Diaphragm ultrasound and measures of respiratory muscle strength in healthy subjects: normal values, theorerical considerations and practical recommendations

Abstract

Background: This study proposed a standardized diaphragm ultrasound protocol to determine reference values for both diaphragm excursion and thickness in healthy subjects, and explored its association with invasively obtained volitional inspiratory muscle strength. Methods: 70 healthy adult subjects (25 men, 45 women; 34±13 years) were included. Diaphragm excursion velocity and amplitude were evaluated by M-mode ultrasound of the right posterior diaphragm during voluntary sniff (VS). Diaphragm thickness was measured in the zone of apposition at total lung capacity (TLC) and functional residual capacity (FRC). All participants underwent invasive measurement of transdiaphragmatic pressure (Pdi) during different voluntary breathing maneuvers. Results: Ultrasound data were successfully obtained in all participants (procedure duration 12±3 min/person). Lower limit of normal (LLN) values for diaphragm excursion were: for VS velocity 7.4 (M) and 5.6 (F) cm/sec. LLN for diaphragm thickness was 0.19 (M) and 0.16 (F) cm at FRC, and 0.56 (M) and 0.43 (F) cm at TLC. There was a close correlation between invasively obtained volitional inspiratory muscle strength and velocity of diaphragm excursion during VS. Conclusions: Diaphragm ultrasound is a feasible diagnostic tool for non-invasive assessment of volitional inspiratory muscle strength in healthy subjects.