Influence of graded hypercapnia on endurance exercise performance in healthy humans

Military and/or emergency services personnel may be required to perform high-intensity physical activity during exposure to elevated inspired carbon dioxide (CO₂). Although many of the physiological consequences of hypercapnia are well characterized, the effects of graded increases in inspired CO₂ on self-paced endurance performance have not been determined. The aim of this study was to compare the effects of 0%, 2%, and 4% inspired CO₂ on 2-mile run performance, as well as physiological and perceptual responses during time trial exercise. Twelve physically active volunteers (peak oxygen uptake = 49 ± 5 mL·kg^-1·min^-1; 3 women) performed three experimental trials in a randomized, single-blind, crossover manner, breathing 21% oxygen with either 0%, 2%, or 4% CO₂. During each trial, participants completed 10 min of walking at ∼40% peak oxygen uptake followed by a self-paced 2-mile treadmill time trial. One participant was unable to complete the 4% CO₂ trial due to lightheadedness during the run. Compared with the 0% CO₂ trial, run performance was 5 ± 3% and 7 ± 3% slower in the 2% and 4% CO₂ trials, respectively (both P < 0.001). Run performance was significantly slower with 4% versus 2% CO₂ (P = 0.046). The dose-dependent performance impairments were accompanied by stepwise increases in mean ventilation, despite significant reductions in running speed. Dyspnea and headache were significantly elevated during the 4% CO₂ trial compared with both the 0% and 2% trials. Overall, our findings show that graded increases in inspired CO₂ impair endurance performance in a stepwise manner in healthy humans.