Ultrasonographic Assessment of the Diaphragm and the Effects of Smoking on Respiratory Function in Individuals Attending a Smoking Cessation Center

<b>Background</b>: Smoking adversely affects pulmonary function and systemic health; however, its impact on diaphragm muscle morphology and its relationship with functional capacity and psychosocial outcomes in individuals without clinically diagnosed respiratory disease remain unclear. This study aimed to evaluate diaphragm muscle thickness in smokers and to investigate its associations with pulmonary function, functional capacity, sleep quality, and depression. <b>Methods:</b> This cross-sectional observational study included 20 smokers and 20 age-matched never-smokers. Pulmonary function was assessed using spirometry. Functional capacity was evaluated with the 6-Minute Walk Test (6 MWT) and the 30 s sit-to-stand test (30 s STST). Sleep quality and depression were assessed using the Pittsburgh Sleep Quality Index (PSQI) and the Beck Depression Inventory (BDI). Inspiratory and expiratory diaphragm muscle thicknesses were measured by ultrasonography. Between-group comparisons and correlation analyses were performed. Results: Smokers exhibited significant impairments in all assessed parameters except expiratory diaphragm thickness compared with controls (<i>p</i> < 0.05). Large to very large effect sizes were observed for FEV₁, FEF_25-75%, functional capacity, and inspiratory diaphragm thickness. Inspiratory diaphragm thickness showed moderate to strong positive correlations with pulmonary function parameters and a very strong positive correlation with functional capacity, while strong negative correlations were observed with sleep quality and depression (<i>p</i> < 0.05). Smoking duration was strongly associated with poorer functional and psychosocial outcomes. <b>Conclusions:</b> Smoking is associated with early and multidimensional impairments in diaphragm muscle morphology, pulmonary function, functional capacity, and psychosocial status, even in individuals without overt respiratory disease. Reduced inspiratory diaphragm thickness may represent an early and clinically meaningful marker of smoking-related respiratory muscle dysfunction.