The ventilatory response to hypoxia in the newborn lamb after carotid body denervation

Abstract

Neonates of various species including lambs respond to hypoxia by a transient hyperventilation followed by a V̇ e depression (diphasic response). To better delineate the role of the carotid chemoreceptors and that of the central depressive/inhibitive effect of hypoxia on minute ventilation, we have studied the V̇ e response of 4-day-iold carotid-deprived lambs (CBD) during successive exposure to moderate and severe (0.12 and 0.07 F i O2) hypoxia. The carotid body denervation was done to abolish most of the chemoreceptor stimulating effect on V̇ e during hypoxia and to allow for central depression/inhibition of V̇ e during hypoxia. In the CBD lambs, baseline V̇ e was 461 ± 81 (SE) ml·(kg·min) −1. It increased to 532 ± 79 ml·(kg·min) −1 and to 541 ± 75 ml·(kg·min) −1, to 0.12 F i O2. These V̇ e increases did not reach level of significance ( P<0.05). After 2–5 min of both levels of hypoxia V̇ e dropped respectively to 460 ± 60 ml·(kg·min) −1 and to 459 ± 38 ml·(kg·min) −1. No marked ventilatory depressions were noted bu V̇ e had only returned to baseline. It is concluded that, in the denervated newborn lamb, the centrally mediated depressive effect of hypoxia is small and not sufficient to explain the diphasic V̇ e response of the intact lamb to steady state hypoxia. Analysis of the magnitude of the hyperventilation and the V̇ e damping pre-hypoxic levels occurring with sustained hypoxi in newborns of various species suggests that the immaturity of the O 2-sensitive chemoreceptor rather than the central effect of hypoxia is the determinant factor of the diphasic response of newborn mammals to hypoxic hypoxia.