CT pectoralis muscle area is associated with DXA lean mass and correlates with emphysema progression in a tobacco-exposed cohort.

INTRODUCTION: Muscle loss is an important extrapulmonary manifestation of COPD. Dual energy X-ray absorptiometry (DXA) is the method of choice for body composition measurement but is not widely used for muscle mass evaluation. The pectoralis muscle area (PMA) is quantifiable by CT and predicts cross-sectional COPD-related morbidity. There are no studies that compare PMA with DXA measures or that evaluate longitudinal relationships between PMA and lung disease progression. METHODS: Participants from our longitudinal tobacco-exposed cohort had baseline and 6-year chest CT (n=259) and DXA (n=164) data. Emphysema was quantified by CT density histogram parenchymal scoring using the 15th percentile technique. Fat-free mass index (FFMI) and appendicular skeletal mass index (ASMI) were calculated from DXA measurements. Linear regression model relationships were reported using standardised coefficient (ß) with 95% CI. RESULTS: PMA was more strongly associated with DXA measures than with body mass index (BMI) in both cross-sectional (FFMI: ß=0.76 (95% CI 0.65 to 0.86), p<0.001; ASMI: ß=0.76 (95% CI 0.66 to 0.86), p<0.001; BMI: ß=0.36 (95% CI 0.25 to 0.47), p<0.001) and longitudinal (ΔFFMI: ß=0.43 (95% CI 0.28 to 0.57), p<0.001; ΔASMI: ß=0.42 (95% CI 0.27 to 0.57), p<0.001; ΔBMI: ß=0.34 (95% CI 0.22 to 0.46), p<0.001) models. Six-year change in PMA was associated with 6-year change in emphysema (ß=0.39 (95% CI 0.23 to 0.56), p<0.001) but not with 6-year change in airflow obstruction. CONCLUSIONS: PMA is an accessible measure of muscle mass and may serve as a useful clinical surrogate for assessing skeletal muscle loss in smokers. Decreased PMA correlated with emphysema progression but not lung function decline, suggesting a difference in the pathophysiology driving emphysema, airflow obstruction and comorbidity risk.