Background and Objectives Carotid endarterectomy under cervical plexus block offers the advantage of awake neurologic assessment. The hypothesis was tested that the addition of clonidine 5 μg/mL to lidocaine 1.5% for the block is as effective clinically as the addition of epinephrine 5 μg/mL but without the associated tachycardia.

Methods In a double-blind, randomized, prospective trial of 40 patients, local anesthetic solutions of lidocaine 1.5% containing either clonidine 5 μg/mL or epinephrine 5 μg/mL were compared for cervical plexus block in patients undergoing carotid endarterectomy. Each solution was administered to 20 patients with a total lidocaine dose of 7 mg/kg. The electrocardiogram, heart rate, and arterial pressure (radial artery catheter) were continuously monitored. Blood samples were drawn for determination of serum lidocaine levels during the first hour.

Results The block onset time (8.4 ± 0.6 minutes for epinephrine, 8.8 ± 0.8 minutes for clonidine) and duration (139 ± 6.7 minutes for epinephrine, 148 ± 5.8 minutes for clonidine) were not different between the two groups. During the period from completion of the block until incision there was a significant heart rate increase in the epinephrine group (23% mean rise) as compared with the clonidine group (4% mean rise) (P < .003). There was no difference in blood pressure between the two groups. The maximum plasma concentrations of lidocaine were 2.5-7.6 μg/mL (mean, 4.5 ± 0.3 μg/mL) for the epinephrine group and 4.7-18.4 μg/mL (mean, 7.5 ± 0.7 μg/mL) for the clonidine group (P < .0002). The maximum concentrations were reached 0-30 minutes (mean, 8 ± 1.4 minutes) after injection for the epinephrine group and 0-10 minutes (mean, 4.5 ± 7.1 minutes) for the clonidine group (P < .03).

Conclusions Clonidine 5 μg/mL is a useful additive to lidocaine 1.5% for cervical plexus block to reduce the incidence of tachycardia; however, omission of epinephrine results in higher serum lidocaine levels.