Clonidine and or adrenaline decrease lignocaine plasma peak concentration after epidural injection

Clonidine is an α₂-adrenoceptor agonist increasingly used in combination with lignocaine for spinal or epidural anaesthesia because of a prolonged analgesic effect. Like adrenaline, it may decrease lignocaine peak concentration ( C _max), thus leading to decreased toxicity. However, the effects of clonidine on resorption of lignocaine into the systemic circulation from the epidural space remain to be established. We studied the pharmacokinetics of lignocaine after epidural injection of lignocaine with or without clonidine, adrenaline and both drugs. Total body clearance and apparent volume of distribution were similar in the four groups, but the maximum observed concentration ( C _max) was markedly increased in the plain solution group as compared with the other groups: (plain lignocaine: 7.15±2.04  μg  ml⁻¹, lignocaine+adrenaline: 3.11±136  μg  ml⁻¹, lignocaine+clonidine: 4.48±1.26 μg  ml⁻¹, lignocaine+adrenaline+clonidine: 4.06±1.42  μg  ml⁻¹ [mean±s.d.]). Our results show that, clonidine decreases lignocaine C _max to the same extent as adrenaline.