in children during heart surgery has been extrapolated from the adult literature. The ACT will overestimate the antifactor IIa and Xa effects of heparin in children because the ACT is also prolonged by hypothermia, hemodilution, platelet dysfunction, and low coagulation factor levels. These factors are more significant in children than in adults during CPB because of their small size relative to the CPB circuits. Most institutions use an age- or weight-based protocol to administer the initial pre-CPB dose of heparin. An example of a simple weight-based protocol is all patients weighing less than 30 kg receive 300 IU heparin per kg and patients greater than 30 kg receive 400 IU per kg. The large circuit prime volume to blood volume ratio in infants and children would be expected to decrease plasma heparin levels with initiation of CPB unless an appropriate quantity of heparin is added to the CPB prime. Most institutions add heparin to the CPB prime as follows: 3.0 IU heparin added per mL of CPB prime for patients less than 30 kg and 2.5 IU heparin added per mL of CPB prime in patients greater than 30 kg. Heparin should always be given into a central line through which venous return can be demonstrated easily or directly into the heart by the surgeon. This is necessary to ensure that the heparin dose has reached the central circulation. An ACT can bedrawn within minutes of heparin administration as peak arterial ACT prolongation occurs within 30 seconds and peak venous ACT prolongation within 60 seconds.
How is heparin reversed?
Systemic heparin is reversed with protamine, a polyvalent cation derived from salmon sperm. Protamine normally is given once the patient’s hemodynamics have stabilized after termination of CPB and the team is satisfied with the repair. The cardiotomy suction should not be used and the arterial and venous cannulas should be removed before protamine neutralization of heparin begins. This prevents contamination of the heparinized CPB circuit with protamine so that prompt reinstitution of CPB is possible, if necessary. There are several approaches to the neutralization of heparin with protamine, all with reportedly good clinical results. Some centers titrate the amount of protamine to the amount of heparin determined to exist at the termination of CPB using the in vitro protamine-heparin neutralization ratio of 1.3:1.0. This method had been criticized because the amount of heparin present postCPB is calculated based on the ACT, and the ACT can be affected by factors other than the concentration of heparin, such as CPB-induced platelet dysfunction, hypothermia, and hemodilution. This may result in an overestimation of the heparin present at the termination of CPB and results in a larger than necessary calculated protamine dose. Other centers administer a fixed dose of protamine based on the patient’sweight (3 to 4 mg per kg IV) regardless of the ACT or heparin dose administered, whereas others administer 1.0 to 1.3 mg of protamine for each 100 units of heparin administered prior to and during CPB.
What is the incidence of protamine reactions in children?
The incidence of protamine reactions in children following cardiac surgery is substantially lower than in adults. Acute reactions to protamine may be mild with transient erythema and hypotension or severe with bronchoconstriction, PA hypertension, and cardiovascular collapse. The mechanism has not been fully characterized. A recent retrospective analysis of 1,249 children showed the incidence of hypotension (defined as a 25% decrease in mean arterial pressure) following protamine administration to be 1.76% to 2.88%, depending on the stringency of criteria linking the hypotensive episode to protamine administration. In this series, no episodes of pulmonary hypertension or RV dysfunction were noted. Clinical experience indicates that pulmonary hypertensive episodes in children following protamine administration are very rare.
What is the role of transesophagealechocardiography (TEE) in this patient?
TEE has a major impact on post-CPB decision making, such as return to CPB to repair residual lesions in approximately 15% of cases when it is used nonselectively in children. In the subset of patients undergoing valve repair and outflow tract reconstruction (such as this patient), the impact of TEE would be expected to be greater by providing immediate assessment of the adequacy of the operative procedure and directing the revision, if necessary. Although detection of retained intracardiac air is facilitated by the use of intraoperative TEE, it remains to be determined what role TEE will play in improving cardiac deairing algorithms particularly in neonates/infants. The role of TEE in the detection of residual VSDs following repair of both simple and complex defects deserves some discussion. Residual defects smaller than 3 mm are detectable by TEE but generally do not require immediate reoperation because they are hemodynamically insignificant. The majority (75%) of these small defects are not present at the time of hospital discharge as determined by transthoracic echocardiography. Residual defects greater than 3 mm detected by TEE require immediate reoperation only when intraoperative assessment suggests that the defect will be hemodynamically significant. Examples of concerning findings are elevated left atrial pressure and/or PA pressure in the presence of good ventricular function, right atrium to PA oxygen saturation step-up with FIO2 ≤0.50, and oximetric measurement of QP:QS >1.5
What determines the oxygen saturation in patients with D-TGA?
Arterial saturation (SaO2) is determined by the relative volumes and saturations of the recirculated systemic and effective systemic blood flows reaching theThe anatomic left-to-right shunt produces effective systemic blood flow, which is the volume of pulmonary venous blood reaching the systemic circulation. The anatomic left-to-right shunt is the result of intercirculatory mixing at the atrial, ventricular, and arterial levels and must equal the amount of the right-to-left shunt at the same sites. The greater the effective systemic blood flow (intercirculatory mixing) relative to the recirculated systemic blood flow, the greater the SaO2. For a given amount of intercirculatory mixing and total systemicblood flow, a decrease in systemic venous or pulmonary venous saturation will result in a decrease in SaO2. 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 makes up the largest portion of total pulmonary and systemic blood flow, with effective blood flows contributing only a small portion of the total flows. This is particularly true in the pulmonary circuit where the total pulmonary blood flow and the volume of the pulmonary circuit (LA, LV, PA) is three to four times larger than the total systemic blood flow and the volume of the systemic circuit (RA, RV, aorta). The net result is transposition physiology, where the PA oxygen saturation is greater than the aortic oxygen saturation. Figure 39.1 further elucidates these concepts.
What are the preoperative issues pertaining to the coronary arteries in D-TGA?
As in normally related great vessels, the coronary arteries in DTGA arise from the aortic sinuses that face the PA. In normally related vessels, these sinuses are located on the anterior portion of the aorta, whereas in D-TGA, they are located posteriorly. In most D-TGA patients (70%), the right sinus is the origin of the right coronary artery, whereas the left sinus is the origin of the left main coronary artery. In the remainder of cases, there isconsiderable variability, with the most common variations being shown in Figure 39.2.Patients with certain types of coronary anatomy (intramural coronaries, single coronary artery) are at risk for postoperative myocardial ischemia and early mortality because reimplantation can result in the distortion of the coronary ostia or the narrowing of the artery itself. Patients with intramural coronaries generally require resuspension of the posterior leaflet of neopulmonary valve once the coronaries and a surrounding tissue cuff are excised. The presence of a single coronary artery or intramural coronary arteries is a risk factor for mortality, and this risk has persisted over the last two decades despite improvement in surgical technique.
What are the clinical subsets of D-TGA?
The four clinical subsets of D-TGA are characterized by differences in anatomy, amount of pulmonary blood flow, and sites of intercirculatory mixing. These are summarized in Table 39.1.
What is the differential diagnosis of D-TGA and how is the diagnosis made?
D-TGA may be associated with either cyanosis or CHF. Cyanosis is severe in patients with limited intercirculatory mixing. Infants with a large quantity of intercirculatory mixing have increased pulmonary blood flow, mild cyanosis, and symptoms of CHF. Chest radiographs may appear normal in the first few weeks of life in infants with D-TGA and IVS. Eventually, the triad of an enlarged egg-shaped heart (large RA and RV), narrow superior mediastinum, and increased pulmonary vascular markings evolve. In patients with D-TGA and VSD without LVOT obstruction, a large cardiac silhouette and prominent pulmonary vascular markings are seen at birth. Right axis deviation and right ventricular hypertrophy are the electrocardiographic (ECG) findings in D-TGA with IVS, whereas right axis deviation, LV hypertrophy, and RV hypertrophy are seen with D-TGA and VSD. Two-dimensional echocardiography is the diagnostic modality of choice in the diagnosis and assessment of infants with D-TGA. It accurately establishes the diagnosis of D-TGA and reliably identifies associated abnormalities, such as VSD, mitral valve, and tricuspid valve abnormalities, and LVOT obstruction. It also reliably delineates coronary artery anatomy. Echocardiographic analysis of the IVS position or LV geometry is also used to noninvasively assess the LV to RV pressure ratio and IV mass in neonates with D-TGA and IVS who are being evaluated as candidates for an ASO. In institutions with high-level echocardiography, a comprehensive cardiac catheterization is no longer routinely performed in neonates with D-TGA. A limited catheterization may be performed in conjunction with a balloon atrial septostomy. In the rare instance where coronary anatomy cannot be clearly delineated by echocardiography, coronary angiography may be indicated. During catheterization of infantsD-TGA may be associated with either cyanosis or CHF. Cyanosis is severe in patients with limited intercirculatory mixing. Infants with a large quantity of intercirculatory mixing have increased pulmonary blood flow, mild cyanosis, and symptoms of CHF. Chest radiographs may appear normal in the first few weeks of life in infants with D-TGA and IVS. Eventually, the triad of an enlarged egg-shaped heart (large RA and RV), narrow superior mediastinum, and increased pulmonary vascular markings evolve. In patients with D-TGA and VSD without LVOT obstruction, a large cardiac silhouette and prominent pulmonary vascular markings are seen at birth. Right axis deviation and right ventricular hypertrophy are the electrocardiographic (ECG) findings in D-TGA with IVS, whereas right axis deviation, LV hypertrophy, and RV hypertrophy are seen with D-TGA and VSD. Two-dimensional echocardiography is the diagnostic modality of choice in the diagnosis and assessment of infants with D-TGA. It accurately establishes the diagnosis of D-TGA and reliably identifies associated abnormalities, such as VSD, mitral valve, and tricuspid valve abnormalities, and LVOT obstruction. It also reliably delineates coronary artery anatomy. Echocardiographic analysis of the IVS position or LV geometry is also used to noninvasively assess the LV to RV pressure ratio and IV mass in neonates with D-TGA and IVS who are being evaluated as candidates for an ASO. In institutions with high-level echocardiography, a comprehensive cardiac catheterization is no longer routinely performed in neonates with D-TGA. A limited catheterization may be performed in conjunction with a balloon atrial septostomy. In the rare instance where coronary anatomy cannot be clearly delineated by echocardiography, coronary angiography may be indicated. During catheterization of infantswith PAH, a trial of ventilation at an fraction of inspired oxygen (FIO2) of 1.0 may be used to determine whether PVR is fixed or remains responsive to oxygen-induced pulmonary vasodilation.
Is CPB in infants and children different from adults?
Flows of 2.0 to 2.5 L/min/m2 are commonly used for infants, children, and adults during mild to moderate systemic hypothermia. Flow rates (expressed in mL/kg/min) will be substantially higher in the neonate than in the adult because of age-related differences in surface area to weight ratios. The recommended full flow rates for pediatric CPB are summarizedThe vast majority of operative procedures in children require the use of either total CPB or DHCA. Arterial and venous cannulation can be particularly challenging in neonates and infants. Venous cannulation for DHCA is usually accomplished with a single large venous cannula in the RA. Once cooling is complete, CPB flow rates are reduced to zero and the venous cannula is removed to allow for maximum surgical exposure. In cases requiring total CPB, the superior and inferior cavae and other sources of systemic venous return are cannulated and surgical tourniquets or tapes are passed around the externalcircumference of the vessels. When these tapes are tightened around the cannulae, the right heart can be isolated completely and opened without entrainment of air into the venous circuit or obscuration of the surgical field by venous blood. At least one tape must be left untightened during delivery of antegrade cardioplegia to allow egress of cardioplegia solution from the coronary sinus without distention of the RA. The superior vena cava (SVC) is larger than the inferior vena cava (IVC) in neonates and infants, so the larger venous cannula is usually placed in the SVC. The opposite is true in older children where two-thirds of systemic venous return is from the IVC. Aortic cannulation in children may be complicated by a hypoplastic ascending aorta or aortic arch, as in patients with hypoplastic left heart syndrome. In these cases, systemic circulation is ductal dependent, so it may be necessary to establish systemic perfusion through the ductus arteriosus by cannulating the PA. If the aortic arch is interrupted, cannulation of the aorta proximal and distal to the interruption is necessary. In infants and children, the arterial cannula is large relative to the size of the aorta, so constant vigilance is necessary to avoid complete or partial obstruction of the aorta by the cannula. Signs of this include dampening of the arterial pressure tracing, distension of the systemic ventricle, and an increase in the aortic line pressure during placement or manipulation of the aortic cannula. The surgeon must reposition the line to ensure adequate perfusion. Some centers utilize near-infrared spectroscopy and transcranial Doppler ultrasonographic technology as adjunctive methods to assess cerebral blood flow during CPB.
