Cardiovascular response of the neonatal lamb to hypoxia and hypercapnia

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Abstract
The effects of hypoxia and hypercapnia on the circulatory dynamics of the unanesthetized newborn lamb were evaluated. Hypoxia produced by inhalation of 8% O2 caused a marked increase in pulmonary arterial pressure and pulmonary vascular resistance and a lowering of systemic arterial pressure with little change in cardiac output. A decrease in the volume of blood in the pulmonary vascular bed was also demonstrated during hypoxia. Hypercapnia induced by inhalation of 8% CO2 was accompanied by a 45% increase in cardiac output, with marked fall in systemic vascular resistance and a moderate fall in pulmonary vascular resistance; aortic and pulmonary arterial pressures remained constant. Inhalation of 100% O2 had no effect on the circulation when compared to room air base-line values. The effects of hypoxia per se on the new-born, in the presence of a patent ductus arteriosus and open foramen ovale, could lead to shunting of a large amount of blood away from the pulmonary vascular bed and inadequate pulmonary perfusion. These effects might be exaggerated by the further lowering of systemic vascular resistance induced by hypercapnia. Adequate oxygenation and ventilation might reverse these effects if not prolonged.