Oral clonidine premedication attenuates the hypertensive response to ketamine

Clonidine diminishes sympathetic nervous system activity via a central action. To test if the haemodynamic responses to ketamine, a centrally acting sympathomimetic drug, are attenuated by clonidine, we studied arterial pressure (AP) and heart rate (HR) changes after ketamine 1 mg kg-1 in 40 normotensive patients undergoing general anaesthesia. They were allocated randomly to receive clonidine 5 micrograms kg-1 and famotidine 20 mg (n = 20) or to a control group (n = 20) which received only famotidine 20 mg orally 90 min before induction of anaesthesia. After administration of ketamine 1 mg kg-1 and vecuronium 0.2 mg kg-1, ventilation of the lungs was controlled with 67% nitrous oxide in oxygen to maintain end-tidal carbon dioxide at 4-4.7 kPa. AP and HR were measured noninvasively at 1-min intervals for 10 min after ketamine. After induction of anaesthesia, AP was increased significantly from resting values in the control group, but remained unchanged in the clonidine group (P < 0.05). Maximum changes in mean AP were also significantly greater in the control group compared with the clonidine group (29.2 (12.8) vs 19.5 (13.1) mm Hg, P = 0.02). However, no change in HR was noted throughout the 10-min study.