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

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