Correlation Between Ultrasound-Derived Diaphragmatic Thickness, Inspiratory Muscle Strength and Peak Expiratory Flow in Institutionalized Older Adults: A Cross-Sectional Exploratory Study

Background: Diaphragm thickness may affect both inspiratory and expiratory function. The ultrasound assessment of diaphragm thickness could be a valuable tool for respiratory muscle evaluation in older adults, although additional research is needed to clarify its association with respiratory functional parameters. Objectives: The primary objective was to explore the relationship between diaphragm thickness (DT), maximal inspiratory pressure (MIP), and peak expiratory flow (PEF) in institutionalized older adults. A secondary objective was to compare MIP and PEF values between participants with normal and thin diaphragm thickness. Methods: A cross-sectional exploratory study was conducted among older adults (n = 36) residing in long-term care facilities (LTCFs) in Spain. DT was measured at the end of maximum inspiration (DTi) and at the end of expiration at rest (DTe) in the supine position. The diaphragm thickening difference (DTD) and diaphragm fraction (DTF) were calculated. A thin diaphragm was defined as a diaphragm thickness (DTe) <1.6 mm in men and <1.4 mm in women. The maximum inspiratory pressure (MIP) was measured using a Powerbreath KH1 device, and the peak expiratory flow (PEF) was measured using Datospir Peak-10. Spearman’s rank correlation coefficient and linear regression models were used to explore the association between diaphragm thickness and respiratory muscle function. Statistical analysis was performed using IBM SPSS Statistics 22.0. Results: The mean age of the participants was 82.4 ± 7.9 years (24 females–12 males). Significant correlations were observed among most ultrasound-derived measures. DTi was positively associated with MIP (ρ = 0.403, 95% CI 0.065–0.658; q = 0.033) and PEF (ρ = 0.534, 95% CI 0.235–0.741; q = 0.003), DTe with MIP (ρ = 0.445, 95% CI 0.116–0.686; q = 0.020), and DTD with PEF (ρ = 0.475, 95% CI 0.158–0.703; q = 0.012). Compared with participants with normal diaphragm thickness, those with a thin diaphragm exhibited lower MIP (β = −18.43, 95% CI: −32.76 to −4.18; p = 0.013). Conclusions: Diaphragm thickness was positively associated with inspiratory muscle strength and peak expiratory flow. Participants with a thin diaphragm demonstrated lower respiratory muscle performance than those with normal diaphragm thickness. However, these findings should be interpreted with caution, and additional studies are needed to confirm these associations.