MOTOR POWER PHARMACODYNAMICS OF SUBARACHNOID HYPERBARIC 5-PERCENT LIDOCAINE IN THE SITTING POSITION

Background: Repetitive dynamometric measurement using a plantar flexio n power device (PFPD) provides detailed data describing the onset and offset of motor block following spinal administration of lidocaine. Th e aim of this study was to evaluate administration of two doses of spi nal lidocaine in the sitting position to determine whether our dynamom etric model produces data consistent with our current understanding of the pharmacokinetics of subarachnoid, hyperbaric, 5% lidocaine. Metho ds: Twenty male patients (54 to 80 yr) undergoing cystoscopy received spinal anaesthesia with either 75 mg (n=10) or 100 mg of hyperbaric li docaine 5%, in the sitting position, under standardised conditions. Pl antar flexion muscle power was recorded during onset and offset of ana esthesia using a load cell interfaced with a computer (PFPD). Results: Onset of paralysis following spinal block in the sitting position was rapid and complete with motor power declining exponentially to 5% of preoperative values by 8.5 min in all patients. There was no differenc e in decay or recovery of plantar flexion motor power data between dos age groups in the sitting position. Measurement using the PFPD shows t hat onset of motor paralysis is described by an exponential decay and that motor recovery occurs at a fixed rate. Extent of block to cold an d pinprick was similar in both dosage groups in the sitting position ( median T-4). Conclusion: This study shows that in the sitting position , doses less than 75 mg of 5% hyperbaric lidocaine are required to sig nificantly improve ambulatory times. (C) Acta Anaesthesiologica Scandi navica 41 (1997).