Minimum effective volume 90% for ultrasound-guided selective trunk block: A quantal bioassay.

Background: Selective trunk block (SeTB) targets the three trunks of the brachial plexus and produces surgical anaesthesia of the entire upper extremity, including the shoulder. However, the optimal dose of local anaesthetic (LA) required for a SeTB is not known.

Objective: This study aimed to evaluate the minimum effective volume 90% (MEV90) of LA required for a SeTB.

Design: Quantal bioassay.

Setting: Single-centre, University hospital in Hong Kong S.A.R, China.

Patients: After ethical approval, 25 ASA I to III patients, aged 18 to 75 years, undergoing upper extremity surgery under an ultrasound-guided (USG) SeTB were included.

Intervention: The volume of the LA (1 : 1 mixture of 2% lidocaine with 5 μg ml-1 epinephrine and 0.5% levobupivacaine) used, starting with 21 ml, was assigned by either increasing or decreasing 3 ml (1 ml each trunk), based on the modified Narayana sequential design.

Main outcome measures: Readiness for surgery (sensory score ≤30 and motor score ≤1) at or within 30 min after the injection was the primary outcome measure. A successful block was defined as achieving readiness for surgery within 30 min and completing the intended surgery without rescue analgesia or conversion to general anaesthesia. The MEV90 was estimated using centred isotonic regression.

Results: The MEV90 of the LA mixture for USG SeTB was 24.7 [95% confidence interval (CI), 23.8 to 28.9] ml. The median [IQR] time to 'readiness for surgery' and 'complete anaesthesia' was 6.6 [5 to 12.3] and 15 [7.3 to 18] minutes, respectively.

Conclusion: The MEV90 of a 1 : 1 LA mixture for USG SeTB to produce surgical anaesthesia of the entire upper extremity (except for the T2 dermatome), using readiness for surgery as the success criterion, is 24.7 ml (95% CI, 23.8 to 28.9) ml.

Trial registration: ClinicalTrials.gov, Trial Registration No: NCT04773405, Registered prospectively on 26 February 2021. Date of patient enrolment: 10 March 2021 (https://classic.clinicaltrials.gov/ct2/show/NCT04773405).