Inspiratory fraction as a marker of skeletal muscle dysfunction in patients with COPD

Background: An inspiratory capacity to total lung capacity (IC/TLC) ratio of ≤25% has emerged as a better marker of mortality in chronic obstructive pulmonary disease (COPD) patients. The relationship among the IC/TLC ratio to lower extremity skeletal muscle function remains unknown. Methods: Thirty-five men with moderate to severe COPD were divided into those with IC/TLC ≤ 25% (n = 16) and >25% (n = 19). The subjects were tested for thigh muscle mass volume (MMT), maximal strength, power output of the lower extremities, and physical activity. Results: Total MMT in the IC/TLC < 25% group was significantly lower (413.91 ± 89.42 cm3) (p < 0.001) than in the IC/TLC > 25% group (575.20 ± 11.76 cm3). In the IC/TLC ≤ 25% group, maximal strength of the lower extremities and muscle peak power output of the lower extremities were 36---56% lower (p < 0.01) than among the patients in the IC/TLC > 25% group. Conclusion: IC/TLC ≤ 25% is associated with reduced maximal strength and peak power output of the lower extremities. IC/TLC ≤ 25% may have an important clinical relevance as an index to determine peripheral muscle dysfunction.