The preliminary results of a study into perioperative equine fatalities suggested that horses still carry a high risk of mortality as a result of general anaesthesia (GA), with the total mortality from perioperative complications found to be 1 percent (Gozalo-Marcilla et al., 2021). This is still high in comparison to mortality rates reported for cats and dogs; however, the preliminary data from the study suggested that the current mortality rate for horses is lower than 20 years ago. Veterinary nurses (VNs) involved with equine anaesthesia require equine-specific knowledge in order to reduce the risks for their patients.
Preoperative patient preparation
It is important to take the time to prepare equine patients correctly prior to GA. A complete history should be taken from the owner, and a complete physical examination should take place. The following should also be considered:
- First, the patient should be weighed. Ideally, this should be done using an electronic weighbridge, but if this is not possible, a weigh tape can be used. Weighing the horse is important as anaesthetic doses are calculated based on body weight (Murrell and Ford-Fennah, 2012)
- The need to starve horses before undergoing GA is currently under discussion. It has been agreed that access to water should be allowed up to premedication. For elective procedures, it is suggested that restricting access to concentrates and large meals of forages for four to six hours prior to premedication may be prudent (Dugdale et al., 2020)
- Shoes should be removed to protect the horse and the recovery box floor during induction and recovery from the anaesthetic. The feet must then be pared and thoroughly scrubbed clean (King, 2015)
- The horse should be groomed thoroughly to prevent contamination of the theatre (King, 2015). A tail bandage should also be applied
- The horse’s mouth should be rinsed out to prevent the patient from aspirating food material on placement of the endotracheal tube (King, 2015)
- An intravenous (IV) catheter should be placed. The catheter should be placed into the left jugular vein if the horse is to be positioned in dorsal recumbency or the uppermost vein if the horse is to be positioned in lateral recumbency (King, 2015)
- Ideally, a large area around the surgical site should be clipped. There should be at least a hand-span width clipped around where the incision is going to be made (King, 2015)
- A padded headcollar should be applied to help to protect the superficial nerves of the head from potential damage caused by the metal components of the headcollar during the GA process (Dugdale et al., 2020)
Preoperative medication
Premedication is the administration of an appropriate medication prior to anaesthesia to facilitate induction, maintenance and recovery (Murrell and Ford-Fennah, 2012). Premedication and induction often involve the use of multiple types of medication together to give a balanced overall effect. This is known as multimodal anaesthesia (see Table 1 for medications used).
Preoperative analgesia is a critical component of any anaesthetic regime and can contribute to improved pain management in the post-operative period
Preoperative analgesia is a critical component of any anaesthetic regime and can contribute to improved pain management in the post-operative period (Murrell and Ford-Fennah, 2012). Analgesic strategies are commonly multimodal, and the VN should be familiar with different classes of analgesic drugs used. They should also work with the veterinary surgeon (VS) responsible for the anaesthetic to ensure that the correct analgesic medication is given prior to induction. The VS in charge of the anaesthetic and the surgeon should discuss and agree on the antibiotics required prior to surgery, if any, and these should be given preoperatively. Antibiotic protocols should be in place and be based on the British Equine Veterinary Association (BEVA) “Protect ME” campaign, aimed at promoting responsible antimicrobial use in practice.
| Class of agent | Mechanism of action | Clinical effects | Side effects | Metabolism |
|---|---|---|---|---|
| Phenothiazines (eg acepromazine maleate) | Dopamine agonist Inhibition of catecholamine activity in central nervous system (CNS) | Calming Anti-arrhythmic Antihistamine | Hypotension due to blockade of peripheral alpha-1 receptors and a direct vasodilator effect Hypothermia Use reduced doses in foals and animals in liver dysfunction/cardiovascular compromise | Liver metabolism and renal excretion |
| Alpha-2 agonists (eg detomidine, romifidine and xylazine) | Activation of alpha-2 adrenoreceptors in the CNS Analgesia is mediated by action at central and peripheral adrenoreceptors | Potent sedation Muscle relaxation Analgesia | Initial hypertension followed by normotension Bradycardia Respiratory system depression Hypothermia Sweating | Liver metabolism and renal excretion |
| Non-steroidal anti-inflammatories (NSAIDs) | Peripheral action to inhibit the production of tissue prostaglandins by inhibition of the enzyme cyclooxygenase, which mediates production of prostaglandins CNS action mediating analgesia and antipyretic actions | Analgesia Anti-inflammatory Antipyretic | Gastrointestinal ulceration Renal toxicity Hepatotoxicity Blood dyscrasias | Liver metabolism |
| Opioids | Act on receptor sites in CNS and other tissues Act as agonists, antagonists or a combination of both | Analgesia Synergistic effect when used in combination with other classes of drugs | Increased locomotor activity Respiratory depression Mydriasis Reduced gastrointestinal motility Euphoria | Liver metabolism and renal excretion |
| Benzodiazepines (eg diazepam and midazolam) | Potentiate the action of inhibitory neurotransmitters (particularly GABA in the CNS) Specific benzodiazepine receptors adjacent to the GABA receptor complex | Not sedative in adult horses Calming Hypnosis Muscle relaxation Anticonvulsant action Useful for sedation of young foals | Minimal cardiovascular or respiratory side effects | Liver metabolism and renal excretion |
| Dissociative anaesthetics (eg ketamine) | Hypnotic effects are largely mediated by blockade of NMDA and HCN1 receptors, but cholinergic, aminergic and opioid systems appear to play both a positive and negative modulatory role in both sedation and analgesia (Sleigh et al., 2014) | Analgesia Anaesthesia | Horses induced without a muscle relaxant will remain stiff during the operation | Liver metabolism and renal excretion |
Induction
All GA medications used for a patient must be prescribed by a VS, who can then delegate the administration of these medications to a VN. Generally, a tranquiliser such as acepromazine is given first, either IV or intramuscularly (IM). This helps to calm the patient. Once this has taken effect, the patient is walked to the induction box – a padded room used to reduce the risk of injury to the horse during the induction process (Figure 1). An alpha-2 agonist is then administered to sedate the patient before the induction medication is given.
Once the patient is deemed ready by the anaesthetist, ketamine and diazepam are given IV to induce GA. There are different methods for trying to control induction for equine patients, including free fall, support from handlers, swing door and sling induction. There are advantages and disadvantages to each induction method, and the VN should discuss the most appropriate method to be used with the anaesthetist.
Maintenance
GA maintenance in horses can be achieved by administering IV or inhalational anaesthetic agents. Total intravenous anaesthesia (TIVA) is generally only used for short procedures or situations where orotracheal intubation has proved problematic. Inhalational anaesthesia is the most common way of maintaining GA. This technique involves administering inhalational anaesthetic agents, but the patient must be intubated for this.
Total intravenous anaesthesia (TIVA) is generally only used for short procedures or situations where orotracheal intubation has proved problematic
Orotracheal intubation in the horse should be performed as described below (Murrell and Ford-Fennah, 2012):
- The endotracheal tube should be lubricated to reduce trauma to soft tissues during intubation
- The mouth of the patient should be opened with a gag
- The head and neck of the patient should be extended
- The tongue should be pulled to one side of the mouth and the tube slid between the dental arcades to the back of the pharynx
- Correct positioning of the tube should be checked by observing for breathing through the tube
- It is important to inflate the cuff of the tube to ensure a seal in the trachea and prevent the horse from breathing around the tube
Once the horse has been intubated, a circle anaesthetic circuit is connected to the endotracheal tube and an inhalation anaesthetic agent (usually isoflurane) is delivered along with oxygen to maintain GA. The patient is then attached to the winch and transported into theatre to be positioned on the operating table.
Positioning equine patients for surgery
Careful positioning of a horse under GA is vital to reduce the risk of post-anaesthetic complications such as myopathy and neuropathy. The VN should make sure that the patient is positioned correctly for the procedure in question.
Positioning in lateral recumbency (Murrell and Ford-Fennah, 2012):
- The lower limb should be pulled forward to prevent compression of the triceps muscle by the full weight of the patient’s body and the upper limb
- Padding should be placed between the limbs so that the upper limb is supported
- The limbs should not be in full flexion or extension
Positioning in dorsal recumbency (Murrell and Ford-Fennah, 2012):
- Square positioning on the table is essential so that the weight distribution is uniform between the different muscle groups
- The neck and head should be raised slightly to reduce the risk of nasal oedema developing. As horses are obligate nasal breathers, this could cause a significant problem during recovery
- The legs should be relaxed and slightly flexed
- The neck should be straight but not overextended, to prevent stretching of the recurrent laryngeal nerves
Monitoring techniques
Monitoring of the anaesthetised horse is carried out to ensure that physiological function and the depth of anaesthesia are adequate (Taylor and Clarke, 2007). Table 2 contains information regarding the most commonly used monitoring techniques. Human senses and the power of data interpretation and integration should never be underestimated when it comes to anaesthetic monitoring (Dugdale et al., 2020). Modern equipment can assist and often provide early warning signs to help to prevent a critical incident from occurring; however, machines should never be fully relied upon (Dugdale et al., 2020).
The VN can assist the anaesthetist with all monitoring techniques whether it be preparing and setting up the machines, inserting an arterial line or manually taking clinical parameters
The VN can assist the anaesthetist with all monitoring techniques whether it be preparing and setting up the machines, inserting an arterial line or manually taking clinical parameters. Although the final decision regarding any treatment will remain with the anaesthetist, good communication is required between the VS and the VN. This will ensure that any abnormalities are detected and responded to promptly and effectively. A written record of every equine anaesthetic should be kept detailing anaesthetic agents given and intraoperative monitoring carried out. Once the operation is complete, the horse should be transported via the winch into a recovery box.
Observation/monitoring technique | Details |
|---|---|
| Palpebral reflex | The palpebral reflex is elicited by gently running a finger along the free margin of the upper eyelid. This reflex is usually retained during surgical anaesthesia |
| Lacrimation and rapid nystagmus | Lacrimation and rapid nystagmus are associated with lightening of anaesthesia |
| Pulse rate | The pulse should be regularly palpated (at least every five minutes) throughout anaesthesia. The heart can also be auscultated at regular intervals using a stethoscope |
| Mucous membranes | Mucous membrane colour and capillary refill time give some guide to oxygenation and the adequacy of perfusion. A capillary refill time of more than two seconds is a cause for concern (Taylor and Clarke, 2007). The mucous membranes should be pale pink in colour |
| Arterial blood pressure (ABP) | ABP provides a great deal of information on the cardiovascular system and is the most important aid to monitoring the anaesthetised horse (Taylor and Clarke, 2007). ABP can be measured using direct or indirect methods, although direct monitoring is more accurate Mean ABP should be maintained at or above 70mmHg throughout the anaesthetic to reduce the risk of the patient developing a post-anaesthetic myopathy (PAM) The RVN should prepare a dobutamine drip for use in case hypotension occurs IV fluids should be administered to horses under GA as standard, to help to maintain adequate perfusion and help to prevent dehydration |
| Electrocardiogram (ECG) | The ECG provides information about the electrical activity of the heart and can be useful in the diagnosis of arrhythmias. The ECG will not, however, give any information on cardiac output. Therefore, blood pressure monitoring along with regular palpation of the pulse should be carried out to create an overall picture of patient progress |
| Respiratory rate | Respiratory rate and rhythm: movement of the chest wall and rebreathing bag should be monitored regularly (at least every five minutes). If intermittent positive pressure ventilation (IPPV) is being used, this should be monitored closely |
| Arterial blood gas analysis | Arterial blood gas analysis provides information on respiratory system and metabolic function. Arterial carbon dioxide, oxygen concentration and pH are the most useful indicators of respiratory function |
| Pulse oximetry | Pulse oximetry measures the pulse rate and provides information about the adequacy of arterial oxygenation and peripheral perfusion. A haemoglobin saturation greater than 94 to 95 percent is desirable during anaesthesia |
| Capnography | Capnography measures the concentration of carbon dioxide in a sample of gas drawn from the end of the endotracheal tube. This indirectly reflects arterial carbon dioxide concentration. Capnography is useful for measuring trends, particularly in horses that are being ventilated (Murrell and Ford-Fennah, 2012) |
Recovery
Horses should be placed in the recovery box so that there is adequate space for them to move into sternal recumbency and to stand safely. Horses that have been placed in lateral recumbency during GA should be placed in the same position in the recovery box. The lower forelimb should also be pulled forward to release pressure and reduce the risk of a myopathy or neuropathy developing. Horses that have been placed in dorsal recumbency during GA should be placed in whichever lateral recumbency facilitates access to the IV catheter, or so that the operated limb is left uppermost (Dugdale et al. 2020).
Some horses attempt to stand too early following GA. Such attempts are often uncoordinated and can increase the risk of traumatic injury (Murrell and Ford-Fennah, 2012). Sedation with an alpha-2 agonist will delay recovery from anaesthesia and give the patient time to regain coordination before attempting to stand. The VN should discuss the type and dose of sedative required with the anaesthetist before drawing up and labelling the medication in advance to ensure that it can be administered without delay. Extubation does not need to be delayed until the horse is swallowing (Murrell and Ford-Fennah, 2012). The VN should also make sure that the cuff is deflated so that removal is not delayed. A nasal tube should be inserted so that oxygen can be administered throughout the recovery process. As horses are obligate nasal breathers, it is good practice to maintain a patent airway.
Sedation with an alpha-2 agonist will delay recovery from anaesthesia and give the patient time to regain coordination before attempting to stand
There are different recovery methods used in equine patients. Some horses are left to recover unassisted but remain closely monitored throughout the process. Manual restraint is sometimes used with foals or small patients. Assisted recoveries can be used to reduce the risk of fractures and other injuries occurring during recovery. A rope recovery system using head and tail ropes pulled through rings high in the wall is most commonly used. This system allows assistance to be given in lifting the horse without risk of injury to human handlers, as they remain safely outside the recovery box (Taylor and Clarke, 2007). The assisted recovery technique is something that VNs should consider and discuss with the anaesthetist. It is essential that the horse is monitored closely during the recovery period, and the VN should ensure that a “crash box” is located outside the recovery box in case complications occur.
Nursing care following GA
Once the horse is deemed ready, it can be walked around to a stable. Depending on the surgical procedure, the VN can prepare the stable with fluids, heat lamps and a deep clean bed. If the horse has sweated in recovery, a breathable rug should be applied. The patient’s clinical parameters should be monitored. Following elective surgery, a sloppy feed such as a warm fibre-based mash can be given. Following colic surgery, or other emergency surgery, the patient should be fed according to the case VS’s instructions. The patient should be observed closely, and a pain score should be carried out. Any abnormalities should be reported to the case VS. Nursing care should then be applied based on the case VS’s instructions, the individual patient and the surgery undertaken.
Conclusion
Horses are high-risk candidates for GA. Any VNs assisting with equine anaesthesia should have a good knowledge of the risks involved and measures that can be taken to reduce them. VNs should be involved in equine anaesthesia and are an asset when working closely alongside the anaesthetist to ensure high standards of patient care and safety.
