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INTRODUCTION

Although prenatal detection of congenital heart defects (CHDs) is attempted during routine fetal ultrasonography, it is estimated that, at present, only 36% of CHDs are detected antenatally, with a wide variation among different localities and states.1 Since the incidence of CHDs is approximately 8 per 1000 live births, and only a third are detected prior to birth, the proposition to perform echocardiography in all neonates for detecting CHDs2 is a huge and costly undertaking in absence of appropriate infrastructure and adequate funding. However, the initial echocardiogram obtained in any neonate (for any indication) should be as complete as possible and interpreted by a pediatric cardiologist who is competent in the interpretation of echocardiograms, or alternately, if it is performed by a non-cardiologist physician, the physician should have enough training to at least suspect the presence of a CHD, which will then require referral to the pediatric cardiology service for further evaluation and management. Understanding fetal and transitional circulatory physiology, as it is affected by a CHD, is essential in anticipating the presentation of a CHD in the neonate. It is important to remember that CHDs that would have significantly affected fetal systemic or placental circulation would likely have led to a fetal demise. Therefore, almost all critical congenital cardiac defects (CCHDs) that present at the time of birth were compatible with a fetal circulation that had adjusted to provide normal or near normal fetal systemic and placental circulation in utero; in the few instances in which fetal circulation is compromised, it is not to such degree as to cause fetal demise prior to birth.

Review of Fetal and Transitional Physiology

In fetuses with a normal heart, the right and left ventricles work in parallel; thus, fetal cardiac output is nearly equal to the combined output of both ventricles and approximates 430 ml/kg/min.3 The right ventricle is dominant, accounting for approximately 56% of total cardiac output and pumps against a slightly greater resistance than the left ventricle (Figures 15-1 and 15-2). The fetal circulation in cases of aortic or pulmonary atresia are illustrated for comparison. In each case, the fetal systemic and placental circulations are preserved because of increases in the right or the left ventricular flow which will carry the function of combined ventricular outputs (Figure 15-3). Immediately following birth, ventilation of lungs, in association with increased arterial oxygen tension, leads to a sharp decrease in pulmonary vascular resistance and a marked increase in pulmonary blood flow. At the same time, separation of umbilico-placental circulation decreases inferior vena cava flow which in conjunction with increased pulmonary flow and venous return to the left atrium, results in the functional closure of the foramen ovale. Closure of ductus arteriosus during the first 48 hours of life,4 completes separation of the pulmonary and systemic circulations. Postnatally, the two ventricles work in series, so neonatal cardiac output represents the volume of ...

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