Prospective development of disability was a common occurrence in our cohort of younger adults with COPD during the two year follow-up period (approximately 1 in 10 subjects). Although respiratory impairment increased the longitudinal risk of disability, the development of non-respiratory impairment and functional limitations in body systems remote from the lung had a greater impact on disablement. Muscle strength, lower extremity function, exercise performance, and mobility-related dyspnea were potent risk factors for disability, even after taking lung function impairment into account. These results require a paradigm shift in COPD: the assessment and treatment of airway obstruction, which have been the cornerstones of treatment, will not be sufficient to prevent the development of COPD-related disability.
Although cross-sectional studies have found a high prevalence of activity restriction in COPD, longitudinal estimates of disablement are rare. 4–10
The SUPPORT study reported that more than half of adults hospitalized for COPD exacerbation subsequently had diminished ability to perform activities of daily living.10
In addition, most studies of COPD-related disability have focused on a restricted range of daily activities, such as activities of daily living which are necessary for survival.4–10
Other studies are limited by small sample size and focus on severe COPD.4–10
Consequently, our adds important new information by prospectively elucidating the development of disability using a broad measure of daily activities in a cohort with a wide range of disease severity.
Our study advances the field because it systematically evaluated the impact of extra-pulmonary impairment and functional limitations on the prospective risk of disability in COPD, after accounting for respiratory impairment. Other studies have individually found that lung function, muscle strength, or exercise capacity are related to performance of daily activities.30, 62–66, 67
But none of these studies evaluated disability of a broad range of daily activities, comprehensively evaluated a functional limitations, and ascertained prospective disability endpoints. Consequently, our work builds on these previous studies and establishes that non-respiratory impairment and physical functional limitations are the main drivers of the disablement process in COPD.
Our study has several limitations. There is some possibility of misclassification of COPD, although we performed rigorous steps to avoid it. The inclusion criteria required a physician diagnosis of COPD, health care utilization for COPD, and dispending of COPD medications, which was designed to increase the accuracy of case ascertainment. We also previously demonstrated the validity of our approach using medical record review.16
Nonetheless, we acknowledge this potential limitation.
Although we had excellent cohort retention (90% of living subjects were re-interviewed), it is possible that selection bias could have been introduced by losses to follow-up by death or other factors. For example, there were some differences in sociodemographic characteristics by follow-up status. There were no differences in age or lung function, but the retained cohort had somewhat higher socioeconomic status. Because lower social class is associated with a greater risk of poor health status and disability, our results likely underestimate the development of COPD-related disability. To the extent that functional limitations are greater in the group without follow-up, the analysis would also underestimate the impact of functional limitations on the risk of disability. Consequently, any bias introduced would be conservative.
Because our goal is ultimately disability prevention, we intentionally recruited younger subjects with COPD (aged 45–65 years). As a result, this age range may limit conclusions about elderly persons with COPD. Moreover, our patients were all insured with access to health care services. Our results may not fully apply to persons who are not receiving treatment for COPD. The demographic and socioeconomic characteristics of Northern California Kaiser Permanente members, however, are similar to those of the regional population.68
There is also no evidence of systematic inclusion or exclusion of healthy persons into the KP system.69
Overall, KPMCP members are likely similar to the general U.S. population.
By elucidating the pathway to COPD-related disability, our goal is to provide a scientific basis for the screening and prevention of COPD-related disability. Although measurement of lung function, which is a cornerstone of clinical practice guidelines, predicts disability, it does not by itself fully characterize disability risk. Development of non-respiratory impairment and functional limitations, which reflect the systemic nature of COPD, are critical determinants of disablement. Consequently, medical management may need to be complemented by comprehensive rehabilitative strategies aimed at the diverse extra-pulmonary manifestations of COPD to prevent disability and restore of function.