Crural diaphragm activation during dynamic contractions at various inspiratory flow rates.

The purpose of this study was to evaluate the influence of velocity of shortening on the relationship between diaphragm activation and pressure generation in humans. This was achieved by relating the root mean square (RMS) of the diaphragm electromyogram to the transdiaphragmatic pressure (Pdi) generated during dynamic contractions at different inspiratory flow rates. Five healthy subjects inspired from functional residual capacity to total lung capacity at different flow rates while reproducing identical Pdi and chest wall configuration profiles. To change the inspiratory flow rate, subjects performed the inspirations while breathing across two different inspiratory resistances (10 and 100 cmH2O . l-1 . s), at mouth pressure targets of -10, -20, -40, and -60 cmH2O. The diaphragm electromyogram was recorded and analyzed with control of signal contamination and electrode positioning. RMS values obtained for inspirations with identical Pdi and chest wall configuration profiles were compared at the same percentage of inspiratory duration. At inspiratory flows ranging between 0.1 and 1.4 l/s, there was no difference in the RMS for the inspirations from functional residual capacity to total lung capacity when Pdi and chest wall configuration profiles were reproduced (n = 4). At higher inspiratory flow rates, subjects were not able to reproduce their chest wall displacements and adopted different recruitment patterns. In conclusion, there was no evidence for increased demand of diaphragm activation when healthy subjects breathe with similar chest wall configuration and Pdi profiles, at increasing flow rates up to 1.4 l/s.