9 May 2023
Ilaria Petruccione DVM, MRCVS concludes this article by discussing the risk factors associated with general anaesthesia, as well as patient preparation, recovery and sustainable processes.
Ilaria Petruccione
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As mentioned in part one of this article (Vet Times Equine 9.1), the preliminary results of the Confidential Enquiry into Perioperative Equine Fatalities (CEPEF) 4 show a net improvement of the mortality rate for non‑colic cases (0.6% versus 0.9% of CEPEF 2)1,2.
This improvement may be attributed to several factors:
To establish the best protocol, factors that may affect the risks associated with general anaesthesia in horses need to be taken into account.
Body mass should be measured, or estimated as accurately as possible.
Large horses present higher risks of postoperative myopathy/neuropathy syndrome. They are also more susceptible to developing hypoxaemia.
Weight overestimation may lead to drugs overdose, while the contrary may lead to hypoalgesia/awareness.
Both conditions can affect the recovery phase.
Draught horses present increased risk of developing spinal cord malacia and they are prone to occult myopathy (equine polysaccharide storage myopathy), which may become apparent only during the recovery phase.
Hyperkalaemic periodic paralysis is a genetic condition reported in the Quarter Horse, which can affect preoperative, intraoperative and postoperative choice.
Atrial fibrillation and laryngeal paralysis are more frequent in large/obese horses.
Horses are prey species; they tend to “run away” from any new situation.
Their fear may increase the amount of circulating catecholamines, which, in turn, may increase the sedation requirement. Horses in exacerbating pain may be difficult to handle.
During recovery, some horses may try to regain standing too early as they may feel safer on their feet.
Sex can affect temperament, but also drug choice – a stallion is likely to be more nervous compared to a mare.
The use of acepromazine carries a low risk of priapism or paraphimosis (1:10,000) in breeding stallions; therefore, in these circumstances its use is better avoided.
The use of alpha‑2 agonists is controversial in the first and third trimester for pregnant mares. More care and expertise may be required to anaesthetise pregnant mares, bearing in mind the risk of hypoxaemia is higher due to the reduced venous return to the right heart and cranial splinting of the diaphragm, and the fact the majority of the anaesthetic drugs will cross the placenta.
The anaesthetic risk is higher in foals, probably due to the immaturity of the various apparatus, which may also lead to alterated drug metabolism.
Geriatric horses may suffer from coexisting morbidities – such as Cushing’s disease – which makes anaesthesia more challenging.
Anaesthetic time of longer than 90 minutes is linked to an increased risk of perioperative morbidity/mortality.
Trying to minimise this time – for example, by clipping the horse whenever possible – is, therefore, very important.
Out-of-hours anaesthesia and surgery carries greater risk – even in healthy patients.
According to the CEPEF 2, horses undergoing elective surgeries and premedicated with acepromazine have lower mortality rates compared to horses not receiving it, whereas a lack of adequate sedation is associated with an increased risk.
A good anamnesis and clinical examination will help clinicians to assign the horse to an American Society of Anesthesiologists physical status, according to the one to five classification system. This will help decisions on whether anaesthesia can be performed or further investigations are required.
Before proceeding to general anaesthesia, the patient must be prepared to minimise risk factors.
Horses cannot vomit, but reflux/regurgitation of gastric content may occur during induction, endotracheal intubation or during tracheal extubation. However, food restriction in horses is primarily aimed at reducing cranial splint of the diaphragm and vena cava compression (which predispose respectively to hypoxaemia and reduced cardiac output).
To obtain a significant reduction of the gut/stomach content, at least 18 hours of fasting is required. To date, no consensus exists regarding how long the preoperative fasting should last, although this has been extensively discussed in a meeting of the Association of Veterinary Anaesthetists in March 2019. From the discussion, it was agreed access to water should be allowed until the time of premedication. Until further evidence will be provided, the final “advice” was that access should be restricted to concentrates and large meals of forages for four to six hours before premedication is administered13.
If access to food is allowed until the time of premedication, the mouth must be washed properly to avoid aspiration of material during endotracheal intubation.
For the safety of the horse and handler, shoes should be removed and any sharp hoof rasped smooth before induction. Anaesthesia time will be prolonged if carried out post-induction.
Vascular access should be secured. A wide‑bore cannula in a jugular vein should always be placed before general anaesthesia, even if the procedure is meant to be short.
Some antibiotics – for example, sodium penicillin – can cause severe bradycardia and hypotension. Therefore, their administration should be performed very slowly – and ideally 20 minutes prior to induction of general anaesthesia – to maintain haemodynamic function.
The contemporary use of trimethoprim sulphonamides and detomidine may cause fatal arrhythmias14.
Sedation is covered in part one of this article; however, it is always good to remember alpha‑2 agonists should be always administered before opioids to avoid undesired effects.
It is equally important to remember every drug has a lag time, so before topping up a minimum time should be allowed for the drug to elicit the desired effect.
No major updates have been recently reported regarding induction agents for the equine population. The choice of anaesthetic induction may influence the quality of the induction, but the environment also plays an important role – a calm and silent situation is preferred to loud music or noise.
A lot of attention has been given to the maintenance of general anaesthesia with volatile agents, to reduce gas emission.
In a recent editorial15, some suggestions were made about how to make equine anaesthesia more sustainable. These include:
Two important systematic reviews have been published regarding recovery:
The decision to perform a rope-assisted recovery must be made considering:
Several steps forward have been made in the past years to reduce the mortality rate in horses undergoing general anaesthesia, but it still remains higher compared to small animals. Continuous effort is needed to allow further improvement.
