HIGH-FREQUENCY DIAPHRAGMATIC FATIGUE DETECTED WITH PAIRED STIMULI IN HUMANS

Purpose: The purpose of this study was to determine whether high frequ ency fatigue was present in the diaphragm after intense whole body end urance exercise. Methods: We used bilateral phrenic nerve stimulation (BPNS) before and during recovery from whole body exercise to detect f atigue in the diaphragm. To detect high frequency fatigue we used pair ed stimuli at 10, 20, 50, 70, and 100 Hz frequency and determined the transdiaphragmatic pressure (Pdi) response to the second stimulation ( T-2). Results: The subjects (N = 10) exercised at 93.3 +/- 2.3% of the ir (V) over dot O-2max for 9.9 +/- 0.5 min. The Pdi response to ''twit ch'' and 10 Hz ''tetanic'' stimulation was decreased immediately after exercise versus pre-exercise values (-23.4 +/- 3.3%). The T-2 amplitu de was substantially reduced at all frequencies immediately after exer cise (-28.0%), but by 30 min into recovery the T-2 amplitude at 70 and 100 Hz was not different from pre-exercise values. In contrast, at 10 and 30 Hz the T-2 response was still significantly reduced. Conclusio ns: We interpret these data to mean that high frequency fatigue as wel l as low frequency fatigue were present in the diaphragm after intense whole body endurance exercise.