General Anaesthesia in the horse

Horses are large animals and their size and weight brings a different perspective to the challenges of anaesthesia. Bespoke equipment, facilities and preparation are essential to performing safe and effective general anaesthesia, allowing for surgeries on horses to take place. All general anaesthetics carry a small risk of mortality and horses are one of the most difficult species to anaesthetise. This is mostly due to their size, but also because of their temperament and fact that they have a strong ‘flight’ instinct. These characteristics make the process of induction, general anaesthesia and recovery difficult. The description given below is the basic procedure that occurs at Clyde Vet Group Equine Hospital which is typical of most leading equine facilities in the UK.

Planning, case selection and preparation

Planning anaesthesia will depend on the case presenting for surgery. An elective surgery is pre-planned and carries a lower risk compared to an emergency surgery on a sick horse, for example a colic surgery.

Prior to surgery the horse, donkey, pony or foal will be examined by the vet carrying out the anaesthetic. Vital statistics will be recorded and the heart will be listened to with a stethoscope to check for any murmurs or arrhythmia that may increase the risk of anaesthesia. The vet will check that the horse has no additional issues to factor into the risk of performing a general anaesthesia, for example a previous injury to a limb which may affect ability to get up after surgery, or a fever and possible infection which could increase the risk of anaesthesia and also of recovery post surgery.

Some surgeries will be categorised as high risk, but go ahead because it is the only chance to save the horses life. These are usually emergency surgeries where the horse may be very sick and dehydrated, such as colic surgery

Pre anaesthetic

Before the surgery the horse should be starved for up to 12 hours and water removed 4 hours before surgery. This helps to reduce the amount of food sitting in the stomach and lowers the risk of regurgitation and food getting into the windpipe during the procedure which could result in aspiration pneumonia. The horse will ideally be settled at the equine hospital the night before surgery so they are not travelled and coming off the trailer just prior to surgery. If possible the horse will be weighed on arrival to allow for accurate drug calculations to be made. At this time shoes are also removed to prevent the horse from injuring themselves when recovering from the anaesthetic, and from damaging the recovery room floor.

Acepromazine is a drug which effectively calms the horse and is given around half an hour prior to the general anaesthetic. At this time the horse is often clipped in the surgical site and an intravenous catheter is placed for administration of medication and anaesthetic drugs, as well as fluids during the anaesthetic.

In order to reduce the risk of surgery in horses time under general anaesthesia needs to be kept to a minimum. For this reason as much preparation for surgery as possible is done prior to induction.

The anaesthetic equipment in the theatre is set up in advance of the general anaesthetic, ensuring that all the equipment is working well and in place before embarking on induction of the horse. Drugs are drawn up specifically for the weight of the horse, and an anaesthetic sheet is filled out ready to record details of the case, drugs administered and for monitoring during anaesthesia.

At the time of induction, the horse will have the mouth washed out to remove any trapped food which may interfere with the endotracheal tube which is placed in the windpipe to administer the gas anaesthetic. The feet are also picked out, and the horse is lead into the induction room. The induction room is a padded, enclosed room which has access to the theatre from the stables area of the hospital. Here the horse is sedated and the anaesthetist will wait until the horse is sleepy, with the head dropped and the horse standing positioned in the correct place next to the wall of the induction room. Often the head collar will be replaced with a soft padded head collar at this stage.

Around 5-10 minutes after the horse is sedated, general anaesthesia will be induced by administration of ketamine and diazepam. The horse takes about a minute to circulate the drugs and then ideally gently drops to the floor guided by the anaesthetist and veterinary colleagues.

A long tube is placed through the mouth into the windpipe to allow administration of gaseous anaesthetic and oxygen. The horse has hobbles placed on the lower limbs and is hoisted onto the surgical table in the theatre. Pads are used to cushion the horse into position and prevent any pressure points on the muscles.

Maintenance of anaesthesia

During surgery the horse is monitored closely. An ECG is placed to measure the heart rate and rhythm. An arterial catheter is placed to measure blood pressure. The anaesthetist is responsible for checking the depth of anaesthesia by monitoring the horse closely along with information from the anaesthetic monitor. The horse is given a constant infusion of fluids to maintain fluid volume and prevent dehydration, and a urinary catheter is placed to collect urine. Due to the size of horses, any change noted during anaesthesia needs to be reacted to immediately. Drugs are always available to increase the horses heart output. If there is a need the horse can be mechanically ventilated if required. Communication between the anaesthetist and surgeon is important throughout the anaesthetic so each knows how the horse is doing, how long the surgery is taking and what medication the horse requires.

Recovery

Once surgery is finished the horse is hoisted into the recovery room and placed in a safe position. The lower legs are stretched and the urinary catheter is removed. Ear plugs are placed and a towel covers the eyes to ensure a dark quiet environment. The endotracheal tube is removed and a nasal tube is placed to maintain the airway in case the nasal passageways swell. This is a critical time for the horse as recovery is the most dangerous time when the horse can get injured. At Clyde veterinary hospital we have a rope pulley system where a head rope and tail rope are placed to help stabilise the horse when it attempts to stand. In other cases the horse is left to get up itself. Whichever decision is made the horse is monitored closely from the other side of the locked door until it is standing safely. Most horses take around 45 minutes to wake up. In the ideal situation they will wake up in stages first sitting on their sternum l before attempting to stand.

Anaesthesia in horses has improved dramatically in recent times and modern facilities and skilled veterinary personnel can result in very low rates of complications occurring and many successful surgeries being performed.