403CHAPTER 36 Critical Care After Surgery for Congenital Cardiac Disease have two good sized ventricles (unless accompanied by other le sions, such as an unbalanced AV septal defect) This disease can[.]
CHAPTER 36 Critical Care After Surgery for Congenital Cardiac Disease 403 have two good-sized ventricles (unless accompanied by other lesions, such as an unbalanced AV septal defect) This disease can exist in a spectrum with confluent central pulmonary arteries and a short-segment main pulmonary artery atresia to discontinuous pulmonary arteries without a true central pulmonary artery In the former situation, the pulmonary artery arborization is often close to normal, with few MAPCAs When antegrade flow is established from the RV into the main pulmonary artery by a reparative procedure, the left-to-right shunt via collateral flow will impose a diastolic load on the LV Preoperative occlusion of these collateral vessels can be accomplished by interventional techniques in the cardiac catheterization laboratory but may leave the child precariously cyanotic in the hours before operation The alternative is to complete the surgical procedure in a hybrid suite This spectrum of the lesion is completely repaired in the neonatal period As mentioned, MAPCAs may be present to varying degrees, supplying some or all segments of the lung They can be associated with a large left-to-right shunt, contributing to volume overload and pulmonary hypertension Larger collateral vessels supplying significant portions of the lung can be anastomosed or “unifocalized” to the native pulmonary arteries, with the ultimate aim being to establish full antegrade pulmonary blood flow Smaller vessels to some segments of the lung can be coiled in the cardiac catheterization laboratory provided that there is antegrade flow from the native pulmonary arteries to those lung segments (dual supply) When the pulmonary arteries are diminutive, it is important to establish early antegrade flow from the RV to the pulmonary artery in an effort to promote growth and establish a pathway to the pulmonary arteries for subsequent balloon dilation A modified BT or Mee shunt may initially be necessary to provide sufficient pulmonary blood flow if the pulmonary arteries are exceedingly small Initially, the VSD can be left open; postoperative management of cyanosis or CHF will be determined by the size of and resistance offered by the pulmonary circulation The course in these patients can be dynamic and demanding for even the most experienced practitioners When collaterals are unifocalized in the operating room and RV to diminutive pulmonary artery continuity is established, cyanosis may ensue, and therapy is aimed at lowering PVR or (re)establishing adequate pulmonary blood flow On the other hand, if the child is fully saturated in the aorta with elevated pulmonary artery oxygen saturation and LA pressure, then a left-to-right shunt through the VSD may be developing, which will produce a volume load on the LV and an unstable postoperative course, dictating VSD closure When the patient is not fully saturated in the aorta but is suffering from a volume-loaded LV with low cardiac output and high LA pressure postoperatively, excessive systemic-to-pulmonary collaterals may be the culprits, requiring catheterization laboratory investigation and occlusion or immediate reoperation noncompliant postoperatively This may manifest as requirements for high filling pressures and consequent right-to-left shunting through a purposefully patent foramen ovale With growth and improved compliance of the RV, the right-to-left shunting diminishes and the infant’s oxygenation improves substantially If hypoxemia persists, a PGE1 infusion can be reinitiated to temporize either until accommodation of the restrictive RV physiology or for a BT shunt In patients with long-segment pulmonary atresia, the need for a conduit to bridge the gap between the RV and pulmonary artery complicates the repair Again, RV failure may occur postoperatively, especially when there is a residual VSD or RV outflow obstruction The conduit may obstruct acutely during chest closure, further elevating pressure in the RV The relationship of the conduit to either normal or variant coronary anatomy is vital, as impingement of these structures can present as sudden inexplicable deterioration at the time of sternal closure After the VSD is closed and blood flow from the RV to the pulmonary arteries is enhanced, there may be excessive pulmonary blood flow (Qp/Qs 1) as a result of the combined flow into the pulmonary arteries from the RV and from aortopulmonary collaterals If this occurs, the patient develops CHF and requires intraoperative inotropic support and an extended period of postoperative mechanical ventilation With large collateral flow, the pulse pressure is wide and diastolic pressure low The patient may require surgery to ligate the collateral vessels or may require embolization Critical Care Management Critical care management of patients with pulmonary atresia is similar to that for TOF Maintaining the patency of the ductus for the perioperative treatment of neonates with pulmonary atresia and critical pulmonary stenosis is essential prior to additional interventions to establish more permanent pulmonary blood flow Following pulmonary valvotomy (interventional or surgical), the goal of therapy is to improve oxygenation and decrease RV afterload Because the LV may be more pressure and volume loaded in this lesion than in the normal fetus, the RV may be relatively Pathophysiology In this condition, an imperforate TV and hypoplasia of the RV are present, often accompanied by a VSD of variable size and by pulmonic stenosis The most common form of tricuspid atresia has normally related great arteries; when associated with transposition of the great vessels, the clinical presentation is similar to that of hypoplastic left heart syndrome In the usual type of tricuspid atresia, an obligatory atrial-level shunt exists from the RA through the patent foramen ovale or ASD into the LA, where complete mixing takes place The degree Critical Care Management for Late Postoperative Care Patients with TOF and pulmonary atresia are subject to the same late problems and complications as patients with TOF alone In addition, they will require conduit revisions over their lifetime Some patients have accelerated conduit obstruction after surgery, which is often related to the presence of a porcine valve.204 Consequently, many experts now prefer bovine-valved conduits or homografts except in certain situations, such as when the distal pulmonary vascular impedance is high, resulting in central pulmonary artery hypertension In these situations, a valveless conduit that permits pulmonary insufficiency (and thus relieves central pulmonary artery hypertension) is preferred In cases when pulmonary regurgitant volume load to the RV is undesirable, some favor the placement of a heterologous bovine jugular vein bioprosthesis with a trileaflet venous valve.205 This has been the valved conduit of choice in patients younger than 18 years whenever pulmonary regurgitant volume load to the RV is undesirable.205 Small-caliber bovine jugular vein conduits may result in significantly improved freedom from dysfunction at and 10 years’ follow-up compared with pulmonary homografts in patients who received the operation during the first years of life.206 Tricuspid Atresia 404 S E C T I O N I V Pediatric Critical Care: Cardiovascular of hypoxemia depends on the amount of pulmonary blood flow, which is regulated by the severity of the pulmonic stenosis This, in turn, is regulated by the size of the VSD; patients with an extremely small/restrictive VSD will have pulmonary atresia in addition to tricuspid atresia The common presentation is characterized by significant hypoxemia caused by decreased pulmonary blood flow Critical Care Management The ultimate palliative surgical course for tricuspid atresia is a modified Fontan procedure Neonatal hypoxemia prior to surgery can be effectively stabilized with a PGE1 infusion An initial palliative procedure may be required to improve pulmonary blood flow (modified BT shunt) if the child becomes severely hypoxemic In contrast, if there is excessive pulmonary blood flow, a pulmonary artery band may be needed The critical care management and complications are those discussed in the sections on shunts, banding, and modified Fontan procedures Complications of chronic hypoxemia and cyanosis are also present Left-Sided Obstructive Lesions Pathophysiology This category includes valvar, subvalvar, and supravalvar mitral and aortic stenosis, aortic coarctation, IAA, and HLHS Although these lesions can occur as isolated defects, they often coexist (Shone syndrome) or are accompanied by other congenital cardiac defects Identification of additional structural defects is necessary for optimal preoperative, surgical, and postoperative treatment Patients with LV outflow tract obstruction tend to present either as neonates or as young infants with significant LV dysfunction and CHF or later in childhood with subclinical LV hypertrophy The dramatic presentation of a neonate with circulatory collapse typically occurs with lesions that obstruct systemic blood flow so severely that right-to-left shunting at the ductus arteriosus is required to perfuse the body As the ductus significantly narrows or closes, the LV proves inadequate to support the systemic circulation or fails, leading to pulmonary edema and respiratory distress When systemic perfusion becomes inadequate, the patient may quickly develop cardiogenic shock Classic examples of this physiology include severe (or “critical”) valvar AS, coarctation of the aorta, and HLHS (see the earlier single-ventricle discussion) If the obstruction is less severe, the child can make the transition through ductal closure without notable LV dysfunction and maintain an adequate cardiac output Over time, however, the pressure overload on the LV stimulates generalized hypertrophy If untreated and significant, long-term pressure overload can cause LV diastolic dysfunction (compliance falls and end-diastolic pressure rises, causing pulmonary edema), LV systolic dysfunction, and episodic myocardial ischemia Clinical manifestations of these changes can include reduced exercise tolerance, exertional chest pain, ventricular dysrhythmias, syncope, and sudden death In this situation, significant LV dilation or clinical signs of CHF are ominous findings associated with a poor prognosis and an increased surgical mortality rate Aortic Stenosis Of the three anatomic subtypes of AS, valvar AS occurs more frequently than subvalvar or supravalvar AS The newborn with critical valvar AS who develops hypotension and acidosis as the ductus arteriosus closes requires resuscitation with PGE1 to restore aortic flow plus mechanical ventilation and inotropic support to achieve stabilization before an intervention is performed Currently, balloon dilation of the stenotic aortic valve during cardiac catheterization is the preferred intervention at most centers A surgical valvotomy under direct vision using CPB is the surgical alternative Despite successful relief of obstruction, significant LV dysfunction and low cardiac output often persist for days after the procedure and require continued treatment with mechanical ventilation and vasoactive drugs Until LV function recovers and can support the entire cardiac output, a PGE1 infusion may be necessary to maintain a PDA Patients should be carefully evaluated after balloon aortic valvuloplasty for residual AS and aortic insufficiency, the chief potential complication of valve dilation, especially if cardiac output does not improve over several days Older infants, children, and adolescents with moderate (pressure gradient 50–70 mm Hg at catheterization) or severe (pressure gradient 70 mm Hg at catheterization) valvar AS also are generally good candidates for balloon aortic valvuloplasty Usually, if more than mild aortic regurgitation coexists with AS, surgical intervention is preferred to balloon valvuloplasty The pathophysiology produced by all types of aortic outflow obstruction is similar—that is, the pressure-overloaded LV becomes progressively hypertrophied and develops reduced compliance and an abnormally elevated end-diastolic pressure Initial assessment of obstruction relief can occur when the patient is still in the catheterization laboratory or operating room by either direct pressure measurements or echocardiography Nevertheless, reevaluation for residual obstruction by physical examination or echocardiography in the ICU as patients recover from anesthesia and baseline physiology returns is important because outflow gradients can change A significant residual obstruction should be suspected in any patient with persistent low cardiac output following the intervention Poor recovery of LV function after surgery can occur secondary to inadequate myocardial protection with cardioplegia in hearts with significant ventricular hypertrophy Patients with marked hypertrophy are also at greater risk for developing VT and ventricular fibrillation early after surgery In patients with preserved LV systolic function who undergo an uncomplicated procedure, such as aortic valvuloplasty or subvalvar membrane resection, myocardial recovery after CPB is typically rapid, and inotropic support is usually not required Systemic hypertension is more common following relief of LV outflow obstruction, especially during emergence from anesthesia and sedation Antihypertensive therapy in the initial 24 to 48 hours may be necessary to prevent an aortic suture line and reconstructed valve leaflet disruption from excessive stress and to allow adequate hemostasis Initially, both titratable b-blockers (e.g., labetalol and esmolol) and vasodilators (e.g., nitroprusside), alone or in combination, are effective in lowering the blood pressure for these patients However, when using vasodilators, caution must be taken to maintain adequate coronary perfusion for the hypertrophied ventricle In addition to assessing aortic valve and LV function, an evaluation for complications specific to each procedure is required For example, if a myectomy is performed as part of the resection of fibromuscular subvalvar AS, the possibility of a new VSD, mitral valve injury, and left bundle branch block should all be assessed Following the Ross procedure, it is important to assess patients for RV outflow tract and LV outflow tract obstruction, because the CHAPTER 36 Critical Care After Surgery for Congenital Cardiac Disease RV outflow tract is also reconstructed with a valved conduit This procedure involves reimplantation of the native coronary arteries into the new pulmonary autograft placed in the aortic position; signs of coronary ischemia, unexplained hemodynamic collapse, or ventricular arrhythmia should prompt aggressive reevaluation of coronary flow adequacy Coarctation of the Aorta Coarctation of the aorta is a narrowing in the descending aorta located at the level of insertion of the ductus arteriosus (juxtaductal or contraductal coarctation) Narrowing of the aortic lumen is asymmetric, with the majority of the obstruction occurring because of posterior tissue infolding, leading to the common description of a posterior aortic shelf Depending on the severity and rapidity of development of the narrowing, patients can present as neonates with severe obstruction (a critical coarctation of the aorta) upon ductal closure, as infants with CHF, or as children/ adolescents with asymptomatic upper extremity hypertension (especially with exercise) Neonates presenting with critical coarctation of the aorta can often be distinguished clinically from patients with critical AS by their clearly discrepant upper versus lower body pulses, perfusion, and blood pressures Other features at presentation are similar, including evidence of CHF and inadequate blood flow to tissues Because ductal narrowing or closure is common after hospital discharge, these patients often become critically ill and suffer endorgan damage before the ductus arteriosus can be reopened and resuscitation complete Intestinal and renal ischemia leading to necrotizing enterocolitis and renal failure, respectively, are wellknown complications of critical coarctation of the aorta Echocardiography often reveals additional left-sided defects, such as bicuspid aortic valve, valvar AS, aortic arch hypoplasia, and VSD Preoperative stabilization and management in this clinical scenario include initiation of PGE1, mechanical ventilation, and inotropic agents as needed In addition, these patients require adequate time for end-organ recovery before performing an intervention In rare situations, when the PGE1 infusion is unable to open the ductal and periductal arch tissue to diminish the afterload on the ventricle and facilitate cardiac and end-organ recovery, the patient should be taken to the operating room urgently for coarctation repair Coarctation of the aorta also occurs in association with complex defects, such as D-transposition of the great arteries, single ventricle, and complete AV canal defect If the ductus arteriosus is patent during echocardiographic evaluation of a neonate with suspected CHD, it often is not possible to predict the severity of coarctation of the aorta with confidence A patient can have an abnormally narrowed aorta just proximal to the site of ductal insertion (i.e., the aortic isthmus) and a posterior shelf but still not develop a severe coarctation of the aorta following ductal closure Therefore, evaluation of the potential severity of coarctation of the aorta in the ICU often involves discontinuing the PGE1 infusion to allow for the ductus to close, followed by close clinical and echocardiographic reassessment for clinical manifestations of coarctation An intervention to reduce aortic arch obstruction is indicated in any neonate with clinical or echocardiographic evidence of reduced ventricular function or impaired cardiac output related to coarctation development These indications are more important than the systolic blood pressure difference between upper and lower body per se, although differences greater than 30 mm Hg often are accompanied by diminished ventricular function The postoperative management of patients following surgical repair of coarctation of the aorta can vary depending on the age 405 at intervention However, the key issues for assessment in all patients are adequate relief of obstruction and preservation of spinal cord function Upper and lower body blood pressures and pulses should be compared serially and the lower extremities monitored closely for the return of sensation and voluntary movement in the early postoperative period Equal pulses and a reproducible systolic blood pressure difference less than 10 to 12 mm Hg between upper and lower extremities indicate an excellent repair Neonates and young infants typically require to days of mechanical ventilation postoperatively Older children and adolescents often can be extubated in the operating room and rarely require inotropic support In fact, these patients repaired at an older age are increasingly likely to have significant hypertension,207 which should be treated aggressively early after surgery to reduce the risk of aortic suture disruption and bleeding b-Blockers and vasodilators, along with adequate analgesia and sedation, are effective Patients with longstanding coarctation of the aorta frequently have persistent systemic hypertension despite adequate surgical repair; continued treatment with angiotensin-converting enzyme inhibitors is advocated to achieve normal blood pressures Due to the challenges in managing this cohort of older patients after coarctation repair and the advent of interventional catheterization approaches, including the use of covered stents, a surgical repair strategy is increasingly falling out of favor Four uncommon complications are associated with surgical repair of coarctation of the aorta Postcoarctectomy syndrome manifests as abdominal pain or distension in older patients and is presumably caused by mesenteric ischemia from reflex vasoconstriction after restoration of pulsatile aortic flow Recurrent laryngeal nerve and phrenic nerve trauma can cause vocal cord paralysis and hemidiaphragm paresis or paralysis, respectively, with neonates and infants at highest risk Disruption of lymphatic vessels or thoracic duct trauma can produce a chylous effusion that may require treatment by drainage, dietary modification, or surgical ligation of the ductus Catheter-directed balloon angioplasty is used to treat both native and residual coarctation of the aorta.208 The results of native coarctation of the aorta dilation after early follow-up appear similar to published surgical results, but aortic aneurysm formation has been reported Balloon angioplasty of recurrent coarctation of the aorta after surgery is effective and is generally preferred to reoperation Interrupted Aortic Arch Patients with IAA typically present as neonates with either a loud systolic murmur or circulatory compromise as the ductus arteriosus closes Therefore, patient presentation can be similar to other critical left-sided obstructive lesions Unlike either critical AS or coarctation of the aorta, however, severe pressure overload on the LV does not occur in the presence of an unrestrictive VSD, which functions as a “pop-off” for LV outflow The approach to resuscitation is similar to that described for the other ductal-dependent left-sided obstructive lesions, with attention to the possibility of pulmonary overcirculation as for HLHS Postoperative management issues specific to patients with IAA include assessment of possible residual left-sided obstruction both in the aortic arch and in the subaortic region, shunting across a residual VSD, hypocalcemia (related to the 22q11 syndrome), dysrhythmias, and LV dysfunction with low cardiac output secondary to global effects of CPB and deep hypothermic circulatory arrest Left-lung hyperinflation on postoperative chest radiographs suggests compression of the left main stem bronchus This 406 S E C T I O N I V Pediatric Critical Care: Cardiovascular complication tends to occur after difficult arch reconstructions when tension on the aorta causes it to press on the anterior surface of the bronchus, thus producing distal air trapping Hypoplastic Left Heart Syndrome Pathophysiology Among the congenital heart lesions, perhaps the most controversial has been HLHS Left untreated, HLHS is a uniformly fatal disease; debate continues regarding the optimal management strategy (i.e., staged palliation, neonatal transplantation, or comfort care) Currently, the 14-month transplant-free survival ranges from 65% to 75%, making the transplantation waitlist mortality unacceptable for these patients The results of surgical management vary among institutions and are clearly dependent on expertise and experience,209 the clinical condition of the neonate at presentation,210 degree of prematurity, associated congenital anomalies, presence of an intact atrial septum, age at presentation,211,212 and degree of hypoplasia of left heart structures.212,213 This common example of single-ventricle physiology also represents the most severe form of obstructive left heart lesion An anatomic spectrum of disease is implied for the lesion; however, in its most severe and common presentation, there is atresia or marked hypoplasia of the aortic and mitral valves with critical underdevelopment of the left atrium, left ventricle, and ascending aorta A 1or 2-mm diameter ascending aorta gives rise to the coronary circulation and to the head vessels before convergence with the ductus arteriosus, where the aorta becomes larger and supplies the circulation to the lower body Pulmonary venous blood returns to the diminutive left atrium and cannot cross the atretic mitral valve It is instead directed to the right atrium and RV, where it mixes with systemic venous return and is ejected into the pulmonary artery Systemic blood flows from the pulmonary artery, across the PDA, to the aorta As the PDA constricts in the neonatal period, systemic blood flow decreases and all ventricular output is directed to the lungs The Qp/Qs ratio approaches infinity as Qs nears zero The patient consequently presents with a high Pao2 (70–200 mm Hg), shock, and profound metabolic acidosis When the ductus arteriosus is reopened with PGE1, systemic perfusion is reestablished, the acidosis resolves, and the Pao2 returns to the range of 40 to 60 mm Hg, representative of a Qp/Qs ratio between and Critical Care Management Preoperative resuscitation with PGE1, correction of metabolic acidosis, and recovery from end-organ dysfunction are crucial to the stabilization and management of patients with this lesion Resuscitation is enhanced by the judicious use of inotropic agents, which optimize cardiac output and end-organ perfusion However, excessive delay in the timing of surgical intervention may result in a gradual decline in PVR, excessive pulmonary blood flow, and inadequate systemic perfusion The surgical reconstructive approach to this lesion now commonly entails three operations intended to provide the child with a reconstructed aortic arch and Fontan-type single-ventricle physiology by to years of age.214,215 In the first stage of the reconstruction (Norwood operation),215 the pulmonary artery is transected at the bifurcation and an anastomosis is performed connecting the pulmonary trunk to the ascending aorta (above the coronary arteries) to form a neoaorta, connecting RV, diminutive LV, coronary arteries, and systemic circulations in an unobstructed fashion Pulmonary blood flow is established via a modified BT shunt (usually 3.5 mm in diameter) or a valveless RV-PA (Sano) conduit The atrial septum is excised to ensure free flow of pulmonary venous return to the TV The Norwood operation may also be performed to repair other complex single-ventricle defects that include systemic outflow obstruction or hypoplasia.216 The critical care considerations are the same as those outlined for patients with single-ventricle physiology Perioperative management requires optimization of combined ventricular output, minimization of systemic oxygen consumption, and careful manipulation of Qp/Qs Postoperative Management Evolution of Treatment Strategies. Common teaching has held that postoperative mortality and hemodynamic instability result from myocardial dysfunction and the physiologic burden imposed by shunt-dependent pulmonary blood flow parallel to the systemic circulation Treatment strategies have emphasized factors that affect the balance between pulmonary and systemic blood flow Immediately following a Norwood operation, PVR may be transiently elevated but soon declines Once PVR falls, treatment is aimed at raising resistance to blood flow through the lungs and redirecting cardiac output to the systemic circulation High inspired concentration of oxygen, hyperventilation, alkalosis, systemic vasoconstriction, and anemia will cause a further increase in pulmonary blood flow and should be avoided Therapies designed to raise PVR and thereby direct aortic blood flow to the systemic circulation have focused on lowering the Fio2 or on allowing the Paco2 to rise and the pH to fall toward 7.3 Ventilation with hypoxic gas mixtures or added carbon dioxide has been advocated by some centers and has been intermittently embraced and abandoned by others Validation of the effectiveness of these techniques to balance the pulmonary and systemic circulations has been difficult.217,218 The clinical focus of care after Norwood palliation for HLHS has evolved since the 1990s The early emphasis was on manipulating the PVR by optimizing mechanical ventilation (i.e., mean airway pressure, tidal volume, rate and inspiratory time, Fio2, and Paco2) The objective was to raise PVR and lower pulmonary blood flow while increasing systemic blood flow Like any therapeutic strategy, manipulating gas exchange and inspired gases had its own set of adverse effects Delivering low Fio2 (below room air concentrations) leads to alveolar hypoxia and can be life-threatening Furthermore, if treatment to elevate PVR is prolonged in preparation for reconstructive or transplantation surgery, PVR may be persistently elevated during and after weaning from CPB In centers where neonates are allowed to awaken and breathe spontaneously in the immediate postoperative period, pulmonary blood flow may become excessive Adding carbon dioxide to the inspired gas may reverse the tendency toward respiratory alkalosis and stabilize the relative balance of the pulmonary and systemic circulations if the forces that drive minute ventilation are suppressed by sedation or analgesia.217,218 However, the metabolic cost of carbon dioxide breathing in an awakening child may discourage the widespread application of this technique until the physiologic advantage over conventional means of controlling alveolar ventilation and Paco2 has been demonstrated This is especially true in unsedated preoperative patients for whom factors controlling respiration during carbon dioxide breathing may limit the change of Paco2 yet substantially increase the respiratory rate and work of breathing By the late 1980s it was apparent that (Norwood) palliated circulation was needed for only 10 to 12 weeks until the infant reached adequate size for a second stage of palliation CHAPTER 36 Critical Care After Surgery for Congenital Cardiac Disease (bidirectional Glenn) The introduction of a 3.5-mm systemicto-pulmonary shunt (rather than a 4-mm shunt) and the appreciation of the surgical complexity of an appropriately placed shunt did as much to reduce excessive pulmonary blood flow in infants as did manipulation of the ventilator Although the smaller shunts were associated with a rare but real incidence of shunt thrombosis, those involved in postoperative care found patient management with a small shunt substantially easier than struggling with the tendency for pulmonary overcirculation with a 4-mm shunt While a relative increase in cyanosis (from a smaller shunt) was believed to be justified by the greater early postoperative stability, mortality rates did not plummet with recognition of the benefits of smaller shunts and PVR manipulation In the early 1990s it was appreciated that arterial oxygen saturation was only one variable in the assessment of Qp/Qs A perfectly acceptable arterial oxygen saturation of 80% in this disease may represent severe pulmonary overcirculation if the mixed venous oxygen saturation is very low (e.g., 20%) Hence, interest emerged in measuring and monitoring both arterial and mixed venous oxygen saturations Thus, in patients with excessive pulmonary blood flow despite a small (3.5-mm) fixed-diameter shunt from the subclavian artery, there was less emphasis on micromanagement of PVR and enhanced interest in pharmacologic support of cardiac output by reducing systemic afterload and diminishing the driving pressure across the shunt Some have advocated the use of the alpha-blocker phenoxybenzamine 407 to blunt systemic vascular reactivity and dilate the peripheral circulation,219 but its potent, long-lasting hypotensive effects and its limited reversibility by alpha-agonists has proven challenging The phosphodiesterase inhibitors then came to enjoy widespread use in pediatric critical care These agents lower SVR, increase cardiac output, and reduce filling pressures The new strategy was to monitor (arteriovenous) Do2, support cardiac output, and reduce SVR Later in the decade, it was observed that some patients had limited coronary reserve, low mixed venous oxygen saturation, rising lactate, and collapsed in the first 48 hours after operation This emphasized the fundamental limitation of myocardial function and cardiac output of these infants in the early postoperative period The morphologic RV and TV seem ill-suited to support both adequate systemic and optimal pulmonary blood flow after the Norwood procedure Several centers then embraced temporary mechanical support of the circulation for the failing Norwood patient in the early postoperative period Specific Considerations for the Norwood Operation. Management of patients following a Norwood-type operation is complex Intensive monitoring is essential because the patient’s clinical status can change abruptly with rapid deterioration Persistent or progressive metabolic acidosis is an ominous prognostic sign that must be aggressively managed Considerations in the assessment of the circulation following the Norwood operation are given in Table 36.5 TABLE Management Considerations for Patients Following a Norwood Procedure 36.5 Scenario Etiology Management Sao2 ,80% Svo2 ,60% Normotensive Balanced flow (Qp Qs) No intervention Sao2 90% Hypotension Overcirculated (Qp Qs) Low PVR Large BT shunt Residual arch obstruction Raise PVR • Controlled hypoventilation • Low Fio2 (0.17–0.19) • Add CO2 (3%–5%) • Increase systemic perfusion • Afterload reduction, vasodilation • Inotropic support • Surgical shunt revision Sao2 ,75% Hypertension Undercirculated (Qp , Qs) High PVR Small, kinked, thrombosed BT shunt Lower PVR • Controlled hyperventilation • Alkalosis • Sedation/paralysis Increase cardiac output • Inotropic support Hematocrit 40% Surgical intervention Sao2 ,75% Hypotension Low Svo2 Low cardiac output Ventricular failure Myocardial ischemia Residual arch obstruction Atrioventricular valve regurgitation Minimize stress response Inotropic support Surgical revision Consider mechanical support Consider transplantation BT, Blalock-Taussig; Fio2, inspired oxygen concentration; PVR, pulmonary vascular resistance; Qp, pulmonary blood flow; Qs, systemic blood flow; Sao2, arterial oxygen saturation; Svo2, mixed venous oxygen saturation ... analgesia.217,218 However, the metabolic cost of carbon dioxide breathing in an awakening child may discourage the widespread application of this technique until the physiologic advantage over conventional... and Paco2 has been demonstrated This is especially true in unsedated preoperative patients for whom factors controlling respiration during carbon dioxide breathing may limit the change of Paco2... perfusion becomes inadequate, the patient may quickly develop cardiogenic shock Classic examples of this physiology include severe (or “critical”) valvar AS, coarctation of the aorta, and HLHS (see