Cardiopulmonary and Gas-Exchange Responses during the Six-Minute Walk Test in Patients with Chronic Obstructive Pulmonary Disease

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Abstract
Background: The 6-min walk test (6MWT) is representative of daily life activities and reflects the functional capacity of patients with chronic obstructive pulmonary disease (COPD). Information on the cardiopulmonary and gas exchange responses to the 6MWT is limited. Objectives: We aimed to analyze the breath-by-breath cardiopulmonary and gas exchange responses of patients with COPD during the 6MWT. We also investigated the extent to which parameters reflecting cardiopulmonary and gas exchange function are associated with exercise capacity. Methods: The oxygen uptake (VO2) kinetics of patients were obtained using mobile telemetric cardiopulmonary monitoring during a 6MWT. A new mean response time (MRT) index was developed to quantify VO2 on-kinetics by correcting MRT for work rate (wMRT). Multiple linear regression analysis was performed to assess the association between variables reflecting cardiopulmonary and gas exchange function and exercise capacity [6-min walking distance (6MWD) and VO2 at steady state (VO2SS)]. Results: In 72 COPD patients (29 females) with a mean (SD) age of 65 (10) years, FEV1 44 (14) % predicted exercise capacity as assessed by VO2SS (p = 0.003) was significantly reduced across the stages of COPD. The criteria for maximal effort during the 6MWT were fulfilled by 82% of the patients. After adjustment for covariates, wMRT was independently associated with 6MWD (p = 9.7 × 10-5) and VO2SS (p = 5.5 × 10-10). Conclusions: As wMRT mostly depends on the rate of increase of pulmonary blood flow, our results underline the fact that cardiocirculatory function may play a significant role in exercise tolerance in patients with COPD. Our findings imply that modification of cardiocirculatory function may be beneficial in the treatment of COPD patients and improve their outcome more than anticipated previously.

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