JM Mador, A Rodis and J Diaz
Division of Pulmonary and Critical Care Medicine, State University of New York at Buffalo, USA.

The purpose of this study was to determine whether diaphragmatic fatigue occurs after voluntary hyperpnea to task failure. Ten male subjects were asked to breathe at minute ventilation (V1) equal or slightly greater than 60% of their 12-s maximum voluntary ventilation (MVV) until task failure. Transdiaphragmatic pressure (Pdi) was measured during bilateral supramaximal stimulation of the phrenic nerves before and 10, 30, 60, and 120 min after hyperpnea. For the group, V1 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 displayed at least a 10% reduction in twitch Pdi during transcutaneous stimulation, while all 10 subjects had a greater than 10% reduction in twitch Pdi during cervical magnetic stimulation. For the group, transcutaneous twitch Pdi was significantly decreased from 27.0 +/- 1.9 at baseline to 21.4 +/- 1.7 cm H2O (p < 0.0001) at 10 min posthyperpnea. Magnetic twitch Pdi was also significantly decreased from 36.0 +/- 2.1 at baseline to 28.7 +/- 1.9 cm H2O (p < 0.0001) at 10 min posthyperpnea. Twitch Pdi remained significantly decreased from baseline for at least 1 h after hyperpnea. After hyperpnea, the mean percentage decrease from baseline in twitch Pdi was virtually identical with the two stimulation techniques. The percentage fall in twitch Pdi after hyperpnea with the two techniques was not significantly correlated (r = 0.4). In conclusion, long-lasting contractile fatigue of the diaphragm reliably occurs after voluntary hyperpnea at levels sufficient to induce task failure. Cervical magnetic stimulation can detect diaphragmatic fatigue after a fatiguing task, but the results obtained with this technique may differ from those obtained with transcutaneous stimulation in individual subjects.

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