The Pulmonary Arterial Bed in Ventricular Septal

go a variable clinical course largely determined by the size of the septal defect and the level of pulmonary vascular resistance.'-7 In perhaps half of the cases, the septal defect closes or becomes smaller.' Of those infants who retain large septal defects, some may maintain a high pulmonary vascular resistance from birth; in most, the resistance probably undergoes at least a transient decrease.1 2 These changes in vascular resistance influence the size of the intracardiac shunts and the development of cardiac failure. -3 It is not easy to correlate structural change in the pulmonary arteries with observed changes in vascular resistance because appropriate morphological data on the pulmonary vessels is scanty during this early period.3' To help fill this gap, I undertook