Development of disability in chronic obstructive pulmonary disease: beyond lung function

Abstract

Background: COPD is a major cause of disability, but little is known about how disability develops in this condition.

Methods: The authors analysed data from the Function, Living, Outcomes and Work (FLOW) Study which enrolled 1202 Kaiser Permanente Northern California members with COPD at baseline and re-evaluated 1051 subjects at 2-year follow-up. The authors tested the specific hypothesis that the development of specific non-respiratory impairments (abnormal body composition and muscle strength) and functional limitations (decreased lower extremity function, poor balance, mobility-related dyspnoea, reduced exercise performance and decreased cognitive function) will determine the risk of disability in COPD, after controlling for respiratory impairment (FEV(1) and oxygen saturation). The Valued Life Activities Scale was used to assess disability in terms of a broad range of daily activities. The primary disability outcome measure was defined as an increase in the proportion of activities that cannot be performed of 3.3% or greater from baseline to 2-year follow-up (the estimated minimal important difference). Multivariable logistic regression was used for analysis.

Results: Respiratory impairment measures were related to an increased prospective risk of disability (multivariate OR 1.75; 95% CI 1.26 to 2.44 for 1 litre decrement of FEV(1) and OR 1.57 per 5% decrement in oxygen saturation; 95% CI 1.13 to 2.18). Non-respiratory impairment (body composition and lower extremity muscle strength) and functional limitations (lower extremity function, exercise performance, and mobility-related dyspnoea) were all associated with an increased longitudinal risk of disability after controlling for respiratory impairment (p<0.05 in all cases). Non-respiratory impairment and functional limitations were predictive of prospective disability, above-and-beyond sociodemographic characteristics, smoking status and respiratory impairment (area under the receiver operating characteristic curve increased from 0.65 to 0.75; p<0.001).

Conclusions: Development of non-respiratory impairment and functional limitations, which reflect the systemic nature of COPD, appear to be critical determinants of disablement. Prevention and treatment of disability require a comprehensive approach to the COPD patient.

Figure 1

Non-respiratory impairment/functional limitations increase the risk of incident disability in COPD. Receiver operating characteristic curves for the prospective risk of incident disability defined as an increase in the proportion of activities that the subject is unable to perform. Blue curve is for sociodemographic characteristics and smoking status (age, sex, race, height, educational attainment, household income, smoking status). Red line also includes respiratory impairment (FEV₁ and oxygen saturation). Green line includes all previous variables plus non-respiratory impairment (muscle strength, body composition) and functional limitations (lower extremity function, balance, exercise performance, mobility-related dyspnea, and cognitive impairment). Each curve was statistically different from the others (p<0.001). Figure 1a depicts the entire cohort. Figure 1b depicts the cohort with COPD GOLD Stage II or greater (more severe COPD).

Figure 1

Non-respiratory impairment/functional limitations increase the risk of incident disability in COPD. Receiver operating characteristic curves for the prospective risk of incident disability defined as an increase in the proportion of activities that the subject is unable to perform. Blue curve is for sociodemographic characteristics and smoking status (age, sex, race, height, educational attainment, household income, smoking status). Red line also includes respiratory impairment (FEV₁ and oxygen saturation). Green line includes all previous variables plus non-respiratory impairment (muscle strength, body composition) and functional limitations (lower extremity function, balance, exercise performance, mobility-related dyspnea, and cognitive impairment). Each curve was statistically different from the others (p<0.001). Figure 1a depicts the entire cohort. Figure 1b depicts the cohort with COPD GOLD Stage II or greater (more severe COPD).