Clinical impact of time of day on acute exercise response in COPD

The purpose of this pilot study was to determine the impact of time of day on the acute response to incremental exercise in chronic obstructive pulmonary disease (COPD). Fourteen subjects (nine men) aged 71 ± 7 years with moderate to severe airflow obstruction (FEV1: 58 ± 13% predicted) followed a counterbalanced randomized design, performing three symptom-limited incremental cycling tests at 8:00, 12:00, and 16:00 hours on different days, each preceded by a spirometry. COPD medications were withdrawn prior to testing. No overall time effect was found for peak exercise capacity (p = 0.22) or pulmonary function (FEV1, p = 0.56; FVC, p = 0.79). However, a large effect size (f = 0.48) was observed for peak exercise capacity and several pulmonary function parameters. For peak exercise capacity, the average within-subject coefficient of variation was 5.5 ± 3.9% and the average amplitude of change was 7 ± 5W. Seven subjects (50%) showed diurnal changes at levels equal to or beyond the minimal clinically important difference for both peak exercise capacity and pulmonary function. In this sub-group, peak exercise capacity was greatest at 16:00 hours (p = 0.03, ƒ = 1.04). No systematic time-of-day effect on peak exercise capacity was obtained in COPD patients in the present pilot study. However, based on the observed effect size and on the average amplitude of change and within-subject variations seen across testing times, the guidelines recommendation that time of day be standardized for repeat exercise testing in COPD should be maintained.