Minimum local anesthetic volumes for a selective saphenous nerve block: a dose-finding study

BACKGROUND: Saphenous nerve block contributes to analgesia after knee and lower leg surgery. However, literature reports a wide range of volumes of local anesthetic being used for this block. METHODS: A non-randomized controlled trial in a single university hospital in March 2015. Eighteen healthy volunteers (ASA 1 status, aged 27-43 years; male-to-female ratio 11/7) were needed to determine the minimum local anesthetic volume (MLAV) of mepivacaine 2% using the Dixon up-and-down method to achieve a selective ultrasound-guided saphenous nerve block. The primary endpoint MLAV (ED50 and ED95) for an ultrasound-guided saphenous nerve block were determined. The secondary endpoints were the position of the saphenous nerve, block onset and duration of action, cutaneous spread of the block, and the occurrence of femoral nerve motor block. RESULTS: The measured MLAV dose that was effective in 50% of cases (ED50) for a complete saphenous nerve block was 1.5 mL; the calculated MLAV dose for 95% of cases (ED95) was 1.9 mL. The saphenous nerve was encountered in almost all cases on the anterior/anteromedial aspect of the femoral artery. We found no correlation between local anesthetic volume and the onset or duration of the block. Cutaneous spread of the nerve block was observed on the anteromedial aspect of the lower leg, with considerable individual variation between individuals in the study. No femoral motor block was observed. CONCLUSIONS: For a selective ultrasound-guided saphenous nerve block, the ED95 MLAV of mepivacaine 2% is 1.9 mL.