Drs Jo Rainger PhD, BVSc, BVSc(Vet),
FANZCVA, Dip Clin Stud (Veterinary Anaesthesia) & Allison J. Stewart BVSc(hons), MS, DACVIM, DACVECC Equine Specialists Hospital, School of Veterinary Medicine, University of Queensland, Gatton Campus.

Anaesthesia is defined as the temporary loss of sensation and awareness, induced by the controlled (and reversible) suppression of nervous function with drugs. It may be regional (a particular area of the body is numbed, like is done for a nerve block in a lame leg), or general (the whole body is affected). When under general anaesthesia, not only does the horse feel no pain, but it is also unconscious, making this technique useful in situations where it is critical the horse does not move during an operation. Horse anaesthesia is associated with some risk, so an understanding of the process can alleviate some of the anxiety owners are likely to experience. Numerous precautions are taken by your veterinarian and 99% of the time the anaesthetic is uneventful and the surgery or imaging procedure is successfully performed.

Why might your horse need an anaesthetic?

Horses require anaesthesia for a variety of problems. It may be needed for an elective procedure such as a castration, for a horse that has sustained a wound, or for emergency surgery to treat colic. Many advanced diagnostic imaging techniques, such as CT scans (computed tomography), require immobility to obtain high quality images and therefore horses must be anaesthetised to achieve this. It is also essential to prevent damage to the multi-million dollar CT equipment.

What happens when a horse is anaesthetised?
How a horse is anaesthetised depends on where it is and the procedure it is to undergo. Many veterinary procedures requiring anaesthesia are done in a paddock or sand arena. This is often referred to as field anaesthesia by veterinarians and is most often achieved by injecting anaesthetic drugs intravenously directly into the bloodstream (e.g. ketamine). Alternatively, horses may be anaesthetised in a hospital setting where they are placed on a machine that uses gas anaesthetics and supplemental oxygen to maintain anaesthesia. This is often called general, or inhalant, anaesthesia.

Field anaesthesia is normally used for shorter elective or emergency procedures; the most common one being the castration of colts. Examples of emergencies that may require field anaesthesia are wounds that cannot be safely repaired standing and mares that are having difficulty foaling.

Prior to a horse being anaesthetised, the first thing a veterinarian will do is to take a thorough history; it is important that the horse owner alert the veterinarian to any abnormalities in behaviour or health. The veterinarian will then examine the horse, which usually involves taking its temperature, listening to the heart and lungs, and inspecting the mucous membranes of the gums - in addition to an examination of the particular anatomical structures associated with the horse’s injury or illness. Most veterinarians will then place an intravenous catheter into the jugular vein to make it easier for the safe administration of injectable anaesthetic drugs.

Before the point at which the horse ‘goes under’ anaesthesia (a process called induction) the horse will always be sedated. If it is uncooperative, as they often tend to be when injured, ill, or stressed by receiving injections, they may be sedated before the catheter is placed. Once the horse is deeply sedated, anaesthesia can be induced. Since most of the drugs used to do this act quickly and leave the body rapidly, they need to be given throughout the surgery/ procedure. For short procedures, the horse may be ‘topped up’ with smaller doses of injectable anaesthetics; for longer procedures drugs are given continuously, rather than injected intermittently, to make sure that the body receives the ideal concentration of drugs throughout, and so that the level of anaesthesia (unconsciousness and loss of sensation) is kept more constant. This is called a ‘constant rate infusion’.

Horse anaesthesia may also be performed in a hospital environment. This usually occurs when a sterile operating room or ‘OR’ is available for the procedure, or if the procedure is going to take a prolonged period of time. Examples include colic surgery, throat surgery, arthroscopy, or surgical repair of wounds or fractures.
The duration that a horse is unconscious is important, since the longer they are lying down (recumbent) the greater the incidence of muscle and nerve damage due to pressure and poor blood flow. This is more likely if the horse is anaesthetised on the ground compared to on a padded surgical table. When a horse is anaesthetised for a lengthy procedure in the hospital, a blood sample may be collected (in addition to the assessment performed for field anaesthesia) to check indicators of infection or disease of the liver and kidneys, the function of which could be further worsened by the anaesthetic drugs. For very sick horses preparing for emergency surgery (such as colic surgery), the blood work can help vets decide how best to rehydrate the horse and balance its electrolyte levels using intravenous fluids. Sometimes, plasma (the liquid portion of the blood containing proteins and nutrients) or blood transfusions may even be required before or during anaesthesia.

The sedation and induction of a horse in the hospital is similar to that performed in the field, except that it happens in a special padded room (called a ‘drop-down stall’ or induction/recovery box) with a door which restrains the horse (Figure 1.), or up against a table that tilts. It is vital that staff are well trained in horse anaesthesia to avoid injury to personnel and the horse, since as they become unconscious they lose the ability to coordinate their body movements and can stagger and/or fall. Similar to the situation in human anaesthesia, in the hospital environment, once the horse is unconscious a flexible, plastic tube (endotracheal tube) is placed though the mouth into the trachea or ‘wind pipe’ (Figure 2.). This allows the horse to be attached to a machine which administers oxygen and also the gas that maintains anaesthesia - allowing them to pass freely to-and-from the lungs. It can also allow the anaesthetist to ‘breathe’ for the horse if needed, using manual or mechanical ventilation.

Once the tube is inserted, ready for it to be hooked up to the anaesthetic machine, the horse is moved from the drop-down stall into the OR. Moving the horse usually involves placing hobbles below the fetlocks and hoisting the horse in an upside down position using a mechanical hoist (Figure 3). This allows the horse to be positioned correctly (and easily!) for the upcoming procedure. Once in the OR, the horse will be lowered onto a padded, hydrolic surgery table; the padding is very important for the prevention of muscle and nerve damage. The horse will be attached to the anaesthetic machine, intravenous fluids and monitoring equipment so that the anaesthetist can monitor its breathing, heart rate and blood pressure (Figure 4.). Blood samples can be collected from an artery to check the lungs and heart are working properly. This can be very important in severely ill horses such as colics.

Just like in human operations, the anaesthetist will aim to monitor the horse’s vital signs so that these may be kept close to normal values. If the blood pressure drops, then intravenous fluids and drugs that specifically increase blood pressure can be used to return the blood pressure to normal.

This is critical for preventing muscle and nerve damage. Once the procedure is completed the horse will be returned to the padded, drop-down stall where it will be closely observed by the anaesthetist until it is standing. Careful monitoring is vital during this stage as horses can obstruct their windpipe and suffocate due to reduced control of their airway. They can also be at risk of stress, and falls/stumbling due to incoordination, during recovery from anaesthesia.

Why is horse anaesthesia associated with risk?
Compared to other pets such as dogs and cats, horses have an increased risk of injury, and occasionally even death, associated with anaesthesia, mainly due to their large body size. When lying down, their body weight compresses the muscles and nerves on the underside of the horse. If the horse is anaesthetised for long periods, muscle and nerve damage can result and may lead to the horse experiencing pain, limb incoordination and an uncontrolled recovery, with potential further injury. Breathing is also compromised when a horse is positioned on its side or on its back. As horses are flight animals, if not sufficiently sedated they can panic during the induction and recovery from anaesthesia - leading to injury and occasionally catastrophic limb fractures. Sedatives and pain relief may be given to horses to help them stand after coming round from anaesthesia and to allow a smoother recovery.

How can horse owners lessen the risks?
One of the main ways of decreasing the risk associated with your horse being anaesthetised is to prevent its requirement. The anaesthetic and recovery risks related to colic surgery are much higher than those of horses having elective hospital procedures such as arthroscopy. Preventative health care such as worming and dental care will decrease the likelihood of colic and therefore the need for a horse to undergo this treatment. If your horse does develop colic call your veterinarian promptly. The longer a horse is suffering from colic, the more likely it is to develop dehydration and shock. The sicker the horse, the higher the risk associated with anaesthesia; every hour can make a difference.

Property maintenance reduces the likelihood of injuries, and therefore the need for wound repairs, and the potential for emergency anaesthesia.

Well-handled, calm horses will sedate more deeply on lower doses of drugs and have smoother inductions and recoveries, which lessens the likelihood of injury. Early and frequent handling of young horses is vital to improving their chances of successful anaesthesia and also decreases the risk for the people involved. Anaesthetising unhandled horses is a recipe for disaster, but is a risk vets have to take when a horse is in urgent need of this procedure.

When scheduling elective field
anaesthesia, careful timing can decrease the chance of complications. It is best not to schedule these towards the end of the day so that the horse can be easily monitored during daylight hours. The drugs used to anaesthetise horses often disrupt their ability to regulate their temperature, so schedule for early morning during summer and during the middle of the day in winter. Ensure that your horse is caught and calm prior to arrival of the veterinarian since excited horses, or those that have been running around immediately prior, often sedate poorly and need higher doses of anaesthetic drugs - which can lead to more complications. Excited horses also often have less-smooth inductions and recoveries.

Choosing the area in which to anaesthetise a horse in the field is also very important. The area should have no objects that could injure it during this time (and during recovery). If possible, the horse should be on a soft surface such as a grassy paddock or sand arena. This will lessen the likelihood of muscle and nerve damage especially if the procedure is lengthy. Horses can be placed on mattresses and padding once they are induced. It is also important to pad the underside of the lower elbow and support the upper legs. Buckle halters should not be left on during anaesthesia with the horse on its side as the buckle can press on the facial nerve and result in a dropped upper lip following the operation (flat rope halters are often the safest to use on anaesthetised horses). The eyes should be protected from the ground and the sun to prevent eye ulcers and retinal damage.

Like when we ourselves are faced with the prospect of being anaesthetised for an operation, owners are often worried about the safety of their horse and anxious about what to expect both before and after. This is especially true when we don’t understand what happens during the procedure, or what can be done beforehand to make sure it all goes as smoothly as possible. Take the time to discuss with your vet all aspects of your horse’s health, so they have the full picture, and don’t be afraid to ask questions…the more information you have at your fingertips, the more rational you will be about what anaesthesia means for your horse!

Complications associated with anaesthesia
Muscle damage
Nerve damage resulting in paralysis
Pneumonia
Cardiac arrest
Leg fracture
Suffocation
Trauma during recovery
Eye ulcers

What is a specialist veterinary anaesthetist?
A specialist anaesthetist is a veterinarian that has undergone a 3-to-5 year supervised training program specifically in anaesthesia. During this time the veterinarian works under the tutelage of one or more specialist anaesthetists, and training programs are closely monitored by a governing body. Once the training program is complete, in Australasia, the veterinarian has to pass an extensive examination process set by the Australian and New Zealand College of Veterinary Specialists to become a Fellow and be awarded the initials FANZCVA. Only then can they be registered as a specialist, or even use the term ‘specialist’.

About the Authors
Dr Jo Rainger trained in veterinary anaesthesia at the University of Sydney’s Veterinary Teaching Hospital, Camden, passed specialty examinations and is a registered specialist in this area. For the last four years she has worked at UQVETs, Gatton. Jo anaesthetises each horse as if it were one of her own.
Allison J. Stewart is a registered specialist in Equine Internal Medicine and Large Animal Emergency and Critical Care. She recently joined UQVETS after teaching in the USA for 15 years. Allison enjoys teaching veterinary students and training specialists and is a member of Equine Veterinarians of Australia.