Phrenic neural output during hypoxia in dogs: constant-flow ventilation vs. spontaneous breathing.

We studied the effects of removing cyclic pulmonary afferent neural information on respiratory pattern generation in anesthetized dogs. Phrenic neural output during spontaneous breathing (SB) was compared with that occurring during constant-flow ventilation (CFV) at several levels of eucapnic hypoxemia. Hypoxia caused an increase in both the frequency and the amplitude of the moving time average (MTA) phrenic neurogram during both SB and CFV. The change in frequency as arterial saturation was reduced from 90 to 60% during SB was significantly higher than that during CFV [SB, 32.3 +/- 10.9 (SD) breaths/min; CFV, 10.3 +/- 5.8 breaths/min; P = 0.001]. By contrast, the increase in the amplitude of the MTA phrenic neurogram was smaller (SB, 0.62 +/- 0.68 units; CFV, 1.35 +/- 0.81 units; P = 0.01). The changes in frequency with hypoxia during both modes of ventilation resulted primarily from a shortening of expiratory time. Both inspiratory time and expiratory time were greater during CFV than during SB, but their change in response to hypoxia was not significantly different. We conclude that the amplitude response of the MTA phrenic neurogram to hypoxia is similar to that seen during hypercapnia; in the presence of phasic afferent feedback the MTA amplitude response is decreased and the frequency response is increased relative to the response observed in the absence of phasic afferents.