In some cases, keeping a horse on his feet for surgery is safer and more effective than general anaesthesia. Patrick Pollock FRCVS outlines what’s involved
While general anaesthesia is a wonderful technique, allowing types of surgical intervention that veterinary surgeons of the past could only dream about, it is not without its risks. A ground-breaking study performed in 62 clinics across the globe, including 41,824 horses, over six years, determined that the risk of death in horses undergoing general anaesthesia for elective surgery was 0.9%. Put simply – and this is what I say to every owner whose horse requires a general anaesthetic – around 1 in 100 normal horses subjected to a general anaesthetic do not survive. Once you add in sick horses, including those with pre-existing risks such as the severe compromise that goes alone with colic, or the risks of pregnancy, the risk of mortality increases further. By contrast, the anaesthetic mortality rates in dogs is around 0.02% – and in humans the rate is closer to 1 in 100,000.
There are many reasons for death or injury during induction, maintenance and recovery from anaesthesia. These include an abnormal reaction to the medications used to induce and maintain anaesthesia, an unforeseen event leading to severe depression of the heart or lungs, or a significant musculoskeletal injury such as a fracture or dislocation during recovery.
In addition to these complications, horses are not really designed for recumbency. Once anaesthetised, lots of body systems become compromised. The heavy abdominal organs compress the thoracic cavity and lungs, reducing the amount of oxygen getting into the blood, and the lungs gradually collapse so that as the anaesthetic progresses it is possible for the horse to become hypoxic (starved of oxygen).
At the same time, the great blood vessels (the aorta and vena cava) are compressed, reducing the amount of blood returning to the heart and increasing the work required by the heart to pump the blood around the body. This combination of reduced perfusion (blood flow) and reduced oxygenation (ventilation) is a perfect storm and referred to as a ‘ventilation/perfusion mismatch’. Alongside hypoxia, this can lead to poor blood flow to the muscles, causing further problems such as myopathy (muscle damage) and occasionally even the development of abnormal heart rhythms (arrythmias) which can have fatal complications.
Although the science of anaesthesia and the skill of anaesthetists has never been more advanced, anaesthesia will never be a procedure which can be performed totally without risk.
Some surgical procedures can be performed only during general anaesthesia, so work goes on to reduce the risks. However, in recent years, an increasing number of procedures have been attempted with the horse standing and sedated. Some of this is due to the development of sedative and local anaesthetic protocols that allow the horse to be restrained in a welfare-friendly manner that is safe for both the patient and the vets. But, as vets transitioned to undertaking more and more surgery with the horse standing, it became clear that many procedures were easier, quicker, more successful and safer for the horse when performed with the horse standing, sedated and with appropriate pain control.
Pioneers of standing surgery
As a surgical resident, I was lucky enough to work with two of the pioneers of standing surgery, Professor Jim Schumacher and Dr Tom Russell. Jim realised that a number of surgical procedures could be accomplished more safely and at less cost with horses standing and undertook the first standing surgery procedure in the 1990s. This was followed by reproductive, airway, ocular and even some colic surgery performed on the standing horse. However, it was probably the area of orthopaedics and specifically fracture repair that was the most transformative. Tom was concerned that following the repair of medial condylar fractures (a type of cannon bone fracture which is inherently very unstable), up to 25% of horses suffered a catastrophic and fatal breakdown of the leg while returning to their feet after surgery. Jim and Tom were pioneers – it was amazing to be in the room when the first screws were placed into the limb of a standing horse, or a large bone flap was created and opened on the front of the horse’s head to gain access to the sinuses.
Multiple different procedures are now routinely performed with the horse standing and sedated. These include enucleation (removal of the eye) and other ocular surgery, removal of the thyroid, laryngoplasty (“tie-back”), dental surgery, sinus surgery, wound reconstruction, skin grafting, foot surgery, reproductive surgery, laparoscopic (keyhole) thoracic and abdominal surgery – and that’s only the tip of the iceberg.
These procedures are possible because we have some fantastic sedative and pain relief protocols, including the development of continuous rate infusion (CRI). During this technique, an intravenous catheter is placed into the jugular vein in the horse’s neck and the anaesthetist drips in the medication mixture throughout the procedure. A combination of medications used that not only reduces movement, but also eliminates anxiety in the horse. It is fair to say that we can never really know what a horse thinks or feels, but careful observation and communication between anaesthetists and surgeons has led to protocols that keep the horse quiet and non-reactive. Movement, heart rate, facial expression and other reactions are monitored throughout the procedure and the quality of the sedation is scored to allow us to continually refine these protocols.
As more and more procedures were performed with the horse standing, vets began to notice that post-operatively these horses had lower pain scores and returned to normal behaviours, such as eating and interacting with other horses in the hospital, more quickly. They generally made a faster recovery. But sedation is only part of the story; it is the local anaesthetic protocol that really makes these procedures possible. Anyone who has ever had a lameness examination performed on their horse will know that nerve blocks are a big part of equine veterinary practice. This technique involves injecting a small volume of local anaesthetic over a nerve, eliminating sensation to a specific area. Equine vets pioneered these techniques, ahead of our small animal and even human medical colleagues, so it was therefore easy to adapt these diagnostic procedures into nerve blocks to allow specific surgery to be performed.
In addition, the horse’s ears are plugged prior to surgery to reduce the stimulation that may be caused by sound. One eye may also be covered.
The surgical procedure
As a surgeon, I will always discuss details of the surgical procedure with the horse’s owner. This includes the risks, success rates and aftercare involved, but now it may also include the fact that this procedure could be performed standing. Many horse owners understand the risks of general anaesthesia, but it is not unusual to be asked “Could my horse come around from the sedation during surgery?” or “Will he feel anything?” These are understandable concerns, but the answer to both questions is no. Recovery from sedation and local anaesthesia is gradual, and, because the horse is carefully monitored throughout the procedure, any changes noticed by the anaesthetist that might suggest that the effects are reducing can be quickly corrected. The appearance of most horses during these procedures suggests that they are almost totally unaware of their surroundings, and most will not react to stimulus. Humans undergoing these types of procedures often have difficulty recalling what happened and there is no reason to suspect this is not also the case in horses.
There are of course some risks with any type of sedation and surgery. The medication combinations used have effects on many different body systems; one of these is the motility (movement) of the intestines. What that means is that horses sedated for prolonged periods are at a slightly increased risk of developing an impaction and showing colic signs. To mitigate against this, we generally place a stomach tube at the end of a standing surgical procedure and administer a mixture of water and electrolytes to encourage normal intestinal movement and soften the ingesta (gut contents).
Lots of things must be taken into account before undertaking any surgery, such as how to clean and prepare the horse so that the surgical site is sterile and doesn’t become contaminated during the procedure. For some of our surgery, the horse will wear a compression suit with zips that allow access to specific areas. This reduces the chance of hair and dust falling into the surgical incision. Because we cannot use clamps to attach the surgical drapes, as would be common during general anaesthesia, we use adhesive drapes and often sterilise bandages to keep areas clean.
Accomplished by feel
Recently, we began to undertake more complex procedures in the standing sedated horse, one of these being prosthetic laryngoplasty or “tie-back”. During this complex and technically tricky surgery, which is a treatment for laryngeal paralysis, one side of the larynx (voice box) is permanently pulled out of the airway to allow the horse to breathe more easily. With the horse anaesthetised and lying on his side, the surgery was performed through a small incision with much of the procedure accomplished by feel. It sometimes felt like operating down a cave. In contrast, the same procedure in the standing sedated horse is almost easy by comparison – the horse’s body is in natural, upright alignment and his neck is stretched out and at the surgeon’s eye level, so surgical access is more straightforward. Because the horse is “awake”, an endoscope placed in his airway allows the surgeon to work out perfectly exactly how much to “tie-back” the paralysed side of the larynx.
Perhaps the pinnacle of standing surgery has been the move to undertake re-innervation of the paralysed larynx. During this procedure, the larynx is literally re-plumbed and given a new nerve supply by implanting two nerves into the paralysed muscle. We began doing this in the standing horse last year – it is easier and the success rates are higher than when performed during general anaesthesia.
Ref Horse & Hound; 20 August 2020