Effect of pressure and timing of contraction on human diaphragm fatigue

The relationship between the mean transdiaphragmatic pressure swing developed with each inspiration (Pdi) and the fraction of the breathing cycle time spent in inspiration (TI/Ttot) (Pdi X TI/Ttot) was related to the maximal time that such a run could be sustained (Tlim). Four normal subjects breathed with a constant breathing pattern for 45 min or until Pdi could no longer be sustained, whichever came first. The breathing patterns included Pdi of 0.15-0.90 of Pdimax and TI/Ttot of 0.15-1.0. Pdi was obtained by adjusting an inspiratory resistance, and the timing by monitoring tidal volume with time base from an oscilloscope. The Tlim of a run was found to be inversely related to both Pdi and TI/Ttot and hence inversely related to their product, following a quadratic hyperbole function. Pdi X TI/Ttot represents an index of the tension time of the diaphragm (TTdi). The breathing pattern that could be sustained more than 45 min was found to have a TTdi of about 0.15, which was termed critical TTdi. Above that value Tlim decreased as a function of TTdi. The results are consistent with Tlim being related to diaphragmatic blood flow limitation.