Hypoplastic left heart syndrome

Congenital heart disease resulting in defects of the heart and circulatory system affects more infants than any other type of birth defect and occurs in approximately 1 of every 115 children born. (1,2) The defects range from mild to severe, and nearly half of the affected infants require intervention within the first year of life. (2) Great strides have been made in the surgical correction and palliation of congenital heart defects since open heart repairs for infants and children first became possible in the 1950s. For children with the most complex heart defects, this success has been very recent.

Hypoplastic left heart syndrome (HLHS) is the fourth most common congenital cardiac defect and the most common form of congenital heart disease that results in a functional single ventricle. HLHS is a constellation of cardiac abnormalities that includes severe stenosis or complete atresia of both the left ventricular inflow and outflow tracts. (3) Before 1980, multiple attempts at palliative surgical treatment of HLHS were mostly unsuccessful. (4) Most infants with this syndrome die by 1 month of age. The advent of the Norwood procedure (5) and advances in medical management have, for the first time, allowed patients with this diagnosis to have a reasonable hope for survival.

Currently, centers specializing in the treatment of congenital heart disease offer either a multistaged surgical palliation or orthotopic cardiac transplantation to families of infants with HLHS. Center-specific preference for surgical palliation versus transplantation has evolved on the basis of results with the individual procedures. The option of supportive care only continues to be offered at some centers because the mortality rate for HLHS is higher than that for other congenital heart defects and little information on long-term follow-up of HLHS survivors is available. (6)

Beginning in 1994, a targeted effort to reduce mortality after the Norwood procedure was undertaken at the Children's Hospital of Wisconsin, a large pediatric tertiary care hospital in Milwaukee that specializes in the treatment of congenital heart disease. Staged surgical palliation of HLHS became the preferred approach to infants with this syndrome in part because of the scarcity of infant heart donors and the encouraging results with early surgical palliation. (7) Researchers focused on strategies to improve systemic oxygen delivery and optimize cardiac output during preoperative, intraoperative, and postoperative management. This effort produced several changes in patients' care as well as a global improvement in understanding the physiological processes involved in infants with HLHS. Early survival (to 30 days or discharge from the hospital) after the Norwood procedure improved greatly, from 53% before 1996 to 95% in 139 patients who had surgery between July 1996 and September 2004. Survival to stage 2 palliation in these 139 patients was 88% through September 2004.

In this article, we review the clinical manifestations of HLHS and current research and management strategies used from diagnosis through surgical recovery to improve patients' survival and outcomes. The experience at Children's Hospital of Wisconsin is used as an example of successful implementation of these strategies.

Prevalence and Etiology

HLHS is estimated to occur in 1 to 3 per 10 000 live births or approximately 1500 infants in the United States each year. HLHS accounts for 7.5% of the infants born with congenital heart disease. A total of 25% of the cardiac deaths within the first week of life and 15% of the deaths in the first month of life are due to HLHS. Infants born with HLHS are typically full-term and have a normal birth weight. Marked extracardiac malformations are present in approximately 2.3%. (3) Two thirds of the reported cases occur in boys. The recurrence rate of congenital heart disease in siblings of a child with HLHS is estimated at 0.5% for HLHS and 2.2% for any other type of congenital heart disease. (2,8)

The embryological cause of HLHS is not fully understood. Most likely multiple developmental anomalies occur during fetal maturation that result in limited left ventricular inflow and outflow and culminate in this complex syndrome.

Anatomy

The classic and most common form of HLHS includes aortic valve atresia with resultant hypoplasia of the ascending aorta, aortic arch, and the left ventricle (Figure 1). Most patients with aortic valve atresia also have mitral valve stenosis or atresia. During intrauterine development and immediately after birth, the right ventricle provides systemic and pulmonary blood flow through the pulmonary artery and a large unrestrictive patent ductus arteriosus (PDA).

The grossly underdeveloped left ventricle and aorta cannot support systemic circulation, leaving infants with HLHS with a single functioning ventricle. Deoxygenated blood returns to the heart via the superior and inferior vena cavas just as it does in the normal heart, but this is the point where similarities end between infants with normal hearts and those with HLHS. In infants with HLHS, the deoxygenated blood enters the right atrium, where it mixes with oxygenated blood from the lungs that has crossed from the left atrium via the patent foramen ovale or an atrial septal defect. From the right atrium, the mixed arteriovenous blood flows into the right ventricle. Blood is ejected through the main pulmonary artery and flows to either the pulmonary circulation through the right and left pulmonary arteries or to the systemic circulation through the PDA to both the transverse and descending aorta. The brachiocephalic vessels and coronary arteries depend on retrograde blood flow from the ductus arteriosus into the transverse arch and hypoplastic ascending aorta. The blood ejected from the right ventricle follows the path of least resistance and is partitioned on the basis of the ratio of pulmonary resistance to systemic resistance. For any given cardiac output, the greatest circulatory efficiency is achieved when equal parts of blood are routed to the pulmonary ([dot.Q.sub.p]) and systemic ([dot.Q.sub.s]) circulations. This type of blood circulation is referred to as parallel circulation (Figure 1).

Pathophysiology

After birth, 3 factors affect the hemodynamic status of infants with HLHS (10): a decrease in pulmonary vascular resistance (PVR), the size of the interatrial communication, and involution and closure of the PDA. With parallel circulation, pulmonary and systemic blood flow is determined by the ratio of PVR to systemic vascular resistance (SVR). The ratio of pulmonary blood flow to systemic blood flow ([dot.Q.sub.P]/[dot.Q.sub.s]) describes how the cardiac output from the single ventricle is partitioned.

[FIGURE 1 OMITTED]

After birth, the PVR gradually decreases during the first several days of life, resulting in increasing pulmonary blood flow. With this increase in pulmonary blood flow, the volume load on the right ventricle also increases. An infant may have increases in systemic oxygen saturation but be at risk for progressive congestive heart failure and decreased systemic perfusion because of the inadequate size and function of the left-sided cardiac structures, including the mitral valve, left ventricle, aortic valve, and ascending aorta. These multiple sites of left-sided obstruction cause diminished aortic blood flow and subsequent reduced coronary artery flow. In HLHS, coronary flow depends on retrograde flow during diastole from the PDA into the small ascending aorta.

With the ductus arteriosus open, most infants with HLHS can maintain a balance between PVR and SVR, resulting in appropriate pulmonary and systemic perfusion. (10,11) However, if a marked discrepancy occurs in blood flow to the pulmonary and systemic circulations, rapid onset of hemodynamic instability may occur. The most common scenario resulting in an increase in the [dot.Q.sub.p]/[dot.Q.sub.s] ratio is excessive pulmonary flow at the expense of systemic flow. Conversely, low SVR or high PVR results in decreased pulmonary blood flow and a marked decrease in oxygen saturation. Without prompt attention, either of these conditions can result in rapid decompensation.

The character of the interatrial communication is a major determinant of pulmonary blood flow. (11) The left ventricle accepts little to no blood flow, so the interatrial communication provides the only route for the pulmonary venous blood to exit the left atrium. If an infant has a small, restrictive interatrial communication, the result is increased left atrial and pulmonary venous pressures. In extreme cases, the restrictive interatrial communication will result in profound hypoxemia and decreased pulmonary blood flow. In infants with an unobstructed, large interatrial communication, the blood flow from the left atrium to the right atrium increases as the PVR decreases, and volume overload of the right ventricle and systemic hypoperfusion may occur.

The PDA is a normal fetal structure that typically constricts and closes shortly after birth. As the PDA begins to close, blood is diverted from the systemic circulation to the pulmonary circulation. Increased flow to the lungs results in pulmonary congestion and a corresponding increase in the Pa[O.sub.2] and oxygen saturation. Because the ductus arteriosus provides the source of blood flow to both the coronary arteries and the descending aorta, as it continues to constrict, progressive systemic and coronary hypoperfusion occurs, ultimately resulting in a profound shock state.

Diagnosis and Clinical Features

Early diagnosis of HLHS may be difficult because infants with this syndrome may not have overt signs of the abnormality before discharge from the hospital. The first sign that a newborn may have HLHS is tachypnea and poor feeding. (12) Pulse oximetry may indicate mild hypoxemia.

Postnatally, HLHS is most commonly diagnosed 2 to 3 days after birth when an infant has respiratory distress and cyanosis. Infants with these characteristics have an open ductus arteriosus, increased pulmonary blood flow, and an adequate atrial communication that is allowing left-to-right shunting of blood. Indications of increased respiratory effort are due to a progressive increase in pulmonary blood flow and pulmonary congestion caused by a gradual decrease in PVR during the first days of life. Mild cyanosis is due to the mixing of pulmonary venous blood and systemic venous blood at the atrial level.

The second most common clinical indication of HLHS is vascular collapse, which is often mistaken for septic shock. In infants with this presentation, the ductus arteriosus is markedly constricted. Clinical signs of congestive heart failure are due to closure of the PDA and diversion of excessive blood flow to the pulmonary bed, resulting in right ventricular overload and decreased peripheral perfusion. The vascular collapse leads to poor renal, cerebral, and coronary perfusion. Rapid intervention is required to prevent cardiogenic shock and death.

The least common indication of HLHS is severe cyanosis in the first hours of life. Infants with this presentation have profoundly diminished pulmonary blood flow from an obstructed interatrial communication or persistence of high PVR. The prognosis for these infants is the least favorable.

Prenatally, HLHS can be diagnosed via fetal echocardiography as early as 16 weeks of gestation. (10) Prenatal diagnosis, which is becoming increasingly common, provides the opportunity for counseling and education of the families of infants who have this syndrome. In addition, prenatal diagnosis allows planning for delivery and care at an institution with a treatment program for congenital heart disease. Delivery at such an institution dramatically reduces the likelihood that an infant with HLHS will be subjected to the stressors of shock. For mothers of infants with suspected HLHS, labor and delivery are typically allowed to proceed normally, although the delivery is attended by neonatology specialists. If the infant's condition is stable, the parents are provided time with the baby before he or she is transferred to the neonatal intensive care unit. Diagnosis is confirmed by echocardiography after admission and stabilization of the infant's condition in the neonatal intensive care unit.

Regardless of when signs of HLHS are detected, 2-dimensional echocardiography with Doppler imaging is required to make the definitive diagnosis. Infants with HLHS rarely require cardiac catheterization, and because of their tenuous condition, catheterization is typically done only when anatomical elements cannot be identified by using echocardiography.

Family Counseling

The diagnosis of HLHS, whether made prenatally or postnatally, is a crisis for parents and families. Although the need to treat infants with the syndrome is urgent, family members should be allowed to experience the shock and grief that accompany such a significant diagnosis. Families need time to rally coping skills and support systems and to process the large amount of information that is presented. Providing multiple opportunities for parents to learn about their infant's heart disease is important.

At most centers specializing in the care of children with congenital heart disease, parents are presented the options of (1) surgical palliation with the Norwood procedure and its subsequent surgical stages or (2) cardiac transplantation. Some centers continue to offer the option of supportive care only for these infants. (6,13,14) Reasons cited for this practice include questions focused specifically on long-term functionality and quality of life for survivors.

In order to make informed decisions, families of children with HLHS should be presented with the latest information available on the outcomes of each treatment option. For the first time in history, patients with HLHS are surviving to school age and adolescence. (15) Recent advances in the care of children with HLHS are expected to have a significant impact on long-term outcomes. Therefore, findings in the oldest group of survivors may be quite different than those in more recent cohorts. In the most recent studies, (15,16) investigators detected growth and neurodevelopmental delays, with the greatest delays in communication and gross motor skills, in children with HLHS who had the Norwood procedure. However, delayed development does not necessarily equate with poor quality of life, and parents of children who had the Norwood procedure have reported good health-related quality of life for their children. (17)

Most importantly, families have reported that they benefitted greatly from accurate information and the support of other families and children who had HLHS. At our institution, every family is given printed information on anatomy, surgical procedures, and the outcomes for HLHS. They are also provided with a list of names of local families who are willing to be contacted and a brochure with information on Internet resources on congenital heart disease. Online support groups for parents are available via informational Web sites. (18)

Preoperative Management

Once the diagnosis of HLHS in an infant is confirmed and the infant's family has chosen to proceed with surgical palliation, a modified Norwood procedure is scheduled. Perioperative mortality is a significant concern, and patients with HLHS are at risk preoperatively, intraoperatively, and especially postoperatively. The limited reserve of the single right ventricle and the inherent inefficiency of parallel pulmonary and systemic circulation create pathophysiological conditions that challenge the intuitive perception of patients' needs by the many healthcare professionals who participate in the care of these infants. A fundamental understanding of the role of pulmonary and systemic resistance in determining blood flow and adequate interpretation of systemic arterial and venous oxygen saturations (Sa[O.sub.2] and SV[O.sub.2], respectively) are crucial to the optimal clinical management of infants with HLHS from diagnosis through discharge. (7,19-22)

As always, preoperatively the goal is to achieve adequate systemic oxygen delivery. In order to achieve this outcome, the ductus arteriosus must be patent and blood flow to the pulmonary and systemic circulations should be nearly balanced (goal [dot.Q.sub.p]/[dot.Q.sub.s] ratio of 1). (22) The immediate therapy for all infants with HLHS is an intravenous infusion of prostaglandin [E.sub.1] to maintain ductal patency. A continuous infusion of the prostaglandin is initiated, preferably through a central catheter, at a rate of 0.05 to 0.1 [micro]g/kg per minute. Prostaglandin [E.sub.1] has several dose-related side effects, including apnea, fever, increased secretions, and capillary leak. (12) An audible murmur and adequate peripheral perfusion provide evidence of ductal patency; however, Doppler echocardiography is needed to confirm flow. Once the ductus is open, the rate of infusion may be reduced to decrease the risk for potential adverse effects. Unrestricted blood flow through the ductus arteriosus is necessary for systemic perfusion.

The [dot.Q.sub.p]/[dot.Q.sub.s] ratio preoperatively is dictated by the adequacy of the interatrial communication. An infant with a mildly restrictive interatrial communication may have balanced circulation and remain in a clinically stable condition as long as the ductus arteriosus remains open. Oxygen saturations of 75% to 85% by pulse oximetry suggest adequate balance between systemic and pulmonary blood flow. (11,22) Ventilatory support may be needed for apneic episodes or tenacious secretions, both common adverse effects of treatment with prostaglandin [E.sub.1]. Judicious use of inotropic support is initiated if evidence of low cardiac output is detected. Infusion of dopamine at a rate of 3 to 5 [micro]g/kg per minute usually results in improved ventricular function. High-dose inotropic support should be used with caution because it can result in increased SVR and cause a shift in the [dot.Q.sub.p]/[dot.Q.sub.s] ratio to greater than 1. Diuretics may be necessary to help alleviate the increased volume load on the right ventricle. (11)

Infants with an unrestrictive interatrial communication may be in stable condition initially, but signs of congestive heart failure may develop as the PVR decreases. When oxygen saturations are approximately 90%, systemic blood flow may be reduced, resulting in tissue hypoperfusion, metabolic acidosis, and a low cardiac output state. (7,8,19) In infants with high oxygen saturation and evidence of tissue hypoperfusion, controlled mechanical ventilation is often initiated to improve the [dot.Q.sub.p]/[dot.Q.sub.s] ratio and systemic cardiac output.

The [dot.Q.sub.p]/[dot.Q.sub.s] ratio can be manipulated by increasing PVR by increasing the PaC[O.sub.2], PaC[O.sub.2] can be increased by adding supplemental inspired carbon dioxide, a potent pulmonary vasoconstrictor, to the ventilator circuit. This approach for increasing PaC[O.sub.2] is preferred over hypoventilation, which may lead to atelectasis. PVR can also be increased by decreasing the concentration of inspired oxygen by adding supplemental nitrogen gas to attain a fraction of inspired oxygen of 0.17 to 0.19. (12) PVR can also be increased by maintaining the hematocrit at greater than 0.40, a state that optimizes oxygen-carrying capacity and increases the viscosity of the blood. (8) Although these medical management strategies may provide temporary palliation, infants with marked pulmonary overcirculation and systemic hypoperfusion benefit from early surgical correction, because the methods to reverse this situation have limited effectiveness.

Infants with HLHS who are born with a severely restricted or no interatrial communication, a rare occurrence, have profound hypoxemia. (11) The severe restriction of blood flow across the atrial septum results in a life-threatening situation. Management strategies include hyperventilation and supplemental oxygenation and an emergent intervention to relieve the interatrial obstruction. Relief of the obstruction can be achieved by a balloon atrial septostomy or blade septostomy at the time of cardiac catheterization or a surgical atrial septectomy. The tenuous condition of these infants makes each of these interventions high risk. The choice of intervention depends on the severity of the obstruction, the infant's cardiac anatomy and physiology, and the experience of the available medical and surgical team.

The role of nurses in the preoperative management of infants with HLHS is pivotal. Consistency in nursing caregivers promotes the ability to detect subtle changes in an infant's condition. Small changes in vital signs, appearance, or behavior can foretell drastic changes in the infant's physiological status. Preoperatively, infants with HLHS are at risk for congestive heart failure and low cardiac output. Nurses who are diligent in monitoring will be able to detect these changes and intervene before an infant is subjected to the stress of low cardiac output. A nurse's knowledge of the side effects of medications such as prostaglandin [E.sub.1] can allow timely interventions before complications occur. The condition of an infant preoperatively has a direct link with how well the infant will respond to surgical palliation and the infant's postoperative course.

Care for infants with HLHS is complex, and often multiple specialists are involved. Coordination of care facilitated by bedside nurses can promote effective communication between medical services and infants' families. A key role of nurses is to support an infant's family while the infant awaits surgery. (23) Large amounts of information are being delivered in a setting that is unfamiliar and intimidating to most families. Stress and anxiety levels are extremely high. Nurses can help alleviate families' stress by providing anticipatory guidance, clarifying information, assisting the families in finding effective coping mechanisms, and rallying appropriate institutional resources such as social work or chaplaincy services.

Intraoperative Management

Typically, infants have stage 1 surgical palliation, the Norwood procedure, when they are approximately 5 to 7 days old. Infants who were in shock must be adequately resuscitated and have evidence of organ recovery before surgery. Timing of surgery is determined on the basis of input from the cardiology, cardiovascular surgery, and critical care staff. Prenatal diagnosis has increasingly enabled elective induction of labor and scheduling of surgery.

Goals

Anatomically, the goal of surgical reconstruction is to relieve obstruction to systemic blood flow, unrestrict blood flow from left to right atrium, and create a source of adequate pulmonary blood flow. Other strategies such as modified ultrafiltration and afterload reduction are implemented to reduce the systemic inflammatory response to cardiopulmonary bypass and to optimize single-ventricle performance after surgery. (7) When these objectives are achieved, the workload of the single ventricle in the vulnerable postoperative period is greatly reduced. Narrowing or coarctation of the aorta after reconstruction causes residual obstruction to systemic blood flow and is poorly tolerated. In addition, as in the preoperative period, both undercirculation and overcirculation of the pulmonary vasculature put infants at risk for hemodynamic instability.

Procedures

At Children's Hospital of Wisconsin, the operative course proceeds as follows. Once the infant arrives in the operating room, anesthesia is induced with high doses of an opioid. The infant is intubated with a nasotracheal tube, and adequate vascular access is obtained. Adequate vascular access includes an arterial catheter and peripheral intravenous catheters if they are not already present. Cerebral and posterior flank oxygen saturations are monitored noninvasively by using near-infrared spectroscopy throughout the procedure. The surgeon opens the chest via a median sternotomy. The proximal end of the shunt connecting the systemic circulation to the pulmonary circulation is attached to the innominate artery. The shunt is used for arterial cannulation for cardiopulmonary bypass; the right atrial appendage is used for venous cannulation. Continuous cerebral perfusion is achieved via the shunt. Only brief periods of circulatory arrest are required for the atrial septectomy and recannulation. The atrial septectomy unrestricts blood flow from left atrium to right atrium. Once cardiopulmonary bypass is established, the infant is cooled to a core body temperature of 18[degrees]C to 22[degrees]C in a period of 20 to 30 minutes.

In order to effectively blunt the normal sympathetic response to stress and reduce afterload on the single ventricle, phenoxybenzamine, a nonselective, irreversible [alpha]-adrenergic-blocking agent, is added to the circuit at a dose of 0.25 mg/kg immediately after bypass is started. Phenoxybenzamine facilitates systemic vasodilation, allowing uniform cooling and a sustained reduction in SVR. This use of phenoxybenzamine is not clinically indicated by the manufacturer; therefore, informed consent from the infant's parent and Food and Drug Administration investigational drug status are required. Other medications such as phentolamine and sodium nitroprusside have also been used for afterload reduction. At Children's Hospital of Wisconsin, phenoxybenzamine is preferred because of its long half-life (approximately 24 hours) and stabilization of the systemic vasoconstrictor response. (21) It has been used as part of an institutionally approved research protocol in all subjects undergoing the Norwood procedure since December 1996.

During the cooling process, the surgeon continues the vascular dissection, dividing and patching the pulmonary artery. All ductal tissue from the native aorta is completely resected, and the anastomosis of the aorta to the pulmonary root is augmented by using a single patch of homograft material (Figure 2). After reconstruction, a 4F oximetric catheter is placed directly in the superior vena cava near the junction of the superior vena cava and the right atrium (Figure 3). This catheter allows direct, continuous measurement of systemic SV[O.sub.2], which becomes a key focus in the postoperative period.

[FIGURE 2 OMITTED]

The shunt connecting the systemic circulation to the pulmonary circulation is the single largest component of resistance to blood flow in the pulmonary circuit. Polytetrafluoroethylene shunts ranging in size from 3 to 4 mm are connected from the proximal innominate artery to the right pulmonary artery. The proper shunt size for each patient is determined initially by the patient's size and is confirmed by hemodynamic data and oxygen saturations. The [dot.Q.sub.p]/[dot.Q.sub.s] ratio is calculated after cardiopulmonary bypass is discontinued. As in preoperative management, the goal is to achieve a [dot.Q.sub.p]/[dot.Q.sub.s] ratio of 1.0.

Medical Management

Several strategies are aimed at reducing the postoperative inflammatory response. Corticosteroids (methylprednisolone [Solu-Medrol] 10 mg/kg intravenously) are administered the night before and the morning of surgery. Aprotinin, an antifibrinolytic agent effective in reducing blood loss after cardiac surgery, also has anti-inflammatory properties and is administered intraoperatively. (24,25) Patients also benefit from modified ultrafiltration, a perfusion technique that removes excess fluids and circulating proinflammatory cytokines. (26) Most recently, biocompatible coatings for components of the cardiopulmonary bypass machinery have become available. Most likely, these coatings will also contribute positively to reducing the postoperative systemic inflammatory response.

Inotropic support is started during rewarming, as the infant is weaned from cardiopulmonary bypass. The use of catecholamines after the Norwood procedure can be a problem because of their systemic vasoconstricting properties. With the use of phenoxybenzamine, the vasoconstricting effects of the catecholamines are limited. (7) Phosphodiesterase inhibitors, such as milrinone, are well suited for use after the Norwood procedure because of their combined inotropic and vasodilator properties. Patients routinely receive milrinone (a loading dose of 50 [micro]g/kg and then an infusion at a rate of 0.5 [micro]g/kg per minute) and dopamine at a rate of 3 to 5 [micro]g/kg per minute. The dosages of other inotropic agents are adjusted to achieve a mean arterial pressure near 40 mm Hg. If necessary, norepinephrine is used to counteract the potent vasodilatory effects of phenoxybenzamine. Epinephrine must be used with caution with simultaneous alpha-blockade, because the beta effects of epinephrine will be unopposed and can result in a paradoxical reduction in blood pressure. (27)

[FIGURE 3 OMITTED]

All inotropic agents are delivered directly into the central venous access via microbore tubing and syringe infusion pumps. The individual infusion sets are connected to a standardized manifold system that is secured prominently at the head of the patient's bed. Drugs are mixed in concentrations that minimize fluid volume and are stable for 72 hours. Standardization of the drug delivery system and management of the intracardiac catheters have eliminated the need to change the system used to deliver inotropic agents when the infant arrives in the intensive care unit and have greatly reduced the potential for medication errors.

When the patient's hemodynamic status is stable without use of cardiopulmonary bypass and the surgical field is free of active bleeding, preparations are made for surgical closure and transport. All patients receive pleural and mediastinal chest drainage tubes as well as temporary pacing leads affixed to both atrium and ventricle. In most patients, the sternum is left open, and the mediastinum is closed with silicone sheeting secured to the skin margins with adhesive and then covered by a transparent, sterile dressing. This technique allows adequate room for postoperative edema without compromising cardiac output and also allows direct visualization of the mediastinal cavity to assess postoperative bleeding. Delayed sternal closure minimizes the potential for atypical tamponade, a consequence of myocardial and chest wall edema, as well as increases in mean airway pressure. (7) Secondary sternal closure is typically undertaken in the pediatric intensive care unit 1 to 5 days after the primary operation.

Deep sedation is maintained by using fentanyl (5-10 [micro]g/kg per hour), benzodiazepines, and vecuronium (0.1 mg/kg per hour) for neuromuscular blockade. Adequate vascular access is imperative. Vascular access includes an arterial catheter, right and left atrial catheters, and the systemic venous catheter for SV[O.sub.2] monitoring. The patient is transported directly to the pediatric intensive care unit, with monitoring of blood pressure, oxygen saturation, heart rate, rhythm, and central venous pressure maintained during transportation.

Nursing Issues

The critical care nurse responsible for the postoperative care of the patient benefits from a thorough understanding of the infant's intraoperative course. Reports from the operating room staff and anesthesiologist should include information on anesthesia management, colloid/crystalloid fluid balance, blood products available, the duration of cardiopulmonary bypass and circulatory arrest, the size of the shunt placed, and any other pertinent events that occurred in the operating room. This information allows the critical care nurse to anticipate risks the infant may face during the early postoperative period.

Postoperative Management

Planning for postoperative care begins in the preoperative period. Adequate resuscitation from any preoperative injury, effective hemodynamic management, and optimal timing of the surgical intervention all have a positive influence on the postoperative course. Preparing an infant's family for the routine postoperative course as well as potential risks and complications will also help smooth the transition to postoperative care.

Reasons for Postoperative Risks

After the Norwood procedure, infants still have an inefficient parallel circulation and the limited reserve of a single functional ventricle (Figure 4). The main pulmonary artery is connected to the systemic circulation, creating the neo-aorta. The synthetic shunt connecting the systemic circulation to the pulmonary artery provides blood flow to the pulmonary circulation. Excessive pulmonary blood flow at the expense of systemic perfusion and inadequate cardiac output are common and potentially fatal hemodynamic disturbances in the early postoperative period. (7,19,27) Despite increasing experience with the Norwood procedure and the general acceptance of neonatal intervention for HLHS, postoperative mortality remains as high as 25% for infants who have the surgery at centers with experience in this procedure. (28-30)

First 48 Hours

The first 48 hours after the Norwood procedure are the time of greatest physiological vulnerability for several reasons. The myocardium of an infant with HLHS has limited stroke volume that is further compromised after ischemia and reperfusion associated with cardiac surgery. Postoperatively, the myocardium is prone to both systolic and diastolic dysfunction, further diminishing cardiac output. Also, injury to the systemic and pulmonary endothelium causes altered vascular responses. This alteration occurs at a time when the demand for oxygen transport is high. (7,19,27) Metabolic demands are high because of the surgical injury to tissue, the release of catecholamines, the systemic inflammatory response that occurs after cardiopulmonary bypass, fluctuations in body temperature, and a depletion of high-energy phosphate stores during surgery. These physiological challenges occur in the immediate postoperative period at the time when oxygen delivery and availability are often impaired. If impaired oxygen delivery is not recognized and effectively treated, the infant is at risk for multiple organ dysfunction, an exacerbation of the systemic inflammatory response syndrome and, potentially, death. (20)

[FIGURE 4 OMITTED]

Medical Gas Management

Although traditionally manipulation of medical gases was used to control PVR after the Norwood procedure, (31-33) at the Children's Hospital of Wisconsin, we have shifted away from this strategy. The induction of hypoxia and hypercarbia can increase PVR, improve systemic blood flow, and reduce the [dot.Q.sub.p]/[dot.Q.sub.s] ratio. Of note, however, the increased systemic blood flow has decreased arterial oxygen content. For manipulation of medical gases to be successful, the decrease in arterial oxygen content must be more than offset by an increase in systemic blood flow to result in improved systemic oxygen delivery. (27,34) Sa[O.sub.2] values and clinical assessment are generally the criteria used for directing this management strategy. Interventions are used to decrease the saturations when the Sa[O.sub.2] is greater than 80% in order to return the [dot.Q.sub.p]/[dot.Q.sub.s] closer to 1.

Preoperatively, when the ductus arteriosus is present, manipulation of medical gases is fairly effective in preventing pulmonary overcirculation and systemic hypoperfusion. However, after the Norwood procedure, the ductus arteriosus is replaced with a synthetic shunt between the systemic and pulmonary circulations that results in a fixed resistor in the pulmonary circuit. The shunt therefore reduces the efficiency of PVR manipulations on postoperative hemodynamics. (20) Using manipulation of medical gases to manage PVR requires astute ventilator management, which complicates the postoperative care of these infants.

Monitoring of Systemic Uenous Oxygen Saturation

Limitations of medical gas management and deliberate hypoxia led us to look to new strategies for the postoperative management of infants who had the Norwood procedure. Routine placement of 4F oximetric catheters in the superior vena cava for measurement of SV[O.sub.2] after the Norwood procedure was begun in 1996 (Figure 3). Oximetric measurement of SV[O.sub.2] provides an early indication of changes in oxygen utilization and delivery. (20,35) Because infants with HLHS do not have a true area of mixed venous blood, the oxygen saturation of blood in the superior vena cava is used as an approximation of SV[O.sub.2]. Continuous SV[O.sub.2] monitoring has allowed earlier detection of decreased systemic cardiac output before adverse effects occur.

Continuous monitoring of SV[O.sub.2] allows estimation of the [dot.Q.sub.p]/[dot.Q.sub.s] ratio by using a modified Fick equation (Table 1) and helps to determine oxygen supply-demand relationships, such as the arterial-venous oxygen content difference (21) (Table 2). By using these values, acute decreases in SV[O.sub.2] in the early postoperative period can be detected that might not be recognized by using the typical postoperative monitoring of blood pressure, heart rate and Sa[O.sub.2]. Figure 5 is an example of a multichannel recording of hemodynamic monitors that was obtained during one of these episodes. In the example, an abrupt decrease in SV[O.sub.2] occurred approximately 2 hours after admission to the intensive care unit, with a corresponding subtle increase in mean arterial pressure and Sa[O.sub.2]. The cause of this event was a sustained increase in SVR, resulting in decreased cardiac output and a systemic-to-pulmonary blood flow tradeoff that allowed maintenance of arterial blood pressure and Sa[O.sub.2]. In this case, these changes were detected only by SV[O.sub.2] monitoring and were effectively reversed by increasing sedation, analgesia, and inotropic support.

Continuous SV[O.sub.2] monitoring has indicated that many infants have episodes of sustained elevations of SVR after the Norwood procedure. (19,21) Decreasing the SVR after the procedure by using afterload-reducing agents such as phenoxybenzamine has important benefits, including decreased total resistance that results in increased cardiac output and a more balanced [dot.Q.sub.p]/[dot.Q.sub.s] ratio. With parallel circulation, as SVR and PVR values become more equal, blood flow is more equally divided between the pulmonary and the systemic beds, allowing the maximum circulatory efficiency possible. (27)

With increasing levels of afterload reduction, the [dot.Q.sub.p]/[dot.Q.sub.s] ratio is determined by the relative resistance of the systemic vasculature and the shunt between the systemic and pulmonary circulations, which acts as a fixed resistor. When the [dot.Q.sub.p]/[dot.Q.sub.s] ratio is fixed pharmacologically, as with phenoxybenzamine, increases in Sa[O.sub.2] correspond to increases in SV[O.sub.2]. (22,27) Efforts to balance the circulations are generally unnecessary, and a low SV[O.sub.2] can be treated solely by efforts to increase cardiac output. Effective lowering of SVR unlinks mechanical ventilation and circulatory balance by allowing the synthetic shunt to become the primary source of pulmonary resistance, thus "fixing" the PVR (36,37) and simplifying postoperative management markedly. Ventilatory management can then be directed toward maintenance of pulmonary function and efficiency of gas exchange. The fraction of inspired oxygen is generally maintained at 0.30 or greater, and mechanical ventilation is used to achieve normal PC[O.sub.2] levels.

Successful postoperative management of infants after the Norwood procedure depends on achieving adequate systemic oxygen delivery. Adequate delivery can be achieved by targeting an SV[O.sub.2] of 50% or greater, a [dot.Q.sub.p]/[dot.Q.sub.s] ratio of approximately 1 to 1.5 (Table 1), a mean arterial blood pressure of 45 mm Hg or greater, an arterial-venous oxygen content difference of less than 5 or a difference in arterial-to-venous oxygen saturation of 20% to 30% (Table 3), arterial and alveolar normocapnia, and a fraction of inspired oxygen that will promote adequate oxygen saturation. For sedation, the infants are given neuromuscular blocking agents and benzodiazepines (lorazepam 0.1 mg/kg every 4 hours) for the first 12 to 24 hours. A fentanyl infusion of 5 to 10 [micro]g/kg per hour is used for pain control.

[FIGURE 5 OMITTED]

Cardiac output is further optimized by evaluating heart rate, rhythm, preload, afterload, and contractility. Normal atrioventricular conduction results in improved cardiac output with a decrease in myocardial oxygen consumption. (18) If an infant is not in synchronous atrioventricular rhythm, measures such as atrial pacing or atrial and ventricular pacing should be used to reestablish such a rhythm.

The filling pressure (preload) of the heart is monitored via the common atrial catheter. The atrial pressure gives information about fluid status and about the diastolic function of the single ventricle. The optimal preload is the one that augments cardiac output to the greatest degree but minimizes the end-diastolic pressure, myocardial wall tension, myocardial oxygen consumption, and venous pressure. Elevations in central venous pressure that are not correlated with an increase in mean arterial pressure will result in a lower end-organ perfusion pressure. As cardiac function improves, the preload needed to achieve an adequate cardiac output decreases. The improvement is reflected by decreases in filling pressures. (27)

Nursing Issues

Attentive nursing care after the Norwood procedure has a marked impact on the postoperative course. The trauma of the surgical procedure leaves infants at risk for complications, including low cardiac output, hypoxemia, bleeding, infection, and side effects of medications. Nurses should know the appropriate ranges for the physiological variables being measured. An understanding of the medications an infant is receiving and knowledge of their actions and potential side effects are important. Nurses must be able to perform a physical assessment of a critically ill infant, monitor the infant's response to therapy, and assess the adequacy of cardiac output. Indirect indications of adequate cardiac output should be assessed, including pink color, warm extremities, capillary refill time less than 2 seconds, an adequate heart rate, urine output of at least 1 mL/kg per hour, and the absence of metabolic acidosis. However, accurate assessment of cardiac output after a Norwood procedure requires incorporation of objective assessment parameters such as SV[O.sub.2] and the [dot.Q.sub.p]/[dot.Q.sub.s] ratio. Astute monitoring by bedside nurses, observance of changes in an infant's physiological status, and early notification of the medical staff of changes is crucial for the successful recovery of infants after the surgery.

In addition, nurses are responsible for the care and maintenance of the intracardiac catheters used for monitoring because the data obtained via these catheters are providing the moment-to-moment hemodynamic information that assessments and interventions are based on. The monitoring systems require proper calibration, including leveling and zeroing of the transducers. The SV[O.sub.2] monitoring system should be calibrated frequently in the early post-operative period by comparing values obtained via the monitor with SV[O.sub.2] values obtained by the laboratory. Although they provide access for administration of medications and fluids, the catheters can be a risk for air embolism if they are not checked meticulously. The obligatory right-to-left shunt puts infants at risk for an air embolism if air should enter the blood stream via an intravenous or a monitoring catheter.

As in the preoperative period, coordination between healthcare services and patients' family members is needed during the postoperative period. Nurses are in a key position to coordinate information. During this stressful postoperative period, support for the family is essential. An opportune way to support the family and provide accurate information is to provide consistency in nursing care whenever possible.

Criteria for Discharge From the Pediatric Intensive Care Unit and the Hospital

After the Norwood procedure, infants are ready to be transferred out of the pediatric intensive care unit when they are breathing without mechanical ventilation and are maintaining oxygen saturations of 75% to 85% with minimal oxygen requirements. The infants' hemodynamic status must be stable, and all intracardiac and arterial catheters must have been removed. All continuous intravenous medications, including inotropic infusions, are tapered off before an infant is transferred to the cardiac step-down unit; however, treatment with oral digoxin, administration of a diuretic, and afterload reduction are continued if necessary. The infants are also given aspirin or subcutaneous heparin to reduce the risk of thrombosis.

Feeding is often a problem in the postoperative period. (39,40) Nutrition and feeding consultants follow up all infants who have undergone the Norwood procedure. Oral feeding on demand is promoted with a goal of achieving a daily energy intake of 460 to 540 kJ/kg (110-130 kcal/kg). Standard formulas and breast milk are supplemented with additives to achieve higher energy density perounce. Parents are encouraged to play an active role in their infant's feedings. If adequate energy intake cannot be achieved with full oral feedings, supplementation via a gastrostomy tube is added. Rarely, in infants with reflux and/or aspiration, all feedings are given via a gastrostomy tube.

Increased parental involvement in care is encouraged as infants make progress toward discharge. Rooming-in is available for parents, and education about the infant's medical and basic care and developmental needs is ongoing. Before discharge, all medications are thoroughly reviewed with parents. The most common discharge medications include digoxin to support the function of the single ventricle, a diuretic to prevent fluid retention, an after-load-reducing agent such as captopril to decrease the workload of the single ventricle, and aspirin or enoxaparin sodium (Lovenox) to decrease the risk of thrombotic complications. Primary caregivers complete certification in cardiopulmonary resuscitation of infants, and signs or indications of concern are reviewed. The parents are trained in the use of an infant scale and pulse oximeter before the infants are discharged to go home. (41) Parents are given a notebook to record their infant's daily weights and oxygen saturations and are given guidelines about when to notify their healthcare providers with concerns. All infants are followed up by both a pediatric cardiologist and a pediatrician. In addition, the infants are referred to local early developmental screening programs to monitor physical and neurological development.

Long-Term Follow-Up

After discharge from the hospital after first-stage surgical palliation, infants with HLHS are monitored closely. Their cardiac anatomy, resulting in parallel circulation, leaves them at risk for serious compromise and inability to tolerate acute episodes of decreased cardiac output. As many as 15% of infants who survive their first hospitalization may die at home before they return for their second stage of surgery. (7,41) Parents are instructed to observe their infants carefully in order to become accustomed to the infants' normal appearance and behavior. Parents are provided with contact numbers and instructions to call if their infant appears ill. Infants are typically seen by a cardiologist and/or a pediatrician every 2 to 4 weeks.

The second stage of surgery, the bidirectional cavopulmonary shunt,

is generally performed when an infant is 4 to 6 months old or when the infant begins to have decreases in systemic oxygen saturations after the Norwood procedure. Second-stage palliation has been performed in infants as young as 6 weeks of age with excellent results. (42) This second step in surgical palliation of HLHS alters the infant's anatomy by providing pulmonary blood flow from the superior vena cava directly to the pulmonary artery (Figure 6). This procedure allows a source of pulmonary blood flow that will grow with the patient, unlike the synthetic shunt in the Norwood procedure, and greatly reduces the volume load on the single right ventricle. For these reasons, the bidirectional cavopulmonary shunt is generally very well tolerated, and infants leave the hospital after this operation in a much more stable situation than before. Home monitoring of weight and pulse oximetry is typically discontinued, and the need for healthcare visits is reduced.

The final surgical stage in single-ventricle palliation, the Fontan procedure, is generally completed when the child is between 18 months and 3 years of age. This step completes the separation of the pulmonary and systemic circulations by directing blood flow from the inferior vena cava directly to the pulmonary artery. Multiple variations of the completion Fontan procedure are used, (43-45) depending on the child's situation (a detailed review of the variations is beyond the scope of this article).

[FIGURE 6 OMITTED]

After completion of the 3 stages of surgical palliation for HLHS, children still require ongoing cardiac care and follow-up. Areas of concern include arrhythmias and long-term function of the single right ventricle. Neurodevelopmental outcomes and quality of life are also being closely followed up in these children. (15,17) Increasingly, encouraging results for children with HLHS offer hope that the outlook for children with this syndrome is favorable.

Conclusion

Infants born with HLHS present several unique challenges to the multidisciplinary management team. Critical care nurses play a fundamental role in both the preoperative and the postoperative management of these patients. Thorough understanding of the physiological consequences of single-ventricle anatomy and parallel circulation is necessary. Improved survival for infants with HLHS has been achieved by using strategies that target optimizing systemic oxygen delivery through the use of direct monitoring of venous oxygenation, stabilization of SVR, and reduction in the postoperative inflammatory response. Integrating these research findings into practice enables critical care nurses to recognize problems early and reduce response time. This nursing care in turn can result in dramatically improved early survival of children with HLHS.

Acknowledgments

We thank Drs James Tweddell, George Hoffman, Raymond Fedderly, and Nancy Ghanayem for their commitment to this population of patients and their unrelenting quest for improved outcomes for these infants. Without their contributions and support, this article would not be possible. In addition, we thank Eliot May, PA-C the critical care nursing staff, and all members of the heart center team who supported this work.

References

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2. Hoffman J. Incidence, prevalence, and inheritance of congenital heart disease. In: Moller J, ed. Pediatric Cardiovascular Medicine. Philadelphia, Pa: Churchill Livingstone: 2000:257-262.

3. Report of the New England Regional Infant Cardiac Program. Pediatrics. 1980:65 (2 pt 2):375-461.

4. Bernhard WF, Litwin SB, Williams WW, Jones JE, Gross RE. Recent results of cardiovascular surgery in infants in the first year of life. Am J Surg. 1972;123:451-460.

5. Norwood WI, Kirklin JK, Sanders SP. Hypoplastic left heart syndrome: experience with palliative surgery. Am J Cardiol. 1980;45:87-91.

6. Caplan WD, Cooper TR, Garcia-Prats JA, Brody BA. Diffusion of innovative approaches to managing hypoplastic left heart syndrome. Arch Pediatr Adolesc Med. 1996;150:487-490.

7. Tweddell JS, Hoffman GM, Mussatto KA.et al. Improved survival of patients undergoing palliation of hypoplastic left heart syndrome: lessons learned from 115 consecutive patients. Circulation. 2002;106(12 suppl 1): 182-189.

8. Wernovsky G, Bove EL. Single ventricle lesions. In: Wessel D. ed. Pediatric Cardiac Intensive Care. Baltimore, Md: Williams & Wilkins; 1998:271-288.

9. May L. Pediatric Heart Surgery: A Ready Reference for Professionals. 2nd ed. Milwaukee, Wis: Maxishare; 2001:41, 78.

10. Rosenthal A. Hypoplastic left heart syndrome. In: Moller J. ed. Pediatric Cardiovascular Medicine. Philadelphia, Pa: Churchill Livingstone; 2000:594-606.

11. Nicolson S. Steven JM, Jobes DR. Hypoplastic left heart syndrome. In: Wetzel R, ed. Critical Heart Disease in Infants and Children. St Louis. Mo: Mosby; 1995:863-884.

12. Fedderly RT. Left ventricular outflow obstruction. Pediatr Clin North Am. 1999;46:369-384.

13. Gutgesell HP, Massaro TA. Management of hypoplastic left heart syndrome in a consortium of university hospitals. Am J Cardiol. 1995;76:809-811.

14. Storch TG. Passive euthanasia for hypoplastic left heart syndrome. Am J Dis Child. 1992;146:1426.

15. Mahle WT, Clancy RR, Moss EM, Gerdes M, Jobes DR, Wernovsky G. Neurodevelopmental outcome and lifestyle assessment in school-aged and adolescent children with hypoplastic left heart syndrome. Pediatrics. 2000;105:1082-1089.

16. Kern JH, Hinton VJ, Nereo NE, Hayes CJ, Gersony WM. Early developmental outcome after the Norwood procedure for hypoplastic left heart syndrome. Pediatrics. 1998;102:1148-1152.

17. Williams DL, Gelijns AC, Moskowitz AJ, et al. Hypoplastic left heart syndrome: valuing the survival. J Thorac Cardiovasc Surg. 2000;119(4 pt 1):720-731.

18. Congenital Heart Information Network. Online support. Available at: http://www.tchin.org/support/index.htm. Accessed September 13, 2004.

19. Tweddell JS, Hoffman GM, Fedderly RT, et al. Patients at risk for low systemic oxygen delivery after the Norwood procedure. Ann Thorac Surg. 2000;69:1893-1899.

20. Hoffman GM, Ghanayem NS, Kampine JM, et al. Venous saturation and the anaerobic threshold in neonates after the Norwood procedure for hypoplastic left heart syndrome. Ann Thorac Surg. 2000;70:1515-1521.

21. Tweddell JS, Hoffman GM, Fedderly RT, et al. Phenoxybenzamine improves systemic oxygen delivery after the Norwood procedure. Ann Thorac Surg. 1999;67:161-168.

22. Kampine JM HG, Tweddell JS. Arterial oxygenation saturation does not accurately predict adequacy of systemic oxygen delivery in neonates following Norwood palliation of hypoplastic left heart syndrome. Paper presented at: Midwest Pediatric Cardiology Society 22nd Annual Meeting; September 25, 1998; Indianapolis, Ind.

23. O'Brien P. Boisvert JT. Current management of infants and children with single ventricle anatomy. J Pediatr Nurs. 2001;16:338-350.

24. Tweddell JS. Berger S, Frommelt PC, et al. Aprotinin improves outcome of single-ventricle palliation. Ann Thorac Surg. 1996;62:1329-1336.

25. Jaquiss RD, Ghanayem NS, Zacharisen MC, Mussatto KA, Tweddell JS, Litwin SB. Safety of aprotinin use and re-use in pediatric cardiothoracic surgery. Circulation, 2002;106 (12 suppl 1);190-194.

26. Davies MJ, Nguyen K, Gaynor JW, Elliott MJ. Modified ultrafiltration improves left ventricular systolic function in infants after cardiopulmonary bypass. J Thorac Cardiovasc Surg. 1998;115:361-370.

27. Tweddell J, Hoffman, GM. Postoperative management in patients with complex congenital heart disease. Semin Thorac Cardiovasc Surg Pediatr Card Surg Annu, 2002;5:187-205.

28. Gutgesell HP, Gibson J. Management of hypoplastic left heart syndrome in the 1990s. Am J Cardiol. 2002;89:842-846.

29. Chang RK, Chen AY, Kiltzner TS. Clinical management of infants with hypoplastic left heart syndrome in the United States 1988-1997. Pediatrics. 2002;110(2 pt 1):292-298.

30. Gaynor JW, Mahle WT, Cohen MI, et al. Risk factors for mortality after the Norwood procedure. Eur J Cardiothorac Surg. 2002;22:82-89.

31. Mora GA, Pizarro C, Jacobs ML., Norwood WI. Experimental model of single ventricle: influence of carbon dioxide on pulmonary vascular dynamics. Circulation, 1994;90(5 pt 2):1143-1146.

32. Weldner PW, Myers JL, Gleason MM, et al. The Norwood operation and subsequent Fontan operation in infants with complex congenital heart disease. J Thorac Cardiovasc Surg. 1995;109:654-662.

33. Barnea O, Santamore WP, Rossi A, Salloum E, Chien S, Austin EH. Estimation of oxygen delivery in newborns with a univentricular circulation. Circulation. 1998;98:1407-1413.

34. Francis DP, Willson K, Thorne SA, Davies LC, Coats AJ. Oxygenation in patients with a functionally univentricular circulation and complete mixing of blood: are saturation and flow interchangeable? Circulation. 1999;100:2198-2203.

35. Rossi AF, Sommer RJ, Lotvin A, et al. Usefulness of intermittent monitoring of mixed venous oxygen saturation after stage I palliation for hypoplastic left heart syndrome. Am J Cardiol. 1994;73:1118-1123.

36. Tweddell JS, Hoffman GM, Ghanayem NS, et al. Ventilatory control of pulmonary vascular resistance is not necessary to achieve balanced circulation in the postoperative Norwood patient [abstract]. Circulation. 1999;100:1671.

37. Migliavacca F, Pennati G, Dubini G, et al. Modeling of the Norwood circulation: effects of shunt size, vascular resistances, and heart rate. Am J Physiol Heart Circ Physiol. 2001;280:H2076-H2086.

38. Janousek J, Vojtovic P, Chaloupecky V, et al. Hemodynamically optimized temporary cardiac pacing after surgery for congenital heart defects. Pacing Clin Electrophysiol. 2000;23:1250-1259.

39. Rudd NA, Zlotocha JR, Mussatto KA, Frisbee SJ, Pelech AN, Frommelt PC. Growth velocity of infants with hypoplastic left heart syndrome: a comparison of enteral feeding strategies [abstract]. Cardiol Young. 2001;11(suppl 1);149.

40. Pillo-Blocka F, Miles C, Beghetti M. Nutrition after surgery for hypoplastic left heart syndrome. Nutr Clin Pract. 1998;12:81-83.

41. Ghanayem N, Hoffman, GM, Mussatto, KA. et al. Home surveillance program prevents interstage mortality after the Norwood procedure. J Thorac Cardiovasc Surg. 2003;126:1367-1377.

42. Bradley SM, Mosca RS, Hennein HA. Crowley DC, Kulik TJ, Bove EL. Bidirectional superior cavopulmonary connection in young infants. Circulation. 1996;94(9 suppl):115-1111.

43. Azakie A. McCrindle BW, Benson LN, et al. Total cavopulmonary connections in children with a previous Norwood procedure. Ann Thorac Surg. 2001;71:1541-1546.

44. Kumar SP, Rubinstein CS. Simsic JM, et al. Lateral tunnel versus extracardiac conduit Fontan procedure: a concurrent comparison. Ann Thorac Surg. 2003;76:1389-1396.

45. Mosca RS, Kulik TJ. Goldberg CS, et al. Early results of the fontan procedure in one hundred consecutive patients with hypoplastic left heart syndrome. J Thorac Cardiovasc Surg. 2000;119:1110-1118.

RELATED ARTICLE: Case Study

MH was a full-term infant girl who weighed 3.5 kg at birth. She went home with her parents on day 2 of life. On her 9th day of life, she was brought to the local emergency department with a 1-day history of poor feeding, increasing lethargy, hypothermia, and increasing respiratory effort. Respiratory failure quickly developed, and intubation was required. The initial pH on arterial blood gas analysis was 6.6, and she was given 4 doses of sodium bicarbonate. Cardiac echocardiography revealed hypoplastic left heart syndrome with aortic atresia and mitral atresia. Infusions of prostaglandin [E.sub.1], 0.1 [micro]g/kg per minute, and dopamine, 10 [micro]g/kg per minute, were started. MH was transported via helicopter to Children's Hospital of Wisconsin for management.

Shortly after her arrival, high-output renal failure developed, with urine output of 17 mL/h and a serum creatinine level of 150 [micro]mol/L (1.7 mg/dL). A subsequent echocardiogram revealed depressed right ventricular function. Treatment with dobutamine, antibiotics, and total parenteral nutrition was started. MH was allowed time to recover and went to the operating room 6 days after admission, day 15 of life, for a Norwood procedure. In the operating room, a 3.5-mm systemic circulation-to-pulmonary artery shunt was placed, and the aorta was reconstructed. Cardiopulmonary bypass time was 3 hours; aortic cross clamp time, 40 minutes; and circulatory arrest time, 9 minutes. Aprotinin and phenoxybenzamine were administered in the operating room.

MH was admitted to the pediatric intensive care unit. She was receiving dopamine 2 [micro]g/kg per minute, epinephrine 0.15 [micro]g/kg per minute, milrinone 0.5 [micro]g/kg per minute, vecuronium 0.1 [micro]g/kg per hour, and fentanyl 5 [micro]g/kg per hour, and her chest was left open. Her vital signs at the time of admission were heart rate 155/min, blood pressure 65/30 mm Hg, mean arterial pressure 38 mg Hg, body temperature 36.2[degrees]C, and right atrial pressure 7 mm Hg. Settings for mechanical ventilation were as follows: fraction of inspired oxygen 0.80, positive pressure 10 mm Hg, positive end-expiratory pressure 5 cm [H.sub.2]O, and respirations 24/min. End-tidal carbon dioxide was 22 mm Hg, arterial oxygen saturation (Sa[O.sub.2]) 78%, and venous oxygen saturation (SV[O.sub.2]) 42%.

What is her [dot.Q.sub.p]/[dot.Q.sub.s] ratio?

[dot.Q.sub.p] = Sa[O.sub.2] - SV[O.sub.2] = 78 - 42 = 36

[dot.Q.sub.s] = Pulmonary capillary oxygen saturation (SC[O.sub.2]; assumed to be 96%) - Sa[O.sub.2] = 96 - 78 = 18

[dot.Q.sub.p]/[dot.Q.sub.s] ratio = 36/18 = 2:1.

Diagnosis: Excessive pulmonary blood flow, twice as much as to the systemic circulation

What is her arterial to venous oxygen saturation difference?

Sa[O.sub.2] - SV[O.sub.2] = 78% - 42% = 36%

Diagnosis: Large arterial-venous oxygen saturation difference, indicating reduced cardiac output

Four hours later, after MH was given fluids to augment her preload and the dose of inotropic agent was adjusted to improve cardiac output, her vital signs were heart rate 165/min, blood pressure 64/32 mm Hg, mean arterial pressure 43 mm Hg, body temperature 37.4[degrees]C, and right atrial pressure 12 mm Hg. The ventilator settings were the same as before. End-tidal carbon dioxide was 29 mm Hg, Sa[O.sub.2] 80%, and SV[O.sub.2] 59%.

What is her [dot.Q.sub.p]/[dot.Q.sub.s] ratio?

[dot.Q.sub.p] = 80 - 59 = 21

[dot.Q.sub.s] = 96 - 80 = 16

[dot.Q.sub.p]/[dot.Q.sub.s] ratio = 21/16 = 1.3:1

Diagnosis: Within the identified range of 1:1 to 1.5:1

What is her arterial-venous oxygen saturation difference?

80% - 59% = 21%

MH had her chest closed on postoperative day 3, and she was extubated on postoperative day 5. She was transferred out of the pediatric intensive care unit on postoperative day 12 and discharged to home on postoperative day 29.

When she was 5 months old, MH had surgery to place a bidirectional cavopulmonary shunt, with a hospital stay of 5 days. When she was 2 years old, she had a Fontan procedure, with a hospital stay of 8 days.

RELATED ARTICLE: Addendum/Update

Surgical Modifications: The Right Ventricle-to-Pulmonary Artery Shunt

Since the original preparation of the manuscript for this article, an important modification of surgical technique for the Norwood procedure has received increasing attention. As stated in the "Intraoperative Management" section, an integral part of the procedure is placement of a synthetic shunt to provide a pathway for blood flow to the pulmonary circulation. Traditionally, this shunt has been placed from the innominate or other central artery to the pulmonary artery. Because it is placed distal to the aortic valve, the shunt allows perfusion to the pulmonary circulation during both systole and diastole. This situation creates the potential for coronary artery steal, the phenomenon of blood flowing away from the coronary arteries and into the low-pressure pulmonary arteries during diastole.

Recently, several groups have presented data on the use of a shunt placed directly from the right ventricle to the pulmonary artery (RV-PA shunt). The technique involves making a small right ventriculotomy, under-mining of the right ventricular muscle, and placing a polytetrafluoroethylene shunt directed to the pulmonary artery confluence. The technique was popularized by Sano et al (1) in 2001, when they reported an improvement in early survival from 50% with the modified Blalock-Taussing shunt to 89% with the RV-PA shunt. Other groups (2,3) have reported similar results and report a smoother early postoperative course. Theoretical advantages and potential disadvantages of the RV-PA shunt are presented in the Table.

Some of these disadvantages, including shunt stenosis or thrombosis and inadequate pulmonary artery growth, also occur with the classic systemic-to-pulmonary shunt.

Although the RV-PA shunt may improve coronary artery perfusion and cardiac output in the early postoperative period, the overall strategy for managing patients after the procedure remains unchanged. These patients still have a form of parallel circulation and require monitoring of systemic oxygen delivery. Currently, a randomized clinical trial is being planned to study both the short- and long-term outcomes of each surgical technique.

References

1. Sano S, Ishino K, Kawada M, et al. Right ventricle-pulmonary artery shunt in first-stage palliation of hypoplastic left heart syndrome. J Thorac Cardiovasc Surg. 2003; 126:504-510.

2. Mahle WT, Cuadrado AR, Tam VK. Early experience with a modified Norwood procedure using right ventricle to pulmonary artery conduit. Ann Thorac Surg. 2003; 76:1084-1088.

3. 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(suppl 1); I1155-I1160.

Deborah Soetenga, RN, MS

Kathleen A. Mussatto, RN, BSN

Authors

Deborah Soetenga is the advance practice nurse for the pediatric intensive care unit and the cardiovascular surgery program at Children's Hospital of Wisconsin, Milwaukee, Wis.

Kathleen A. Mussatto is the research manager for the Herma Heart Center at Children's Hospital of Wisconsin.

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