Control of the respiratory cycle in conscious humans

Rafferty, G. F., and W. N. Gardner. Control of therespiratory cycle in conscious humans. J. Appl.Physiol. 81(4): 1744-1753, 1996.We studied inconscious humans the relative strength of mechanisms controlling timingand drive components of the respiratory cycle around their resting setpoints. A system of auditory feedback with end-tidalPCO₂ held constant in mild hyperoxiavia an open circuit was used to induce subjects independently to change inspiratory time (TI) andtidal volume(VT_I)over a wide range above and below the resting values for every breathfor up to 1 h. Four protocols were studied in various levels ofhypercapnia (1-5% inspiredCO₂). We found thatTI (and expiratory time) could be changed over a wide range (1.17-2.86 s,P < 0.01 forTI) and VT_Iincreased by 500 ml (P < 0.01)without difficulty. However, in no protocol was it possible to decreaseVT_I belowthe free-breathing resting value in response to reduction of auditoryfeedback thresholds by up to 600 ml. This applied at all levels ofchemical drive studied, with restingVT_I valuesvarying from 1.06 to 1.74 liters. When reduction inVT_I wasforced by the more "programmed" procedure of isocapnic panting,end-expiratory volume was sacrificed to ensure that peak tidal volumereached a fixed absolute lung volume. These results suggest that theimperative for control of resting breathing is to prevent reduction ofVT_I belowthe level dictated by the prevailing chemical drive, presumably tosustain metabolic requirements of the body, whereas respiratory timingis weakly controlled consistent with the needs for speech and othernonmetabolic functions of breathing.