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

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Abstract
Postnatal circulatory changes were examined in 3 groups of lambs at 1, 4 and 6 wk 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 i.v. infusion of 0.9% NaCl solution (25 mg/kg per min) for 2 min into the superior vena cava to increase mean left atrial pressure to about 25 mm Hg. Resting heart rate fell progressively, from 210 .+-. 27/min (mean .+-. SD) at 1 wk to 141 .+-. 26 at 6 wk; arterial pressure increased from 71 .+-. 8 mm Hg during the 4th wk to 80 .+-. 10 at 6 wk. Aortic flow per kilogram body weight fell from the high level of 425 .+-. 86 ml during the 1st wk to 147 .+-. 28 ml by the 6th wk. This reduction in cardiac output probably is associated with alterations of O2 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 1-wk-old lambs, but rose more (58%) in the other 2 groups. The maximal cardiac output achieved during saline infusion was greater during isoproterenol and less after propranolol administration in each group. Since the neonatal lamb has a high resting cardiac output it apparently has less capacity to respond to volume loading than does the older lamb.

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