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Articles in PresS, published online ahead of print February 22, 2002
J Appl Physiol, 10.1152/jap.00612.2001
Submitted on June 14, 2001
Accepted on February 16, 2002
1 Department of Preventive Medicine, University of Wisconsin, Madison, WI, USA
* To whom correspondence should be addressed. E-mail: mbabcock{at}wlu.ca.
We previously compared the effects of increased respiratory muscle work during whole body exercise and at rest on diaphragmatic fatigue and showed that the amount of diaphragmatic force output required to cause fatigue was reduced significantly during exercise (Babcock et al, J Appl Physiol 78: 1710, 1995). We now used positive pressure proportional assist ventilation (PAV) to unload the respiratory muscles during exercise in order to determine the effects of respiratory muscle work, per se, on exercise-induced diaphragmatic fatigue. Following 8-13 min of exercise to exhaustion under control conditions at 80-85% VO2MAX, bilateral phrenic nerve stimulation (BPNS) using single "twitch" stimuli (1 Hz) and paired stimuli (10-100 Hz) showed that Pdi was reduced by 20-30% for up to 60 min following exercise. Use of PAV during heavy exercise reduced the work of breathing by 40-50% and VO2 by 10-15% below control. PAV prevented exercise-induced diaphragmatic fatigue as determined by BPNS at all frequencies and times post exercise. Our study has confirmed that high- and low- frequency diaphragmatic fatigue result from heavy intensity whole body exercise to exhaustion; furthermore, the data show that the workload endured by the respiratory muscles is a critical determinant of this exercise-induced diaphragmatic fatigue.
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