Cardiovascular responses to hypoxemia and acidemia in the intact anesthetized lamb.

In the newborn lamb acute hypoxia generally induces little or no increase of cardiac output, while the same magnitude of hypoxia in the adult sheep is associated with very large increases.' These findings included lambs up to 60 days of age. This is curious, unless the newborn is normally operating at the top of its cardiac output capacity for several weeks after birth.' It would seem unlikely that an animal which engages in substantial physical activity within a few minutes of birth would have a fixed cardiac output. Furthermore, many of the autonomic reflex mechanisms which may contribute to circulatory regulation are operating in the immediate postnatal period."' Although the influence of exercise upon the cardiac output of the newborn lamb is not known, it is clear from the present studies that transfusions of maternal blood are followed by large increases of systemic blood flow. This demonstrates that the lamb can increase its cardiac output, at least transiently. It therefore seemed germane to ascertain if under similar experimental conditions acute hypoxia prior to and during metabolic acidemia, could influence systemic blood flow. A preliminary report on these experiments has been given.'