Anaesthesia and analgesia for the orthopaedic patient

The past 15 to 20 years have seen a large increase in the number of equine practices that have invested in developing the facilities and personnel to undertake surgeries that, previously, would have been referred. Indeed, a number of these practices have become referral centres.

The drivers for this change are many and probably include:

• the increase in the number of trained equine surgeons
• the desire of practices to improve their service provision and the job satisfaction they provide to their staff
• the increase in the number of surgeons who choose to undertake peripatetic work
• client expectations

As a result, it is likely the number of surgeries being undertaken in horses is higher than ever. What is more certain is that many more practices are undertaking surgeries.

However, while the number of trained equine surgeons working in practices has significantly increased, the number of trained equine anaesthetists has lagged behind – and this can mean, in many cases, the person delivering the anaesthetic may have had little training and/or experience.

One justification that may be offered for a practice investing in a trained equine surgeon, but not a trained equine anaesthetist, is that the practice is likely to be focused almost exclusively on orthopaedic procedures (for example, rather than colic cases), and orthopaedic surgery is more likely to be undertaken in healthy animals with a predictable surgery length, so the anaesthesia should also be routine. Therefore, any veterinary surgeon should be able to perform the anaesthesia.

Regardless of whether I agree with this argument, I do think anaesthesia and analgesia in orthopaedic patients have their challenges. Therefore, the purpose of this article is to highlight aspects of anaesthesia and analgesia for the orthopaedic patient that I hope will be useful for colleagues involved in anaesthetising horses to consider when about to anaesthetise their next patient.

Anaesthetic risk

When considering risk of anaesthetic-related morbidities or mortalities, or discussing such risks with clients, particular risks exist that may be more relevant in some orthopaedic patients.

Mortalities

At vet school, we are taught young, fit and healthy animals are the lowest anaesthetic risk. However, in horses, I think this statement is not necessarily true.

Firstly, I think evidence is growing that certain causes of equine anaesthetic-related mortality are over-represented in young animals – for example, in spinal cord myelomalacia (SCM), some of the risk factors include an animal being younger than four years old and dorsal recumbency (Hughes et al, 2019).

One common reason, other than castration, for anaesthetising a horse younger than four years old is to perform orthopaedic surgery – and I have observed at least two cases of SCM in young horses anaesthetised for orthopaedic surgery over the years.

Secondly, I think fit horses provide particular challenges for the anaesthetist in two respects:

• The speed drugs distribute around the body and into the brain as a consequence of the low heart rate.
• If the horse is presented having recently trained or competed (for example, off-track) then the autonomic balance and circulating catecholamines of that animal can also cause problems, particularly with arrhythmias.

Morbidities

Many orthopaedic procedures are carried out with the horse in lateral recumbency, and the anaesthetist should be mindful some morbidities are more common in lateral recumbency – for example, myopathies (Johnston et al, 2004).

Additionally, evidence exists that other types of morbidity may be over-represented in orthopaedic patients – for example, non-septic orthopaedic surgery may be a risk factor for post-anaesthetic colic (Senior et al, 2006).

Apart from the aforementioned risk factors that may be more relevant in orthopaedic patients, epidemiological studies into equine post-anaesthetic mortalities and morbidities highlight not only the importance of minimising anaesthesia time to reduce risk, but also the importance of careful consideration of the timing of anaesthetic induction – both in the sense of where out-of-hours induction is less likely to result in survival (Johnston et al, 2002; Milner et al, 2014), but also to bear in mind “normal” hours timing and risk.

For example, when compared to inducing anaesthesia (in non-colic cases) between 6am and 1pm, anaesthesia induced between 1pm and 6pm carries about 1.5 times the risk of death (Johnston et al, 2002). Therefore, should we endeavour to undertake as many procedures as possible in the morning?

In terms of minimising anaesthesia time, apart from routine tasks – such as taking care to do as much of the preparation of the surgical site as possible prior to induction of anaesthesia – the anaesthetist should also take a proactive role in case planning and reminding the surgical team of the time elapsed in surgery, as, in my opinion, it may be more prudent to undertake two or more shorter surgeries in a horse, than try to undertake one very long surgery.

The recovery period is when most anaesthetic-related deaths occur. Nowadays, fractures are the most common cause of anaesthetic-related mortality (Dugdale and Taylor, 2016) – with one purported reason for this being the use of isoflurane and sevoflurane, rather than halothane. If catastrophic injuries are becoming more prevalent since isoflurane and sevoflurane replaced halothane, efforts should be made to mitigate against these injuries.

Horses recovering from anaesthesia that have significant pre-existing pathology, or have had significantly invasive orthopaedic surgery, need careful consideration as to how to minimise the risk of fatal complications in recovery. One answer is to not anaesthetise horses – and the increasing range and number of standing surgeries is likely a good thing in terms of reducing mortality.

Another way to mitigate against recovery injuries is to intervene in the recovery period through chemical (for example, sedation) or physical (for example, assisted recovery) restraint. Another mitigation is to use partial IV anaesthesia (PIVA) to reduce the amount of inhalation agent used during anaesthesia. These mitigations are briefly discussed later in the article.

Protocol

In terms of perianaesthetic protocol, the main area I think the management of cases and outcomes can be improved is through the provision of sufficient multimodal and preventive analgesia.

Multimodal analgesia is the practice of using different drugs that act on as many parts of the pain pathway (transduction, transmission, modulation, perception) as necessary. Preventive analgesia is ensuring, where possible, analgesia is present before and during the time the horse may experience pain – preoperatively, perioperatively and postoperatively.

Broadly speaking, six main classes of analgesic drugs are used in veterinary medicine, for which we use the acronym NOLANP:

• N: NSAIDs
• O: opioids
• L: local anaesthetics
• A: α2-adrenoreceptor agonists
• N: N-methyl-D-aspartic acid antagonists
• P: paracetamol

Many anaesthesia protocols will likely involve four of the six classes (NSAIDs, α2-adrenoreceptor agonists, opioids and ketamine), but how you further use these classes of drugs – and more locoregional analgesic techniques – is worth considering. Two examples would be PIVA and use of local anaesthetics orlocoregional techniques.

PIVA (using drugs with analgesic effects)

The idea of infusing IV analgesic agents – for example, opioids, local anaesthetics, α2-adrenoreceptor agonists and ketamine – throughout anaesthesia and surgery is threefold:

• you provide analgesia with reduced side effects, as long as the infusion (and whatever lag period) lasts
• some agents may provide additional benefits attractive to some equine anaesthetists – for example, stable mean arterial blood pressures and reduced movement
• in theory, less inhalation agent should be needed, which may improve recovery quality and recovery times

Many protocols and combinations exist, but some of these techniques have disadvantages. Therefore, I encourage readers to look at two excellent reviews by Gozalo-Marcilla et al (2014; 2015).

Local anaesthetics and locoregional techniques

Administration of agents – mainly local anaesthetics, but other agents, such as opioids, α2-adrenoreceptor agonists and ketamine – using locoregional techniques can greatly facilitate pain relief.

Drugs can be administered in a variety of ways depending on the case requirements – for example, perineurally, intrasynovially, epidurally, through IV local anaesthetic techniques (Bier’s block), and through direct application on to tissue, such as splash blocks or the use of wound catheters.

As usual, these techniques can have various disadvantages, depending on the technique and drug chosen, but if we always consider what locoregional techniques could be used in every case then, perhaps, they may be used more.

It is worth noting the local anaesthetic agents are the only drugs we can label as analgesics in the (Greek) literal sense (an- without; algesic- pain/discomfort), as they are the only drugs capable of completely obtunding nociceptive stimuli. Therefore, they can greatly facilitate surgery, anaesthesia and rehabilitation.

Recovery

In general, I prefer to recover horses with their injured/affected leg uppermost; however, occasionally, risk/benefit decisions may have to be made if that means the horse is placed in the opposite lateral recumbency than it had spent a significant length of time in on the table – for example, weighing up whether it would be worse for a horse to have to lie on an injured or affected leg and have to use that leg to try to move from the recumbency it is in, against asking the horse to be able to have efficient gas exchange and inhalation agent elimination from a congested lung.

In our clinic, we use a rope recovery system in the majority of cases, as not all horses are suitable candidates for rope recoveries on the basis of size, temperament and procedure (for example, head/eye surgery).

As aforementioned, the recovery period is where most anaesthetic-related deaths occur and many of these are as a result of fractures – and certain (orthopaedic) cases, such as fracture repair, are probably at much higher risk of catastrophic injury in recovery.

Standing surgeries

As anaesthetic-related mortalities have remained at about 1% for elective procedures in horses, more surgeries are being undertaken in a standing horse for some orthopaedic cases – and much merit to this approach exists for suitable horses and surgeries.

However, it should be noted prolonged sedation for standing surgeries can produce its own morbidities – for example, eye lesions and colic.

In summary, anaesthesia and analgesia for equine orthopaedic patients carries particular challenges for anaesthetists. An understanding of these – and how to identify and mitigate them – is likely to improve the anaesthetic-related outcome in these cases.