Comparative assessment of the anaesthetic and analgesic effects of intramuscular and epidural clonidine in humans

Purpose

The aim of the study was to assess and compare in analogous controlled experimental conditions, the anaesthetic sparing and analgesic effects of the same dose of clonidine administered by the intramuscular (im) and epidural (ep) routes.

Methods

We used a randomized, double blind and placebo controlled protocol. Sixty patients undergoing abdominal hysterectomy were distributed into three groups who, 30 min before surgical incision, received: 300 μg ep clonidine plus im saline; ep saline plus 300 μg im clonidine; or ep and im saline (ss). General anaesthesia was maintained with 60% N₂O in O₂, and isoflurane administered at concentrations to maintain mean arterial pressure (MAP) and heart rate (HR) within 20% of basal values. Isoflurane requirements (mass spectrometry), cardiovascular variables (MAP, HR), and plasma concentrations of glucose, cortisol and prolactin were evaluated at critical time points. In the recovery room (RR), sedation (Ramsay) and pain intensity (VAS) were estimated at the time of analgesia request (TAR).

Results

Intramuscular and ep clonidine decreased isoflurane requirements similarly by about 85% (P < 0.001). Patients in the ep group had lower MAP (P < 0.03) and HR (P < 0.001) than in the im group, but im and ep clonidine similarly blunted the plasma prolactin increase induced by intubation. In RR, ep but not im clonidine (P < 0.01) induced postoperative analgesia demonstrated by a prolonged TAR 80.8 ± 7.3 (ep) 35.9 ± 3.2 (im) and 44.5 ±5.1 (ss) min and a lower VAS (P < 0.05).

Conclusions

Epidural and intramuscular clonidine decreased isoflurane requirements similarly, but only the epidural route provided postoperative analgesia, suggesting a spinal site for the analgesic action.