The impact of local anesthetic distribution on block onset in ultrasound-guided interscalene block

Background Recent investigations of local anesthetic distribution in the lower extremity have revealed that completely surrounding the sciatic nerve with local anesthetic provides the advantage of more rapid and complete anesthesia in the territory served by the nerve. We hypothesized that a pattern of distribution that entirely envelops the targeted nerve roots during interscalene block would provide similar benefits of more rapid anesthesia onset. Methods During interscalene block guided by ultrasound with nerve stimulator confirmation, the pattern of local anesthetic distribution was recorded and later classified as complete or incomplete envelopment of the visible nerve elements in 50 patients undergoing ambulatory shoulder arthroscopic surgery. The pattern was then compared with the extent of block setup at pre‐determined intervals, as well as to post‐operative pain levels and block duration. Results Twenty‐two patients (44%) had complete envelopment of the nerves in the plane of injection during ultrasound imaging of the interscalene block. There was no difference in the fraction of blocks that were fully set‐up at 10 min with regards to complete or incomplete envelopment of the nerves by local anesthetic. All of the patients had complete setup of the block by 20 min. In addition, the post‐operative pain levels and duration of block did not vary among the two groups with complete vs. incomplete local anesthetic distribution around the nerves. Conclusion The presence or absence of complete envelopment of the nerve elements in the interscalene groove by local anesthetic did not determine the likelihood of complete block effect at pre‐determined time intervals after the procedure.