Reduced tidal volume increases 'air hunger' at fixed P_CO2 in ventilated quadriplegics

The act of breathing diminishes the discomfort associated with hypercapnia and breath-holding. To investigate the mechanisms involved in this effect, we studied the effect of tidal volume (V_T) on CO₂-evoked air hunger in 5 high-level quadriplegic subjects whose ventilatory capacity was negligible, and who lacked sensory information from the chest wall. Subjects were ventilated at constant frequency with a hyperoxic gas mixture, and end-tidal P_CO2 was maintained at a constant but elevated level. V_T was varied between the subjects' normal V_T and a smaller V_T. Subjects used a category scale to rate their respiratory discomfort or 'air hunger' at 30-40 sec intervals. In 4 of 5 subjects there was a strong inverse relationship between breath size and air hunger ratings. The quality of the sensation associated with reduced V_T was nearly identical to that previously experienced with CO₂ alone. We conclude that afferent information from the lungs and upper airways is sufficient to modify the sensation of air hunger.