Dose-related prolongation of tetracaine spinal anesthesia by oral clonidine in humans.

The prolonging effects of oral clonidine on sensory block during tetracaine spinal anesthesia were studied in 47 healthy patients scheduled for urologic or gynecologic surgery. All patients received 15 mg tetracaine intrathecally in isobaric saline. The patients were randomly allocated into four groups. Group 1 (n = 13) was administered 0.25 mg triazolam orally. Group 2 (n = 12), Group 3 (n = 12), and Group 4 (n = 10) received 75 micrograms, 150 micrograms, and 300 micrograms of oral clonidine, respectively. These drugs were administered 1 h before anesthesia. Sensory block was evaluated by pinprick. All regression times in Groups 2, 3, and 4 were significantly longer than those in Group 1. The prolonging effect of oral clonidine increased in a dose-dependent manner and reached a maximal effect at 150 micrograms. Four patients in Group 4 developed bradycardia (heart rate < 45 bpm), suggesting that the dose of 300 micrograms of oral clonidine may promote bradycardia during spinal anesthesia.