Pharmacodynamics of subarachnoid hyperbaric 5% lignocaine

Twenty male patients (55 to 85 yr) undergoing cystoscopy received spinal anaesthesia with either 75 mg (n = 10) or 100 mg of hyperbaric lignocaine 5% under standardised conditions. Plantar flexion muscle power was recorded during onset and offset of anaesthesia using a load cell interfaced with a computer (PFPD) and these data were compared with intermittent clinical assessments of spinal anaesthesia. Onset of paralysis was rapid and complete with motor power declining exponentially to 5% of preoperative values by 5 minutes in all patients. Extent of block to cold and pinprick was similar in both dosage groups (median T4), as was the rate of block onset. Block regression was complete by three hours in all patients and restitution of plantar flexion motor power was associated with normal thermosensibility at L1 and recovery of the patient's ability to walk and micturate. Recovery of plantar flexion motor power occurred at 95.5 +/- 7.38 min in patients given 75 mg compared with 129 +/- 9.5 min in those given 100 mg lignocaine (P < 0.05). The time between onset and full motor recovery in the 75 mg group (7.2 +/- 1.2 min) was less than the 100 mg lignocaine group (29 +/- 5.1 min) (P < 0.001). The larger dose of lignocaine did not confer any clinical advantage in block onset or intensity and made the onset of recovery less predictable. The Bromage grading, while clinically appropriate during anaesthesia onset, does not provide data relating to the density of block and the PFPD was therefore useful for describing the anaesthesia recovery phase.