Thoracic ratio and diaphragmatic excursion predict lung function impairment in COPD patients

To investigate the correlation between the thoracic transverse diameter/anteroposterior diameter (TTD/ATD) ratio and diaphragmatic excursion during deep breathing (DE-DB) in patients with chronic obstructive pulmonary disease (COPD), to evaluate their predictive value for lung function impairment, and to elucidate the underlying mechanism and clinical utility using mediation and decision curve analysis (DCA). This cross-sectional study enrolled 120 COPD patients from a tertiary hospital between June 2024 and June 2025. All pulmonary function measurements were obtained pre-bronchodilation, with each maneuver repeated at least three times to ensure reproducibility. The TTD/ATD ratio and ultrasonographic diaphragmatic parameters were measured. Analyses included Pearson correlation, multiple linear regression, logistic regression, mediation effect analysis, and DCA. Both the TTD/ATD ratio and DE-DB significantly decreased with increasing COPD severity ( P < 0.001). A significant positive correlation was observed between TTD/ATD and DE-DB ( r = 0.569, P < 0.001). Both parameters were also significantly positively correlated with FEV1/FVC and FEV1% predicted (r values ranging from 0.73 to 0.81, all P < 0.001). Mediation analysis revealed that DE-DB partially mediated the relationship between TTD/ATD and FEV1% predicted, accounting for 26.2% of the total effect. The combined predictive model (TTD/ATD + DE-DB) achieved an area under the curve (AUC) of 0.963, which was significantly superior to either parameter alone ( P < 0.05). DCA demonstrated that the combined model provided a higher net benefit across a wide range of threshold probabilities. Joint variable analysis stratified patients into four risk tiers. The dual-high-risk group (Q1) had a 20.83-fold higher risk (OR = 0.048, P < 0.001) of severe COPD compared to the dual-low-risk group (Q4). Thoracic configuration is closely associated with diaphragmatic function in COPD patients, and both are independent predictors of lung function. Diaphragmatic function plays a significant partial mediating role in the impact of thoracic configuration on lung function. The combined assessment of these two non-invasive measures enables effective risk stratification, offering a powerful tool for the early identification and personalized management of COPD.