Vet Insight: General Anaesthesia

David Blakey BVetMed CertAVP(EM) MRCVS started playing polo with The Wilton Hunt Pony Club at the age of 10 and went on to both play and groom in South Africa and the U.K, before setting off for The Royal Veterinary College. He graduated in 2010, and currently works at the Three Counties Equine Hospital in Gloucestershire, the practice that looks after the Polo Times string. He deals with everything from miniature Shetlands to performance sport horses, although he does have a particular interest in anesthesia and older horse management. David regularly plays polo at Edgeworth’s Wednesday evening chukkas and once held a 1 goal handicap, but lack of time and money has curtailed his polo habit

General Anaesthesia

Why, and what are the risks? Photography courtesy of David Blakey A catheter is paced into an artery for direct blood pressure monitoring

An increasing number of veterinary procedures can be performed on horses under heavy sedation, such as wound repair, dentistry, stem cell injections into tendons, wind surgeries and even the repair of some fractures. It is aided by the administration of local anaesthetics, either around nerves to block out whole regions (such as the lower limb) or by injecting under the skin to desensitise the immediate area.

If it is possible to perform so many procedures under sedation with the horse remaining on its feet, why would you need to perform general anaesthesia?

What is General Anaesthesia?

General anaesthesia (G.A.) involves giving a patient a combination of drugs so that they become so deeply asleep that their protective reflexes are abolished – a medically induced coma. This is ideal for surgical procedures as the patient is immobile and cannot consciously feel or remember the procedure.

There are two types of G.A. – field and hospital. A field anaesthetic is usually used for relatively quick procedures on yard such as a castration. Typically, the horse is heavily sedated and then given another intravenous injection containing ketamine which induces G.A. lasting for 20-30 minutes. This is a very safe form of anaesthesia as it is for a short period and the horse is allowed to recover to its feet in a wide, open space.

Hospital anaesthetics are used for longer and more complicated surgeries. The horse is “induced” in the same way as a field anaesthetic, but a tube is placed via the mouth into the trachea and the horse connected to breathing circuit. This delivers a mixture of oxygen and an anaesthetic gas (isoflurane or sevoflurane) that maintains the G.A. once the ketamine has worn off. At the end of surgery, the horse is moved to a padded recovery box where it can breathe off the anaesthetic gas, waking up before getting to their feet.

Why?

Colic surgery is one procedure that requires general anaesthesia, as the horse must be placed on its back so that the surgeons can open up the abdomen to explore its contents to its fullest extent and correct the cause of the pain.

Surgical exploration of delicate structure, such as joints, is only really possible under anaesthesia. The patient can be positioned optimally to allow the joint to be fully assessed with an arthroscope (key-hole camera) and the joint flexed and extended to open up different areas of the joint. This is particularly important when trying to find fragments of bone or foreign objects due to a wound.

Some horses will not tolerate procedures even after very heavy sedation. For instance, I know of two horses who require general anaesthesia for hock medications as they cannot be prevented from kicking the clinician any other way. Additionally, the risk of sudden movement in some sedated horses presents a danger to the operating team, equipment and the horse itself – a bit like trying to change a tyre if the car might drive away at any moment.

The horse is supported behind a padded gate as it is induced to reduce the risk of injury

Once the horse is induced, an ET tube will be placed into the trachea via its mouth

Once induced, the horse is readied to be moved into theatre from the padded box

The Risks

A study performed 15-years-ago collating data from clinics across the UK concluded that the risk of death for a healthy horse undergoing G.A was 1 in 100. With advances in techniques, in our and many Hobbles are placed around the pasterns to lift the horse into theatre The horse is hoisted into theatre and carefully positioned on an air-pad and connected to a ventilator

Recovery floor: after surgery, the horse is moved into the padded box and ropes attached to the head and tail

Procedures requiring G.A.

• Colic surgery • Removal of multiple or difficult to access sarcoid • Key-hole joint surgery • Complicated wound repairs • Most fracture repairs • Most wind surgeries • Some forms of castration • Caesarean section • Surgeries on foals • Electric Atrial Fibrillation treatment • Clinician preference

Recovery standing: the horse has regained its feet and is being steadied by the ropes

other clinics, it appears to be now closer to 1 in 400. In humans, it’s 1 in 100,000.

The main reason for this is due to size. Horses lying on their sides or back for long periods can lead to a myopathy developing, a severe form of damage to muscles due to restriction of blood flow where they are compressed. To prevent this, the horse is positioned on air- pads and the limbs supported. Blood pressure is monitored via a catheter placed into an artery and we regulate drug infusions to maintain blood flow to the muscles.

Recovery of the horse to its feet is the most risky time during the whole process. The incoordination seen and the horse’s size can mean there are chances of injury, including neck and limb fractures, not seen in humans. In our clinic, we utilise ropes attached to the head and tail to help stabilise the horse as it recovers to its feet, and a small recovery box means the horse cannot gain momentum before meeting the padding. In field G.As, this is less of problem if an open space with a good footing is used.

Conclusion

Though it carries risk to the patient, many surgical procedures are best performed under general anaesthesia. The drug combinations, techniques and experience gained by equine veterinary anaesthetists in the last 15 years means that it is a significantly safer process, and facilitates better results for horses and owners.

David Blakey BVetMed CertAVP(EM) MRCVS Three Counties Equine Hospital Email: info@tceh.co.uk Tel: 01684 592099 Web: www.tceh.co.uk