Evaluation of respiratory muscle strength by diaphragm ultrasound: normative values, theoretical considerations and practical recommendations

Background This prospective study proposed a standardized diaphragm ultrasound protocol to determine reference values for both diaphragm excursion and thickness in healthy subjects, and explored the association between volitional tests of respiratory muscle strength and diaphragm ultrasound parameters.

Methods 70 healthy adult subjects (25 men, 45 women; 34 ± 13 years) were included. After spirometric lung function testing and determination of maximal inspiratory (MIP) and expiratory pressure (MEP), diaphragm excursion and thickness of the right hemidiaphragm were evaluated by ultrasound. M-mode ultrasound of the posterior diaphragm was performed during tidal breathing (TB), voluntary sniff (VS) and deep breathing. Data were collected for amplitude and velocity of diaphragm displacement. 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: 1.5 cm (males; M) and 1.3 cm (females; F) for amplitude during TB, with velocity of 1.0 (M) and 1.0 (F) cm/sec; and 2.5 (M) and 2.1 (F) cm for amplitude during VS with velocity of 7.4 (M) and 5.6 (F) cm/sec. LLN for amplitude at TLC was 8.2 (M) and 7.0 (F) cm. 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. Values for males were consistently higher than for females, independent of age. LLN for the thickening ratio was 2.7 with no difference between genders. There was a close correlation between volitional inspiratory muscle strength and velocity of diaphragm excursion during VS. Normative data for Pdi during different breathing maneuvers are presented; statistical correlation between Pdi and diaphragm excursion velocity and amplitude during forced inspiration was documented.

Conclusions Diaphragm ultrasound was a feasible, reproducible and valid diagnostic tool for non-invasive assessment of volitional inspiratory muscle strength in healthy subjects.