Air hunger induced by acute increase in PCO₂ adapts to chronic elevation of PCO₂ in ventilated humans

Brief increases in arterial PCO₂ (Pa(CO₂)) (lasting several minutes) produce a sensation of respiratory discomfort (air hunger). It is not known whether air hunger adapts to chronic changes in Pa(CO₂). This study tested whether the level of end-tidal PCO₂ (PET(CO₂)) required to evoke air hunger would increase with chronic elevation of PET(CO₂) (lasting several days). Four ventilator-dependent subjects participated in a 2-wk study during which they were ventilated with air (placebo) or air rich in CO₂ (CO₂ exposure). Average resting PET(CO₂) during control periods was 25 Torr (typical for such patients); PET(CO₂) was 15 Torr higher during CO₂ exposure. Ventilation and arterial PO₂ did not differ between conditions. Periodically, we performed tests in which subjects rated the intensity of air hunger induced by brief increases in PET(CO₂). The increase in PET(CO₂) required to elicit a given air hunger rating during CO₂ exposure also increased by ~15 Torr. That is, subjects' sensation of air hunger fully adapted to the chronic increase in PET(CO₂). Arterial pH did not fully return to control values during CO₂ exposure. Accommodation in the chemoreceptors and neural pathways that subserve air hunger sensation may explain the adaptation of air hunger.