Anesthesia is a set of techniques that allows the realization of a surgical, obstetrical or medical act by suppressing or reducing pain, with attenuation or complete loss of the state of consciousness, Equine patients are characterized by their diversity: from the young foal to the geriatric horse, through the working horse or the competition “crack”, from the miniature horse to the draught horse, from ponies to zebras through donkeys, To this diversity are added differences in pharmacological sensitivity according to the individual, age, breed, species. We can therefore better understand the following adage: Every equine patient is different, there may be routine surgeries… But there is no routine anesthesia.
There are two main types of anesthesia for horses
- General anesthesia, which is a state comparable to sleep produced by injecting drugs and/or breathing anesthetic vapors. The drug injection can be adjusted so as not to induce a total loss of consciousness but simply its attenuation, this is called tranquilization.
- Loco-regional anesthesia allows the patient to be put to sleep only in the part of the body where the operation will take place. The principle is to block the nerves of this region by injecting a local anesthetic near them. A general anesthesia may be associated or may become necessary, especially in case of inadequacy of the local-regional anesthesia.
From a vigorous horse to an anesthetized horse, several steps
During the initial consultation, a general veterinarian or anesthetist verifies the horse’s registration and vaccination status, takes a history, and performs a thorough clinical examination or additional tests. During the consultation, you are invited to answer the veterinarian’s questions and to ask those you deem useful for your information. The horse is then reassessed the day before and/or the day of the anesthesia. The choice of anesthesia is determined according to the planned procedure and the horse’s state of health.
The horse is then unshod, to limit the risk of injury to the animal or the staff. If the horse is to be put to sleep, it is put on an empty stomach 8 to 10 hours before the procedure (access to water is maintained), except if the anesthesia is performed in an emergency or if it is a foal (the latter can continue to feed until the last moment).
An intravenous route is used, which allows for a rapid and adjustable action of the molecules used during the procedure, unlike the intramuscular route, which is usually reserved for uncontrollable or dangerous animals.
Before surgery, the horse is bandaged, its feet cleaned, and if it must lie down, its mouth washed to remove any food particles.
Photo: Epidural catheter placed between the first two coccygeal vertebrae for optimal pain control of an extensive posterior wound. Equine clinic, E.N.V.L.)
Anesthesia of the standing horse
Standing anesthesia or tranquillization, aims at obtaining an immobility of the horse, its indifference towards the external environment (sedation), while ensuring a prevention of the pain (analgesia) and by preserving the safety of the people surrounding the animal.
Physical restraint can be achieved by taking a forelimb, a fold of skin at the neck, an ear or by putting in place a nose cone. It used to be de rigueur during interventions on standing horses but is now relegated to the background by the advent of molecules capable of providing the elements previously mentioned, while limiting the stress and danger caused by physical restraint.
Three families are commonly used nowadays for the tranquilization of the horse: phenothiazines (acepromazine), aj-agonists (xylazine, detomidine, romifidine) and morphinics (morphine, butor- phanol). Benzodiazepines (diazepam), which are of little interest for sedation in adults, are the molecules of choice for foals.
It is particularly interesting to combine these families, and a neuroleptanalgesia is then achieved: the sedative and/or analgesic effects of the agents are potentiated, their respective doses are reduced, and their own side effects are attenuated.
However, the perception of pain with this type of protocol is not completely abolished and a local or loco-regional anesthesia, which interrupts the transmission of nerve messages, is often necessary.
Some basic principles must be respected to obtain a quality anesthesia on a standing horse: work in a calm environment, limiting as much as possible any visual or auditory stimulation of the animal, leave a person at the horse’s head to reassure it. Nevertheless, if the procedure or the surgical approach is too complex, if the animal’s character is delicate, a general anesthesia may be necessary.
General anesthesia for the horse
General anesthesia can be performed in the field as well as in the operating room. It always takes place in 4 stages: premedication, induction, maintenance, and recovery. Pre-medication: It aims to tranquilize the horse, to provide a preventive analgesia, to potentiate the general anesthetic agents to limit their doses and therefore their side effects. At the same time, it allows to favor a soft bedding. We use here the molecules described for the anesthesia of the standing horse.
The induction: it consists in provoking a loss of consciousness and a myorelaxation which results in the horse lying down. Different molecules, administered intravenously, are used, the most frequent being dissociative anesthetics (ketamine, tiletamine) and thiobarbiturates (thiopental) associated or not with guaifenesin; propofol, rarely used in adults, is more frequently used in foals.
If the anesthesia is performed in a room, the horse lies in a padded induction box and is supported by lanyards attached to its tail and head or by a low flank. If the anesthesia is performed in the pasture, the horse is simply held with a lead rope at the head. Maintenance: This consists of maintaining the horse in a state of anesthesia by various procedures, the choice being guided by the duration of the intervention and/or its location. Short-term anesthesia (in the pasture)
Anesthesia maintenance is performed with intravenous anesthetic agent. Given the size of the animal, any cardio-respiratory depression, facilitated on an animal in lateral or dorsal recumbency, can be deleterious to the horse and it is considered that general anesthesia without supplemental oxygen in the inspired air should not last longer than one hour.
Even in the pasture, a minimum of equipment is necessary to deal with complications: an endotracheal tube to maintain airway patency and allow effective administration of oxygen, an oxygen cylinder connected to a demand valve or tubing, and restraints to limit any inappropriate movement of the horse.
How to limit the negative effects of anesthesia in the horse
To limit the deleterious effects of prolonged respiratory depression, supplemental oxygen in the inspired air is recommended as a minimum, but assisted or controlled ventilation with a respirator is still preferred. In this setting, halogenated volatile anesthetic agents (halotane, isoflurane) in combination with oxygen (transport gas) are preferentially used for maintenance, using a volatile anesthesia machine. The horse is first orotracheally intubated and then meticulous care is taken in moving the animal and positioning it on the surgical table.
In all cases, monitoring of the anesthetized horse is necessary: it aims to verify the depth of anesthesia: an animal anesthetized too deep risks a major cardiovascular or respiratory depression that can go as far as cardiac arrest, an animal anesthetized too shallow risks moving. The monitoring of anesthesia is therefore based on the evaluation of the depression of the central nervous system and of the cardiovascular and respiratory functioning thanks to the interpretation of clinical signs and parameters requiring a more or less complex equipment.
The awakening: the awakening phase represents a delicate period, the horse can indeed try to get up prematurely when the administration of anesthetics is stopped, which can be dangerous for the horse and the anesthetist. It is up to the anesthesiologist to decide whether to let the horse stand up; if he considers that the horse is still under the effect of the anesthetics, he will use physical and/or pharmacological restraint.
For procedures performed in the pasture, the limited trauma associated with the low level of the procedure and the short duration of the operation allow the horse to wake up with minimal assistance. For more extensive surgeries in the hospital, the horse is taken to a recovery box at the end of the procedure where it can be assisted in its effort by head and tail lanyards.
During this phase, it is interesting to leave an endotracheal tube in place until the horse is standing to supplement the inspired air with oxygen and limit any obstruction of the upper airways. Finally, the quiet, semi-darkness and absence of external or painful stimuli will limit the occurrence of too early attempts to get up.
Risks of anesthesia in horses
Anesthetizing a horse has become a frequent and common act in French veterinary practice, but this is also associated, as in all species, including humans, with a certain level of anesthetic risk potentially leading to serious injury or even death. In horses, this risk of mortality or serious morbidity is very high: this species does not tolerate prolonged decubitus on its side and even less on its back, due to its consequences on the cardiovascular, respiratory, and neuromuscular systems. Nevertheless, the current conditions of monitoring of anesthesia and the recovery period allow for rapid detection and treatment of abnormalities.
Whether it is during general or loco-regional anesthesia, complications
Unpredictable life-threatening complications such as severe allergy, cardiac arrest, asphyxia, neurological damage is extremely rare.
A study conducted between 1991 and 1996 showed a mortality rate for perioperative complications of 0.9%, with the following main causes: cardiac arrest (32.8%), fracture on awakening (23.3%), myopathy (7.1%). The risk is increased in the case of significant impairment of vital functions, such as colic (risk of 7.9%).
The statistically significant risk factors are
- age (lower risks between 2 and 7 years of age);
- the choice of anesthetics;
- the time of day (higher risk during the evening and night).
- and finally, of course, the type of procedure undertaken and the clinical setting in which the operation is performed.
Therefore, the veterinarian will explain to the owner the risks involved in the anesthesia, have the owner sign a consent form and inform the insurance company of the procedure.
Anesthetizing a horse is never a trivial matter. The evolution of the art of equine anesthesia has focused on the conditions under which the procedures are performed and, between 1970 and 1990, on the monitoring of anesthesia and the recovery period, thanks to an ever-increasing knowledge of basic and applied veterinary sciences. The benefits of such progress were soon felt with a decrease in perioperative morbidity/mortality and an increase in the number and complexity of procedures performed on horses.
The last decade has seen the advent of a new issue for the veterinary anesthesiologist: pain control. Often neglected, pain is a source of major complications that, in the best of cases, slows down the horse’s recovery and, in the worst of cases, leads to its death. However, analgesia is a simple therapeutic practice that undoubtedly optimizes the quality of care provided to animals and improves the prognosis of their condition. Such an awareness will allow the veterinarian of this new millennium to offer to his patients an environment more in conformity with the definitions of animal welfare and comfort and to come closer to a more humanitarian vision of this profession that he chose… also for that.
Equine surgery at the turn of the century was dependent on physical restraint. Note the holding of the horse’s head and the shackles.
The primary reasons for inducing anesthesia in any living species are to prevent or relieve pain and to reduce an animal to immobility to facilitate medical or surgical procedures made necessary by its condition. The “heroic” times of equine veterinary practice favored a so-called “physical” immobilization, which lasted until the mid-1970s. From that time on, the science of equine anesthesiology caught up to and surpassed the art as evidenced by the plethora of publications describing and detailing drug agents, anesthetic techniques, equipment, and monitoring methods specifically designed for use on the horse. As a result, training programs dedicated solely to comparative veterinary anesthesiology have emerged in most veterinary schools around the world. These now advocate the merits of tranquilization, analgesia, and “pharmacological” anesthesia in horses and, most importantly, offer an appreciation of the art of equine anesthesia that is grounded in sound knowledge of basic and applied veterinary science. The development of this veterinary discipline has been instrumental in the successful technological evolution of equine medical and surgical practice. Indeed, modern anesthesia allows the realization of diagnostic or curative acts in conditions of increased safety for the manipulators and the animal, by optimizing the control of the pain and finally by facilitating the cooperation of the horses regarding the procedures carried out, which minimizes the stress always deleterious in a patient.
