PHARMACODYNAMICS OF SUBARACHNOID HYPERBARIC 5-PERCENT LIGNOCAINE

Twenty male patients (55 to 85 yr) undergoing cystoscopy received spin al anaesthesia with either 75 mg (n=10) or 100 mg of hyperbaric lignoc aine 5% under standardised conditions. Plantar flexion muscle power wa s recorded during onset and offset of anaesthesia using a load cell in terfaced with a computer (PFPD) and these data were compared with inte rmittent clinical assessments of spinal anaesthesia. Onset of paralysi s 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 T-4), 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 L(1) and recovery of t he patient's ability to walk and micturate. Recovery of plantar flexio n motor power occurred at 95.5+/-7.38 min in patients given 75 mg comp ared 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 advan tage in block onset or intensity and made the onset of recovery less p redictable. The Bromage grading, while clinically appropriate during a naesthesia onset, does not provide data relating to the density of blo ck and the PFPD was therefore useful for describing the anaesthesia re covery phase.