Haemodynamic effects of clonidine injected epidurally in halothane-anaesthetized dogs

The haemodynamic effects of clonidine administered in the epidural space were studied in 16 halothane-anaesthetized dogs. The animals were randomly assigned to two groups: Group I received three doses of 3 ml of normal saline, Group II received three doses of 3 micrograms X kg-1 of clonidine, through an epidural catheter, whose tip was located between L2-T11. Control haemodynamic measurements were taken one hour after completion of the surgical preparation (period P1); they were repeated every 45 minutes after each incremental dose (periods P2, P3, P4) and 105 minutes after a total cumulative dose of 9 micrograms X kg-1 of clonidine or 9 ml of saline were given (period P5). No significant changes over time were observed in Group I. In Group II clonidine produced statistically significant reductions of systemic blood pressure (BP), mean left ventricular pressure (LV), heart rate (HR), cardiac output (CO) and peak LV dP/dt only after a total clonidine dose of 9 micrograms X kg-1 and these changes were sustained. BP fell 15 per cent, CO 21 per cent, HR 25 per cent, LV 20 per cent and peak LV dP/dt 30 per cent when P5 measurements were compared to control values within Group II (p less than 0.05). These haemodynamic effects of clonidine are likely due to minimal systemic absorption and/or cephalad spread of the drug towards its site of action in the brain stem. The reductions of HR, CO, BP, and isovolemic indices of contractility are likely explained by a reduction of sympathetic outflow at the spinal cord and medulla oblongata levels as well as increased parasympathetic tone.(ABSTRACT TRUNCATED AT 250 WORDS)