Quantification of muscle tone using shear-wave velocity during anaesthesia induction: a prospective observational study

Abstract Purpose Shear-wave elastography is a non-invasive ultrasound method that provides quantitative information on the elasticity of soft tissues. However, a universal velocity scale for quantification has yet to be developed. This study aimed to determine the shear-wave velocities (SWVs) of the abdominal muscles during anaesthesia induction for future development of a universal scale. Methods We enrolled 75 adult patients who received elective surgery with an American Society of Anesthesiologists physical status of I–III from December 2018 to March 2021. We measured and calculated the SWVs before and after opioid administration (baseline at rest and opioid-induced rigidity condition) and after muscle relaxant administration (zero reference condition). The SWV value was adjusted for subcutaneous fat thickness using our proposed corrections. The SWVs after the adjustment were compared among the values at baseline, opioid-induced rigidity, and complete muscle relaxation using one-way repeated-measures analysis of variance and post-hoc Tukey–Kramer tests. Significance was set at p < 0.05. Results The SWVs in the baseline, opioid-induced rigidity, and complete muscle relaxation conditions after adjustment were 2.08 ± 0.48, 2.41 ± 0.60, and 1.79 ± 0.30 m/s, respectively (p < 0.001 for all comparisons). Conclusion The SWV as reference was 1.8 m/s, and the SWVs at rest and opioid-induced rigidity were ~ 10% and ~ 30% higher than the reference, respectively. SWV adjusted for subcutaneous fat thickness may be a scale point for the assessment of muscle tone. Clinical trial registration: UMIN Clinical Trials Registry identifier UMIN000034692, registered on October 30th, 2018