Partial intravenous anaesthesia in horses: alpha-2 adrenoreceptor agonist infusions

Partial intravenous anaesthesia has become a popular technique for maintaining general anaesthesia in horses and a variety of agents have been investigated in the literature. The technique uses an inhalational agent plus the infusion of an intravenous agent such as an alpha-2 adrenoreceptor agonist, optimising a balanced anaesthesia approach to produce the desired level of anaesthesia and analgesia. A balanced anaesthesia approach involves the administration of a number of different drugs in the perioperative phase, each with different actions and effects to produce general anaesthesia and provide unconsciousness, muscle relaxation and analgesia. In general, advantages of partial intravenous anaesthesia with an alpha-2 adrenoreceptor agonist can include reduction in inhalational agent requirements which in turn, reduces associated cardiopulmonary depression, provision of additional analgesia, improved intraoperative conditions, superior recovery quality and less environmental pollution. Potential disadvantages associated with the technique can include the cardiovascular effects associated with alpha-2 adrenoreceptor agonists, which may be more apparent in compromised horses, increased sweating, increased urine production, a reduction in gastrointestinal motility and additional cost associated with extra drugs and infusion equipment. This review describes the modern evidence for partial intravenous anaesthesia with alpha-2 adrenoreceptor agonists to maintain general anaesthesia in horses.