Effect of acute hypoxia on respiration and brain stem blood flow in the piglet.

Changes in local brain stem perfusion that alter extracellular fluid Pco2 and/or [H+] near central chemoreceptors may contribute to the decrease in respiration observed during hypoxia after peripheral chemoreceptor denervation and to the delayed decrease observed during hypoxia in the newborn. In this study, we measured the changes in respiration and brain stem blood flow (BBF) during 2-4 min of hypoxic hypoxia in both intact and denervated piglets and calculated the changes in brain stem Pco2 and [H+] that would be expected to occur as a result of the changes in BBF. All animals were anesthetized, spontaneously breathing, and between 2 and 7 days of age. Respiratory and other variables were measured before and during hypoxia in all animals, and BBF (microspheres) was measured in a subgroup of intact and denervated animals at 0, 30, and 260 s and at 0 and 80 s, respectively. During hypoxia, minute ventilation increased and then decreased (biphasic response) in the intact animals but decreased only in the denervated animals. BBF increased in a near linear fashion, and calculated brain stem extracellular fluid Pco2 and [H+] decreased over the first 80 s both before and after denervation. We speculate that a rapid increase in BBF during acute hypoxia decreases brain stem extracellular fluid Pco2 and [H+], which, in turn, negatively modulate the increase in respiratory drive produced by peripheral chemoreceptor input to the central respiratory generator.