IN THE fetal lamb the pulmonary vascular resistance is higher than systemic vascular resistance so that most of the blood entering the pulmonary artery passes through the ductus arteriosus to the aorta.1 The initial ventilation of the lungs results in a marked fall in pulmonary resistance and allows adequate perfusion of the lung for oxygenation.2 The prompt fall in pulmonary resistance with a newborn lamb's first breath is thought to be the result of arteriolar vasodilation in response to an elevation of the PO2 and fall of the Pco2 in the alveolus.3,4
The pulmonary vascular resistance in the fetal lamb can also be reduced by the exogenous administration of acetylcholine, isoproterenol, and histamine. Further, the stimulation of the vagi results in pulmonary vasodilation of the fetal lungs.5 Thoracic sympathectomy has also been shown to cause pulmonary vasodilation indicating the presence of sympathetic tone.5