Developmental changes in the pattern of the hypoxic ventilatory response in rabbits

Abstract

The ventilatory response to hypoxia in mature mammals consists of an initial increase in ventilatory minute volume with a subsequent decline to a level that remains elevated compared to normoxia, but in neonatal mammals this decline often results in apnea. We investigated the maturation of the hypoxic ventilatory response in rabbits. New Zealand White rabbits between age 14 and 22 days were anesthetized with 1–2 minimum alveolar concentration (MAC) of sevoflurane, tracheotomized, cannulated and paralyzed and mechanically ventilated. Phrenic nerve recordings were performed during normocapnia under 1 MAC sevoflurane anesthesia. Inspiratory O2 concentration (FiO2) was decreased from 21% to 10% for 4–5 minutes or until phrenic nerve activity stopped (apnea). All animals showed an initial increase in peak phrenic nerve activity (PNA) and in phrenic rate. With sustained hypoxia, peak PNA subsequently decreased in all animals. In some animals phrenic rate slowed slightly but a regular rhythm was maintained (adult pattern) while other animals developed apnea or displayed PNA at a slow and irregular rate (immature pattern). The pattern of the hypoxic response was consistent for repeated exposures within each animal. We conclude that in rabbits the change from the immature to adult response pattern to hypoxic exposure occurs during the third week of life.Supported by departmental funds.