Efficacy of long-acting local anesthetics versus their mixture with shorter-acting local anesthetics for peripheral nerve blocks guided by ultrasound: a systematic review with meta-analysis of randomized controlled trials.

Background/importance: Local anesthetic (LA) mixtures are used in peripheral nerve blocks (PNB) to improve onset, though study results remain conflicting.

Objective: This systematic review and meta-analysis compared the efficacy outcomes of long-acting LA to their mixture with shorter-acting LA in ultrasound-guided PNB. The primary outcome was sensory block onset.

Evidence review: We searched WoS, Scopus, MEDLINE, EMBASE, BVS/LILACS, and Cochrane databases from 1998 to 2024 for randomized controlled trials (RCTs). We conducted a random-effects meta-analysis, evaluated the risk of bias (RoB) with RoB 2.0, performed sensitivity analyses, assessed non-reporting bias with DOI plots and Luis Furuya-Kanamori index, and evaluated strength of evidence with Grading of Recommendations Assessment, Development and Evaluations.

Findings: We included 10 RCTs (516 participants). Mixture of LA may have no effect on sensory block onset (mean difference (MD) -1.62 min, 95% CI: -4.04 to 0.81; I²=81.50%, 95% CI: 62.82% to 90.80%; prediction interval (PI)=-7.78 to 4.55; very low certainty) and motor block onset (MD -5.60 min; 95% CI: -14.54 to 3.33, I²=98.89%, 95% CI: 98.50% to 99.18%; PI=-31.90 to 20.69; very low certainty), while it may reduce the duration of sensory block (MD -2.16 hours, 95% CI: -4.16 to -0.17; I²=90.77%, 95% CI: 84.22% to 94.60%; PI=-7.24 to 2.92; very low certainty).

Conclusions: LA mixtures may not affect sensory and motor block onset in ultrasound-guided PNB but could shorten the duration of sensory blockade.