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:
- The present tendency to use an inhalant agent in combination with an IV constant‑rate infusion – the so-called partial IV anaesthesia (PIVA) – aiming to reduce the volatile anaesthetics and to improve the quality of recovery. Since the publication of the CEPEF 2, numerous reports about the use of PIVA techniques using different drug combinations have been published3-5.
- It has become common practice to sedate horses with an alpha‑2 agonist before the recovery period, either when the patient is still on the surgical table or once in the recovery box. This benefit was demonstrated firstly by Santos et al6 and confirmed later by other authors7,8.
- As suggested from other studies, improvements in monitoring have contributed to a reduced risk of anaesthetic mortality9. The CEPEF 4 indicated about 90% of the horses undergoing general anaesthesia had at least an ECG and a pulse oximeter.
- The induction of general anaesthesia was meanly assisted, with either personnel (61%) or using a gate (25%)1, although the quality of the induction was not investigated.
- A total of 51% of recoveries were free, 41% were assisted with ropes and 8% were manually assisted (mostly foals). Unfortunately, these data didn’t answer yet the controversy regarding the best method of recovery from general anaesthesia. Head and tail rope systems cannot completely prevent fractures during recovery10-12, but this technique may be beneficial for some cases. The use of slings was also very limited and, again, did not ensure a good recovery.
Risk factors
To establish the best protocol, factors that may affect the risks associated with general anaesthesia in horses need to be taken into account.
Size
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.
Breed
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.
Temperament
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
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.
Age
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.
Surgical time
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.
Time of day
Out-of-hours anaesthesia and surgery carries greater risk – even in healthy patients.
Anaesthesia protocol
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.
Patient status
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.
Preparation
Before proceeding to general anaesthesia, the patient must be prepared to minimise risk factors.
Food restriction
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.
Mouth wash
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.
Shoe removal
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
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.
Preanaesthetic medications
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
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.
Induction agents
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.
Sustainable anaesthesia
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:
- Reduction of fresh gas flow (up to 2L/min in a 500kg horse).
- Consider total IV anaesthesia for short procedures.
- Choice of anaesthetic agent: sevoflurane (with the shortest atmospheric lifetime) has a lower impact than isoflurane – and both have a significantly lower impact than desflurane.
- Reduce anaesthetic time: where appropriate, consider clipping surgical sites prior to anaesthesia. Ensure the theatre team is present and coordinated, and minimise avoidable delays during anaesthesia.
Recovery
Two important systematic reviews have been published regarding recovery:
- A review regarding risk factors and influence of interventions during the recovery period concluded that recovery quality can be improved by the administration of an alpha‑2 adrenoreceptor agonist immediately prior to recovery16.
- Lloyd and Murison17 concluded the evidence available was insufficient to always recommend rope assistance during recovery from general anaesthesia in equids. They concluded that rope assistance may improve time and quality of recovery in some patients.
Rope-assisted recovery
The decision to perform a rope-assisted recovery must be made considering:
- Individual patient: temperament is one of the main factors affecting the choice of recovery method; for example, a yearling will hardly tolerate ropes compared to an older horse used to being handled. Anecdotally, some breeds – such as Arabian horses – do not tolerate ropes; again, this should be evaluated individually for the individual patient.
- Clinical condition: recovery quality is likely to be affected from clinical conditions such as perioperative hypotension, hypothermia and hypoxaemia – as well as the duration of anaesthesia (longer than 90 minutes) – regardless of rope assistance.
- Team and facilities: it is essential for personnel to be familiar with the rope system, and an adequately designed recovery stall is essential to avoid injuries in the postoperative period.
Conclusion
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.
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