Introduction

Equine anesthesia has always been associated with greater patient risk compared to other common domestic species. Delivery of anesthesia is complicated by many factors including the large patient size range, the need to induce controlled recumbency during anesthetic induction, specialized equipment requirements, perfusion issues related to large body mass, and instinctive behaviors. In addition, drug dosages are often extrapolated from those of other species and many of the drugs are used in an extra‐label manner. Understanding the unique factors associated with equine anesthesia compared to other species is the first step in improving patient safety.

Anesthetic risk

The risk of anesthetic mortality is greater in equine species compared to other domestic animals. Studies have reported overall mortality rates between 0.24% and 2.24% for horses [1–7] compared to the widely reported rates for dogs (0.05%) and cats (0.11%) [8]. However, other smaller studies from other countries have reported higher small animal mortality rates that are similar to those reported for equine patients [9–11]. A mortality rate of 1% is commonly referenced in most surgical facilities when discussing the risk of anesthesia with horse owners; however, the percentage can increase considerably with metabolically unstable patients such as those with colic, where mortality has been reported to be as high as 38.3% [12]. The primary complications associated with death are cardiac arrest, orthopedic injuries in recovery, and myopathy or neuropathy in the postanesthesia period. Complication rates as high as 17.5% have been reported [13]. Postanesthetic colic is an additional concern. The use of opioid analgesics, time of surgery, choice of anesthetics, breed, and concurrent use of certain antibiotics have been implicated as risk factors for developing postoperative colic, although specific mechanisms have not been definitively determined [13–19].

Variation in size

Horses come in a broad range of sizes. A miniature horse foal can weigh less than 10 kg, while a draft horse used for competition pulling can weigh up to 1300 kg. Because of this size variation, specific equipment must be purchased to move these animals, deliver anesthetic, and assist or control ventilation. A small animal anesthetic machine can be used for most miniature horses, neonates, and foals up to 150 kg, but large animal anesthetic systems must be purchased for older foals and horses. The limiting factors with most small animal systems include the volume of the carbon dioxide absorbent canister, the volume of the reservoir bag, and the diameter of the breathing system tubing and adaptor that connects to the endotracheal tube. Endotracheal tubes, reservoir bags, and anesthetic tubing are specifically designed for equine patients and are, therefore, more expensive than equipment used in small animal patients (much of which is repurposed from the human market). Much of the monitoring equipment designed for human patients can be adapted for use on larger species. One common problem is that human monitors often consider normal heart rates in adult horses to be bradycardic with default alarms set for rates below 40 beats/min.

Surgical tables and hoisting mechanisms must be designed specifically for equine or large animal patients. If a practice regularly sees large draft breed horses, it is wise to purchase a 2‐ton hoist rather than the typical 1‐ton hoist as larger horses can overstress inappropriate equipment. In addition, the typical surgical table designed for an adult equine is not big enough to support the mass of a 1000 kg patient for a procedure requiring lateral recumbency. Therefore, two surgical tables may need to be used together to provide adequate support. If two tables are not available, the use of stacked pads or mats can substitute under limbs, head, and neck. Many of the draft breeds do not have long enough tails to attach a rope for assisted recovery. Fortunately, most draft horses are relatively cooperative regarding recovery.

Size is also a factor in determining peripheral (skeletal muscle) perfusion. Mean arterial blood pressure requirements increase as muscle mass (and muscle compartment/compression pressure) increases [5]. The risk of postanesthetic neuropathy and myopathy is higher in equine patients, particularly draft and large warmblood breeds. While recommendations on minimum mean arterial pressures during anesthesia vary, this author recommends a minimum mean arterial blood pressure in horses greater than 500 kg of at least 80 mmHg based on personal experience. Maintaining a mean arterial blood pressure of greater than 70 mmHg in normal adult horses has been shown to minimize postoperative complications associated with hypotension [20,21]. Neonates and foals have lower normal resting mean arterial blood pressure (47–50 mmHg in neonates and 55–70 mmHg in foals), but there has been a report of a neonate developing postanesthetic myositis after an anesthetic event with average mean arterial blood pressure between 45 and 65 mmHg [22]. Therefore, the goal with all ages and sizes of horses should be to maintain blood pressure above 60 mmHg in foals and 70–80 mmHg in adults depending on body size. Adjuncts to increase mean arterial blood pressure include inotropes, vasopressors, and catecholamines. Dobutamine is used most often in equine patients, but dopamine, ephedrine, phenylephrine, vasopressin, norepinephrine, epinephrine, and calcium salts are alternative options if used appropriately. Of the inotropes, dobutamine has been found to be the most useful for increasing mean arterial blood pressure in neonates, foals, and adult horses under general anesthesia [23–28].

Cardiopulmonary resuscitation

Anatomic variation complicates cardiopulmonary resuscitation. An equine neonate or older foal can be managed similar to a canine but adult equine ribs are narrow and impossible to separate by hand if open chest cardiac compressions are required. A rib resection is required for open chest compressions, and this is not practical in most situations. Therefore, chest compressions on an adult equine patient require use of the anesthetist’s full body weight concentrated on either both knees or feet. External chest compressions must be performed at a minimum of 80 compressions/min in an adult to produce a cardiac output near 50% of that of a deeply anesthetized horse [29]. However, external compressions of 20/min in ponies have been shown to produce a cardiac output of approximately 50% normal baseline values [30]. As an alternative to external compressions, when a horse suffers cardiopulmonary arrest during general anesthesia for abdominal exploration, the surgeon can make an incision through the diaphragm and perform direct cardiac compression.

Although cardiac arrhythmias are common in horses during exercise and after anesthesia, ventricular fibrillation is uncommon in horses prior to arrest [31–33]. Even if ventricular fibrillation is present, most electrical cardiac defibrillators are not designed to deliver a large enough electrical output required to defibrillate an adult equine heart.

Considerations for equine induction

Inducing recumbency in horses from the standing position can be dangerous. In order to minimize the risk, appropriate sedation and muscle relaxation should be administered prior to induction. Ketamine is one of the primary induction drugs used in horses, but it has minimal muscle relaxant properties. Muscle relaxation and induction quality can be improved by increasing the dose of premedication, adding an opioid or phenothiazine to an α₂‐adrenergic receptor agonist, and/or administering ketamine with a coinduction agent (commonly either a benzodiazepine or propofol) [34–37].

There are differences in induction doses between horses, donkeys, and mules. Donkeys are typically less affected by premedication and induction drugs when compared to horses. Mules appear to be intermediate between donkeys and horses. A starting point for dosing drugs for donkeys can be based on 1.5 times the dose of a horse. The exception to this rule is guaifenesin, as donkeys are more sensitive to the central effects of the drug, resulting in hypotension and apnea with bolus administration. The difference in response to drugs in donkeys appears to be due to variation in body water distribution and drug metabolism [38,39].

Effect of recumbency in equine patients

Prolonged recumbency in equine patients is not benign. Ventilation–perfusion mismatch and shunting are inevitable when an equine patient is recumbent. Lateral recumbency results in atelectasis of the dependent lung and dorsal recumbency results in progressive atelectasis of the dorsal lung fields [40]. Controlled ventilation can improve arterial oxygenation, but in a patient with abdominal distension associated with colic, ruptured bladder, or pregnancy, cardiac output can be compromised during periods of high intrathoracic pressure. In contrast to small animal patients, a recent study performed with horses found that decreasing the inspired oxygen concentration to 50% from > 95% resulted in significant reductions in arterial oxygen saturation; therefore, the use of lower inspired oxygen concentrations to reduce atelectasis is not recommended [41–45].

Heart rate considerations

Sedation with an α₂

Stay updated, free articles. Join our Telegram channel

Join