The human ventilatory response to acute hyperoxia during and after
8 h of both isocapnic and poikilocapnic hypoxia.
Tansley, J. G., C. Clar, M. E. F. Pedersen, and P. A. Robbins.
University of Oxford, Laboratory of Physiology, Oxford OX1 3PT,
United Kingdom
APStracts 3:0490A, 1996.
In a recent study (Howard, L.S.G.E., & Robbins, P.A., J. Appl.
Physiol. 78, 1092-1097, 1995; Howard, L.S.G.E., & Robbins, P.A.,
J. Appl. Physiol. 78, 1098-11, 1995) it has been shown that
ventilation (E) rises progressively during 8 h of isocapnic hypoxia.
Rapid alternations in PETO2 suggested that this was associated with
both a rise in hypoxic sensitivity and a rise in baseline or `hypoxia
insensitive' ventilation. Similar changes in respiratory control were
detected in poikilocapnic hypoxia, but were masked by the associated
alkalosis. The purpose of the current study was twofold: (1) to
determine whether baseline changes inE can still be detected
following 5-10 min hyperoxia; and (2) to determine whether any such
changes persist following a return to euoxia. Twelve subjects were
initially studied, although the results from 4 were rejected before
analysis. Three 16 h protocols were compared: (1) 8 h isocapnic
hypoxia (PETCO2 held at pre-study value, PETO2 =55 Torr) followed by
8 h isocapnic euoxia (PETO2=100 Torr); (2) 8 h poikilocapnic hypoxia
(PETO2 =55 Torr) followed by 8 h poikilocapnic euoxia (PETO2 =100
Torr); and (3) control where the inspired gas was air. The
ventilatory response to 10 min isocapnic hyperoxia (PETCO2 held 1-2
Torr above pre-study value, PETO2=300 Torr) was measured before the
experiment, after 20 min, 4h and 8h of hypoxia, and again after 20
min, 4h and 8h after the return to euoxia. There was a significant
rise in hyperoxicE during both forms of hypoxic conditioning
(p<0.05, ANOVA). This rise in hyperoxicE persisted during the
8 h following the relief of hypoxia (p<0.05, ANOVA). The
effects of poikilocapnic hypoxia on hyperoxic ventilation did not
differ from the effects of isocapnic hypoxia. We conclude that 8 h of
hypoxic conditioning, whether isocapnic or hypocapnic, does cause a
sustained rise inE that cannot be completely reversed by hyperoxia,
and which persists for at least 8 h after the exposure.
Received 9 July 1996; accepted in final form 16 October 1996.
APS Manuscript Number A637-6.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 13 November 1996