Epidural clonidine does not decrease blood pressure or spinal cord blood flow in awake sheep

Preliminary data in animals and humans suggest that epidurally administered clonidine produces antinociception and is not neurotoxic. However, clonidine can produce vasoconstriction, and epidurally administered clonidine decreases spinal cord blood flow in anesthetized pigs. To examine the effect of epidurally administered clonidine on spinal cord blood flow in awake animals, the authors inserted lumbar epidural, femoral arterial and venous, pulmonary arterial, and left ventricular catheters in 13 adult sheep. Following a 24-h recovery, the authors injected saline (N = 6) or clonidine, 750 micrograms (17-25 micrograms/kg; N = 7) epidurally, and measured arterial blood gas tensions; temperature; heart rate; systemic and pulmonary arterial, right atrial, and pulmonary capillary wedge pressures; and spinal cord and renal blood flows (by radioactive microsphere injection) before and at 45 min and 4 h following injection. Epidural saline injection did not affect measured variables. Heart rate decreased from 112 +/- 9 to 86 +/- 4 beats/min (mean +/- SE; P = .003) and arterial PO2 decreased from 99 +/- 3 to 78 +/- 6 mmHg (P = .04) 45 min following clonidine injection. Temperature increased from 39.1 +/- .2 to 40.6 +/- 1 degree C (P = .0001) 4 h following clonidine injection. Epidural clonidine administration did not affect cardiac output, pulmonary and systemic pressures, or renal or spinal cord blood flows, except for an increase in mid-thoracic spinal cord blood flow 45 min following injection. The authors conclude that, in sheep, epidural clonidine does not produce dangerous cardiovascular depression or global spinal cord ischemia.