Normal and abnormal transitional circulation in the IUGR infant.

Abstract

A LTHOUGH MOST infants with intrauterine growth retardation (IUGR) have “normal” transitional circulation,’ some develop cardiopulmonary distress requiring appropriate cardiorespiratory support. Diagnosis and optimal management of the IUGR infant with cardiopulmonary distress during the immediate neonatal period is dependent on understanding of the characteristics of fetal, transitional, and neonatal circulation in the healthy neonate and its deviation from normal in the IUGR infant. Much of the information regarding normal transitional circulation has been obtained from studies of fetal and neonatal lamb? that, although differ from the human circulation in certain aspects, is similar to it in general. During fetal life, the right and left ventricles work in parallel; thus fetal cardiac output is nearly equal to the combined output of both ventricles. The right ventricle is dominant, accounting for approximately 60% of total cardiac output and pumps against slightly greater resistance than the left ventricle. Immediately following birth, ventilation of the 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 the umbilicoplacental circulation, decreases inferior vena caval flow, which in conjunction with increased pulmonary blood flow and venous return to the left atrium, results in functional closure of the foramen ovale. Closure of the ductus arteriosus during the first 48 hours of life,’ completes the separation of the pulmonary and systemic circulations. Postnatally, the two ventricles work in series, and the neonatal cardiac output represents the vol of blood pumped by each ventricle. During normal transitional circulation, the vol pumped by the right ventricle increases cjnly slightly, whereas its afterload (pulmonary