Chen X, Ling D, Cao J, et al. - Researchers examined how the minimum effective volume (MEV) of lidocaine 1.5% for an ultrasound-guided popliteal sciatic nerve block is correlated with individual factors including the cross-sectional nerve area, gender, age, BMI, and the depth of the sciatic nerve. Further, they evaluated the safety of combined femoral and sciatic nerve blocks by monitoring the plasma concentration of local anesthetics. Combined single-shot femoral and continuous sciatic nerve blocks were provided to 40 patients. They performed the femoral nerve block with an in-plane technique and 15 mL of lidocaine 1.5%. Between the tibial and peroneal nerves inside the paraneural sheath, positioning of a continuous peripheral nerve block annular tube was done. a They provided a loading dose of 5 mL of lidocaine 1.5% 30 minutes after the femoral nerve block in order to block the sciatic nerve after achieving the maximum compound muscle action potential (CMAP) amplitude using nerve conduction studies. Through the indwelling annular tube, they pumped additional lidocaine 1.5% at a rate of 30 mL/h if, after 8 minutes, the CMAP amplitude was still present. After the administration of lidocaine 1.5%, they recorded the CMAP amplitude monitored by the nerve conduction studies and pinprick tests every 2 minutes . The MEV was recorded when there is a decrease in the CMAP amplitude to nearly 0 mV. Analysis suggest that the cross-sectional area of the sciatic nerve is positively correlated with the MEV. For complete sciatic nerve blocks, the MEV of lidocaine 1.5% ranged from 7 to 15 mL. Lidocaine, monoethylglycinexylidide, and glycinexylidide had maximum concentrations of 1,672.9 (227.6), 265.7 (32.7), and 42.2 (22.4) ng/mL, respectively. Observations support the possible utility of regression equation in predicting the MEV of lidocaine 1.5% for popliteal sciatic nerve blocks. In this study, toxic threshold limits were not touched by the maximum concentrations of lidocaine and its metabolites.