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of health status is generally based on the American Society of Anesthesiologists (ASA) system.

This system uses information from the history, physical examination, and laboratory findings to place horses into one of five categories.

The classification allows for standardization of physical status only.

The ASA system does not classify risk, although increased risk of complication is associated with a high ASA status.

These classifications are not always useful for horses: nevertheless, the system serves as a guide to case management.

Box 1.1 ASA Classification System

ASA 1	Healthy horse does not require intervention (e.g. castration).
ASA 2	Horse with mild systemic disease (e.g. mild anemia, mild recurrent airway obstruction) or localized injury (e.g. wound repair).
ASA 3	Horse with moderate systemic disease (e.g. stable colic, infected joint).
ASA 4	Horse with severe systemic disease (e.g. recent ruptured bladder, endotoxemia).
ASA 5	A moribund horse not expected to survive longer than 24 hours (e.g. unstable colic, ruptured bladder of several days duration).
E	The letter E is added to status 2–5 under emergency conditions.

III Risk factors

A Age and physical status

The risk increases with age, and horses aged 12 years or older are at an increased risk of mortality.

Older horses may be more prone to fracture of a long bone in the recovery period, which could result in euthanasia.

Foals have an increased risk of fatality and this is probably associated with unfamiliarity with neonatal anesthesia, an immature cardiovascular system, and presence of systemic illness.

Pregnant mares have increased risk of mortality in the last trimester of pregnancy and this is probably associated with a need for emergency surgery. Otherwise, there is no difference between sexes.

Horses with a high ASA physical status have increased risk of mortality.

B Type of surgery and recovery

In otherwise healthy horses, the risk of mortality (euthanasia) following fracture repair is highest from repair failure or from fracture of another bone.

However, long periods of anesthesia typical of fracture repair have also been associated with increased mortality, and horses presented for fracture repair may be dehydrated and stressed.Emergency surgery (non‐colic) carries a 4.25 times higher risk of mortality compared with elective surgery, and for emergency abdominal surgery the risk of fatality is 11.7%.Colic surgery is associated with increased mortality because of a higher ASA physical status, emergency procedure with less time for stabilization, and use of dorsal recumbency possibly with episodes of hypotension.Eye surgery, in one institution, resulted in longer recovery times and risk of complications and were associated with long anesthesia time compared to non‐ophthalmic procedures. Fluconazole (microsomal P450 enzyme inhibitor) use was associated with increased risk of postoperative colic and longer recovery time. The use of the lowest effective volume of local anesthetic for a retrobulbar block was also recommended (10 ml/500 kg horse). Ophthalmic procedures have been associated with unsatisfactory recovery quality.

Assisted recovery with ropes can decrease the risk of fracture and dislocation, although the benefit of assisted‐recovery is still debatable.

C Time of day

Performing anesthesia outside of normal working hours carries an increased risk for horses. This increase in risk is separate from the fact that most of these cases are emergency in nature.

Surgeries performed between midnight and 6 a.m. carry the highest risk of mortality. This may be due to the nature of the emergency, as well as to staff shortages and personnel fatigue.

D Body position

Dorsal recumbency was found to increase risk compared to either lateral recumbency, but most “colic” surgeries are performed with the horse in dorsal recumbency.

An increased risk of postanesthetic myelopathy has mostly been associated with draft breeds between 6 and 24 months of age and dorsal recumbency.

E Drug choice

Using total inhalational anesthesia regimen in foals (<12 months of age) without premedication carries the highest risk.

Halothane sensitizes the myocardium to circulating catecholamines. Fewer cardiac arrests occurred when isoflurane was substituted for halothane, although overall mortality did not differ between groups because limb fracture in recovery was prevalent for the isoflurane group.

Isoflurane and sevoflurane may be associated with unsatisfactory recovery, and sedation is often used following anesthesia to reduce the risk of excitable recovery.

Use of isoflurane and sevoflurane was linked with increased mortality, but this was because these drugs are more likely to be selected for sick horses.

Not using any premedication is associated with the highest risk, probably owing to increased circulating catecholamines from stress. It may be prudent to premedicate foals before induction of anesthesia, especially when using halothane.

Acepromazine lowers the risk of mortality when used alone as a premedicant, because it reduces the incidence of ventricular arrhythmias in the presence of halothane.

No particular injectable induction regimen is associated with greater risk when used with inhalational anesthesia.

Total intravenous anesthesia (TIVA) is associated with the lowest risk of all, but TIVA is often used for short procedures. TIVA has been associated with reduced stress response.

F Duration and management of anesthesia

Long periods of anesthesia (>2 hours) with volatile anesthetics are often associated with cardiovascular depression and poor tissue perfusion, leading to problems such as cardiac arrest or postanesthetic myopathy.

Intraoperative hypotension during anesthesia has clearly been associated with postanesthetic myopathy, and can still occur with short anesthetic periods. Direct arterial pressure monitoring should be used during lengthy anesthetic periods.

Postanesthetic myopathy may lead to bone fracture or dislocation.

Postanesthetic airway obstruction and pulmonary edema may be prevented by keeping the head in a normal position thereby reducing the risk of laryngeal nerve paralysis, and with good airway management in the recovery period.