Task failure, breathing discomfort and CO2 accumulation without fatigue during inspiratory resistive loading in humans.

Task failure during inspiratory resistive loading has been attributed to inspiratory muscle fatigue. Six subjects breathed at their own rate and duty cycle through an inspiratory resistive load to a target mouth pressure of 80% maximal inspiratory pressure (MIP) until task failure, when breathing discomfort (measured with a modified Borg scale) was maximal. Six protocols were used to vary ventilatory parameters and hence the rate of rise, of end-tidal CO2. MIP did not decline during resistive loading in any protocol, but there were significant increases in end-tidal CO2 (ranging from 1.0 to 3.9). Time to task failure increased in protocols that enabled subjects to reduce the rate of CO2 accumulation. Differences in endurance times between subjects were related to their sense of breathing discomfort with rising CO2 measured in separate rebreathing studies. Task failure was due to breathing discomfort associated with CO2 accumulation and sensations related to the generation of large inspiratory pressures, rather than inspiratory muscle fatigue.