September 2002 / Volume 47 / Number 9 / Page 1002
Anticipatory Use of Venoarterial Extracorporeal Membrane Oxygenation for a High-Risk Interventional Cardiac Procedure
The use of venoarterial extracorporeal membrane oxygenation (VA ECMO) expanded substantially during the 1990s in centers managing large numbers of pediatric patients with complex congenital and acquired heart defects. The usual indication for mechanical circulatory support with VA ECMO in pediatric cardiac patients is inadequate oxygen delivery. Typical causes include low cardiac output (following cardiac surgery, from acute myocarditis, or from end-stage cardiomyopathy), severe cyanosis (due to intracardiac right-to-left shunting leading to myocardial failure), and severe hypoxemia from acute respiratory failure. VA ECMO has also been used as a rescue strategy for selected cardiac patients who suffer in-hospital cardiac arrest and do not respond to conventional resuscitation measures in the intensive care unit (ICU). As ICU teams have gained more experience with VA ECMO in cardiac patients who suffer life-threatening decompensations of cardiorespiratory function, increased interest has developed in identifying patients at risk of profound cardiac or respiratory failure so that these events can be averted by the proactive use of mechanical support.
This case presents the anticipatory use of VA ECMO in a neonate with severe cyanosis and arrhythmia caused by Ebstein's anomaly. Ebstein's anomaly is a rare (1/100,000 live births) congenital heart defect with a broad spectrum of clinical presentations, ranging from severe cyanosis and congestive heart failure in neonates to mild oxygen desaturation only upon vigorous exercise in adults. Anatomically, Ebstein's anomaly is characterized by downward displacement of the posterior and septal leaflets of the tricuspid valve, together with valve leaflet deformities (Fig. 1). The portion of the right ventricle above or on the atrial side of the displaced valve leaflets becomes abnormally thinned, a process known as atrialization of the right ventricle. When the right atrium contracts, blood is ejected into both the atrialized and distal portions of the right ventricle. During right ventricular contraction, blood in both portions of the right ventricle can be ejected backward (regurgitation) into the right atrium as well as forward into the main pulmonary artery. Substantial regurgitation can elevate the right atrial pressure, leading to a net shunting of systemic venous blood across a coexisting atrial septal defect or patent foramen ovale into the left atrium (right-to-left shunt), causing cyanosis. The right atrium is almost always enlarged, especially in patients with substantial tricuspid regurgitation; this produces an increased heart size on chest radiograph. When a large portion of the right ventricle is atrialized, ventricular function is usually diminished and forward flow into the main pulmonary artery is additionally compromised. These problems generating pulmonary blood flow are exacerbated in newborns who have relatively increased pulmonary vascular resistance in the first hours and days after birth.
Abnormalities of the heart's electrical conduction system are common in patients with Ebstein's anomaly. The electrocardiogram often shows delayed conduction in the right ventricle (right bundle branch block). There is also a higher incidence of accessory atrioventricular conduction pathways, which predispose Ebstein's anomaly patients to sudden episodes of supraventricular tachycardia (SVT) that can lower cardiac output and increase cyanosis. The Wolff-Parkinson-White syndrome is relatively common in Ebstein's anomaly, but occasional patients have a rarer form of accessory pathway known as a Mahaim fiber. SVT caused by either type of accessory pathway is typically managed first with antiarrhythmia medications. If pharmacologic therapy does not produce adequate arrhythmia control or is poorly tolerated, patients are referred for cardiac catheterization. During catheterization the accessory conduction pathway is mapped by intracardiac electrophysiologic techniques, and, if technically feasible, the pathway is destroyed (ablated) by the targeted application of radiofrequency electrical current via a transcatheter approach.
The diagnosis of Ebstein's anomaly is made by echocardiography, and cardiac catheterization is seldom required to define the structural abnormalities. In severe cases the diagnosis can be made in the fetus when a routine 4-chamber view of the heart is obtained as part of a screening obstetrical ultrasound.