Sensation of inspiratory difficulty during inspiratory threshold and hyperinflationary loadings. Effect of inspiratory muscle strength.

Dynamic hyperinflation loads the inspiratory muscles by increasing end-expiratory lung volume (EELV) and imposing intrinsic positive end-expiratory pressure (PEEPi), the latter behaving as an inspiratory threshold load (ITL). The aim of the current study was to examine how induced-inspiratory muscle fatigue affects the independent effects of the imposed ITL and increasing operating lung volume on the perceived inspiratory difficulty. Dynamic hyperinflation in healthy subjects was induced by positive end-expiratory pressure (PEEP). Increasing operating lung volume alone (without PEEPi) and increasing ITL alone (without change in EELV) were induced by continuous positive airway pressure (CPAP) and external ITL, respectively. Inspiratory difficulty was quantified by the modified Borg scale and analyzed by step forward multiple regression, using the imposed ITL, EELV, and end-inspiratory lung volume (EILV) as independent variables. When fresh, the first entered variable was the imposed ITL (r(2), 0.38). Adding EILV into the model increased r(2) to 0.67. After fatigue, the first entered variable became EILV (r(2), 0.50) and the second selected variable was the imposed ITL, which increased r(2) to 0.66. EELV was insignificant under both conditions. The coefficient of EILV increased significantly from 0.039 +/- 0.005 to 0.092 +/- 0.012 (% inspiratory capacity(-)(1)) after fatigue run (p < 0.001), whereas that of the imposed ITL did not change. It is concluded that in the experimental conditions studied, inspiratory muscle fatigue increased the importance of lung volume over that of inspiratory threshold load in determining the perceived inspiratory difficulty.