Effects of hypoxia on respiratory control in neonates at high altitude

The respiratory responses to acute hypoxia were investigated in 21 newborn infants in Leadville, CO. Time-related changes in pulmonary ventilation (VE) described four distinct phases to the hypoxic response characterized by an abrupt initial decrease (phase I), a transient subsequent increase (phase II) and decrease (phase III), and a further transient decrease upon termination of the hypoxic challenge (phase IV). Based on the degree of respiratory stimulation during phase II, the subjects were divided into two groups, "responders" and "nonresponders." In both groups in VE were attributed to variation in tidal volume (VT), inspiratory duration (TI), and breath duration being little affected. The latter indicated an absence of vagal volume-related modulation of respiratory pattern. In all subjects, a fixed linear function described the relationship between the mean inspiratory flows at 0.3 s after inspiratory onset (i.e., V0.3/0.3) ant at end inspiration (i.e., VT/TI). Thus, changes in respiratory output with hypoxia are attributed to variations in both the "gain" and "shape" of the inspiratory drive.