Ultrafast ultrasound coupled with cervical magnetic stimulation for non-invasive and non-volitional assessment of diaphragm contractility.

Measuring twitch transdiaphragmatic pressure (Pdi,tw ) elicited by cervical magnetic stimulation (CMS) is considered as a reference method for the standardized evaluation of diaphragm function. Yet, the measurement of Pdi requires invasive oesophageal and gastric catheter-balloons. Ultrafast ultrasound is a non-invasive imaging technique enabling frame rates high enough to capture transient events such as evoked muscle contractions. This study investigated relationships between indices derived from ultrafast ultrasound and Pdi,tw , and how these indices might be used to estimate Pdi,tw . CMS was performed in 13 healthy volunteers from 30% to 100% of maximal stimulator intensity in units of 10% in a randomized order. Pdi,tw was measured and the right hemidiaphragm was imaged using a custom ultrafast ultrasound sequence with 1 kHz framerate. Maximal diaphragm axial velocity (Vdi ,max ) and diaphragm thickening fraction (TFdi,tw ) were computed. Intra-session reliability was assessed. Repeated-measures correlation (R) and Spearman correlation coefficients (ρ) were used to assess relationships between variables. Intra-session reliability was strong for Pdi,tw and Vdi,max and moderate for TFdi,tw . Vdi,max correlated with Pdi,tw in all subjects (0.64 < ρ < 1.00, R = 0.75; all P < 0.05). TFdi,tw correlated with Pdi,tw in eight subjects only (0.85 < ρ < 0.93, R = 0.69; all P < 0.05). Coupling ultrafast ultrasound and CMS shows promise for the non-invasive and fully non-volitional assessment of diaphragm contractility. This approach opens up the prospect of both diagnosis and follow-up of diaphragm contractility in clinical populations.