Effect of epidural clonidine on spinal cord blood flow and regional and central hemodynamics in pigs.

Epidural clonidine is reported to produce analgesia in humans. To investigate the effect of epidural injection of this alpha 2-adrenoceptor agonist on spinal cord blood flow as well as on regional and central blood flow and hemodynamics, 11 anesthetized pigs were studied. Each pig received clonidine in increments of 3, 10 and 30 micrograms/kg, each dose given in a volume of 5 ml via a lumbar epidural catheter. The tip of the catheter was located in the lumbar epidural space. The microsphere method was used to measure regional circulation. The measurements were made 45 min after each dose. Each pig served as its own control. The lowest dose of epidural clonidine (3 micrograms/kg) did not affect regional blood flow to the spinal cord or to any other organ. The intermediate and high doses were associated with local vasoconstriction in the lumbar and thoracic parts of the spinal cord that produced a statistically significant reduction in flow of 25-35% (P less than 0.05). Blood flow to the brain, cerebellum and the cervical parts of the spinal cord was not significantly changed, nor was renal blood flow. In the adrenal and in skeletal muscles a marked reduction of the blood flow occurred after the high dose, 61% and 78%, respectively. These findings indicate that epidural clonidine 3 micrograms/kg, a dose of clinical interest, is not likely to produce dangerous vasoconstriction in the spinal cord.