Quadratus lumborum block (QLB) has gained traction as a regional anesthesia technique to manage postoperative pain following laparoscopic surgery. However, the 90% minimum effective concentration (MEC90) of local anesthetics for posterior QLB remains undetermined.

We conducted a double-blind, comparative dose-finding study involving 54 women scheduled for elective laparoscopic myomectomy under general anesthesia. Each patient received a bilateral posterior QLB with 20 mL of ropivacaine on each side. The concentration administered varied for each patient and was determined based on the response of the previous participant. The initial concentration was set at 0.20%. Upon successful block, the subsequent patient was assigned to receive either the same (probability of 0.89) or a 0.05% lower concentration (probability of 0.11). In cases of block failure, the concentration was increased by 0.05% for the next patient. The trial concluded when 45 successful blocks were achieved, with block success defined as a pain score of three or fewer 30 minutes after arrival in the post-anesthesia care unit.

The 90% minimum effective concentration (MEC90) of ropivacaine was 0.340% (95% CI 0.329 to 0.344%).

The optimal concentration of ropivacaine for posterior QLB to achieve satisfactory analgesia following laparoscopic myomectomy is a 20 mL volume of 0.340% ropivacaine per side.

Chinese Clinical Trial Registry ChiCTR2200055743.