No abstract

We found that breathing strategies affect measurement of sustainable inspiratory pressure (SIP). After allowing time for learning, the maximum sustainable inspiratory pressure (SIPmax) was 46% greater than SIP. We therefore developed a test of ventilatory muscle performance that used progressive 2-minute increments in threshold inspiratory resistance. Subjects started with a low load and continued to breathe until they could no longer inspire. With increasing load there was a fall in minute ventilation and time of inspiration, and an increase in oxygen consumption and PETCO2-Power was greatest when loads were 55 to 75% of maximum static inspiratory pressure (MIP). The inspiratory mouth pressure corresponding to the greatest load achieved (PmPeak) was the same in trained and naive subjects. PmPeak/MIP was reproducible and was not influenced by fixing subjects' breathing frequency. We concluded that tests of ventilatory muscle performance should allow subjects to develop breathing strategies to handle high inspiratory loads. Two-minute incremental loading is a simple assessment of ventilatory muscle performance and the test may have clinical application where reproducibility is necessary, AM REV RESPIR DIS 1987; 135:919-923

Maximum relaxation rate (MRR) and the time constant of relaxation (tau) of transdiaphragmatic pressure (Pdi) was measured in four 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 X 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 greater than 0.20, MRR and H/L decreased and tau increased, with maximum changes at the highest TTdi. At TTdi less than 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. In this experimental setting, change in relaxation rate was as useful a predictor of diaphragmatic fatigue as fall in H/L of the diaphragmatic EMG.

No abstract