Hemodynamic and analgesic effects of clonidine added repetitively to continuous epidural and spinal blocks

Clonidine in spinal and epidural blocks prolongs anesthesia, but can cause hypotension and bradycardia. The aim of our study was to compare hemodynamic and analgesic effects of spinal versus epidural clonidine alone and after repetitive dosing. In a prospective, randomized, double-blind study, we evaluated 40 patients scheduled for lower extremity orthopedic surgery under continuous spinal or epidural anesthesia with bupivacaine 0.5% (initial dose 5 mg and 50 mg, respectively). In either spinal or epidural technique one-half of patients received clonidine (150 micrograms) in addition to bupivacaine. Repeat doses of the same anesthetic mixture were allowed in cases of subsequent pain. Mean arterial pressure (MAP) and heart rate were recorded for 6 h after each injection. Duration of clinically useful anesthesia was defined as the time from drug administration to first sensation of pain. Intrathecal, but not epidural, clonidine decreased MAP significantly compared with bupivacaine alone. MAP after intrathecal clonidine with bupivacaine was lower than epidural clonidine with bupivacaine 5 and 6 h after injection. Repetitive administration caused no further decrease in MAP. Onset time required to surgical anesthesia (sensory block of T11) did not differ among the four groups. Duration of spinal and epidural anesthesia was increased more than two fold by clonidine. In summary, the addition of clonidine prolongs analgesia by either route. These results may be explained by clonidine's sites of action in hemodynamic control and the density of bupivacaine-induced block.