Changes in relaxation rate with diaphragmatic fatigue in humans

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Abstract
Maximum relaxation rate (MRR) and the time constant of relaxation (.tau.) of transdiaphragmatic pressure (Pdi) was measured in 4 male subjects and compared with the high-to-low frequency ratio (H/L) of the diaphragmatic electromyogram (EMG) as a predictor of diaphragmatic fatigue. Pdi and inspiratory time-to-total breath duration ratios (TI/TT) were varied, and TT and tidal volume were held constant,; inspiratory resistances were used to increase Pdi. Studies were performed at various tension-time indices (TTdi = Pdi/Pdimax .times. TI/TT). Base-line MRR/Pdi was 0.0100 .+-. 0.0004 (SE) ms-1, and baseline .tau. was 53.2 .+-. 3.2 ms. At TTdi > 0.20, MRR and H/L decreased and .tau. increased, with maximum changes at the highest TTdi. At TTdi < 0.20, there was no change in H/L, MRR or .tau.. The time course of changes in H/L correlated with those of MRR and .tau. under fatiguing conditions. Change in relaxation rate was as useful a predictor of diaphragmatic fatigue as fall in H/L of the diaphragmatic EMG.

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