Anesthesia for the patient with a single ventricle

Transcription

1 22 Anesthesia for the patient with a single ventricle Susan C. Nicolson James M. Steven Introduction In the early 1970s, Fontan 1 and Kreutzer 2 independently introduced operative treatment of tricuspid atresia that resulted in nearly normal systemic arterial oxygen saturation and normal volume work for the single ventricle. This procedure, subsequently referred to as the Fontan operation, created a series circulation which requires the single ventricle to pump fully saturated blood only to the systemic circulation, thereby reducing the pressure and volume work to that of a normal systemic ventricle. The systemic venous drainage passes directly through the pulmonary vascular bed without benefit of a pumping chamber. The child s pulmonary vascular resistance (PVR) must be low to maintain the pulmonary circulation and the cardiac output (CO) on which it depends. Since that time, the principle of the Fontan operation has been applied to the full spectrum of cardiac lesions with one functional ventricle. Suitable physiology for ultimate repair by a modification of the Fontan procedure is predicated on carefully planned, appropriately timed and executed palliative operations designed for the specific patient s single ventricle physiology. This chapter will illustrate these principles using hypoplastic left heart syndrome (HLHS), the most common congenital cardiac malformation where there is only one developed ventricle. Hypoplastic left heart syndrome represents the fourth most common defect presenting in the neonatal period and accounts for 7.5% of the newborns with congenital heart disease sufficiently significant to require early therapeutic intervention. A variety of anatomic lesions may be classified as producing single ventricle anatomy or physiology, also referred to as a univentricular heart (Fig. 22.1). Most of these lesions have both atrioventricular (AV) valves committed to a single systemic ventricular chamber, or atresia/severe stenosis of one of the AV valves. Many of these hearts actually have two ventricles, although one of the ventricles is typically small with minimal contribution to cardiac ejection, thus it is functionally considered a single ventricle. Finally, some patients have (a) (b) Fig Basic anatomical features of tricuspid atresia (a) and hypoplastic left heart syndrome (b). In tricuspid atresia, pulmonary blood flow depends on the degree of hypoplasia of the pulmonary valve and artery. In hypoplastic left heart syndrome, there may be atresia or stenosis of the aortic and mitral valves, but all variants have a small or non-existent left ventricle and a hypoplastic ascending aorta. Reproduced with permission from Wernovsky G, Bove EL. Single ventricle lesions. In: Chang C, Hanley FL, Wernovsky G, Wessel DL, eds. Pediatric Cardiac Intensive Care. Baltimore, MD: Williams & Wilkins, 1998: two ventricles of normal size which cannot be separated because a ventricular septal defect ( VSD) is remote from either great vessel, or because of straddling of the AV valve attachments over the VSD, or very large intraventricular communications that cannot be septated. After assessment of the details of the cardiac anatomy, the initial approach to the single ventricle patient is to determine the degree of pulmonary blood flow (PBF) and whether the pulmonary circulation is dependent on the ductus arteriosus. Patients with ductal dependent circulation will require a reliable source of PBF, accomplished by creation of a systemicto-pulmonary shunt. The most common shunt performed is a modified Blalock Taussig (BT) shunt, in which a Gortex tube graft connects the innominate or subclavian to the pulmonary artery (PA) (Fig. 22.2). Some single ventricle patients will have unrestricted flow to both the PA and aorta. Such patients usually develop progressive congestive heart failure as the PVR decreases in infancy, and require PA banding to 356

2 CHAPTER 22 Anesthesia for the patient with a single ventricle Table 22.1 Initial surgical strategy for single ventricle patients. Anatomy Surgical intervention 2 Semilunar valves of adequate size, normal aortic arch Pulmonary artery band 1 Semilunar valve, normal aortic arch BT shunt 1 Semilunar valve, hypoplastic aortic arch Aortic arch reconstruction with BT shunt, or Norwood procedure 2 Semilunar valves, aortic stenosis Damus, Kaye, Stanzel with BT shunt (possible aortic arch reconstruction) or palliative arterial switch 2 Semilunar valves with pulmonary stenosis No initial intervention required BT, Blalock Taussig. (a) (b) Fig (a) Right modified Blalock Taussig shunt. A 3 4 mm Goretex graft is sewn between the right subclavian or innominate artery and the right pulmonary artery. (b) Pulmonary artery banding. An umbilical tape or similar material is placed around the main pulmonary artery to limit pulmonary blood flow. ing aorta. Ductal closure results in inadequate systemic and coronary perfusion, leading to progressive metabolic acidemia, ischemia and death. Although uncommon, ductal narrowing will result in reduced systemic blood flow. The compensatory increase in right ventricular pressure necessary to provide sufficient systemic perfusion may cause an increased PBF (Qp) to systemic blood flow (Qs) ratio (Qp : Qs) thereby mimicking the findings of unrestrictive PBF. With the pulmonary and systemic arteries connected in parallel, the Qp : Qs depends on a delicate balance between the pulmonary and the systemic vascular resistance (SVR). Stage I reconstruction (Norwood operation) limit PBF and to protect the pulmonary circulation from high flow and pressure that can eventually cause irreversible pulmonary disease (Fig. 22.2). Finally, an occasional single ventricle patient will have will have a combination of pathologic abnormalities (restrictive VSD, pulmonary stenosis) providing the appropriate amount of PBF and obviating the need for immediate surgical intervention (Table 22.1). Hypoplastic left heart syndrome pathophysiology (Fig. 22.3) The left ventricle is a non-functional structure in the child with HLHS. Pulmonary venous return must be routed to the right atrium through a stretched foramen ovale, an atrial septal defect (ASD), or rarely by total anomalous pulmonary venous connection. Systemic and pulmonary venous returns mix in the right atrium. The right ventricle (RV) supplies both the systemic and pulmonary circulations in a parallel fashion, since the main PA gives rise to the branch pulmonary arteries as well as the systemic circulation via the ductus arteriosus. Blood flows retrograde from the ductus arteriosus through the transverse aortic arch to its branches, and through the ascending aorta to the coronary arteries. Flow to the lower body is antegrade from the ductus arteriosus via the descend- The basic surgical approach is illustrated in Fig Preoperative management Much of the preoperative management of neonates with HLHS entails optimizing the condition of the cardiovascular and other organ systems. The key to management of HLHS perioperatively rests with the ability to assess and manipulate systemic perfusion and Qp : Qs. While clinicians have traditionally relied on estimates based upon systemic oxygen saturation, data from Rychik et al. 3 comparing accuracy of Doppler flow patterns in the aorta to other methods of estimating Qp : Qs reveal a weak correlation between systemic oxygen saturation and measured Qp : Qs. Although cumbersome for routine evaluations, Doppler flow patterns are substantially more accurate and precise in evaluation of Qp : Qs. When available, the addition of data to quantify systemic output and Qp : Qs, such as mixed venous oxygen saturation 4,5 or Doppler aortic flow patterns, has greatly improved the assessment and appropriate intervention in neonates with HLHS. This information assumes even greater importance in the volatile physiology exhibited in the early postoperative period. In the preoperative period, neonates with HLHS who have been stabilized and who are not impaired by other vital organ system dysfunction, are initially assumed to be able to 357

3 PART 5 Anesthesia for specific lesions Fig Single ventricle pathophysiology. There is complete mixing of systemic and pulmonary venous blood in the ventricle, and oxygen delivery is affected by the balance between the systemic vascular resistance (SVR), the pulmonary vascular resistance (PVR), and the cardiac output (CO). Optimal oxygen delivery is provided by a balance between SVR and PVR, and maintaining good CO. AV valve, atrioventricular valve; HR, heart rate. maintain satisfactory balance in Qp : Qs. The goal for such patients is to allow spontaneous ventilation via a natural airway. The majority of neonates meet this objective. The most common imbalance of Qp : Qs typically manifests itself with signs of inadequate systemic output and relative excess in PBF. These signs might include hypotension, lactic acidosis, and diminished urine flow in the context of relatively high systemic oxygen saturation. Once assured of an adequate circulating intravascular volume, oxygen carrying capacity, and a non-restrictive patent ductus arteriosus, therapeutic measures are often directed at increasing PVR. In order to obtain selective constriction of the pulmonary vasculature, clinicians have employed gas mixtures that either reduced alveolar PO 2, 6 promoting hypoxic pulmonary vasoconstriction, or increased alveolar PCO 2 7 to achieve constriction via local effects on ph or tissue carbon dioxide. 8 Either of these ambient gas manipulations can be accomplished by placing the infant in a hood supplemented with nitrogen or carbon dioxide, respectively. Controversy persists as to the comparative efficacy of these strategies. 9 Caution must be used when altering the inspired gas mixtures in neonates breathing spontaneously while receiving prostaglandin E 1 by infusion. Mild hypoventilation can result in significant hypoxemia in neonates breathing an FIO 2 below Although increased inspired carbon dioxide has been shown to improve oxygen delivery in the anesthetized neonate under conditions of controlled ventilation, 10 the increased oxygen consumption associated with carbon dioxide induced tachypnea in the spontaneously breathing patient might negate the benefits observed in the anesthetized patient. Anatomic variables can have a major impact upon the observed physiology. Some restriction to pulmonary venous return, such as occurs with left-to-right flow across the foramen ovale, is desirable as it tends to balance pulmonary and systemic resistance, and thus Qp : Qs. Infants who lack that restriction, such as those with an ASD or unrestrictive anomalous pulmonary venous return, tend to exhibit high Qp : Qs. In contrast, those who have severely obstructed pulmonary venous return, such as the few who present with an intact atrial septum and no alternative decompressing vein, 358

4 CHAPTER 22 Anesthesia for the patient with a single ventricle (b) (a) (c) (d) (e) (f) Fig The Norwood stage I palliation for hypoplastic left heart syndrome. (a) Incision of hypoplastic ascending aorta and preparation of the native pulmonary valve to become the neoaortic valve. (b) Cutting of a homograft patch to augment the neoaorta. (c,d) Construction of the noe-aorta. (e,f ) Completion of the Blalock Taussig shunt and final anatomy. Reproduced with permission from Casteñeda AR, Jonas RA, Mayer JE, Hanley FL. Hypoplastic left heart syndrome. In: Casteñeda AR, Jonas RA, Mayer JE, Hanley FL, eds. Cardiac Surgery of the Neonate and Infant. Philadelphia, PA: Saunders, 1994: have an extremely low Qp : Qs. Despite therapeutic maneuvers designed to lower PVR and promote PBF, these infants exhibit marked hypoxemia that requires urgent intervention to decompress pulmonary venous return in order to have any hope of survival. Preoperative evaluation and stabilization should also include a survey of other vital organ systems for congenital or acquired abnormalities. Our recent series including 102 newborns with HLHS indicates that approximately 15% have some genetic syndrome or significant non-cardiac malformation. 11 The magnitude and distribution of acquired vital organ dysfunction usually relates to circulatory instability at the time of diagnosis. Infants that have suffered a profound or protracted shock state at the time of diagnosis can demonstrate a wide spectrum of injury to renal, central nervous system (CNS), cardiac, gastrointestinal 12,13 or hepatic systems. These derangements may necessitate a delay in operative intervention to permit recovery. Hypoplastic left heart syndrome is increasingly being diagnosed in utero allowing for planned management at delivery suggesting greater stability through more controlled circumstances. A study of patients with critical left heart obstructive lesions (including HLHS) showed greater hemo- dynamic stability and a lower incidence of preoperative neurologic events in those patients with prenatal diagnosis. 14 Intraoperative management Although the vast majority of neonates presenting for stage I reconstruction (e.g. Norwood or Sano operation) receive an intravenous induction of anesthesia, virtually any anesthetic agent can be used for this purpose with careful attention to the hemodynamic consequences of the technique selected. We prefer the phenylpiperidine-based synthetic opioids (e.g. fentanyl) because they blunt the endogenous catecholamine response to noxious stimuli at doses that are usually tolerated hemodynamically However, even with these hemodynamically neutral agents, large doses may result in significant cardiovascular changes, such as bradycardia and hypotension. These observations suggest that the neonate with HLHS requires some endogenous catecholamine release to sustain satisfactory hemodynamics. Unfortunately, this threshold dose that separates sufficient from excessive varies between patients, necessitating individual titration to arrive at the optimal dose. Although clinical research conducted more than a decade 359

5 PART 5 Anesthesia for specific lesions ago popularized the view that massive doses of opioid analgesics should be administered perioperatively to effect stress hormone suppression in neonates undergoing cardiac surgery, 18 recent efforts to duplicate these findings have not confirmed the original results. 19 The latter demonstrated that massive doses of fentanyl did not completely suppress release of endogenous catecholamines, even in combination with benzodiazepine infusions. However, outcome measures were no different in any of the study groups. We typically employ total intraoperative fentanyl doses between 50 and 75 µg/kg followed by a fentanyl infusion (2 5 µg/kg/hour) begun postoperatively in the cardiac intensive care unit (CICU). Management of the airway and ventilation assumes great importance during induction of anesthesia. Given the propensity for the majority of neonates with HLHS to exhibit excessive PBF, the anesthesiologist must take care not to employ ventilatory maneuvers that lower PVR, such as hyperventilation with high concentrations of oxygen. In an infant with typical HLHS physiology, one might initiate manual ventilation with air or a low concentration of supplemental oxygen. The extent to which the anesthesiologist adjusts FIO 2 prior to laryngoscopy would depend upon the magnitude of hemodynamic response to the initiation of controlled ventilation. Infants who demonstrate significant reduction in systemic arterial pressure despite low FIO 2 may not tolerate prolonged exposure to high FIO 2 without deleterious hemodynamic consequences. Means of increasing PVR, as discussed previously, should be available in the operating room. We favor inspired carbon dioxide for several reasons. It tends to augment systemic arterial pressure immediately. Nor does it require neutralization of all safety systems designed to avoid delivery of a hypoxic gas mixture. Finally, as a gas of some historical importance in anesthesia delivery systems, it is available with flow meters in appropriate clinical ranges. Tabbutt et al. 10 in a recent clinical comparison of intubated, paralyzed, and anesthetized neonates with HLHS, demonstrated that inspired carbon dioxide proved consistently more effective than hypoxic gas mixtures at increasing indices of systemic output, including systemic arterial pressure and oxygen delivery. Hypoplastic left heart syndrome with obstruction of pulmonary venous outflow Management changes significantly in the context of an infant with extremely high PVR and very low Qp : Qs, such as the infant with an intact atrial septum. Despite transient hemodynamic and metabolic stability that might ensue with aggressive maneuvers designed to lower PVR and promote PBF, these infants exhibit marked hypoxemia that requires urgent surgical intervention to decompress pulmonary venous return in order to have any hope of survival. Whether this intervention includes the entire stage I reconstruction, or is limited to an atrial septectomy with a planned return following a period of assessment and recuperation, remains a surgical judgement for which little supporting data exist. 20,21 If tolerated hemodynamically, a higher dose of opioid affords the advantage of blunting the response that any noxious stimulus might have on PVR. Substantial ventilation with high FIO 2 is typically necessary to achieve marginal gas exchange. When systemic oxygenation falls below a threshold value, temporizing measures designed to diminish metabolic demand deserve strong consideration, such as surface cooling of particularly vulnerable organs (e.g. CNS). The use of sodium bicarbonate to address metabolic acidemia in the face of extremely limited PBF offers limited benefit, and may even pose hazard. The elimination of carbon dioxide following bicarbonate hydrolysis is severely impaired. Thus, bicarbonate administration will often result in a shift from metabolic to respiratory acidemia with little change in ph and the attendant prospect of highly undesirable increase in PVR. Intraoperative monitoring consists of invasive continuous arterial pressure in addition to standard cardiovascular, respiratory, and temperature monitors. In order to minimize the hazard of thrombosis in the central thoracic veins in all single ventricle patients, we employ direct transthoracic atrial lines in lieu of percutaneous jugular or subclavian central venous pressure catheters. In addition, an umbilical venous catheter positioned in the orifice of the superior vena cava (SVC) at the time of surgery serves as a valuable monitor of mixed venous oxygen saturation, enabling more precise assessment of systemic CO and Qp : Qs. 4,5,10 At the termination of cardiopulmonary bypass (CPB), the physiologic goals are identical to those expressed preoperatively, although the proclivities are quite different. The pulmonary circulation now resides at either the distal end of a restrictive prosthetic systemic-to-pulmonary shunt or an RV-to-PA tube graft (Sano modificationafig. 22.5). At our institution, we have far more experience with the systemicto-pa shunt arrangement. A variety of subtleties in the technical execution of this shunt, relief of atrial obstruction, and pre-existing condition of the pulmonary circulation can render the ultimate physiology somewhat unpredictable. Technical issues of graft insertion, proper graft size, particularly in the neonate under 2.0 kg, and the ability to delineate the etiology of insufficient PBF on termination of CPB, introduce additional complexities in a number of patients where a Sano modification has been performed. Given our limited experience managing patients with an RV-to-PA tube graft, subsequent discussion will be limited to patients having their PBF provided by a systemic-to-pa shunt, which is far less prone to excessive PBF than the native anatomy. Measures to assure a clear airway and complete reexpansion of the pulmonary parenchyma are performed in the terminal phases of rewarming on bypass. The magnitude of reduction in the mean systemic arterial pressure that occurs with trial opening of the shunt during the terminal 360

6 CHAPTER 22 Anesthesia for the patient with a single ventricle Fig The Sano modification of the Norwood stage I operation. Instead of a right modified Blalock Taussig shunt, pulmonary blood flow is provided by a right ventricle to pulmonary artery conduit, usually a 5-mm Goretex graft. Reproduced with permission from Pizarro C, Malec E, Maher KO et al. Right ventricle to pulmonary artery conduit improves outcome after stage I Norwood for hypoplastic left heart syndrome. Circulation 2003; 108: II phase on CPB can provide qualitative insights as to what PVR one might expect in the early post-bypass period. Initial ventilatory support should be adjusted accordingly. In general, we begin with a pattern of ventilation designed to result in low normal PaCO 2 and a FIO 2 between 0.6 and 1.0, recognizing that adjustments become necessary in all patients as indicated by the individual infant s physiology. In addition, the physiology typically demonstrates dynamic change over time, requiring continuous surveillance and further adjustment. Assuming a technically satisfactory repair and no unusual risk factors, PVR typically falls in the first few hours after surgery. Despite a perfect technical result, the Norwood operation does not result in any reduction of the volume or pressure burden placed on the single ventricle, as the physiology of parallel systemic and pulmonary circulations where a Qp : Qs of unity remains the objective throughout the postoperative period. Yet the heart incurs the cost of insults related to cessation of coronary perfusion, CPB, and hypothermic circulatory arrest (HCA). This may account for the cardiovascular frailty exhibited by these infants in the early postoperative period. However, in the absence of major deficiencies in myocardial protection or persistent anatomic residua, such as significant arch obstruction, coronary compromise, or valvar insufficiency, this magnitude of myocardial dysfunction can usually be ameliorated with relatively modest doses of inotropic agents (e.g. dopamine at 3 5 µg/kg/minute). In a time course characteristic of many major cardiac interventions in neonates and young infants, myocardial performance may deteriorate in the first 6 12 hours postoperatively before they start to improve. As a result, we routinely take measures to reduce metabolic demands by continuous muscle relaxant (e.g. pancuronium mg/kg/hour) and opioid (e.g. fentanyl 2 5 µg/kg/hour) infusions. Infants demonstrating increased SVR during rewarming on CPB often receive a loading dose of milrinone at that time. Zuppa et al. 22 recently described the pharmacokinetics of milrinone in 16 neonates with HLHS given a loading dose of milrinone on CPB at the time of rewarming. These investigators recommend a loading dose of 100 µg/kg, followed by initiation of an infusion (0.2 µg/kg/minute) within 90 minutes of the bolus dose to achieve and maintain plasma concentrations similar to those reported in other therapeutic settings. No data exist to guide the bolus and/or infusions doses when milrinone is begun after termination of CPB. Modified ultrafiltration (MUF) conducted immediately following CPB has been demonstrated to exert beneficial effects upon hematocrit, hemodynamics, hemostasis, pulmonary function, and CNS recovery Perioperative weight gain is reduced significantly as are certain inflammatory mediator levels. Whenever possible, we conduct MUF at the termination of CPB following stage I reconstruction. Occasionally, the position of the bypass cannulae or the continuous flux of blood through the MUF circuit results in unfavorable hemodynamic changes precluding completion of the filtration. In 99 consecutive patients undergoing stage I between September 2000 and August 2002, all tolerated the pertubations of MUF. Stage I reconstruction requires substantial suture lines in creation of the neo-aorta. Thus, rapid restoration of normal hemostasis represents an important early postoperative objective. Following MUF, once satisfied with the technical and physiologic result of the repair, heparin effect is reversed with protamine. Given the risk factors that jeopardize platelet number and function, including deep hypothermia and profound dilution of circulating volume on CPB, 28 replacement of blood loss with fresh whole blood (< 48 hours old) restores hemostasis more effectively than other blood products. 29 Fresh whole blood replacement also serves to minimize donor exposure to those patients who are anticipated to require three open heart surgical interventions. Should these measures fail to achieve adequate hemostasis despite elimination of all surgical bleeding sites, laboratory testing should be conducted to direct component therapy at those elements of the hemostatic pathway most likely to be impaired: platelet and fibrinogen replacement. Reports of antifibrinolytic therapy in the very young are largely limited to aprotinin. 30,31 The 361

7 PART 5 Anesthesia for specific lesions volume of blood loss can be reduced by high dose aprotinin but it may not be clinically important since donor exposures are not necessarily affected. Prompt control of hemostasis resulting in reduced transfusion requirement can be associated with a reduced need for re-exploration for bleeding. Re-exploration in patients under 2 years of age undergoing complex surgery at our institution, which includes all patients with a single ventricle, was reduced (from 3.0% to 0.8%) following the adoption of the routine use of fresh whole blood. Cardiac tamponade can easily occur from a small quantity of mediastinal blood accumulated in the early postoperative period before bleeding has completely ceased. Continuous removal is essential because blockage easily occurs in the relatively small mediastinal drainage tubes of these neonates. A technique of active, continuous aspiration of accumulating blood from the mediastinum has virtually eliminated this complication. 32 Common problems in the early post-bypass period Excessive hypoxemia represents one of the more commonly encountered problems in the early post-bypass period. Although inadequate Qp : Qs becomes the assumed cause, factors that impair systemic oxygen delivery thereby reducing mixed venous oxygen saturation are now known to be more common than previously believed. 3,4,5,33 One typically observes a progressive increase in systemic oxygen saturation during MUF, for example, probably due to the impact that hemoconcentration and the resulting increased oxygen delivery have upon mixed venous oxygen saturation. Thereafter, measures directed at maintaining hematocrit above 40 45% may alleviate excessive demands placed upon the recovering heart to increase systemic output. The distinction between systemic hypoxemia due to low Qp : Qs, low pulmonary venous oxygen saturation, or low mixed venous saturation is a critical one, as the therapies are diametrically different. Measures designed to reduce PVR will impose a further volume load on a heart already struggling to provide marginal systemic perfusion. Patients demonstrating low SvO 2 would be better served with therapies that promote systemic output, such as inotropic agents or vasodilators. Similarly, those with low pulmonary venous oxygen saturation require a strategy of ventilatory support designed to reduce atelectasis and promote gas exchange in impaired alveoli. Unfortunately, the latter diagnosis is rarely made definitively in the OR or CICU, as blood sampling from the pulmonary veins presents logistic challenges. Intraoperatively, expectant measures directed at expansion of the lungs and maintenance of normal functional residual capacity usually suffice to avoid pulmonary vein desaturation. Among the three etiologies of persistent systemic hypoxemia, this was believed to be the least common, but a recent series found pulmonary vein desaturation in as many as 30%. 34 When systemic hypoxemia occurs due to low Qp : Qs, other manifestations provide supporting evidence. Trial opening of the systemic-to-pa shunt during the latter phases of rewarming on CPB fails to demonstrate significant drop in the mean systemic arterial pressure. The early post-bypass hemodynamics reveal a relatively narrow pulse pressure and/or high diastolic pressure. A substantial discrepancy exists between arterial and end-tidal carbon dioxide measurements. These suggestive pieces of inferential evidence can be confirmed by aortic Doppler flow analysis or calculation of a Fick ratio using oxygen saturation determinations. Most commonly, diminished PBF reflects a subtle technical aspect of the arch reconstruction, innominate artery dimension, or the BT shunt. However, certain patient subsets exhibit profound abnormalities in the pulmonary vasculature that cause excessive PVR elevations. Neonates with HLHS routinely demonstrate extremely high and volatile PVR when born with extreme pulmonary venous obstruction due to intact atrial septum without alternative decompressing veins. Even the typical HLHS anatomic constellation is associated with marked abnormalities in the number and muscularization of the pulmonary vasculature by pathologic examination. 35 Hypotheses attribute these changes to chronic fetal pulmonary venous obstruction. 36 One can speculate that these changes become more extreme in the context of the marked obstruction caused by HLHS with intact atrial septum. Fetal echocardiography has confirmed alteration in pulmonary venous flow pattern in relation to the magnitude of restriction at the atrial septum. 37 In the context of hypoxemia due to low Qp : Qs, interventions fall into three categories: technical, pulmonary vasodilation, and systemic vasoconstriction. In the context of patients expected to have unusually elevated PVR, modifications in the surgical technique might entail placement of a larger shunt or interposition between a larger systemic vessel (e.g. aorta) and pulmonary arteries. Pulmonary vasodilator therapy includes the strategies one might employ in any patient demonstrating elevated PVR, such as oxygen, moderate hyperventilation, normothermia, alkali, and nitric oxide Should those measures prove insufficient to result in adequate PBF, the focus might be expanded to include measures designed to increase the driving pressure across the shunt, using higher doses of inotropic infusions or even vasoconstrictors. The latter necessitates careful monitoring to avoid jeopardizing perfusion to other vital organs. Depressed myocardial performance represents another potential problem in the early post-bypass period. As mentioned previously, some degree of myocardial dysfunction typically occurs following this procedure as there is no hemodynamic benefit achieved to offset the cost of CPB and an ischemic interval. When this dysfunction becomes more significant than usual, specific causes should be sought. Even in the context of the typical conduct of stage I reconstruction, the consequences of aortic atresia make routine myocardial 362

8 CHAPTER 22 Anesthesia for the patient with a single ventricle protection measures, such as the infusion of cardioplegia solutions, challenging. Thus inadequate myocardial preservation represents one potential cause for persisting or excessive myocardial depression. Technical considerations represent the predominant cause of myocardial dysfunction following this complex intervention. One of the most intricate aspects of this procedure is the reconstruction of an aortic arch in such a way that the small ascending aorta, which principally serves to provide coronary flow, is not compromised. This subtle finding may not become evident until the cardiac volume is restored in anticipation of terminating CPB. Residual hemodynamic derangement represents another potential cause of myocardial dysfunction. Given that under the best of circumstances, one emerges from the Norwood operation with no appreciable hemodynamic benefit, one would expect a result with newly imposed volume or pressure loads to be poorly tolerated. Examples of such findings would include: residual aortic arch obstruction, AV valve dysfunction, and semilunar valve obstruction or regurgitation. Metabolic disturbances also result in significant myocardial dysfunction. This fragile RV struggling to cope with significantly increased volume output demands at systemic pressure is perhaps more susceptible to what might otherwise be modest metabolic disturbances. As such, one should track and address those variables that have impact upon myocardial performance, such as ionized calcium and lactic acidosis. The rapid administration of blood products, for example, which contain calcium-binding drugs, high levels of potassium and lactic acid, as well as other vasoactive mediators, can result in an acute, profound deterioration in cardiac performance in the early postoperative period. In our experience, myocardial performance will deteriorate when the arterial ph falls below 7.3 and may contribute to further reduction in Qs. The administration of intravenous bicarbonate, calculated to completely eliminate the base deficit, often exerts a beneficial effect on both myocardial performance and Qs. In addition to the inherent cardiac sensitivity, inescapable anatomic peculiarities accentuate this vulnerability. Blood carrying the transfused products from the systemic venous circulation enters the RV and is directed immediately to the reconstructed aorta, whereby the first branch is the coronary circulation. Thus constituents of the transfused blood (e.g. citrate, potassium, lactate) infused into the venous circulation arrive at the coronary arteries with greater speed and concentration than might have occurred had they been dissipated over the course of the pulmonary vasculature before entering the aorta. This effect is further accentuated if central venous catheters are employed to infuse the blood product. We abide by a protocol whereby blood transfused via central lines or rapidly through peripheral catheters is either fresh whole or washed packed cells. Arrhythmias most commonly occur as manifestations of the problems described previously. When they become mani- fest early in the process of rewarming on CPB, coronary insufficiency represents the most common cause, particularly if the arrhythmia is ventricular in origin. Metabolic disturbances produce the same qualitative rhythm changes seen in normal hearts, although the manifestations might be more extreme. Given the predominantly extracardiac nature of the Norwood procedure, acquired heart block rarely follows this operation, unless it existed preoperatively. On rare occasions, a patient presents with HLHS and a primary arrhythmia, such as Wolff Parkinson White syndrome. Excessive PBF may complicate the early postoperative period; however, this diagnosis should be entertained cautiously. In many instances, the apparent excess PBF really reflects a relative imbalance with respect to significantly diminished systemic CO (Qs). The latter should be specifically excluded or addressed before invoking extreme measures to restrict PBF. Of course, subtle technical differences in the conduct of the operation can result in an anatomic propensity to an excessive Qp : Qs, and this can, in turn, jeopardize systemic perfusion. Such patients typically exhibit an extremely wide pulse pressure or low diastolic pressure reflecting pulmonary runoff. If myocardial performance otherwise appears robust, the specific measures employed to increase PVR preoperatively are appropriate in this setting. In most patients, this condition dissipates as the infant recovers from surgery. Should the problem persist beyond the first postoperative day, a cardiac catheterization should be considered to evaluate the need for further surgical intervention aimed at diminishing PBF. The volume work of the single ventricle after stage I reconstruction is equal to the sum of the systemic and PBF (Qp + Qs). After a period of maturation of the pulmonary vasculature, systemic venous return may be directed to the pulmonary arteries, thus placing the two circulations in series. When the Fontan operation was uniformly undertaken months after stage I, an operative mortality of 16 40% occurred. 41 The most common cause of early death was low CO associated with tachycardia, low systolic and diastolic blood pressures, and high ventricular end-diastolic pressures. The majority of patients with signs of low CO demonstrated echocardiographic evidence of an abrupt change in ventricular geometry that resulted in a small, thick-walled cavity with a low diastolic volume when compared to the preoperative state. Although systolic shortening appeared normal, the ventricular compliance was diminished. The physiologic result is impaired diastolic function of the ventricle resulting in increased end diastolic pressure. The resulting increase in pulmonary venous pressure impeded PBF thereby reducing systemic output. Retrospective analysis of the data available preoperatively proved insufficient to predict those children who would develop physiologically important reduction of ventricular compliance associated with rapid contraction of end diastolic volume following single stage Fontan. 363

9 PART 5 Anesthesia for specific lesions Initial management for other univentricular heart malformations Although HLHS represents the most common anatomic constellation resulting in a single functional ventricle, many other forms exist (e.g. tricuspid atresia). In fact, these malformations may have appeared more common than HLHS because they survive without the need for complex reconstructive surgery in the neonatal period. Hence these variants were much more prevalent during childhood before stage I reconstruction was developed as a viable option for neonates with HLHS. The management of other single ventricle malformations strives for the same physiologic goals as stage I: balanced Qp : Qs, unobstructed flow from the single ventricle to the systemic circulation, and conditions in the pulmonary circulation that promote the fall in PVR that normally occurs with maturation. The latter typically entails assurance that no resistance to pulmonary venous return exists and pulmonary arterial flow is subjected to an anatomic restriction that limits Qp : Qs ratio to unity. For example, tricuspid atresia variants may require a range of interventions in the neonatal period, depending on their anatomy. Those with ductal-dependent PBF will require a systemic-to-pa shunt to provide a balanced Qp : Qs. Variants with associated VSD may have adequate PBF without a shunt. A small subgroup with tricuspid atresia and a large VSD may exhibit excessive PBF requiring a PA band to achieve a Qp : Qs of 1. Fig Bidirectional cavopulmonary anastomosis (or bidirectional Glenn shunt). Top, the previous right modified Blalock Taussig shunt is divided and ligated, and (bottom) the superior vena cava is anastomosed to the right pulmonary artery. Reproduced with permission from Casteñeda AR, Jonas RA, Mayer JE, Hanley FL. Single-ventricle tricuspid atresia. In: Casteñeda AR, Jonas RA, Mayer JE, Hanley FL, eds. Cardiac Surgery of the Neonate and Infant. Philadelphia, PA: Saunders, 1994: relation of the SVC to the right atrium is preserved. As bypass is needed, other coexisting anatomical risk factors can be addressed. Thirdly, it simplifies execution of the Fontan itself. Superior cavopulmonary anastomosis Evolution of staged approach to Fontan Many have adopted a systematic staged approach to the Fontan operation for all patients with univentricular hearts in an effort to reduce the volume load of the ventricle as early as possible and to minimize the impact of rapid changes in ventricular geometry and diastolic function that accompany primary Fontan. 42 Two options have gained acceptance as the first step of the staged Fontan: bidirectional Glenn 43 or hemi-fontan. 44 The SVC is divided and anastomosed to the undivided pulmonary arteries, creating a bidirectional cavopulmonary (Glenn) shunt (Fig. 22.6). This source of PBF may be exclusive if the previous shunt is ligated, or additive if it is not. When previous sources are occluded, it provides the same physiologic benefit as hemi-fontan and can be performed without bypass. During hemi-fontan, all systemicto-pa shunts are ligated, and PBF is achieved exclusively via an SVC-to-PA anastomosis. Certain technical features of the hemi-fontan make it, in our opinion, a more logical step in the process of eventual completion of the Fontan. First, it enables elimination of stenosis or distortion of the branch pulmonary arteries and their confluence. Second, the normal Preoperative assessment The conversion from a circulation based upon complete mixing and parallel perfusion of both the systemic and pulmonary vascular beds via an arterial shunt to a series circulation where PBF becomes a diversion of systemic venous return requires certain preconditions. In essence, the flow of blood through the pulmonary circulation must be free of significant impediments in order that systemic venous pressure does not reach physiologically unacceptable levels. These potential impediments take three forms: elevated PVR, AV valve dysfunction, and diminished ventricular compliance. Elevated PVR encompasses two distinct mechanisms: the size of the major branches or the state of the arteriolar resistance vessels. In patients with HLHS, one must also confirm that no obstruction to flow exists at the remnant of the atrial septum. With the caveat that systemic venous pressures of 16 mmhg or less are generally tolerated without significant sequelae, while those 20 mmhg and over are associated with a variety of morbidities, very small differences distinguish those who do well with the operation from those who have a poor outcome. Contemporary non-invasive methods 364

10 CHAPTER 22 Anesthesia for the patient with a single ventricle of assessing the anatomy and physiology of candidates for superior cavopulmonary anastomosis (SCPA) are not capable of distinguishing such small physiologic differences. Thus, we perform cardiac catheterization on all HLHS patients who are considered candidates for SCPA in order to quantitate PVR, ventricular end-diastolic pressure, AV valve function, and obstruction at the atrial septum remnant. In addition, anatomic information about the PA architecture is obtained in conjunction with injections to evaluate the presence of accessory venous communications between the superior venous drainage and the heart or inferior vena cava (IVC) (e.g. left SVC to coronary sinus). Postoperatively, such vessels could serve as a mechanism by which upper body venous return is diverted to the heart without passing through the pulmonary circulation, thereby resulting in unanticipated levels of hypoxemia. These data can be used to estimate the SVC pressure on completion of SCPA. Recognizing that this formula requires several assumptions that render it an oversimplification, one can estimate the postoperative SVC pressure as follows: P svc = 1 (P PA P PV )(Q PA : Q SA ) 5 e3 Q e7 + P LA PB : S PB Where P SVC and P LA represent the pressure determinations in the SVC postoperatively, and the left atrium, respectively Where P PA and P PV are the preoperative pressures in the PA and vein, respectively Where Q PB : Q SB and Q PA : Q SA are the Qp : Qs ratios before and after SCPA, respectively In infants approximately 6 months of age, we estimate the proportion of venous return coming from the upper body to be roughly equal to that from the lower body, although SVC flow may comprise as much as 60 70% of the total venous return in some. In other words, Q PB : Q SB approximates With this assumption, we estimate the change in P PA P PV that is proportional to the reduction in flow (Q PB : Q SB /Q PA : Q SA ). For example, assuming the following hemodynamics measured preoperatively: P PA = 17, P PV = 8, Qp : Qs = 1.5, and P LA = 8, the P SVC postoperatively = ((17 8)(0.5)/1.5) + 8 = 11. Unfortunately, several of the assumptions limit this sort of calculation to the level of a crude estimate. P PA is notoriously difficult to measure accurately when the only source of PBF is a systemic pulmonary shunt. Catheters placed across the shunt probably alter PBF while they are present, while PV wedge pressures to estimate P PA have a variety of limitations, particularly if PVR is elevated. The ventricular compliance is dynamic as well, particularly in the context of significant changes in ventricular volume and pressure loading conditions. In addition, the imposition of CPB and an ischemic interval have a negative impact on ventricular compliance, albeit a transient one if the operation proceeds according to plan. Finally, the Qp : Qs determinations depend upon PVR which might be altered by the medications employed to sedate an infant for catheterization. Despite all the limitations, however, this estimate does help to predict problem patients as well as the type of problem they might encounter, whether PVR, ventricular compliance or AV valve function. Preoperative assessment should also incorporate an evaluation of other vital organ systems with a history of primary or secondary dysfunction. For patients receiving anticoagulant or functional platelet inhibitors, plans for the cessation of those therapies must be formalized. Careful history regarding the child s response to sedative medications should be elicited. Intraoperative management Infants typically return for SCPA between 5 and 8 months of age. Given their developmental stage and prior hospital experiences, many will manifest separation anxiety when taken from the parents. Thus, unless they have some contraindication, sedative premedication is administered orally prior to surgery. Although a variety of sedative potions are available, we prefer pentobarbital 4 mg/kg p.o. because of its potency and duration of action. When administered minutes in advance, a high proportion of patients will be sleeping. This serves to allay the parental anxieties to some extent and also facilitates induction with a volatile inhaled anesthetic agent, if that is the planned technique. Anesthesia can be induced with a variety of intravenous or inhaled agents. Unless the preoperative evaluation has revealed myocardial dysfunction or significant unusual hemodynamic loading conditions (e.g. arch obstruction, AV valve insufficiency), these infants generally tolerate nearly normal doses of anesthetic agents without manifesting untoward cardiovascular effects. We usually employ a combination of inhaled anesthetic, opioid, and muscle relaxant. Most commonly, the total opioid administered for the case is the equivalent of 50 µg/kg of fentanyl. Our goal is sufficient emergence from the anesthetic effect to permit tracheal extubation within a few hours of arrival in the CICU. This period of stabilization enables the infant to thoroughly rewarm, recover from CPB, and demonstrate that bleeding has subsided. Although these infants at the time of induction have the same anatomy and physiology as the newborn following stage I, subtle changes occur in the interval that make them significantly more resilient. Maturation and compensatory mechanisms in myocardial development render the heart more capable of managing the excess volume load of a parallel circulation. In addition, through differential growth, the shunt is more restrictive, protecting the infant from excessive 365

11 PART 5 Anesthesia for specific lesions acute volume loads irrespective of manipulations that lower PVR significantly. Finally, the baseline PVR is low, so even extreme measures cannot produce a substantial reduction in PVR from baseline values, therefore any Qp : Qs change is comparably small. Nevertheless, we try to minimize any additional volume burden that might be placed on the ventricle prior to CPB and planned ischemia by minimizing supplemental oxygen delivery and ventilating to normocapnea. All standard non-invasive monitors are applied for induction. An intra-arterial catheter is placed for continuous monitoring following tracheal intubation. The site selected for this catheter varies according to a variety of considerations related to congenital or acquired vascular anomalies. The placement of a BT shunt may have compromised the ipsilateral subclavian artery. In addition, previous monitoring and catheterization sites may not be viable. There are also a variety of aortic arch branching patterns some of which result in stenosis of the subclavian supply. Non-invasive arterial pressure measurement on all four extremities provides the data necessary to identify the appropriate site(s). Cannulation of the central veins via the jugular or subclavian is avoided out of concern for the implications of thrombosis in those vessels. Unlike stage I, the SCPA provides significant hemodynamic benefit. With occlusion of the shunt, the circulations are no longer connected in parallel, thereby reducing the volume output demands for the RV to that which is necessary to perfuse the systemic circulation alone. Pulmonary blood flow becomes a diversion for venous return from the upper body, in effect a series circulation of a pump and two resistors. Since the volume of blood flow to the upper body is at least as great as that to the lower body at 6 months of age, the mixture of oxygenated and deoxygenated blood remains 1 : 1, or higher. Thus the expected systemic oxygen saturation tends to increase slightly, but the heart need only accomplish half the volume work (Qs) to do so. Most patients exhibit robust hemodynamics on completion of this procedure. Although we usually infuse a low dose of dopamine (3 µg/kg/minute) in the atrial catheter, it may not be necessary in many. Infants exhibiting substantial diastolic dysfunction or valve regurgitation may benefit from an inodilator such as milrinone. When anticipated on the basis of preoperative information, a loading dose will be administered during rewarming on CPB. The strategy for managing PVR changes dramatically as well. With PBF now relying upon passive venous return (i.e. no pump to propel blood through the pulmonary circulation), measures designed to minimize the impediments to PBF assume paramount importance. Since medical therapies are limited in their capacity to produce reliable, substantial improvement in ventricular compliance or AV valve function, attention is focused on minimizing PVR. Shortly before the termination of CPB, the tracheal tube should be cleared of secretions and the lungs completely re-expanded, as PVR will be minimized at normal functional residual capacity (FRC). Both atelectasis and alveolar overdistension increase PVR. A tidal volume designed to achieve a low normal PaCO 2 at a respiratory rate no greater than 20 is selected. Doppler flow studies have demonstrated that PBF occurs preferentially during the expiratory phase of positive-pressure ventilation in patients following cavopulmonary anastomosis, thus we strive to limit rate and inspiratory time to no greater than 1 second. 45,46 Positive end-expiratory pressure (PEEP) is only applied judiciously to preserve normal FRC, based upon investigations in Fontan patients demonstrating significant reduction in cardiac index mediated by an increase in PVR at PEEP values over 6 mmhg. 47 Immediately following termination of CPB, MUF is instituted. Modified ultrafiltration offers significant benefit to patients following cavopulmonary anastomosis. 48 Postoperative blood loss and the proportion of patients demonstrating significant pleural and pericardial effusions are both significantly reduced. Other investigators have shown benefits in pulmonary function across a wider spectrum of patients that may prove particularly crucial in this population. Infants for SCPA represent a high-risk group for postoperative hemorrhage. They have several risk factors which tend to exacerbate bleeding, including age (< 2 years), reoperation, hypoxemia, and deep hypothermic bypass management. Upon completion of MUF, heparin effect is rapidly reversed with protamine. Fresh whole blood comprises the preferred product for blood replacement following protamine administration. As described previously, this product provides restoration of all hemostatic elements, including platelets, and thereby limits donor exposures as well. In the vast majority, SCPA can be performed while limiting patient exposure to a single blood donor. Specific problems in the immediate postoperative period Hypoxemia of greater magnitude than anticipated represents the most common postoperative problem encountered by patients following SCPA. In some instances, this may represent a manifestation of hypovolemia and diminished PBF, while in others it might reflect the mechanical ventilation strategy. In the latter circumstance, PaO 2 may rise as ventilatory support is tapered. In the absence of improvement with manipulation of intravascular volume or ventilation, diagnostic evaluation is indicated to search for connections that enable venous return from the upper body to bypass the pulmonary circulation and enter the heart or lower body venous system (e.g. an unrecognized left SVC draining to the coronary sinus). Often these collateral vessels can be occluded using transcatheter coil embolization, but the hemodynamic impact of occlusion should be tested with a balloon catheter prior to definitive embolization. 366