Respiratory Muscle Strength and Imaging Parameters in Smokers: A Prospective Observational Study

Respiratory muscle strength tests are commonly used in respiratory rehabilitation as an indicator of training in patients with chronic obstructive pulmonary disease (COPD). Although respiratory muscle strength tests are associated with lung function, their relationship with imaging parameters has not been fully investigated. Therefore, we aimed to reveal the relationship between imaging parameters and respiratory muscle strength and determine the usefulness of respiratory muscle strength tests as an indicator of the pathophysiology of dynamic pulmonary hyperinflation in smokers.

In this single-center prospective observational study conducted in Japan, 48 patients with COPD and 10 non-COPD smokers were included in the final analysis. Most of the participants were men and the median age was 77 [interquartile range 70.8-80.1]. Respiratory muscle strength tests, pulmonary function tests, and inspiratory and expiratory chest computed tomography were performed. Quantitative imaging parameters of gas trapping and emphysema were measured using the disease probability measure (DPM), a voxel-wise image analysis.

Maximal inspiratory and expiratory pressures (PImax and PEmax) were negatively correlated with residual volume/total lung capacity (Spearman's rank correlation coefficient (r): -0.37, -0.40, respectively and p-value (p): <0.01), but not with forced expiratory volume in 1 s. PImax and PEmax were negatively correlated with gas-trapping lesions recognized by DPM (DPM_GasTrap) in the single correlation analysis (r: -0.31, -0.37 and p: <0.05, <0.01, respectively), and the values for DPM_GasTrap significantly differed between the high and low PImax and PEmax groups.

Respiratory muscle strength reflects the degree of gas trapping, not the degree of obstructive airflow limitation in smokers. It proves particularly valuable, especially during the initial phases of COPD, before emphysematous changes manifest.