INSPIRATORY MUSCLE STRENGTH AND ENDURANCE IN STEROID-DEPENDENT ASTHMA

The adverse effect of long-term steroid treatment on respiratory muscl e function remains controversial. We evaluated inspiratory muscle stre ngth and endurance in steroid-dependent asthmatics in comparison with other asthmatics or with patients with chronic obstructive pulmonary d isease exhibiting a comparable level of lung hyperinflation. Inspirato ry muscle function was assessed by maximal inspiratory pressure (PImax ) and by an incremental inspiratory threshold loading test in 19 patie nts who had had steroid-dependent asthma (SDA) requiring a mean daily dose of 20.7 +/- 0.8 mg prednisone for 5 +/- 1.4 yr. They were compare d with 16 healthy control subjects, 30 patients with COPD, and 16 pati ents with non-steroid-dependent asthma (NSDA). PImax as percentage of predicted values (%PImax) was not significantly different in patients with SDA (77 +/- 5%) or NSDA (83 +/- 6%) than in control subjects (93 +/- 4%). In contrast, %PImax was lower in patients with COPD (59 +/- 4 .4%) than in those with SDA or NSDA (p < 0.05) or the control subjects (p < 0.0001). A significant correlation was found between %PImax and hyperinflation assessed by the FRC/TLC ratio (r = 0.42; p < 0.0001). I nspiratory endurance, defined as the ratio of maximal peak inspiratory pressure sustained for 2 min to individual PImax (Plim2/PImax), was s ignificantly lower in the SDA (43 +/- 3%; p < 0.0001), NSDA (65 +/- 4% ; p = 0.01), and COPD (55 +/- 3%; p < 0.0001) groups than in the contr ol group (76 +/- 2%). Plim2/PImax was also lower in patients with SDA than in those with COPD (p = 0.0073) or NSDA (p < 0.0001). Hyperinflat ion plays a major role in inspiratory muscle dysfunction associated wi th obstructive lung disorders, but the finding of a significantly decr eased endurance in patients with SDA when compared with patients with COPD, despite a lower level of hyperinflation in the former group, poi nts to a deleterious effect of long-term corticosteroid treatment on i nspiratory muscle function in asthmatics.