SELF-CONTROL OF LEVEL OF MECHANICAL VENTILATION TO MINIMIZE CO2 INDUCED AIR HUNGER

Hypercapnia produces an uncomfortable urge to breathe ('air hunger'), which is alleviated by increasing breathing. It has been postulated th at awake humans control breathing partly to minimize these sensations; such behavioral control presumably involves the forebrain. To test th is postulate, we compared the ventilatory response to hypercapnia when the subject breathed spontaneously to the response when the subject u sed forebrain commands to control ventilation-on the basis of minimizi ng air hunger (achieved with subject-controlled positive pressure vent ilation). In six healthy adults during hypercapnia (46 mmHg), spontane ous ventilation significantly exceeded, by 17%, the level of (mechanic al) ventilation needed to alleviate air hunger. This suggests that spo ntaneous breathing is not behaviorally controlled to minimize discomfo rt. Alternatively, mechanical ventilation confers an additional relief of air hunger beyond that provided by spontaneous breathing. Since me chanical ventilation (with reduced respiratory muscle contraction) was more effective than spontaneous breathing in relieving air hunger, ou r results also suggest afferents that signal the degree of respiratory muscle contraction do not contribute to air hunger relief.