Anesthesia, analgesia and sedation: right option at right time

The confidential inquiries into perioperative equine fatalities have contributed significantly to our understanding of the procedures and challenges in equine anesthesia.

ANESTHESIA

Preliminary results from the fourth confidential inquiry (CEPEF4) were published towards the end of 2021 (Gozalo-Marcilla et al), with data compiled from 6,701 procedures under general anesthesia and 1,955 standing sedations across 69 centers worldwide. The most used premedication for general anesthesia is a combination of alpha-2 adrenergic agonists combined with partial agonist or agonist-antagonist opioids (33%).

The addition of acepromazine to this combination comprises an additional 15.3% of procedures. Alpha-2 adrenergic agonist monotherapy was used in 13% of procedures. Approximately 10% used alpha-2 adrenergic agonists together with pure opioids and another 10% used them with acepromazine. Ketamine combined with a benzodiazepine is used for induction in most procedures (88.3%), and isoflurane is the most commonly used maintenance agent (78.3%).

A continued shift to using partial intravenous anesthesia (PIVA) was demonstrated in 3,718 of the 6,701 general anesthetic procedures recorded (55%). The most used drugs for PIVA include alpha-2 adrenergic agonists alone (47.9%), lidocaine alone (28%), or a combination of lidocaine and ketamine (10.3%). In 63% of general anesthetic procedures, additional alpha-2 adrenergic agonist medications were administered in the immediate recovery period, while 29.9% did not receive additional sedation for recovery.

Individual patient factors and clinician preference play a large role in the selection of anesthetic protocols.

Despite improvements in anesthetic protocols and better monitoring, equine anesthesia still has a high risk of morbidity and mortality compared to other species. CEPEF4 reveals a 1% mortality rate for equine general anesthesia. Colic surgeries have a much higher mortality rate at 3.4%, compared to non-colics at 0.6%, although these numbers are improved from the first CEPEF studies 20 years ago. The increased use of intensive monitoring and the use of partial intravenous protocols may be contributing to decreased fatality rates.

Pulse oximetry and electrocardiograms were used in 90% of procedures. Invasive arterial pressure and end-tidal carbon dioxide are measured in more than 75% of procedures. Additional monitoring in more than 50% of procedures includes end-tidal oxygen and volatile agent concentrations, and arterial blood gas analysis in 70% of colic surgeries. During recovery, a perfect storm exists of increased risk of critical incidents, and the cessation of cardiovascular support and intensive technological monitoring. Vigilant observation by a trained member of staff is imperative to improve recovery outcomes.

Standing sedation

With appropriate case selection, standing sedation may be much safer than general anesthesia, but it is not entirely without risk. Colitis, colic and re-fracture caused a 0.2% fatality rate in preliminary CEPEF4 data. The standing position has many advantages for a horse’s cardiovascular and respiratory function, but perhaps the biggest advantage is that the perfect storm of anesthetic recovery is avoided altogether.

In addition to decreased anesthetic risk, additional advantages to performing certain procedures in a standing patient can be noted. Standing sedation should be performed preferentially in cases where positioning improves surgical access or simplifies a procedure. This is relevant in many throat surgeries, such as laser cautery of the larynx, where the absence of an endotracheal tube and the neutral position of the larynx in a standing horse creates significant advantages.

Limb loading in the standing horse may also make tendon identification and palpation easier for tenotomy procedures. The standing position facilitates access via laparoscopy to structures located dorsally in the abdomen and head or within the thorax. Routinely performed standing surgeries include castration, lateral digital extensor tenotomy/tenectomy for stringhalt, medial patellar desmotomy for severe cases of upward fixation of the patella, nephrosplenic space ablation, ovariectomy, cryptorchidectomy, thyroidectomy, digital neurectomy, annular ligament desmotomy, a variety of eye surgeries, osteostixis and partial dorsal spinous ostectomy/interspinous desmotomy, to name a few.

Many types of orthopedic surgery can also be performed in the standing horse, but should only ever be attempted by surgeons who are already highly skilled in performing the procedures under general anesthesia. Standing surgeries need to be performed efficiently, sometimes awkwardly close to the ground, while contending with increased haemorrhage due to gravitation.

Examples of orthopedic surgeries that are routinely performed standing include removal of specific, small osteochondral fragments or simple chip fractures of dorsoproximal P1, condylar and sagittal P1 fracture lag screw fixation, and implant removal.

It is, unfortunately, much harder to maintain a sterile surgical field in a standing patient. The surgical team’s safety is also a paramount consideration, as horses can still behave unpredictably and kick out violently with remarkable accuracy, despite heavy sedation. The horse must, therefore, tolerate the draping and proximity equipment, which some simply will not, regardless of the degree of sedation.

As with any veterinary procedure, the risk versus benefit for the patient and the team, the patient’s temperament, and the clinician’s comfort and skill need to be weighed up in making the best decision for each individual patient.

Image © skumer / Adobe Stock

Analgesia

For short-term analgesia, particularly in a hospital situation, myriad analgesic options include parenteral NSAIDs, alpha-2 adrenergic agonists, opioids, lidocaine and ketamine, as well as epidural and local infusions of a variety of drugs.

Outside of hospital settings, we are largely reliant on oral NSAIDs for long-term analgesia, which all carry a risk of adverse events such as gastric ulceration and right dorsal colitis. NSAIDs are very effective in reducing hyperalgesia, due to peripheral sensitization (for example, with OA). They effectively inhibit inflammation through inhibition of the metabolism of arachidonic acid to prostaglandins, among other poorly understood mechanisms.

In October 2019, BEVA published primary care clinical guidelines for analgesia based on published literature and the opinion of a panel of experts. Key recommendations for routine castrations were:

  • Intratesticular local anesthesia for all horses, regardless of the castration method.
  • Horses should receive NSAIDs prior to surgery.
  • Butorphanol and buprenorphine are not appropriate as sole analgesics.
  • Analgesia should continue for three days after castration.

Among additional analgesic insights from the panel were:

  • Phenylbutazole provides superior analgesia to meloxicam and firocoxib for hoof pain/laminitis.
  • Flunixin and firocoxib are considered more effective than phenylbutazone or meloxicam in horses with colic.

Multimodal pain management

Wherever possible, multimodal pain management should begin before surgery. This may include an NSAID, opioid and a local block. In cases at risk for severe perioperative pain, epidural analgesia should be considered. In addition to being more effective, pre-emptive analgesia has the added benefit of decreasing drug requirements during anesthetic maintenance and smoothing recovery, thereby decreasing anesthetic risk, and the potential for peripheral and central sensitisation.

Some surgeries are known to be more painful than others. One such surgery is neurectomy of the lateral plantar nerve. The author has seen horses trembling and sweating, hypertensive and tachycardic for many hours after surgery where the analgesic requirement was underestimated.

Manipulation of the already sensitized nerve during surgery can result in extreme pain. Hypertension in response to surgical stimulus should alert the anesthetist to the inadequacy of analgesia in a patient who is in an otherwise appropriate plane of anesthesia. Mistaking inadequate analgesia for inadequate depth of anesthesia will increase cardiovascular instability and increase anesthetic risk during maintenance and recovery.

Catriona Mackenzie published an editorial review on analgesia in the Equine Veterinary Journal in 2021, where several studies are cited that advance our knowledge of equine analgesia. Highlights from recently published papers are:

  • Intratesticular mepivacaine results in improved and faster cremaster muscle relaxation compared to lidocaine (Crandall et al, 2020).
  • Intravenous regional limb perfusion with lidocaine or mepivacaine is safe and provides antinociception to the distal limb for the duration of tourniquet application, and improves discomfort associated with tourniquet application (Mendez-Angulo et al, 2020).
  • Buprenorphine at 5mg/kg to 10mg/kg is superior to butorphanol at 0.03mg/kg to 0.1mg/kg for managing surgical pain in a multicentre, prospective, randomized, blinded clinical trial. Importantly, no association was found between administration of either drug and postoperative colic (Taylor et al, 2016).
  • The pharmacokinetics and safety of repeated doses of paracetamol at 20mg/kg twice daily for 14 days have been described in the horse. Although equine therapeutic concentrations are not known, serum biochemistry, gastroscopy and liver biopsy analysis suggests the drug is safe at this dose (Mercer et al, 2020).

It would be helpful to have more objective data on the efficacy of analgesic protocols for individual cases, but also within the profession at large. A pain scoring system should be repeatable with minimal inter-observer variability, which is particularly relevant in practices where cases are not always followed up by the same veterinarian. Various scoring systems have been developed for the purposes of objective pain quantification, the identification of subtle pain and to monitor response to therapy. One system, a visual analogue scale, involves a 10cm horizontal line on which a trained observer draws a vertical line at a point that represents the perceived degree of pain.

The author prefers a numerical rating scale, one of which is described here. Horses are given a score of 1 to 4 for each of 9 different behavior assessments (Table 1). Increasing scores indicate the presence of increasing pain. This numeric rating scale is easy enough for owners and yard staff to monitor, and provides a good objective assessment of behavioral pain indicators to monitor quality of life, disease progression and adequacy of analgesia. Clinicians can set target outcomes and interventions (for example, continue with twice daily “bute” until the pain score falls below 12, or arrange a follow-up consultation if the pain score is above 16). It can also be adapted to include physiological parameters, such as heart and respiratory rates – particularly in hospitalized patients. The right analgesic option for each patient can then be based on this objective assessment.

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