Extradural clonidine does not potentiate analgesia produced by extradural morphine after meniscectomy

We have studied the ability of clonidine to potentiate morphine analgesia in 28 patients (ASA I) after meniscectomy under general anaesthesia. One hour after surgery, morphine 3 mg (n = 10), clonidine 75 micrograms (n = 8) or morphine 3 mg plus clonidine 75 micrograms (n = 10) was injected extradurally. Morphine alone and in combination with clonidine produced similar and significant analgesia as assessed by verbal analogue pain scores. Pain scores did not decrease significantly in patients given clonidine alone. There were statistically, but not clinically significant decreases in systemic arterial pressure after morphine alone and clonidine alone. No patient developed sensory or motor block. One patient given morphine alone developed retention of urine. It is concluded that, in the dose used in this study, clonidine did not potentiate the analgesia produced by extradural morphine.