Postnatal changes in the circulation and responses to volume loading in sheep.

We examined postnatal circulatory changes in three groups of lambs at 1, 4, and 6 weeks after birth.Five lambs in each group were instrumented chronically with electromagnetic flow transducers on the ascending aorta and catheters in the aorta, left ventricle, left atrium, and superior vena cava.After recovery for 2 days, measurements were made daily at rest and during intravenous infusion of 0.9% NaCI solution ( 25ml/kg per min) for 2 minutes into the superior vena cava to increase mean left alrial pressure to about 25 mm Hg.Resting heart rate fell progressively, from 210 ± 27/min (mean ± SD) at 1 week to 141 ± 26 at 6 weeks, whereas arterial pressure increased from 71 ± 8 mm Hg during the 4th week to 80 ± 10 at 6 weeks.Aortic flow per kilogram body weight fell from the high level of 425 ± 86 ml during the 1st week to 147 ± 28 ml by the 6th week.This reduction in cardiac output probably is associated with alterations of oxygen consumption per kilogram body weight in the neonatal period.During infusion of saline, left ventricular output increased by a modest 35% over control levels in the 1week-old lambs, but rose more (58%) in the other two groups.The maximal cardiac output achieved during saline infusion was greater during isoproterenol and less after propranolol administration in each group.We, therefore, suggest that since the neonatal lamb has a high resting cardiac output it has less capacity to respond to volume loading than does the older lamb.