Diaphragm Thickness Is Correlated To Respiratory Muscle Strength And Whole-body Muscle Mass In Young Adults

Ultrasound (US) is a non-invasive imaging technique that can be used to examine functional and morphological characteristics of the diaphragm. This technique is utilized to predict weaning success in mechanically ventilated patients, however, to what extent US-determined characteristics of the diaphragm are related to respiratory function in healthy adults is not well understood. PURPOSE: To examine associations between US-determined characteristics of the diaphragm and measures of respiratory muscle strength (maximal inspiratory and expiratory pressures). METHODS: Twenty-four healthy individuals volunteered (13 M, 11F; 23 ± 3 yr; 172 ± 9 cm; 74 ± 15 kg). While lying supine, US (Terason 3200 T) images of the right hemidiaphragm were obtained to determine diaphragm thickness (B-mode, mid-axillary line) and diaphragm excursion (m-mode, between mid-clavicular and axillary lines, below right costal margin) during tidal expiration, tidal inspiration, and maximal inspiration. Respiratory muscle strength was assessed by maximal inspiratory (MIP) and expiratory pressures (MEP, Cosmed Pony FX). Body composition was measured via bioelectrical impedance. RESULTS: Collapsed across both sexes, diaphragm thickness at end of expiration and at end of maximal inspiration showed positive correlations with MIP (r = 0.420 and 0.455, respectively, P < 0.05). Diaphragm thickness at end of expiration was also positively correlated with body mass (kg) (r = 0.543, P < 0.01) and estimated whole-body muscle mass (kg) (r = 0.648, P < 0.001). No correlations were found between any measures of diaphragm excursion and respiratory muscle strength (P > 0.05). Interestingly, males showed greater diaphragm thickness at end of expiration compared to females (0.21 ± 0.03 vs. 0.15 ± 0.03 cm, P < 0.001). CONCLUSION: These preliminary findings suggest that US-determined measures of diaphragm thickness are related to respiratory muscle strength in healthy young adults (primarily MIP), and that US may be capable of identifying sex-based differences. Future research is needed to identify the impact of exercise or other interventions on morphological and functional characteristics of the diaphragm.