Journal of Applied Physiology, Vol 70, Issue 5 2045-2051, Copyright © 1991 by American Physiological Society
Phrenic neural output during hypoxia in dogs: constant-flow ventilation vs. spontaneous breathing
S. S. Naqvi, A. S. Menon, B. E. Shykoff, A. S. Rebuck and A. S. Slutsky
Department of Medicine, Mount Sinai Hospital, Toronto, Ontario, Canada.
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.
