Respiratory-Related Quality of Life: Relation to Pulmonary Function, Functional Exercise Capacity, and Sputum Biophysical Properties

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Abstract
One of the difficulties in assessing mucoactive therapy is selecting clinical outcome variables that reflect the impact of clearing airway secretions on quality of life (QOL). Petty and colleagues developed a questionnaire designed to evaluate the clinical impact of mucoactive therapy in patients with chronic bronchitis (CB). We evaluated this questionnaire in a multicenter study of a mucolytic medication used in patients with CB and hypothesized that spirometry, exercise capacity, and sputum clearability changes would correlate with QOL changes. This was a multicenter trial in 159 patients with stable CB (111 completed the 16-week study). Spirometry, plethysmography, the 6-minute walk test (6MWT), and Petty score as a measure of QOL were assessed at each visit. Sputum was collected at each visit. Cough transportability was measured in a cough machine, and mucociliary transportability was measured on the frog palate. Cohesivity was measured in a filancemeter, interfacial tension by de Noüy ring, and wettability by contact angle analysis. Within the entire data set of 694 evaluations, there was no correlation between pulmonary function and QOL. There was an inverse correlation with distance covered in a 6MWT (R2 = 0.041, p < 0.0001). Sputum CTR was directly correlated with QOL (R2 = 0.027, p < 0.0001). Change from baseline (mean of first three visits) was computed and compared the change in the mean of values at the 8- and 12-week visits (n = 108 sets of data pairs). This was analyzed as a percentage of change for continuous measurements, and as QOL is normative, we calculated the absolute change in QOL. There was no relation between QOL and 6MWT changes. There was an inverse relation between change in forced expiratory volume in 1 second and QOL (R2 = 0.092, p = 0.0021) as well as between forced vital capacity and QOL (R2 = 0.05, p = 0.024). There was a direct relation between CTR and QOL (R2 = 0.039, p = 0.048). The relation between QOL and 6-minute walk distance was expected but weak. The consistent relation between CTR and QOL (suggesting that improved CTR of sputum is associated with decreased QOL) is difficult to explain. A change in forced expiratory volume in 1 second and forced vital capacity did correlate with a change in QOL. There is a need for a good QOL tool to evaluate mucus clearance devices or medications. The Petty questionnaire was designed specifically for this task, but the effect on sputum properties by current mucoactive agents may be too small to elicit a significant change in the Petty score.