Effect of Total Dose of Lidocaine on Duration of Adductor Canal Block, Assessed by Different Test Methods: A Report of Two Blinded, Randomized, Crossover Studies in Healthy Volunteers.

BACKGROUND: The binary aims of this study were to investigate the effect of total dose of lidocaine on duration of an adductor canal block (ACB) and to validate different methods used to assess nerve blocks.

METHODS: We performed 2 blinded, randomized, controlled crossover trials, including healthy, young men. In study 1, 14 subjects received 4 ACBs with saline and 40, 80, and 160 mg lidocaine. In study 2, 14 new subjects received 2 ACBs with 100 and 300 mg lidocaine. We kept volume constant at 20 mL for all blocks, only altering concentration. ACB duration was assessed every hour postblock using mechanical (primary outcome) and temperature discrimination; warmth and heat pain detection thresholds; pain during heat stimulation; and tolerance to electrical current in the saphenous distribution. Finally, we measured quadriceps femoris muscle strength (clinical trial registration: NCT02172729).

RESULTS: In study 1, block duration assessed by mechanical discrimination differed significantly when comparing the 40-mg dose with the 80-mg dose (mean difference, 1.15 hours; 99% confidence interval [CI], 0.38-2.09 hours) and with the 160-mg dose (mean difference, 0.92 ours; 99% CI, 0.17-1.62). However, there was no difference between the 80-mg and 160-mg doses (mean difference, -0.23 hour; 99% CI, -1.12 to 0.46 hours). Neither for the secondary outcomes were there any differences between the 80- and 160-mg doses (99% CI including 0). Because of 38% (5/13) failed blocks in the 40-mg group, we decided to perform study 2. In study 2, all but 1 test showed no difference in duration despite a 3-fold increase in dose. The temperature discrimination test showed 100% sensitivity and specificity for differentiating between the presence and absence of block and was the only test with scores >90% for both parameters.

CONCLUSIONS: We did not find evidence that increasing the total dose of lidocaine may prolong duration of an ACB. The temperature discrimination test was the only test with scores >90% for both specificity and sensitivity.