Minimum effective anaesthetic concentration of hyperbaric lidocaine for spinal anaesthesia.

ABSTRACT

PURPOSE: 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. METHODS: 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. RESULTS: 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. DISCUSSION: 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.