MINIMUM EFFECTIVE ANESTHETIC CONCENTRATION OF HYPERBARIC LIDOCAINE FOR SPINAL-ANESTHESIA

Purpose: Minimum effective anaesthetic concentration (MEAC) of lidocai ne for spinal anaesthesia, defined as the concentration at which a spi nal anaesthetic agent produces surgical anaesthesia within 20 min of a dministration in 50% of patients, was determined in a randomised, doub le-blind study in young patients undergoing knee and ankle surgery, Me thods: Using the combined spinal-epidural technique, 48 or 72 mg hyper baric lidocaine containing dextrose 7.5% was administered intrathecall y to 43 patients at concentrations ranging from 0.2-0.9%, The choice o f lidocaine concentration was determined by Dixon's up-and-down method . Complete anaesthesia was defined as: (I) pinprick anaesthesia at or higher than T-12, (2) anaesthesia to transcutaneous tetanic electric s timulation (50 Hz at 60 mA for five seconds) in the knees and (3) comp lete 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% Cl-0.21-0.87), Anaes thetic effect was variable with mean duration of anaesthesia of 29 min (range: 20-50 min) and maximum pinprick sensory level ranging from T- 2-T-10 In the 72 mg group, successful anaesthesia was achieved consist ently concentration 0.3%, i.e., MEAC was < 0.3%. Mean duration of comp lete anaesthesia was 46 min (range: 30-60 min) with maximum sensory le vel from T-3-T-8. Discussion: Spinal anaesthesia can be accomplished w ith very dilute lidocaine solutions (< 0.9%), The value of MEAC is dos e-dependent, i.e., complete anaesthesia can be accomplished with lower concentrations by increasing the dose of spinal anaesthetic administe red.