General clinical considerations for anesthesia of the horse.

The peculiarities of the equine species present a number of unique situations that must be addressed when horses are anesthetized. Perhaps the most troublesome situation is related to the horse's size. Though the horse's large lungs are responsible in part for its sustainable athletic ability, they are detrimental to effective ventilation when the horse is anesthetized and placed in a recumbent position. Of major concern is depression of ventilation and cardiovascular function. Hypercapnia and hypoxemia usually result from hypoventilation, and with time all anesthetized horses suffer from some degree of cardiovascular depression. Decreased blood flow coupled with the horse's weight pressing downward on the undermost tissues frequently disturbs microcirculation and causes injury to muscle tissue. Of major importance is the product of anesthetic depth and anesthetic time. Only through careful observation and initiation of supportive measures can injuries related to anesthesia or surgery be kept to a minimum. Because of the horse's nature, safe anesthesia cannot always be assured, even when state-of-the-art anesthetic techniques are practiced.