Effect of acetazolamide on respiratory muscle fatigue in humans.

Previous studies have demonstrated that carbonic anhydrase inhibition with acetazolamide reduces exercise capacity. The mechanism responsible for this early fatigue is unclear, but may be partly mediated by impaired respiratory muscle function. Inspiratory muscle strength and endurance were assessed in seven healthy men (age 28 ± 5 yrs, ±SD) by measuring maximal inspiratory pressure (MIP) and time to task failure during a constant-load breathing test (CLBT), respectively, under control (CON) and acetazolamide (ACZ; 500 mg/8 h po for 3 days) conditions that were separated by two weeks and randomized between subjects. In addition, MIP was measured before and after moderate-intensity cycling exercise to fatigue while pulmonary gas exchange, plasma pH, and ventilation were measured during exercise. ACZ did not alter pulmonary function (FVC, FEV1, MVV) or MIP measured at rest (CON, -157 ± 47 vs. ACZ, -154 ± 45 cmH(2)O, p>0.05), but decreased time to task failure during the CLBT (CON, 1340 ± 820 vs. ACZ, 698 ± 434 s; p=0.01). Exercise duration during cycling exercise was reduced (p=0.003) with ACZ (1090 ± 254 s) compared to CON (1944 ± 532 s) in the presence of a significantly lower plasma pH and higher ventilation compared to control (p<0.05). Compared to resting values, MIP was reduced (p=0.03) in ACZ but not CON at exhaustion. In conclusion, carbonic anhydrase inhibition with ACZ is associated with impaired respiratory muscle function at rest and following constant load cycling which may contribute to reduced exercise tolerance with carbonic anhydrase inhibition.