Trainee anaesthetist diagnosis of intraneural injection. A study comparing B-Mode ultrasound with the fusion of B-Mode and elastography in the soft embalmed Thiel cadaver model

Background: The incidence of intraneural injection during trainee anaesthetist ultrasound guided nerve block varies between 16% in experts and up to 35% in trainees. We hypothesised that elastography, an ultrasound-based technology that presents colour images of tissue strain had the potential to improve trainee diagnosis of intraneural injection during UGRA when integrated with B-Mode ultrasound onto a single image. Methods: We recorded 40 median nerve blocks randomly allocated to 0.25ml, 0.5ml, 1ml volumes to 5 sites on both arms of two soft embalmed cadavers using a dedicated B-Mode ultrasound and elastography transducer. We wrote software to fuse elastogram and B-Mode videos then asked 20 trainee anaesthetists whether injection was intraneural or extraneural when visualising B-Mode videos, adjacent B-Mode and elastogram videos, fusion elastography videos or repeated B-Mode ultrasound videos. Results: Fusion elastography improved the diagnosis of intraneural injection compared to B-Mode ultrasound, Diagnostic Odds Ratio (DOR) (95%CI) 21.7 (14.5 - 33.3) versus DOR 7.4 (5.2 – 10.6), P<0.001. Compared to extraneural injection, intraneural injection was identified on fusion elastography as a distinct, brighter translucent image, geometric ratio 0.33 (95%CI: 0.16 – 0.49) P<0.001. Fusion elastography was associated with greater trainee diagnostic confidence, OR (95%CI) 1.89 (1.69 – 2.11), P<0.001, and an improvement in reliability, Kappa 0.60 (0.55 - 0.66). Conclusions: Fusion elastography improved the accuracy, reliability and confidence of trainee anaesthetist diagnosis of intraneural injection.