Dosing interval for prolongation of tetracaine spinal anesthesia by oral clonidine in humans.

This study was designed to evaluate the optimal administration time of oral clonidine as premedication for the prolongation of tetracaine spinal anesthesia in humans. Forty male patients scheduled for urologic surgery were studied. Patients were allocated randomly into four groups given 15 mg of 0.5% isobaric tetracaine. Group 1 was given 250 micrograms triazolam orally 1 h before anesthesia. Groups 2, 3, and 4 were administered 150 micrograms oral clonidine just before anesthesia or 1 h and 3 h before anesthesia respectively. Sensory block was assessed by pinprick. Groups 2 and 3 had a significantly prolonged time of sensory block (74%-94%, P < 0.01) when compared with Group 1. However, the prolonging effect of oral clonidine was not apparent in Group 4. We conclude that, when administered within 1 h before anesthesia, oral clonidine, at a dose of 150 micrograms, produced a significant prolongation of tetracaine spinal anesthesia without adverse effects.