COMPUTER-CONTROLLED LIDOCAINE INFUSION FOR THE EVALUATION OF NEUROPATHIC PAIN AFTER PERIPHERAL-NERVE INJURY

Background: Systemic lidocaine has been reported to be effective in tr eating several neuropathic pain syndromes. Few reports relate plasma l idocaine concentration to analgesia and the available studies have bee n complicated by labile plasma lidocaine concentrations. We used a com puter-controlled infusion pump (CCIP) to target and maintain stable pl asma lidocaine concentrations and study the effect of intravenous lido caine on (1) pain scores, (2) current perception thresholds, (3) side effects, and (4) pain distribution in patients suffering from peripher al nerve injury pain. Methods: This study used a randomized double-bli nd placebo-controlled design. Eleven patients suffering from neuropath ic pain after peripheral nerve injury received both a lidocaine and sa line infusion in separate study sessions. The order of the study sessi ons was randomized and separated from each other by 1 week. The CCIP w as programmed to target plasma lidocaine concentrations of 0.5, 1, 1.5 , 2, and 2.5 mu g/ml, each held for 10 min. Pain scores and pain distr ibution were assessed in the painful area, and electrical current perc eption thresholds (CPT) of the ring finger were measured using a cutan eous perception threshold neurometer (Neurometer CPT, Neurotron, Balti more, MD). Side effects were recorded at fixed intervals. Plasma lidoc aine concentrations were measured at 4 and 9 min after each step incre ase in infusion and correlated with the observed effects. Results: Sal ine infusion had no effect. However, with lidocaine there was a signif icant plasma concentration-dependent decrease in pain scores starting at 1.5 mu g/ml. This effect typically corresponded with a decrease in the size of the receptive field to which the pain was referred. For th e electrical stimulus, there was no significant effect on cutaneous pe rception at 2000-Hz stimulation at the highest concentration examined; however, there was a significant increase in thresholds at 250-Hz (st arting at 1.5 mu g/ml) and 5-Hz (starting at 1.0 mu g/ml) stimulation. There were no serious side effects. In all, 54.5% of patients reporte d lightheadedness (average plasma lidocaine concentration: 1.5 mu g/ml ) and one patient reported nausea (2.3 mu g/ml). Discussion: The compu ter-controlled delivery of intravenous lidocaine results in relatively stable plasma concentrations which allows a more thorough evaluation of the relationship between plasma concentration and patient response. This administration methodology for intravenous lidocaine may prove t o be a valuable clinical and research tool.