Resting Hyperinflation Predicts Incremental Shuttle Walk Distance in Chronic Obstructive Pulmonary Disease

Rationale: The incremental shuttle walk test (ISWT) correlates closely with peak oxygen uptake in COPD and relates to important outcomes such as mortality, response to treatment and hospital readmission. Despite this, there is limited data on the physiological determinants of ISWT distance (ISWD) in COPD. Methods: In this exploratory, prospective observational study, spirometry, lung volumes, diffusion capacity (DLCO) and oscillometry were performed in patients with confirmed COPD. Patients then completed two ISWT with the results of the best test, measured by distanced walked taken. The determinants of ISWD and dyspnoea measured by BORG score were evaluated. Results: 25 COPD patients, mean (SD) age 71 (8.82) years, 48% female with a mean (SD) FEV1 Z-score -2.54 (0.83) were recruited. Median (IQR) ISWD was 350 (210-440) metres (mean (SD) 66.4 (27.9)% predicted distance). Most patients (85%) stopped due to inability to maintain walking speed with submaximal mean heart rate of 77.3 (10.1)% predicted and BORG dyspnoea score of 'severe' (median 5/10 (IQR 4-5.5)). Inspiratory capacity to TLC ratio (IC/TLC) correlated strongly with ISWD, even when corrected for age and height (r_s = 0.59 p = 0.02). Oscillatory reactance (Xrs₅) and DLCO were also correlated with ISWD. There were no oscillometric or spirometric predictors of dyspnoea. Conclusion: Resting hyperinflation measured by IC/TLC, predicted ISWD despite submaximal dyspnoea, suggesting that hyperinflation may not be the mechanism that limits exercise performance, but rather reflects overall impairment in COPD.