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Respiratory effects in humans of a 5-day elevation of end-tidal PCO2 by 8 Torr

University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom

Submitted 23 May 2003 ; accepted in final form 2 July 2003

The aims of this study were to determine 1) whether ventilatory adaptation occurred over a 5-day exposure to a constant elevation in end-tidal PCO₂ and 2) whether such an exposure altered the sensitivity of the chemoreflexes to acute hypoxia and hypercapnia. Ten healthy human subjects were studied over a period of 13 days. Their ventilation, chemoreflex sensitivities, and acid-base status were measured daily before, during, and after 5 days of elevated end-tidal PCO₂ at 8 Torr above normal. There was no major adaptation of ventilation during the 5 days of hypercapnic exposure. There was an increase in ventilatory chemosensitivity to acute hypoxia (from 1.35 ± 0.08 to 1.70 ± 0.07 l/min/%; P < 0.01) but no change in ventilatory chemosensitivity to acute hypercapnia. There was a degree of compensatory metabolic alkalosis. The results do not support the hypothesis that the ventilatory adaptation to chronic hypercapnia would be much greater with constant elevation of alveolar PCO₂ than with constant elevation of inspired PCO₂, as has been used in previous studies and in which the feedback loop between ventilation and alveolar PCO₂ is left intact.

hypercapnia; compensatory metabolic alkalosis; chemosensitivity

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