COPD was related to a broad array of physical functional limitations compared to a matched referent group without the disease, including lower extremity functioning, exercise performance, skeletal muscle strength, and self-reported limitation in basic physical actions. These physical functional limitations are directly attributable to COPD, because patients with and without COPD were recruited from the same source population of managed care patients and were matched by age, sex, and race. For distance walked in 6 minutes, the COPD-related decrement amounted to more than the length of a football field.
These findings are important because they indicate that COPD has impacts on diverse body systems remote from the lung. Although previous studies have characterized physical function in COPD patients, they have not been able to estimate a broad array of decrements specifically attributable to the disease. Prior studies have either evaluated a limited spectrum of function or lacked an appropriate referent group.16, 33-46
Moreover, many of these studies included a highly selected group of persons with COPD, often on the severe end of the disease spectrum. These results from the FLOW study advance the field by establishing the multiple physical functional decrements that are directly attributable to COPD among persons with a broad range of disease severity.
In our theoretical framework, which is based on the work of Verbrugge and Jette, the development of functional limitations is the first step in the pathway to developing disability in COPD.5
Poorer lower extremity function, submaximal exercise performance, and muscle strength may result in disability from the disease. We are prospectively following the matched cohort of patients with and without COPD to determine the eventual impact of functional limitations on disability.
The role of cardiovascular comorbities in the development of functional limitations in COPD is complex, because both conditions share risk factors (e.g., smoking) and COPD itself appears to predispose to adverse cardiovascular outcomes.7,32
When we controlled for cardiovascular comorbidities and diabetes, the estimates for functional limitation attributable to COPD were only slightly attenuated. It is likely that these analyses overadjusted the risk estimates. Future longitudinal analyses may better elucidate this complex inter-relationship.
A study strength is the large sample of COPD patients who have a broad spectrum of disease severity, ranging from mild to severe. Our study is, to our knowledge, the largest prospective COPD cohort study to systematically evaluate a broad range of functional limitations compared to an appropriate matched control group. Recruitment from a large health plan should also help ensure genearlizability to patients who are being treated for COPD in clinical practice. Availability of interview data, pulmonary function tests, and extensive physical characterization allows robust conclusions about physical functioning.
Our study is also subject to several limitations. Although the inclusion criteria required health care utilization for COPD, misclassification of asthma could have occurred. Our COPD definition required concomitant treatment with COPD medications to increase the specificity of the definition. In addition, all patients had a physician diagnosis of COPD and reported having the condition. The observed lifetime smoking prevalence was similar to that in other population-based epidemiologic studies of COPD, supporting the diagnosis of COPD rather than asthma.47, 48
We also previously demonstrated the validity of our approach using medical record review.7
When we limited the definition of COPD to more severe disease (GOLD Stage II or greater), however, the results were not substantively affected. In sum, we have a high degree of confidence that our results are not affected by misclassification bias.
Because our ultimate focus is on disability prevention, we intentionally sampled younger adults with COPD. Therefore, these results may underestimate the impact of COPD on functional limitation among older patients. In addition, Kaiser Permanente members, because they have health care access, may also be different than the general population of adults with COPD. Mitigating these limitations, the sociodemographic characteristics of Northern California Kaiser Permanente members are similar to those of the regional population, with some under-representation of income extremes.49, 50
Moreover, selection bias could have been introduced by non-participation in the study. There were some differences among subjects who did and did not participate in the interviews, but they were modest in scope and not likely to affect the results.
In conclusion, a broad array of physical functional limitations could be attributed directly to COPD. Prospective follow-up will determine the impact of these functional limitations on the risk of disability. Strategies to prevent COPD, or to treat it effectively, have potential to reduce physical functional limitation, and perhaps disability, in the general population.