The effects of increasing mean arterial pressure on left ventricular output in newborn lambs

We developed a preparation in which the mean arterial pressure of the newborn lamb is varied with little change in left ventricular preload. Seven lambs underwent balloon atrial septostomy and were instrumented with vascular catheters, an electromagnetic flow transducer on the aorta, and a balloon occluder on the aorta. Four different preload conditions were established by volume infusion or withdrawal. Aortic flow was measured continuously during gradual occlusion of the aorta. Curves relating mean left atrial pressure to stroke volume before balloon inflation showed a plateau, with little increase in stroke volume with increases in atrial pressure above 7 mm Hg. With balloon inflation, stroke volume decreased linearly with increases in mean arterial pressure. Curves relating left atrial pressure to stroke volume constructed at constant mean atrial pressure showed no plateau but showed continued increases in stroke volume as left atrial pressure was increased to 10 mmHg. When mean arterial pressure is constant, the Starling mechanism is operative in newborn left ventricles. The plateau in newborn lamb function curves is due to increases in mean arterial pressure that accompany volume infusion.