The effects of hypoxia in the new‐born lamb before and after denervation of the carotid chemoreceptors*

Abstract

Twenty-seven unanaesthetized new-born lambs, 6 hr-10 days old, responded to 2 levels of inspired oxygen, 125 and 110 mm Hg (alveolar CO, being controlled) with a sustained increase in minute ventilation (V), a small increase in heart rate and a less consistent rise in systemic blood pressure. An increase in V was observed when arterial oxygen tension (Pa, O2) had fallen by 6-15 mm Hg. There appeared to be no fixed threshold of Pa, O2 at which ventilation started to increase. The increase in ventilation caused by these levels of hypoxia was significantly and directly related to the age of the lamb and to its control alveolar CO2 More severe hypoxia caused a progressive increase in V until Pa C2 was about 25 mm Hg when respiration failed. This increase at Pa, O2 25 mm Hg was markedly potentiated when alveolar PCO2 (Pa, CO2) was increased and abolished after bilateral denervation of the carotid chemoreceptors. Significant ( > 10%) left-to-right shunts were found in 10 out of 12 lambs lightly anaesthetized with pentobarbitone sodium, breathing air. Hypoxia diminished the left-to-right pressure gradient largely by its pressor effect on the pulmonary circulation. When inspired O2 tension (PI, O2) was 70 mm Hg, all 7 lambs studied showed a reversal of the pressure gradient and evidnce of right-to-left shunts (11-42%) across the ductus arteriosus. The implications of these findings have been discussed with reference to previous studies of the new born response to hypoxla and it is concluded that the peripheral chemo-receptors are fully active at birth and in the new-born period.