Inspiratory fraction correlates with exercise capacity in patients with stable moderate to severe COPD

Exercise intolerance is the hallmark of COPD. Static lung hyperinflation and increased dynamic hyperinflation during exercise are associated with reduced functional capacity in COPD patients. Inspiratory capacity correction for the total lung capacity, defined as inspiratory fraction (IF), may be functionally more representative than other traditional indices in these patients.

To investigate the association between IF and exercise capacity in patients with stable, moderate to severe COPD.

Fifty COPD subjects and 34 healthy volunteers constituted the study cohort. Pulmonary function and cardiopulmonary exercise testing were performed, and ventilation and gas exchange parameters were measured.

IF was significantly correlated with percent-of-predicted peak oxygen consumption (VO2) in the subjects with COPD (r = 0.52, P < .001). IF was an independent predictor of reduced exercise capacity in the COPD subjects, and was more sensitive and specific than percent-of-predicted FEV1. Statistical analysis generated the equation: percent-of-predicted peak VO2 = 65.9 IF + 0.45 percent-of-predicted FEV1 + 35.8 (R(C2) = 0.39, P < .001). The subjects with IF < 0.23 had more severe lung hyperinflation and less exercise capacity than the subjects with IF > 0.23. At peak exercise, the breathing frequencies of the 2 groups were similar, whereas the low-IF subjects had reduced peak minute ventilation and peak tidal volume, relative to the high-IF subjects.

Compared to FEV1, IF is a robust factor to reflect lung hyperinflation and to estimate the exercise capacity of subjects with stable moderate to severe COPD.