PATTERN OF RECOVERY FROM DIAPHRAGMATIC FATIGUE OVER 24 HOURS

The rate of recovery from diaphragmatic fatigue beyond 1 h is unknown. To investigate this question, we studied 12 healthy subjects and measured transdiaphragmatic twitch pressure (Pdi(tw)) using magnetic stimulation of the phrenic nerves. Measurements were obtained at baseline and after a fatigue protocol consisting of inspiratory resistive loading in which the subjects generated 60% of maximal transdiaphragmatic pressure until task failure. At baseline, Pdi(tw) was 38.9 +/- 1.1 (SE) cmH(2)O and fell to 25.1 +/- 0.6 cmH(2)O 10 min after the conclusion of the fatigue protocol (P < 0.01). Pdi(tw) increased to 27.6 +/- 0.9, 31.6 +/- 1.1, and 32.7 +/- 1.2 cmH(2)O 1, 8 and 24 h, respectively, after the conclusion of the fatigue protocol; the 24-h value was significantly lower than baseline (P < 0.01). The nadir in Pdi(tw) after the protocol was delayed by 10 min. In separate experiments, we showed that this delay was probably due to the development of twitch potentiation as a result of forceful diaphragmatic contractions during the fatigue protocol. In conclusion, induction of diaphragmatic fatigue with this experimental protocol produced a marked decrease in diaphragmatic contractility that persisted for at least 24 h.