In this article Luis Filipe Louro DVM MSc MRCVS, anaesthetist at Scott Dunn’s Equine Clinic, explains the procedure from start to finish.
General anesthesia is a state of controlled, reversible unconsciousness resulting from the administration of anaesthetic drugs to suppress nervous function. It prevents the horse feeling pain and becoming distressed during a surgical procedure.
Unfortunately horses do have a higher anaesthetic risk than any other domestic animal. This is mainly due to their size and weight, as well as their temperament. It is very important for the surgery team to assess each individual case thoroughly before surgery to decrease the possibility of complication.
Because horses are so heavy their lungs do not function as well when lying on their backs or side, reducing the levels of oxygen in the blood around the body. The anesthetist is careful to position the horse correctly on the table and constantly monitor blood pressure to minimise the risk of any muscle damage.
When recovering from anaesthesia the horse’s natural ‘flight instinct’ may cause the horse to panic, possibly injuring itself in the process, however every precaution is taken to reduce the risk, with fully padded recovery boxes and constant supervision.
For routine operations the hospital will usually request that your horse’s shoes be removed before arrival to help prevent the horse injuring itself when being anaesthetised and recovering. In an emergency situation be prepared for your horse to come home without shoes as the hospital will remove them before any general anaesthetic.
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It is often routine to starve the horse overnight before surgery. Usually the horse is admitted in time for this to be undertaken at the hospital, but always check with your vet whether you need to withhold food yourself and for how long.
It is sensible to contact your insurer before any procedure; you will need their permission to operate to ensure it is covered under your policy.
Prior to anaesthesia the horse is weighed to ensure accurate drug dosage and subjected to a thorough pre anaesthetic examination. The heart and lungs are checked to detect any cardiac or respiratory problem as well as other vital signs such as temperature and mucus membrane colour to determine any health issues that might compromise the success of anaesthesia.
A pre anaesthetic blood test is run to insure that no muscle, liver damage or immunosuppression is present. The horse’s mouth is flushed and the area requiring surgery is clipped and prepared. Finally an intravenous catheter is placed in his jugular vein so drugs can be administered safely (Picture 1).
The horse is sedated to keep it calm before induction and it is led to the fully padded induction or ‘knock down’ box. The anaesthetist will administer an induction agent by injection. This is the initial stage of anaesthesia and will cause the horse to become unconscious and recumbent.
The surgical team is present to assist the anaesthesist and make sure the horse goes down quietly. The next step is to insert an Endotracheal Tube, through the mouth of the horse into the trachea (Picture 2). This tube will keep the horse’s airway open whilst delivering the anaesthetic gas to maintain the anaesthesia whilst on the operating table.
After induction as been completed successfully the horse is moved into the operating theatre using an overhead winch (Picture 3) which lifts the horse using hobbles on its legs. This may sound dramatic but it is a safe and effective way of lifting such a heavy animal.
Next the horse is positioned on the operating table (Picture 4). This is one of the most critical aspects of anaesthesia, as the horse must be positioned carefully to allow access to the area that needs attention without compromising blood flow to any muscles.
During surgery the anaesthesist is responsible for monitoring the vital signs, blood pressure, respiratory rate, heart rate and ECG. He must also constantly observe other parameters such as the amount of anaesthetic gas used and artificial ventilation (mechanical breathing assistance) if necessary. Nowadays this is done with the help of very reliable monitoring and anaesthetic machines which reduce the risks of anesthesia significantly. (picture 5)
At the end of the procedure the horse is winched into the padded recovery box with a nasal tube and oxygen to allow him to breathe and recover from the anesthesia. More sedation is also administered to keep the horse calm while the anaesthetic gas is exhaled from his lungs.
The horse is usually left quietly in the padded box to recover on his own but he will be under constant observation by the anaesthetist until he is safely standing. Some hospitals also use assisted recovery by helping the horse to stand with a rope and pulley system. Once the horse is standing and stable on his feet he will be led back to his stable to start his aftercare regime.
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