Journal of Applied Physiology, Vol 68, Issue 3 891-896, Copyright © 1990 by American Physiological Society
Effect of sustained hypoxia on ventilatory response to CO2 in normal adults
D. Georgopoulos, S. Walker and N. R. Anthonisen
Department of Medicine, University of Manitoba, Winnipeg, Canada.
In adult humans the ventilatory response to sustained hypoxia is biphasic,
characterized by an early increase followed by a decline to an intermediate
plateau. Recently, we have shown that this decrease in hypoxic sensitivity
is long lasting, because up to 1 h of room air breathing is required for
complete recovery of the initial hypoxic response (J. Appl. Physiol. 64:
521-528, 1988). It is not known whether this posthypoxia decrease in
ventilatory response is general or specific only to hypoxic stimuli. We
therefore examined responses to CO2 before and after hypoxia. The
ventilatory response to 5 min of normoxic CO2 breathing was evaluated in
eight normal adults on 2 days: 5 min before and after 25 min of normocapnic
hypoxia (arterial O2 saturation +/- 80%) and 5 min before and after 25 min
of room air breathing (control day). During hypoxia, ventilation (VI),
after an initial increase, declined significantly. At the end of hypoxia,
abrupt exposure to room air transiently dropped VI to values that were
significantly below base line. On each experimental day, the first and
second exposure to CO2 increased ventilation by a similar amount,
averaging, respectively, 8.46 +/- 0.9 and 8.84 +/- 0.92 (SE) l/min on the
hypoxic day and 8.24 +/- 0.96 and 7.65 +/- 0.94 l/min on the control day.
All the hypercapnic increases of VI were accomplished through similar
changes of breathing pattern with similar time courses. We conclude that
sustained hypoxia does not affect the ventilatory response to CO2, but it
selectively depresses hypoxic sensitivity.
