Exercise Intolerance in Cystic Fibrosis: Importance of Skeletal Muscle.

PURPOSE Exercise intolerance, evaluated by O2 consumption, predicts mortality in Cystic Fibrosis (CF). People with CF exhibit skeletal muscle dysfunctions that may contribute to an imbalance between O2 delivery and utilization. Sildenafil, a phosphodiesterase type 5 (PDE5) inhibitor, increases blood flow and improves O2 consumption, although the exact mechanisms in CF have yet to be elucidated. Thus, we hypothesized that exercise intolerance in CF is limited primarily by an impaired skeletal muscle O2 utilization, and sildenafil improves exercise tolerance in CF by addressing this mismatch between O2 demand and extraction. METHODS Fifteen individuals with mild-to-moderate CF and eighteen healthy controls completed an incremental exercise test and measurements of gaseous exchange, chronotropic response, hemodynamics, and O2 extraction and utilization. People with CF also completed a four-week-treatment with sildenafil with a subsequent follow-up evaluation after treatment. RESULTS Skeletal muscle O2 extraction and utilization during exercise were reduced in people with CF when compared to controls. Exercise capacity in our CF population was minimally limited by hemodynamic or chronotopic responses, while peripheral O2 extraction was more closely associated with exercise capacity. The study also demonstrated that four weeks of sildenafil improved skeletal muscle O2 utilization during exercise to similar values observed in healthy individuals. CONCLUSIONS Individuals with mild to moderate CF exhibit exercise intolerance secondary to a reduction in O2 utilization by the exercising skeletal muscle. The present study demonstrated that four weeks of sildenafil treatment improves the capacity of the skeletal muscle to utilize O2 more efficiently during exercise. Findings from the present study highlight the importance of targeting skeletal muscle O2 utilization to improve exercise tolerance in CF.