Background: This study aimed to estimate the minimum effective anaesthetic concentrations of lidocaine required to block the femoral nerve under ultrasound (US) guidance in 90% (MEAC90) of patients.
Methods: A minimum of 45 patients who had undergone knee arthroscopy were included in this observational study. All the patients received US-guided sciatic, obturator, and femoral nerve blocks. The femoral nerve block was performed using 15 ml of lidocaine. The lidocaine concentration given to a patient was determined by the response of the previous patient (a biased-coin design up-down sequential method). If a patient had a negative response, the lidocaine concentration was increased by 0.1% w/v in the next patient. If a patient had a positive response, the next patient was randomized to receive the same lidocaine concentration (with a probability of 0.89) or to receive a concentration 0.1% w/v less (with a probability of 0.11). A positive response was defined as complete sensory and motor block. The patients' responses were analysed to calculate the mean MEAC90.
Results: Fifty-two patients were required to complete the study; 45 had a positive response and seven had a negative response. The mean MEAC90 was estimated to be 0.93% w/v [95% confidence interval (CI), 0.8-1.03%]. Lidocaine 0.93% w/v was estimated to produce a successful block in 89% (95% CI, 78-100%) of patients.
Conclusions: Perineural injection of 15 ml of lidocaine 0.93% w/v under US guidance could provide successful femoral nerve block in 90% of patients.
Keywords: anaesthetic techniques, regional, femoral; anaesthetics local, lidocaine; equipment, ultrasound machines.