Practical Tips for Equine Field Anaesthesia
Performing general anaesthesia in the horse carries an inherent risk to the patient. The Confidential Enquiry into Peri-operative Equine Fatalities (CEPEF) reported a mortality rate of one in 100 in non-colic surgical cases (Johnson et al 2002). This makes avoiding general anaesthesia, if possible, in the horse a viable option. Many surgical procedures can be carried out in the standing horse using systemic sedation and analgesia in combination with local anaesthetic techniques, which will significantly reduce patient-associated risks and costs. Therefore, each case should be assessed individually as to whether the procedure can be safely and humanely carried out standing or whether general anaesthesia is essential.
Safety....The location and personnel available for induction, maintenance and recovery
There is the risk of potential injury to the horse during induction, maintenance and recovery of general anaesthesia, particularly in the field situation. These factors should be assessed for each case and need to be minimised.
It is essential to ensure there are the desired number and experience of personnel available to assist.
All patients should have a full history taken and clinical examination performed, if amenable, prior to administration of any drugs. This will attain baseline values for each patient and will alert the Veterinary Surgeon to any factors that may become a concern or affect choice of drugs for general anaesthesia. Several different factors usually need to be considered before a final choice of drug, route of administration and choice of local analgesic techniques can be made. These include the patient’s physiological condition, such as concurrent medical illnesses and pregnancy, the patient’s musculoskeletal status, the duration of surgical anaesthesia required, the experience of available personnel, the facilities available and environment in which the procedure is being performed and your familiarity with the drugs.
It is recommended that anaesthesia without the provision of supplemental oxygen should be limited to a maximum of one hour. Once the horse is recumbent, V/Q mismatching occurs in the lungs, which becomes progressively worse with time, resulting in an increased oxygen requirement. If oxygen supplementation is not supplied, then hypoxaemia can occur. Therefore, any procedure that is likely to require more than one hour of general anaesthesia time should be carried out in the hospital setting, where supplemental oxygen can be administered (in addition; a special licence is required to carry oxygen cylinders in vehicles).
Sedation and analgesia
Sedation (including analgesia) is one of the most important steps prior to anaesthesia, helping achieve a smooth, injury free induction and recovery. Sedating the horse heavily, usually with a combination of acepromazine, an α2 agonist and an opioid will reduce the likelihood of an excitable anaesthetic induction and provision of adequate analgesia will aid a smoother recovery (See table 2 for doses). Using a combination of analgesic and anti-inflammatory agents (multimodal analgesia) will block different areas of the nociceptive pathway and thus reduces ‘wind-up’, therefore it should lead to quicker recovery times and help reduce patient morbidity. It is advisable to give a non-steroidal-anti-inflammatory drug (NSAID) prior to surgery, as well as an opioid and an α2 agonist. Ketamine has also been used by intramuscular injection to provide stunning sedation, if a horse is particularly fractious, or painful.
Achieving good sedation
Following these steps should result in good sedation and analgesia being achieved.
Heavy sedation appropriate for induction of anaesthesia is attained when the horse holds it head low, has a loose ventral lip, ears are relaxed and there is minimal response to sound or movement.
Induction of anaesthesia
Ketamine is licensed for the induction of anaesthesia in the horse. It can either be administered alone or be combined with low doses of benzodiazepines, such as midazolam or diazepam, which will aid muscle relaxation and a smoother induction and intubation (see Table 3 for doses). It is important that the horse is heavily sedated before these drugs are administered to avoid excitation. It takes approximately 60-90 seconds for the horse to become recumbent following the intravenous administration of ketamine. Thiopentone is an alternative agent for induction of anaesthesia, although this is not licensed. It takes approximately 10-20 seconds for the horse to become recumbent following the intravenous administration of thiopentone.
Positioning for induction
Positioning of the horse for induction is of paramount importance to reduce the risk of injury and the following is a guideline on how to hold the horse for induction.
Intubation is advisable in order to ensure a patent airway for the duration of the anaesthetic. An endotracheal tube or a nasotracheal tube can be used, and in general the size of the nasotracheal tube is approximately 10mm smaller than for the endotracheal tube that would be used in that horse. As a guide, a standard 500kg horse would likely need a size 26mm or 30mm endotracheal tube. To intubate with ease, the head and neck need to be fully extended and a gag placed between the incisor teeth.
A suitable lubricant should be applied around the cuff and the tube inserted on the midline into the mouth and over the base of the tongue. One hand should be used to palpate the larynx and the other to guide the endotracheal tube. A twisting motion should be applied to the tube as it is gently advanced through the larynx. Only gentle pressure should be applied, and if moderate resistance is felt, the tube should be retracted before another attempt at advancement is made. It is common for the tube to advance into the oesophagus. If this occurs, retract the tube into the oropharynx, ensure the head and neck are fully extended and repeat advancement of the tube with a twisting motion. To place a nasotracheal tube, advance the tube via the ventral meatus of a nostril and once the tube is rostral to the larynx, the same gentle twisting motion should be used to advance the tube through the larynx. To check the tube is in the correct location, the flow of air can be felt at the proximal end of the tube.
Positioning for surgery
Once induced, the horse can be placed in the correct position for surgery. Positioning needs to be carefully considered in order to prevent myopathies, neuropathies and damage to the cornea. The dependent fore limb should be pulled cranially and dorsally to reduce the pressure on the triceps muscle. The corneas should be moistened with eye lubricant and the lower eye protected from the ground. The headcollar area around the facial nerve should be padded or the headcollar removed, to prevent facial nerve damage and paralysis.
Maintenance of anaesthesia and monitoring
Depth of anaesthesia
In order to monitor the depth of anaesthesia, the cardiovascular system, eye position and muscular tone should all be assessed regularly. The pulse rate in horses does not normally correspond to depth of anaesthesia and will not normally rise in response to noxious stimuli. Indicators of a light depth of anaesthesia during total intravenous anaesthesia (TIVA) include an increased respiratory rate and depth of inspiration, a fast palpebral reflex, nystagmus, lacrimation, swallowing, increased muscle tone and, lastly, spontaneous movement. However, in comparison to volatile anaesthesia in the horse, using TIVA will preserve reflexes. The eye may be central, as occurs with the use of ketamine, the palpebral response will be sharper and faster and respiratory rate will be quicker, when compared to the same depth of anaesthesia when using isoflurane. There is generally no requirement to monitor blood pressure directly when using TIVA (without volatile anaesthesia) as blood pressure normally remains within normal limits.
Bolus ‘Top ups’
The induction dose of ketamine will provide anaesthesia for approximately 10 minutes. After this time, the horse needs to be maintained either on a continuous infusion or repeat boluses given approximately every 10-15 minutes. As a rough guide, the repeat bolus should be approximately 1/4-1/3 of induction dose of ketamine combined with the α2 agonist, which will be required every 10-15 minutes, depending on the depth of anaesthesia. The aim is to not exceed the original dose used for induction of anaesthesia, which could lead to severe ataxia on recovery. This means that giving repeat boluses for maintenance can only be used for short procedures lasting less than 30-40 minutes.
Continuous rate infusion
If a longer duration of anaesthesia is required, then a continuous infusion would be preferable. This can be carried out using a combination of three drugs (known as ‘the triple drip’) that provide anaesthesia, analgesia and muscle relaxation. The ‘triple drip’ is a combination of ketamine, an α2 agonist and the muscle relaxant guaifenesin (GGE). However, due to accumulation of drugs over time, it is recommended that anaesthesia time using a triple drip, even with supplemental inspired oxygen, should be limited to 60-90 minutes to avoid severe ataxia and prolonged recoveries. Many books suggest using a 10% solution of guaifenesin, however, this has been shown to cause thrombophlebitis and significant ataxia on recovery. Instead, the author recommends using a 5% solution (50mg/ml) of guaifenesin to reduce ataxia on recovery.
This is achieved by diluting 15% guaifenesin with either 0.9%NaCl or 5% glucose, and then adding the ketamine and α2 agonist, as described in Table 4. The total dose of guaifenesin also needs to be limited to 100mg/kg to keep ataxia minimal. This would equate to a 500kg horse receiving a maximum of 1L of 5% guaifenesin. At the concentrations suggested in Table 4, the maintenance rate for anaesthesia is 1ml/kg/hr. This equates to using 500ml per hour in a 500kg horse, so the calculated drip rate would be 2-3 drops every second. This of course should not be standardised, but titrated to effect in each individual, by increasing or decreasing the drip rate as required.
If at all possible, a local analgesic technique should be used intra-operatively, such as intra-testicular local anaesthetic for castrations. This will reduce the amount of drugs required to maintain a surgical plane of anaesthesia and aid a smoother recovery.
|VIDEOS 1, 2 and 3: show differing depths of
anaesthesia using the eye and the palpebral
reflex. Using TIVA, nystagmus would indicate
a light depth of anaesthesia. The palpebral
reflex would normally be fast (video 1) to
medium (video 2) to achieve a surgical plane
of anaesthesia. The slow palpebral reflex
(video 3) would indicate a deep plane of
anaesthesia when using TIVA
The area for recovery, as for induction, should be free of dangerous objects or places for the horse to become trapped. The horse should be kept well positioned during recovery to prevent neuropathies, myopathies and damage to the cornea. Recovery time would be expected to be around 20-30minutes for the first attempt to stand, depending on the total amount of drugs administered. However, some horses will try and stand earlier, especially if stimulated. If horses try and stand too early, they will be significantly ataxic, which could result in injury. It would be ideal during a field recovery to keep the horse down for as long as possible to reduce the ataxia on standing. When a horse stands naturally, they push the head into the ground to lift the neck and then move the head ventrally. Thus in order to prevent the horse from standing, the opposite action needs to be performed. This can be achieved by one experienced person restraining the horse as follows:
Once the horse is ready to stand, it can be recovered either with or without further assistance.
One option is to place two long lunge lines, one to each side of the head collar. The assistants, wearing gloves holding the lunge lines, should stand adjacent to the horse and slightly caudally, but a safe distance away, in an arrow formation. The idea is that as the horse tries to stand and falls forwards the lines attached to the head prevent the forward movement and assist the horse in standing. Assisting recovery should be abandoned if it becomes dangerous to the personnel.
It is important to consider what to advise the owner regarding the horse before leaving the yard, including feeding regimes following general anaesthesia, specific clinical signs to monitor, including passing of faeces, evidence of colic, or acceptable degree of haemorrhage from the surgical site. A post operative analgesia plan that can be administered by the owner and monitoring for signs of pain should also be discussed.
Licensing of drugs
Of the drugs mentioned in this article, those that are at present licensed for use in the horse are the α2 agonists (detomidine, romifidine and xylazine), ketamine, acepromazine and the opioids (butorphanol and pethidine). It is possible that buprenorphine will become licensed for use in the horse in the future. However, pethidine is a schedule II controlled drug, thus is under strict purchase, storage and dispensing control, meaning carriage in a car is not considered to be acceptable.
Guaifensin and thiopentone are no longer licensed for use in the horse, but may still be available.