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Level I Evidence: How Much Volume of Local and How Long Should You Wait for an Effective Median Nerve Block?
Lyndsay M. Lovely, BSc, MSc(OT) 1; Donald H. Lalonde, BSc, MSc, MD2
1Dalhousie Medicine New Brunswick, Saint John, NB, Canada; 2Dalhousie University, Saint John, NB, Canada

Introduction: There is little high level evidence to guide us to the ideal volume of local anesthesia we should inject to perform the ideal median nerve block at the wrist. There is a similar lack of high evidence for how long we should wait after injection before we operate. This study provides level I evidence in the comparison of 5cc vs. 10 cc of buffered lidocaine with epinephrine for the efficacy of median nerve blocks. It also provides a surprising answer to the question of how long does it take after injection for the nerve to reach peak anesthesia.

Materials & Methods: This was a prospective double-blinded randomized control trial in which each participant acted as his/her own control. All 14 volunteers underwent blinded randomized bilateral median nerve blocks of 5cc on one side and 10cc of 1% lidocaine HCl with 1:100,000 epinephrine and 1cc 8.4% bicarbonate in the other wrist. One surgeon performed all 28 blocks over 1 minute at 5mm proximal to the wrist crease and 5mm ulnar to the median nerve. He wrapped all wrists with bandages to blind injectees and data collectors. After injections, the blinded data collectors asked participants in 5-minute intervals whether the level of anesthesia had increased in the last 5 minutes. When the level had not increased for 15 consecutive minutes (3 consecutive negative responses), we determined the last increase to be the time to peak anesthesia. After each reported negative response, the data collectors poked the fingertips with a sterile 30-gauge needle to determine where pain sensation remained intact.

Results: Following the median nerve blocks, 7 participants in the 5cc group and 1 participants in the 10cc group failed to reach complete numbness to pin prick in the median nerve distribution. On average, participants reached maximal numbness at 117.86 minutes with 5 cc and required less time to reach maximal numbness with 10 cc (106.07 minutes). A statistically significant difference was demonstrated between the survival curves for 5 cc and 10 cc, ?2 (1) = 4.04, p < .05.

Conclusions: We recommend using a minimum of 10 cc of 1% lidocaine HCl with 1:100,000 epinephrine and 1cc 8.4% bicarbonate for non-ultrasound guided wrist blocks. We also recommend waiting a minimum of 40-45 minutes after injecting the median block to allow nerve penetration by the local anesthetic before making an incision in that nerve distribution.

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