The minimum effective analgesic volume of 0.5% bupivacaine for ultrasound-guided anterior suprascapular nerve block

Abstract

Objective: The interscalene brachial plexus block (ISBB) constitutes the gold standard for analgesia after shoulder procedures. Ipsilateral phrenic nerve block remains the most common adverse effect after ISBB. Alternative nerve blocks are performed in shoulder surgery in order to prevent hemi-diaphragmatic paralysis (HDP). The purpose of the present study was to investigate the minimum effective local anesthetic volume of 0.5% bupivacaine for postoperative analgesia with an anterior suprascapular nerve block (ASSB). The secondary aim was to investigate diaphragm functions with the local anesthetic doses used while conducting effective volume research. Method: This prospective observational study was conducted at the American Hospital of Istanbul, Turkey, from March to July 2022. The initial injected volume of 0.5% bupivacaine was 10 ml. Our clinical experience indicates that this yields a complete sensory block of the anterior suprascapular nerve. In accordance with the up-and-down method, the volume of 0.5% bupivacaine used for a particular patient was determined by the outcome of the preceding block, which represented block success. In case of effective ASSB being achieved, the volume of 0.5% bupivacaine to be administered to the next patient was lowered by 1 ml. In case of block failure, however, the volume of 0.5% bupivacaine to be applied in the subsequent case was increased by 1 ml. Ipsilateral hemi-diaphragmatic movement measurements were taken before (baseline) and 30 minutes after the block. General anesthesia was induced 60 minutes after the completion of the block performance by means of a standardized protocol. Results: Sixty-seven patients were included in the study. The ED50 and ED95 calculated for anterior suprascapular nerve block using probit transformation and logistic regression analysis were 2.646 (95% CI, 0.877-2.890) and 3.043 ml (95% CI, 2.771-4.065), respectively. When complete paralysis was defined as 75% or above, partial paralysis as 25-50%, and no paralysis as 25% or less, volumes of 6 ml or lower appeared to cause no paralysis for the anterior suprascapular nerve block. Conclusion: We, therefore, recommend using a volume of 6 ml or less in order to achieve diaphragm-sparing features for anterior suprascapular nerve blocks.