CONTRIBUTION OF DIAPHRAGMATIC POWER OUTPUT TO EXERCISE-INDUCED DIAPHRAGM FATIGUE

In nine normal humans we compared the effects on diaphragm fatigue of whole body exercise to exhaustion (86-93% of maximal O-2 uptake for 13 .2 +/- 2.0 min) to voluntary increases in the tidal integral of transd iaphragmatic pressure (integral Pdi) while at rest at the same magnitu de and frequency and for the same duration as those during exercise. A fter the endurance exercise, we found a consistent and significant fal l (-26 +/- 2.9%, range -19.2 to -41.0%) in the Pdi response to suprama ximal bilateral phrenic nerve stimulation at all stimulation frequenci es (1, 10, and 20 Hz). integral Pdi . f(B) (where f(B) is breathing fr equency) achieved during exercise averaged 509 +/- 81.0 cmH(2)O/min (r ange 304.0-957.0 cmH(2)O/min). At rest, voluntary production of integr al Pdi . f(B), which was <550-600 cmH(2)O/min (similar to 4 times the resting eupenic integral Pdi . f(B) or 60-70% of Pdi capacity), did no t result in significant diaphragmatic fatigue, whereas sustained volun tary production of integral Pdi . f(B) in excess of these threshold va lues usually did result in significant fatigue. Thus, with few excepti ons (5 of 23 tests) the ventilatory requirements of whole body enduran ce exercise demanded a level of integral Pdi . f(B) that, by itself, w as not fatiguing. The rested first dorsal interosseous muscle showed n o fatigue in response to supramaximal ulnar nerve stimulation after wh ole body exercise. We postulate that the effects of locomotor muscle a ctivity, such as competition for blood flow distribution and/or extrac ellular fluid acidosis, in conjunction with a contracting diaphragm ac count for most of the exercise-induced diaphragm fatigue.