Anesthetic Management of the Horse: Inhalation Anesthesia (31-Jan-2003)

Introduction Management of intermediate and long-term (more than 30 60 minutes) anesthetic procedures most commonly involves the use of inhalational anesthetic methods in many species. Over the last 40 years, this has become true also for equine anesthesia. The delivery of anesthetic drugs via the lungs offers some advantages in part caused by the pharmacokinetics of the volatile agents in clinical practice. Among these advantages, the relatively quick and easy adjustment of anesthetic depth ranks high. Rapid changes in anesthetic delivery and tissue concentrations of volatile anesthetics allow for the anesthetist to quickly react to alterations in organ system functions (central nervous, cardiovascular, and respiratory) and thereby control functionality within physiologic limits. This is even more important when taking clinical conditions and diseased patients into consideration. Furthermore, inhalation anesthesia with the modern volatile anesthetics may produce a relatively short recovery period, a factor of growing interest in the literature of equine anesthesia. Volatile anesthetics are potent drugs with a relatively low margin of safety (therapeutic index: 2 4). They must be used carefully as overdosing may result in severe depression of the cardiopulmonary, central nervous, and other system functions and lead to death. In the past, several surveys on morbidity and mortality rates related to anesthesia even if not specific to horses suggested that a major percentage of anesthetic deaths were avoidable [1,2]. Fortunately, a newer and equine specific study (CEPEF-1) does not show similar results [3], but unwanted effects even though quantitatively reduced with halothane, isoflurane, desflurane, and sevoflurane can still not be excluded. Therefore, understanding of principles and concepts of inhalational anesthesia in general is crucial to the employment of volatile anesthetics. These topics are considered basic to this chapter and are discussed elsewhere [4-12]. The scope of this chapter is, instead, to provide a focused review of selected topics, namely interactions with carbon dioxide absorbers, mechanism of action of inhaled anesthetics, and use of sevoflurane and desflurane in the equine patient.