Abstract
Asthma and chronic obstructive pulmonary disease co-exist in a significant proportion of patients. Whether asthma increases mortality risk among subjects with airflow limitation remains controversial. We used data from 2121 adult participants in the population-based Tucson Epidemiological Study of Airway Obstructive Disease cohort. At enrolment (1972-1973), participants completed questionnaires and lung function tests. Participants were categorised into four groups based on the combination of airflow limitation (AL; forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) <70%) and physician-confirmed asthma at baseline. Vital status as of January 2011 was assessed through the National Death Index. Cox proportional hazards models were used to test differences in mortality risk across the four airflow limitation/asthma groups. In multivariate Cox models, the AL+/asthma+ group had a 114% increased mortality risk during follow-up compared with the AL-/asthma- group (adjusted HR 2.14; 95% CI 1.64-2.79). The corresponding hazard ratios were 1.09 (95% CI 0.89-1.34) and 1.34 (95% CI 1.14-1.57) for the AL-/asthma+ and AL+/asthma- groups, respectively. Among subjects with airflow limitation, asthma was associated with increased mortality risk (HR 1.58, 95% CI 1.17-2.12). However, this increased risk was substantially reduced and no longer significant after further adjustment for baseline FEV1 levels. Similar results were obtained when airflow limitation was defined as FEV1/FVC less than the lower limit of normal. In a population-based cohort, subjects with concomitant airflow limitation and asthma had an increased risk of dying, which was mainly related to their baseline lung function deficits.