Injectable (field) Anesthesia

Pre-operative considerations Pre-operative preparation of the horse may be fairly abbreviated when anesthesia and surgery are occurring in the field, however it is wise to conduct a brief physical exam. The cardiovascular system should be examined by ausculting the heart and palpating the pulse. Murmurs or heart block may or may not be normal, and should be recognized. An elevated heart rate may indicate stress or pain but can also indicate significant blood loss (eg, in the case of a horse with a laceration). The respiratory system should be examined by observing respiratory rate, presence of any nasal discharge, and auscultation of the lungs. Chronic obstructive lung disease may be common in older horses and could be a source of hypoxemia during anesthesia, while respiratory infections are fairly common in younger horses. Subclinical respiratory infections can still cause hypoxemia under anesthesia, so the owner should be questioned about the herd history. Other abnormalities can be observed during a brief physical exam, which might impact on anesthesia. For example, is there any evidence of visual deficits, dehydration or myositis, or poor body condition? What is the medical history of the horse? Is there a history of a chronic problem requiring other medications (eg, hyperkalemic periodic paralysis)? Is the horse insured (which may require permission from the company before anesthesia)? What the horse is used for and the degree of fitness may affect how it responds to anesthesia. Certainly, it is important to assess the horse’s temperament in deciding how best to anesthetize it. Other drugs given to the horse may affect anesthesia; fluphenazine (commonly used as a long-acting tranquilizer) can cause profound hypotension during anesthesia. Pre-operative laboratory data should be conducted as is practical and as indicated by the physical exam. In many instances it may be possible to get a packed cell volume and estimate of total protein. For the horse with a pre-existing disease, it would be wise to have more baseline data. Newer, portable equipment is making it possible to conduct lab work in the field. Generally, it is not recommended to exceed 60-90 minutes of anesthesia with injectables. The type and length of procedure and physical surroundings where you will be working may influence drug choice as well as other equipment needed. . For instance, for a routine castration, padding the animal is probably not a consideration. But if you have a 2-hour procedure to conduct, some type of padding under the animal is probably indicated (eg, inner tubes, mats, mattresses, et al). If the horse is sick, then it may be wise to have oxygen and emergency drugs available. The same holds for preparation of the horse. Withholding feed for 12 hours is recommended for procedures longer than 20-30 minutes to prevent hypoxia caused by distention of the abdomen. While placement of an intravenous catheter is not absolutely essential for a routine castration, it certainly will save headaches for longer cases which will require repeated doses of anesthetics, and is essential in sick cases where complications may occur. Estimation of the horse’s weight may be made more accurate by using a weight tape, thereby allowing more accurate drug dosing.