Management of Infants With Hypoplastic Left Heart Syndrome
2004; American Association of Critical-Care Nurses; Volume: 24; Issue: 6 Linguagem: Inglês
10.4037/ccn2004.24.6.46
ISSN1940-8250
AutoresDeborah J. Soetenga, Kathleen Mussatto,
Tópico(s)Cardiac Structural Anomalies and Repair
ResumoCongenital 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 procedure5 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.6Beginning 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.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.2,8The 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.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 arterio-venous 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 brachio-cephalic 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 (Q̇p) and systemic (Q̇s) circulations. This type of blood circulation is referred to as parallel circulation (Figure 1).After birth, 3 factors affect the hemodynamic status of infants with HLHS10: 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 (Q̇p/Q̇s) describes how the cardiac output from the single ventricle is partitioned.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 Q̇p/Q̇s 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 Pao2 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.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 Pre-natal 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.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.17Most 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.18Once 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. Peri-operative 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 (Sao2 and Svo2, respectively) are crucial to the optimal clinical management of infants with HLHS from diagnosis through discharge.7,19–22As 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 Q̇p/Q̇s ratio of 1).22 The immediate therapy for all infants with HLHS is an intravenous infusion of prostaglandin E1 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 μg/kg per minute. Prostaglandin E1 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 Q̇p/Q̇s 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 E1. 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 μg/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 Q̇p/Q̇s ratio to greater than 1. Diuretics may be necessary to help alleviate the increased volume load on the right ventricle.11Infants with an unrestrictive inter-atrial 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 Q̇p/Q̇s ratio and systemic cardiac output.The Q̇p/Q̇s ratio can be manipulated by increasing PVR by increasing the Paco2. Paco2 can be increased by adding supplemental inspired carbon dioxide, a potent pulmonary vasoconstrictor, to the ventilator circuit. This approach for increasing Paco2 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 inter-atrial 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 E1 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.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.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.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°C to 22°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 α-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 Svo2, which becomes a key focus in the postoperative period.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 Q̇p/Q̇s ratio is calculated after cardiopulmonary bypass is discontinued. As in preoperative management, the goal is to achieve a Q̇p/Q̇s ratio of 1.0.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 pro-inflammatory 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 μg/kg and then an infusion at a rate of 0.5 μg/kg per minute) and dopamine at a rate of 3 to 5 μg/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.27All 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 fentany
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