The effects of alpha2-adrenoceptor agonists on circulation and respiration

In December 1999, the United States Food and Drug Administration approved dexmedetomidine, the active isomer of medetomidine, a veterinary anaesthetic, for use in the ICU. Dexmedetomidine is far more potent and selective as an α2-agonist than clonidine, and it is the first of its class in the USA to be available for intravenous administration to critically ill patients. The clinical approval of dexmedetomidine stimulated this review of the applications of α2-adrenergic agonists in the critically ill to modulate adrenergic output, to provide sedation and to limit narcotic analgesic use in this challenging group of patients. This chapter focuses on the salient features of the effects of α2-adrenergic agonists on cardiovascular and respiratory function, commenting on the special aspects of the dynamic state of critical illness. The bulk of this review is devoted to a discussion of pertinent human trials of dexmedetomidine. The remainder of the chapter briefly addresses the effects of epidural clonidine administration and the comparative circulatory effects of two other α2-adrenergic agents, mivazerol and tizanidine.