Tidal volume perception in normal subjects: the effect of altered arterial PCO2.

We examined the relationship between tidal volume (VT) perception and level of CO2. Ten normal subjects were connected to a volume-cycled ventilator set in control mode, and VT and respiratory rate were adjusted until subjects were comfortable. At 2 levels of CO2 which differed by 6-8 mmHg, subjects rated ten different ventilator tidal volumes which ranged from 40-200% of the VT that made them comfortable at the lower CO2. Despite large differences in inspiratory muscle activity, there was no significant difference between the two levels of CO2 for either the exponent [1.39 +/- 0.35 vs 1.36 +/- 0.49 (low CO2 vs high CO2, mean +/- SD), P > 0.5] or constant (-0.09 +/- 0.12 vs -0.14 +/- 0.17, P > 0.1) for VT perception. For the group, there was no correlation between the hypercapnic ventilatory response (HCVR) measured by rebreathing and the exponent for VT perception. We conclude that: (1) the level of CO2 does not influence magnitude estimation of VT; (2) respiratory muscle activity is not essential to VT perception; and (3) there is no correlation between the HCVR and magnitude estimation of VT.