VENTILATORY RESPONSES TO INSPIRATORY THRESHOLD LOADING AND ROLE OF MUSCLE FATIGUE IN TASK FAILURE

To examine respiratory muscle recruitment pattern during inspiratory l oading and role of fatigue in limiting endurance, we studied seven nor mal subjects on 17 +/- 6 days during breathing against progressive ins piratory threshold load. Threshold pressure (Pth) was progressively in creased 14 +/- 5 cmH(2)O every 2 min until voluntary cessation (task f ailure). Subjects could adopt any breathing pattern. Tidal volume (VT) , chest wall motion, end-tidal Pco(2), and arterial O-2 saturation wer e measured. At moderate loads [50-75% of maximum Pth (Pth(max))], insp iratory time (TI) decreased and VT/TI and expiratory time increased, i ncreasing time for recovery of muscles between inspirations. At high l oads (> 75% Pth(max)), VT/TI decreased, which, with progressive decrea se in end-expiratory lung volume (EELV) throughout, increased potentia l for inspiratory force development. Progressive hypoxia and hypercapn ia occurred at higher work loads. Immediately after task failure all s ubjects could recover at high loads and still reachieve initial Pth(ma x) on reimposition of progressive loading. Respiratory pressures were measured in subgroup of three subjects: transdiaphragmatic pressure re sponse to 0.1-ms bilateral supramaximal phrenic nerve stimulation at e nd expiration initially increased with increasing load/decreasing EELV , consistent with increasing mechanical advantage of diaphragm, but de creased at highest loads, suggesting diaphragm fatigue. Full recovery had not occurred at 30 min after task failure. We demonstrated that pr ogressive threshold loading is associated with systematic changes in b reathing pattern that act to optimize muscle strength and increase end urance. Task failure occurred when these compensatory mechanisms were maximal. Inspiratory muscles appeared relatively resistant to fatigue, which was late but persistent.