DIAPHRAGMATIC FATIGUE FOLLOWING VOLUNTARY HYPERPNEA

The purpose of this study was to determine whether diaphragmatic fatig ue occurs after voluntary hyperpnea to task failure. Ten male subjects were asked to breathe at minute ventilation (VI) equal or slightly gr eater than 60% of their 12-s maximum voluntary ventilation (MW) until task failure. Transdiaphragmatic pressure (Pdi) was measured during bi lateral supramaximal stimulation of the phrenic nerves before and 10, 30, 60, and 120 min after hyperpnea. For the group, VI averaged 107.1 +/- 7.1 L/min (SE) (range, 71-154 L/min), which represented 60 +/- 2% of the MVV. After voluntary hyperpnea, seven of the 10 subjects displa yed at least a 10% reduction in twitch Pdi during transcutaneous stimu lation, while all 10 subjects had a greater than 10% reduction in twit ch Pdi during cervical magnetic stimulation. For the group, transcutan eous twitch Pdi was significantly decreased from 27.0 +/- 1.9 at basel ine to 21.4 +/- 1.7 cm H2O (p < 0.0001) at 10 min posthyperpnea. Magne tic twitch Pdi was also significantly decreased from 36.0 +/- 2.1 at b aseline to 28.7 +/- 1.9 cm H2O (p < 0.0001) at 10 min posthyperpnea. T witch Pdi remained significantly decreased from baseline for at least 1 h after hyperpnea. After. hyperpnea, the mean percentage decrease fr om baseline in twitch Pdi was virtually identical with the two stimula tion techniques. The percentage fall in twitch Pdi after hyperpnea wit h the two techniques was not significantly correlated (r = 0.4). In co nclusion, long-lasting contractile fatigue of the diaphragm reliably o ccurs after voluntary hyperpnea at levels sufficient to induce task fa ilure. Cervical magnetic stimulation can detect diaphragmatic fatigue after a fatiguing task, but the results obtained with this technique m ay differ from those obtained with transcutaneous stimulation in indiv idual subjects.