Stimulus-response characteristics of CO2-induced air hunger in normal

Hypercapnia evokes an uncomfortable sensation, termed 'air hunger'. We examined the relationship between PET(CO2) and ratings of air hunger intensity under three conditions in 16 subjects: 1) mechanical ventilation with hyperoxic gas mixtures at fixed frequency and tidal volume (twice resting ventilation), 2) the same mechanical ventilation, but with hypoxic gas mixture, 3) spontaneous breathing with hyperoxic gas mixture. In each case, PET(CO2) was varied randomly among several levels, each held for 5 min. During hyperoxic mechanical ventilation, the mean threshold for air hunger sensation was 43 Torr, i.e., 4 Torr above resting PET(CO2); intolerable air hunger was evoked by 50 Torr. The threshold and tolerable levels of PET(CO2) varied among individuals, but were not well correlated with their ventilatory responses to CO2. Hypoxia (PET(O2) 60-75 Torr) shifted the PET(CO2) at both threshold and tolerance down by only 2 Torr. Breathing greatly reduced the air hunger experienced at any given PET(CO2) (threshold increased 5 Torr, and sensitivity decreased 50%).