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University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom
During
ventilatory acclimatization to hypoxia (VAH), the relationship between
ventilation (
E) and end-tidal
PCO2 (PETCO2) changes.
This study was designed to determine 1) whether these changes can be seen
early in VAH and 2) if these changes
are present, whether the responses differ between isocapnic and
poikilocapnic exposures. Ten healthy volunteers were studied by using
three 8-h exposures: 1) isocapnic
hypoxia (IH), end-tidal PO2
(PETO2) = 55 Torr and
PETCO2 held at the
subject's normal prehypoxic value;
2) poikilocapnic hypoxia (PH),
PETO2 = 55 Torr; and
3) control (C), air breathing. The
E-PETCO2
relationship was determined in hyperoxia (PETO2 = 200 Torr) before
and after the exposures. We found a significant increase in the
slopes of
E-PETCO2 relationship after both hypoxic exposures compared with control (IH vs.
C, P < 0.01; PH vs. C,
P < 0.001; analysis of covariance with pairwise comparisons). This increase was not significantly different between protocols IH and
PH. No significant changes in the
intercept were detected. We conclude that 8 h of hypoxia, whether
isocapnic or poikilocapnic, increases the sensitivity of the hyperoxic
chemoreflex response to CO2.
hypercapnic sensitivity; ventilatory acclimatization; altitude
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