Equine sedation and anaesthesia – right options at the right time

General anaesthesia in horses still carries a high mortality risk. 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% in CEPEF 2).

For the first time, the study also included standing sedation, for which the mortality rate is reported to be 0.2%. Despite the improvement, the mortality rate is still significantly high compared to human or small animal anaesthesia.

For this reason, the choice of the method should be multifactorial and the owner should be informed regarding the risks carried by general anaesthesia. Standing sedation is also increasing in popularity for this reason.

Sedation

Horses are prey species, driven by their sympathetic nervous system – that is, they will tend to “flight” away from any new situation. Physical restraint alone is often insufficient – especially when painful interventions are required. Therefore, to safely perform surgical and medical procedures, it is often necessary to immobilise the horse.

No single drug produces “ideal” chemical restraint in every horse. Drugs are used in combination to optimising the onset, quality and duration of sedation while minimising side effects.

Sedatives and anaesthetic drugs act on a wide range of organ systems – including the brain, heart, blood vessels and lungs – to achieve their effect.

The interactions between sedative and/or anaesthetic drugs, and these organs and system, are complex and occasionally may be unpredictable. For this reason, before any treatment is administered, a full history – including details of patients, such as age, vaccination status, previous and current disease – should be obtained. Before making a final choice of agent, method and route of administration, the following factors need to be considered:

• Patient physical condition (concurrent disease or condition as pregnancy).
• Patient temperament (this may be affected by breed).
• Patient and staff safety (facilities and available personnel, their experience and ability to help).
• Environment (a field rather than in a clinical hall).
• Procedure (duration preview, major versus minor surgery, degree of pain/nociception expected).
• Familiarity with the drugs/technique.

The efficacy of any sedative or anaesthetic agent can be influenced by individual variation in response to the drug’s actions and also by external factors (such as the level of noise/distraction) which may be difficult to control.

The animal should be left undisturbed. It is important take into account these circumstances and allow sufficient time for the drug to produce an effect. Bear in mind, sedated horses may be still arousable and kick or bite.

Standing sedation is commonly used to perform surgical and medical procedures in horses when general anaesthesia is not warranted or desired. Advantages of standing sedation over general anaesthesia include:

• lower complications rates
• less cost
• simpler monitoring
• reduced time to complete the procedure in most cases
• reduced risk of mortality imposed by general anaesthesia

Disadvantages of standing sedation could include:

• excessive sedation, which may result in ataxia and recumbency (on rare occasion)
• not ideal surgical conditions, as the horse may still move, and potential danger to personnel and horse if the sedation level is inadequate

A multimodal, balanced approach by supplementing sedatives and tranquillisers with systemic analgesic or loco-regional anaesthetic techniques will facilitates standing procedures. Therefore, the choice of the appropriate drug combination is essential to achieve an adequate level of sedation for the procedure.

Patient evaluation and preparation

Patient evaluation should always be performed as it would be prior to general anaesthesia. Horses not used to being handled or in exacerbated pain may not be the best candidates for standing sedation. Specific preparation may be required (such as fasting prior to laparoscopic surgery).

The procedure should be carried out in a quiet location and the patient should become accustomed to this environment before any drug administration.

This is important when drugs are administered by the oral or IM route to avoid delayed onset of action. An IV catheter should always be aseptically placed to facilitate drugs administration and avoid perivascular or intracarotid injection.

A urinary catheter may be helpful for procedures longer than 60 minutes, as α2-agonists promote urinary production. The head should be supported with a stand placed in front of the horse.

Demeanour, cardiovascular (heart rate, capillary refill time, peripheral pulse quality), respiratory parameters and degree of ataxia should be monitored to detect excessive sedation or drug-induced side effects.

To improve adjustment of infusion, the Ghent algorithm has been recently published. Albeit, it is strongly advised to be always ready to convert to general anaesthesia in case it is needed.

Drug administration techniques and combinations

Drugs are either administered by IM, IV bolus or constant rate infusion during the procedure. IM injection may be performed 30 to 45 minutes before the start of the procedure to reduce the horse’s stress, improve sedation and decrease infusion requirement.

Sedation is mainly based on α2-agonist. An opioid should be included to improve quality of sedation/analgesia. Low doses of either ketamine or lidocaine can be added as part of a balanced protocol to improve sedation and analgesia.

A loco-regional nerve block and epidural injection should be performed wherever appropriate, as they reduce the required level of sedation and augment the patient’s tolerance to the procedure. Recent highlights regarding standing procedures include the following:

• Olivera et al (2021) proposed and validated a sedation scale in horses that offers the advantage to differentiate between high level of tranquilisation and low sedation in horses sedated with acepromazine and/or detomidine.
• Loomes and Louro (2022), in a clinical commentary, highlight the fact that even if performing fracture repair in standing sedation removes the risk associated with recovery from general anaesthesia, it still involves several considerations, among which are safety of the patient and staff in an adverse or unexpected event.
• Haunhorst et al (2022) compared effects of butorphanol or buprenorphine in combination with detomidine and diazepam on the sedation quality, surgical conditions, and postoperative pain control after cheek tooth extraction in horses. A bolus of detomidine (15μg/kg IV) was followed by either buprenorphine (7.5μg/kg IV) or butorphanol (0.05mg/kg IV). After 20 minutes, diazepam (0.01mg/kg IV) was administered and sedation was maintained with a detomidine IV infusion (20μg/kg/h). All horses received either a maxillary or mandibular nerve block, and gingival infiltration with mepivacaine. Sedation quality was assessed by the surgeon from 1 (excellent) to 10 (surgery not feasible). A pain scoring system (EQUUS-FAP) was used to assess postoperative pain. Serum cortisol concentrations and locomotor activity (pedometers) were measured. They found that buprenorphine combined with detomidine provided more reliable sedation than butorphanol. However, after surgery, stimulatory effects of buprenorphine caused excessive central stimulation, which interfered with facial pain scoring.
• Beazley et al (2022), using electrical impedance tomography, showed that the use of carbon dioxide insufflation in standing horses, sedated with acepromazine, detomidine and butorphanol, and sedation maintained with a detomidine infusion causes a significant decrease of tidal volume with an increase of the intra-abdominal pressure.

The second part of this article focuses on general anaesthesia.

• Use of some drugs in this article is under the cascade.