Sensory assessment and block duration of transmuscular quadratus lumborum block at L2 versus L4 in volunteers: a randomized controlled trial.

Background: The efficacy of an ultrasound guided transmuscular quadratus lumborum block (QLB) for perioperative analgesia of the upper and lower abdomen remain debatable. The purpose of this study was to compare the cutaneous sensory blocked area (CSBA) between QLB blocks performed at the L2 vs. L4 levels.

Methods: Twenty-two healthy volunteers were randomized 1:1 to receive an ultrasound guided right transmuscular QLB at the L2 level (group QL2) or L4 level (group QL4). A cold stimulus was applied for testing of the CSBA at 30 minutes after the blockade was performed. The CSBA was mapped and then calculated. Three hours after the QLB, a cold stimulus was applied once every hour until sensation returned normal and the effective block duration for each volunteer was determined and recorded.

Results: The maximum cephalad dermatome level reached was T7 in group QL2 vs. T11 in group QL4, respectively. Caudally, both groups reached the L2 dermatome level. The QL2 block primarily affected dermatomes T9 to L1, while the QL4 block affected T11 to L1. The total CSBA was larger in QL2 group than that in QL4 group (748 [171] cm2 vs. 501 [186] cm2, P=0.004). The effective duration of the QLB was significantly longer in group QL2 than in group QL4 (18.5 [2.0]h vs. 14.1 [4.7]h, P=0.012). The number of affected dermatomes assessed by cold test was significantly larger for the volunteers in groups QL2 (4.6 [0.81] vs. 2.1 [0.30], P<0.001).

Conclusions: Ultrasound guided transmuscular QLB injection of 0.375% 20 mL ropivacaine at the L2 level produced a widespread cutaneous sensory blockade and a prolonged sensory block to cold sensation compared with the L4 level.