Keteamine has hit the headlines because of its use as a recreational drug, but its greater role in society is as an anaesthetic. It has been in veterinary use since the 1970s and is probably the most widely used equine anaesthetic.
In horses, ketamine is used in nearly all cases requiring anaesthesia — whether for major surgery in the operating theatre or for shorter, simpler procedures such as castrations carried out in the field.
When anaesthesia is needed in an emergency, perhaps if a horse is trapped or injured, a ketamine-based anaesthetic is always chosen. Its reliability and limited side effects help to ensure the safety of the vet and handlers, as well as the patient.
Why ketamine works
It is well known that anaesthesia in horses carries a much higher risk of death or serious complication than it does in humans or small animals.
In spite of numerous hi-tech advances in veterinary medicine, the anaesthetic-related death rate in horses remains at just under 1%. There has, however, been a shift in the cause.
While earlier publications indicated that failure of the heart and circulation was responsible in some way for the majority of deaths, a recent study from a large individual clinic suggests that devastating limb or neck fractures occurring during recovery from anaesthesia are now responsible for a greater proportion of the fatalities.
One explanation for the increased fracture risk could be that more reliable anaesthesia enables vets to carry out more prolonged and complex surgery.
Alternatively, fewer heart and circulatory failures may simply indicate that we are now better at supporting the cardiovascular system during anaesthesia.
The introduction of ketamine as an equine anaesthetic agent more than 30 years ago was seen as a major breakthrough, producing a smooth induction of anaesthesia and — more importantly — a calm, controlled recovery.
The use of ketamine alone does not result in particularly good muscle relaxation. It is generally used after premedication with sedatives such as detomidine, romifidine or xylazine, as well as a valium-like drug. The resulting drug combination produces a state of effective anaesthesia appropriate for surgery.
Among the general anaesthetics used for horses, ketamine is remarkably safe. Unlike others, it does not depress vital respiratory and cardiac function. This is a particularly valuable attribute with horses and may well have contributed to the decrease in cardiovascular-related mortality.
Unfortunately, ketamine is not suitable for maintenance of very prolonged anaesthesia because it accumulates in the body. Inhaled volatile anaesthetics, administered with a purpose-built anaesthetic machine, are routinely used instead. These volatile anaesthetics have their own set of unwanted side effects, however, and infusions of injectable drugs — including ketamine — have more recently been used to limit the dose of these anaesthetic gases.
At this stage, it is too soon to know whether this approach reduces mortality, but ketamine may well have another valuable role in equine anaesthesia here.
A future at risk?
Ketamine is used worldwide to provide anaesthesia and pain relief to both horses and humans.
In many low-income countries it is often the only anaesthetic available. Ketamine has particular value where skills and facilities are limited. Its phenomenal safety record has led to its recognition as a valued field anaesthetic, for trauma, for any situation without sophisticated anaesthetic equipment and where only untrained personnel are available to manage anaesthesia.
Yet concerns about the abuse of ketamine as a recreational drug have led to calls for international control of its availability. The proposed measures would make it more difficult to obtain ketamine for medical and veterinary use.
The resulting restrictions could harm our equine patients — a potential scenario that has spread alarm throughout the veterinary world, and one that should be of concern to all horse owners.
Ref: Horse & Hound; 10 December 2015
