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 ventila
tion with hyperoxic gas mixtures at fixed frequency and tidal volume (
twice resting ventilation), 2) the same mechanical ventilation, but wi
th hypoxic gas mixture, 3) spontaneous breathing with hyperoxic gas mi
xture. In each case, PET(CO2) was varied randomly among several levels
, each held for 5 min. During hyperoxic mechanical ventilation, the me
an 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 th
reshold and tolerable levels of PET(CO2) varied among individuals, but
were not well correlated with their ventilatory responses to CO2. Hyp
oxia (PET(O2) 60-75 Torr) shifted the PET(CO2) at both threshold and t
olerance down by only 2 Torr. Breathing greatly reduced the air hunger
experienced at any given PET(CO2) (threshold increased 5 Torr, and se
nsitivity decreased 50%).