Myocardial function in the newborn animal

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The roles of hypoxia and hypercapnia in elevating arterial pressure and resistance were evaluated in normal unanesthetized newborn lambs. Nineteen Iambs without ductus shunting were studied in room air 4 hours to 10 days after birth with blood gases, pH, pulmonary, systemic, and atrial pressures, and indicator dilution cardiac output determinations. and systemic resistance and pulmonary blood volume were calculated. Eleven lambs were exposed to 8 per cent 02, 13 to 100 per cent O.~, and 6 to 8 per cent CO2 inhalation. artery pressure and resistance on room air were highest in younger lambs, reaching near normal levels by 72 hours. All lambs on 8 per cent O5 showed striking elevation of artery pressure and resistance unassociated with increased cardiac output. Pulmonary blood volume decreased. These changes were abolished by 100 per cent 02 inhalation. No other significant changes on 02 occurred. Eight per cent COs inhalation, producing an increase in Pace2 and fall in pH, was associated with increased cardiac output, slight fall in systemic resistance, and a moderate rise in artery pressure. resistance remained unchanged. It appears that hypoxia is a powerful stimulus to maintain pressure and resistance at fetal levels and would tend to maintain the fetal pattern of circulation in the newborn, contributing to further hypoxemia. This may be reversed by adequate oxygenation.