Balanced Anesthesia in Horses

Introduction The phrase “balanced anesthesia” originated from the coadministration of drugs to produce an “ideal” anesthetic state. Today the ideal anesthetic state implies adequate hypnosis, analgesia, muscle relaxation, and relief from stress. The last of these attributes, relief from stress, has been the focus of several investigations conducted in horses in order to determine the magnitude of the stress response; anesthetic drugs and anesthetic techniques, which either limit or promote the stress response, and therapies which are effective in limiting or preventing the stress response. Toward this end the development and clinical use of total intravenous anesthesia (TIVA) techniques have been advocated as an alternative to inhalant anesthesia or used in conjunction with inhalant anesthesia to produce the ideal anesthetic state in adult horses. One of the principle advantages of intravenous anesthesia (IVA) in modifying or limiting the stress response derives from the maintenance of a higher arterial blood pressure during the maintenance phase of anesthesia. This factor combined with added analgesia may account for the majority of the stress alleviating benefits reported for most IVA preparations. Most, if not all, IVA drug combinations are prepared by combining an alpha-2 agonist (xylazine, detomidine, medetomidine, romifidine) with a centrally acting muscle relaxant (guaifenesin) and a dissociative anesthetic (ketamine, Telazolt). Since this anesthetic cocktail is generally prepared from three separate drugs they are commonly referred to as “triple drip.” It is important to remember that not all triple drip drug combinations contain the same drugs or drug concentrations. One triple drip drug combination, xylazine, guaifenesin, and ketamine has been demonstrated to produce relatively rapid and uneventful induction to anesthesia, a stable maintenance phase of anesthesia and quiet uncomplicated recovery from anesthesia. It is important to note, however, that most experimental studies and clinical evaluations of IVA have been for relatively short periods of time (,1 hr) and that longer periods of anesthesia would result in greater total drug load, which could predispose to lingering drug effects and prolonged and potentially poor quality recovery from anesthesia. We decided to investigate the advantages and disadvantages of a “triple drip” drug combination administered in conjunction with inhalant anesthesia for maintenance of anesthesia in the adult horse.