Horse Operations
Sickness in horses is a concern, but illness that requires surgery is worse.
Not only do you have the condition itself to think about, there's the additional worry of what could go wrong under general anaesthetic
So, exactly what risk does anaesthesia pose to horses?. In humans, the chances of dying due to the anaesthetic itself are approximately 1 in 100,000, in small animals the figure is roughly 1 in 1,000 for active dogs. In healthy horses, free from colic, a large study puts the risk at 1 in 100.
Before considering the possible reasons for the high mortality rate in horses, lets look first at what happens when an anaesthetic is given.
Preparing The Horse For Surgery
Before an operation, the horse will be weighed to ensure accurate dosing. This is important as anaesthetic drugs are potentially lethal for any patient. They reversibly depress the nervous systems. An added complication is that the same, or similar, anaesthetics developed for humans are used in horses and they affect the two species in different ways.
Underestimation of weight will result in underdosing. If the animal is only partly anaesthetised he could become excited and, with an accompanying loss of muscle coodination, the situation could become hazardous for the handlers and the horse.
Overestimation of weight is equally dangerous, with the real possibility of overdosing and an excessive depression of body systems as a result.
For an elective procedure, when you are able to choose the timing of an operation in a non-emergency, the animal will not be allowed to eat for 12 hours prior to the anaesthetic.
In people, a period of starvation is needed to reduce the risk of vomiting during induction of anaesthesia. Horses cannot vomit but a full gut will prevent the diaphragm from moving properly, particularly if the horse has to be placed on its back for the surgery, and can hinder its breathing.
There is also a suggestion that a full gut may predispose a horse to colic following the anaesthetic. Water should be freely available before surgery. It is absorbed quickly and horses are unlikely to drink copiuos amounts. Ideally, the animal should be thoroughly groomed before surgery. This will reduce dust and dirt brought into the operating theatre, which lowers the risk of infection.
Prior to premedication, the animal's mouth will be washed out, either with a hose or syringes of water. This is to clear away any food material which could be pushed or pulled into the trachea by the endotracheal tube, or inhaled if a tube is not being used.
The endotracheal tube, inserted in the windpipe, allows oxygen to be delivered along with any anaesthetic gasses that are necessary.
Finally, a catheter will be inserted into the horse's jugular vein under local anaesthetic to allow the drugs to be administered safely and to provide an open vein in case of emergencies.
Horse Going Under Anaesthetic
The anaesthetic process is generally broken down into four phases:
Induction
Maintenance
Recovery
The premed is generally a drug, or drugs given before the anaesthetic to calm the horse in strange surroundings. A non-steroidial,
anti-inflammatory drug, such as phenylbutazone, might be given to provide analgesia or pain relief.
Anaesthetic induction generally takes place in a well-padded box, usually of rubber matting, although straw is used sometimes. This
padding prevents the horse from damaging himself during the induction and recovery phases.
Ideally, the animal will gently sit and then lie down as the drugs take effect. Occasionally, however, some horses grow excited
during the process and fall more abruptly or stagger a little before becoming recumbent. This can be risky for handler and horse. To limit
the danger, some clinics will section off the box to contain the animal against a wall.
Once the animal is on the ground, the endotracheal tube is put into his
windpipe via the mouth or nose.
His legs will be restrained by hobbles and ropes, which also help to position him for surgery. In most clinics a hoist is attached to the hobbles
and the horse is lifted into the operating table. Although this looks dramatic, the unconscious horse does not come to any harm.
To keep horses asleep during surgery, it is common for an anaesthetic machine to deliver gases. Intravenous drugs, used for some procedures which
last more than one hour seem to have fewer associated problems than drugs which are inhaled.
The horse should be closely watched throughtout the time he is anaesthetised. Often an ECG (electrocardiograph) will be attached to record
the electrical activity of the heart. Blood pressure is also monitored as problems can arise if it drops too low. In some clinics, the
amount of oxygen in the horse's blood is also measured to assess the animals breathing
Once the operation is over the maintenance drugs are stopped. After being hoisted back onto the padded box,
the horse is laid on his side and the hobbles removed.
The horse's legs will be put in a neutral position, with the underneath foreleg pulled forward to ease the weight on the muscles of that leg.
The endotracheal tube is removed and another tube may be placed in the nose to allow more oxygen to be given. The horse is then left quietly to regain
consciousness. The intention is for him to roll onto his brisket and get up within an hour of the operation's close.
However, as horses are flight animals, nearly all
injuries are sustained during the recovery phase.
What Can Go Wrong
Complications During Surgery Although vets do not want to frighten any horseowners, or deter anyone from agreeing to vital surgical procedures, general anaesthesia in horses is not to be undertaken lightly.
The major causes of death while under anaesthetic are cardiac arrest during the operation, post-anaesthetic lameness (nerve and/or muscle damage sustained in anaesthetesia which may appear up to seven days after surgery) and fractured limbs or neck in recovery.
Some of these problems may be the result of major dysfunction of the heart, circulation or respiratory systems while anaesthetised, hence the need to monitor these systems closely.
The drugs which maintain anaesthesia are generally delivered as a vapour, which is inhaled by the animal. These drugs not only render the brain and the heart. There is a fine line between anaesthesia that is enough to prevent movement and deaden the animal's awareness and quantities that could result in cardiac arrest. Some occur even with good monitoring and experienced anaesthetists in attendance.
The heart is monitored with regular checks on electrical activity (using ECG) and blood pressure. However, these are not direct indicators of how well the heart is functioning. Developments in monitoring cardiac function are in place, but they are still extremely expensive and require a degree of expertise which is not widely available.
Post Anaesthetic Lameness
After cardiac arrest, the second major cause of death due to anaesthesia is post anaesthetis lameness (PAL), or a myopathy/neuropathy
syndrome.
This has similarities to tying-up or exertional rhabdomyolysis, but in this scenario the muscle damage is due to a depression of the
circulatory and respiratory systems and the weight of the unconscious animal pressing on the muscle masses.
This pressure will be exacerbated if a horse has lain
on a hard surface or been positioned awkwardly. Under general anaesthesia, the blood is not pumped from the heart with enough driving
force or pressure to reach all the muscles.
The weight of the animal pressing down on these makes it even more difficult for the blood
to flow. Without blood flow, oxygen and nutrients cannot be efficiently delivered to the muscle cells and waste products cannot be removed.
This can me made worse by respiratory depression, which results in less oxygen being absorbed into the blood stream. In extreme cases
this can kill off the muscle cells, causing widespread damage. If limited to just one muscle mass, the horse may be able to get to his feet
after the anaesthetic but will be lame. If the damage is widespread he may never stand again.
A midpoint can be that the horse manages to get up but the muscles or nerve damage makes it difficult to bear weight on the affected limb
(similar to us when a leg goes to sleep). This produces uncoordinated movement and may result in a broken leg or, if the horse tips forward
trying to stand, a broken neck.
Minimising The Risk
Studies have shown if we support the circulation and heart with intravenous fluids and specific drugs, reduction in the incidences of myopathies are less. Also long periods of anaesthesia induce more problems than short procedures.
Improved pain relief may lessen the need for inhalational drugs. Alternative means of maintaining anaesthesia with intravenous agents are being explored. In the meanwhile, careful monitoring can spot potential problems.
Some clinics artificially ventilate their patients to ensure that sufficient oxygen is taken up into the blood stream. While this appears to smooth the progress of the anaesthetic and reduces the amount of inhalational drugs required, its effectivemness in reducing death rates is till unproven and may cause other problems.
With proper facilities (including means of monitoring and resuscitation) and staff experienced in equine anaesthesia and surgery, the process can be managed effciently and quickly to minimise the risks to the horse.