DIAPHRAGM FATIGUE FOLLOWING MAXIMAL VENTILATION IN MAN

When highly motivated normal subjects perform maximal isocapnic ventil ation, a substantial fall in ventilation is observed during the first minute associated with slowing of the maximum relegation rate (MRR) of the inspiratory muscles. This suggests that these muscles are excessi vely loaded, raising the possibility that overt contractile failure of the diaphragm contributes to the fall in ventilation. We therefore in vestigated the effect of maximal isocapnic ventilation (MIV) on twitch transdiaphragmatic pressure (Pdi,Tw) elicited by cervical magnetic st imulation. We measured Pdi,Tw before and after 2 min MIV in nine norma l subjects. Initial mean (so) ventilation for the nine subjects was 19 6 (15) L . mia(-1) falling by 35% at 1 min. Pdi,Tw fell following MIV, at 10 min was reduced by 24%, and remained substantially reduced 90 m in after MIV. No change in Pdi,Tw was observed during control studies in which subjects were studied with the same protocol but omitting MIV . We conclude that diaphragmatic contractility is reduced after 2 min maximal isocapnic ventilation and diaphragmatic fatigue may be a limit ing factor in maximal ventilation in man.