Effects of willful ventilatory control on respiratory sensation during hypercapnia

Remarkable augmentation of breathing discomfort has been noted when ventilation is constrained to the steady state level during progressive hypercapnia. However, the effect of willful enhancement of ventilation on breathing discomfort remains to be evaluated. The present study examined the effects of moderate willful increases or decreases in ventilation during progressive hypercapnia on breathing discomfort in 12 subjects. There were a total of 5 rebreathing trials. In the first (F1) and the fifth trials the subjects rebreathed freely. In the other trials subjects breathed by tracking a target to achieve hypercapnic ventilatory responses that were the same (HCVR-S), 25% higher (HCVR-H) and 25% lower (HCVR-L) than in the F1 trial. Breathing discomfort was assessed every 30 s by a 150-mm visual analog scale (VAS). The sensational response (dVAS/dPCO2) during HCVR-S [3.8 +/- (SE) 0.8 mm/Torr] was significantly smaller (p < 0.01) than that during the F1 (6.3 +/- 0.8 mm/Torr) trial. HCVR-H resulted in a further decrease in dVAS/dPCO2 to 3.1 +/- 0.7 mm/Torr as compared to HCVR-S (p < 0.05). HCVR-L significantly increased dVAS/dPCO2 to 4.9 +/- 0.7 mm/Torr compared to HCVR-S (p < 0.05). The final free rebreathing ventilatory response was significantly larger than the initial free rebreathing response (2.7 +/- 0.5 as compared to 2.1 +/- 0.4 liters/min/Torr, p < 0.01). However, the sensational response did not change (6.3 +/- 0.8 vs. 5.8 +/- 0.7 mm/Torr). These rebreathing studies indicate that willful control of respiration decreases respiratory sensation even at comparable levels of ventilation. In particular, moderate willful increases in ventilation produce an ameliorating effect on the sensation of breathing discomfort.