Minimum effective anaesthetic concentration of hyperbaric lidocaine for spinal anaesthesia

Minimum effective anaesthetic concentration (MEAC) of lidocaine for spinal anaesthesia, defined as the concentration at which a spinal anaesthetic agent produces surgical anaesthesia within 20 min of administration in 50% of patients, was determined in a randomised, double-blind study in young patients undergoing knee and ankle surgery.

Using the combined spinal-epidural technique, 48 or 72 mg hyperbaric lidocaine containing dextrose 7.5% was administered intrathecally to 43 patients at concentrations ranging from 0.2-0.9%. The choice of lidocaine concentration was determined by Dixon's up-and-down method. Complete anaesthesia was defined as: (1) pinprick anaesthesia at or higher than T12, (2) anaesthesia to transcutaneous tetanic electric stimulation (50 Hz at 60 mA for five seconds) in the knees and (3) complete leg paralysis; all occurring in both lower extremities within 20 min. Epidural anaesthesia was initiated if anaesthesia was incomplete.

In the 48 mg group, MEAC was 0.54% (95% CI-0.21-0.87). Anaesthetic effect was variable with mean duration of anaesthesia of 29 min (range: 20-50 min) and maximum pinprick sensory level ranging from T2-T10. In the 72 mg group, successful anaesthesia was achieved consistently at a concentration of 0.3%, i.e., MEAC was < 0.3%. Mean duration of complete anaesthesia was 46 min (range: 30-60 min) with maximum sensory level from T3-T8.

Spinal anaesthesia can be accomplished with very dilute lidocaine solutions (< 0.9%). The value of MEAC is dose-dependent, i.e., complete anaesthesia can be accomplished with lower concentrations by increasing the dose of spinal anaesthetic administered.