Effect of hypercapnia on maximal voluntary ventilation and diaphragm fatigue in normal humans

Relatively little is known about the combined effects of hypercapnia and fa tigue on the human diaphragm. We examined the effects of acute hypercapnia and fatigue in seven subjects by measuring changes in transdiaphragmatic pr essure (Pdi) elicited by cervical magnetic stimulation after 2 min maximal voluntary ventilation (MW) while breathing air and also with the inspired P co(2) increased to 8% for 12 min before and during the MW. Diaphragm streng th was assessed before and at 0, 20, 40, 60, and 90 min after the MW in bot h studies with the subjects breathing air. There was no difference in the l evel of ventilation for each run. Mean (+/- SD) twitch Pdi (TwPdi) fell sig nificantly (p < 0.01) at 20 min after the control and hypercapnic MW; (30.4 [:7.8] to 27.0 [8.1] cm H2O control and 30.3 [4.1] to 27.3 [5.0] cm H2O CO 2 and remained significantly (p < 0.01) below baseline. The changes in TwPd i at 20 to 90 min were not significantly different between the control and CO, runs. The decrease in TwPdi at 0 min after MW, however, was greater (15 %) in the hypercapnic run than in the control run (8.1%) (p < 0.05) when co mpared with baseline valves. Hypercapnia does not intensify long lasting fa tigue but may reduce diaphragm contractility immediately after MW.