3D ultrasound estimation of the effective volume for popliteal block at the level of division

Abstract

Abstract Local anaesthetic injection between the tibial and commmon peroneal nerves within connective tissue sheath results in a predictable diffusion and allows for a reduction in the volume needed to achieve a consistent sciatic popliteal block. Using 3D ultrasound volumetric acquisition, we quantified the visible volume in contact with the nerve along a 5 cm segment. Methods We included 20 consecutive patients scheduled for bunion surgery. Ultrasound guided popliteal block was performed using a posterior, out of plane approach at the level of division of the sciatic nerve. Thirty mL of mepivacaine 1.5% and levobupivacaine 0.5% were slowly injected while assessing the injection pressure and the diffusion of the local anaesthetic. Volumetric acquisition was performed before and after the block to quantify the volume of the sciatic nerve and the volume of the surrounding hypoechoic halo contained inside the connective tissue in a 5 cm segment. Results All blocks were successful within 20 min after the injection. The total estimated volume contained inside the common connective tissue sheath was 6.8 ± 2.6 cm 3 . Of this, the volume of the halo surrounding the nerve was 4.4 ± 1.7 cm 3 and the volume inside the sciatic nerve was 2.4 ± 1.7 cm 3 . Conclusions The volume of local anaesthetic in close contact with the sciatic nerve can be estimated by volumetric acquisition. Our results suggest that the effective volume of local anaesthetic needed for a successful sciatic popliteal block could be reduced to less than 7 mL.