title: Effects of CO2 breathing on ventilatory response to sustained hypoxia in normal adults.
abstract: The relationship between CO2 and ventilatory response to sustained hypoxia was examined in nine normal young adults. At three different levels of end-tidal partial pressure of CO2 (PETCO2, approximately 35, 41.8, and 44.3 Torr), isocapnic hypoxia was induced for 25 min and after 7 min of breathing 21% O2, isocapnic hypoxia was reinduced for 5 min. Regardless of PETCO2 levels, the ventilatory response to sustained hypoxia was biphasic, characterized by an initial increase (acute hypoxic response, AHR), followed by a decline (hypoxic depression). The biphasic response pattern was due to alteration in tidal volume, which at all CO2 levels decreased significantly (P less than 0.05), without a significant change in breathing frequency. The magnitude of the hypoxic depression, independent of CO2, correlated significantly (r = 0.78, P less than 0.001) with the AHR, but not with the ventilatory response to CO2. The decline of minute ventilation was not significantly affected by PETCO2 [averaged 2.3 +/- 0.6, 3.8 +/- 1.3, and 4.5 +/- 2.2 (SE) 1/min for PETCO2 35, 41.8, and 44.3 Torr, respectively]. This decay was significant for PETCO2 35 and 41.8 Torr but not for 44.3 Torr. The second exposure to hypoxia failed to elicit the same AHR as the first exposure; at all CO2 levels the AHR was significantly greater (P less than 0.05) during the first hypoxic exposure than during the second. We conclude that hypoxia exhibits a long-lasting inhibitory effect on ventilation that is independent of CO2, at least in the range of PETCO2 studied, but is related to hypoxic ventilatory sensitivity.
authors.affiliation: Department of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada.
authors.forename: D
authors.initials: D
authors.lastname: Georgopoulos
authors.forename: D
authors.initials: D
authors.lastname: Berezanski
authors.forename: N R
authors.initials: NR
authors.lastname: Anthonisen
mesh_terms: D000328:Adult; D002245:Carbon Dioxide / Q000494:pharmacology*; D005260:Female; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D012016:Reference Values; D012119:Respiration / Q000187:drug effects*; D012129:Respiratory Function Tests; D013990:Tidal Volume
chemical_list: D002245:Carbon Dioxide