Postnatal change of cardiac function in lambs: effects of ganglionic block and afterload

This study characterizes the maturational change of cardiac function in unanaesthetized lambs from 1 day to 6 months of age. The cardiac function curve, the relationship between left ventricular output (Qco) and left ventricular end diastolic pressure, was studied in lambs of ages 1 day, 2-3 days, 7-8 days, 1 month, 3 months and 6 months. Cardiac output was measured by thermodilution while left ventricular end diastolic pressure was either raised by infusion of 5% glucose, or lowered by haemorrhage. At elevated left ventricular end diastolic pressure, cardiac output tended to reach a plateau of approximately 300 ml/min per kg in lambs less than 1 week of age and 200 ml/min per kg in lambs older than 1 month. Cardiac function was depressed by total ganglionic block in 1-3 days-old lambs, but not in lambs older than 1 week. Heart rate decreased in response to ganglionic block in lambs up to 1 week of age, but it increased in lambs 3-6 months old. Increases in afterload with methoxamine infusion during ganglionic block further depressed cardiac function in all age groups of lambs. The stroke work curve was shifted downward by hexamethonium and returned back to control level by methoxamine. We conclude that newborn cardiac output is near maximum even at 1 month of age. Sympathetic activity and circulating catecholamines help maintain cardiac function in 1-3 days-old lambs. Cardiac function is sensitive to afterload in lambs of all ages studied.