Anesthesia: general - overview | horses | Vetlexicon Equis from Vetlexicon

Introduction

Can be defined as a reversible state of unconsciousness, produced by a controlled intoxication of the nervous system.
There is a decreased sensitivity to environmental stimuli and reduced motor response to stimuli.
Anesthetic agents may be delivered via a variety of routes, but are most commonly administered by intramuscular or intravenous injection and/or by inhalation.
Starvation for up to 18 h pre-operatively is routine to reduce the risk of hypoxemia developing due to the effect of a full intestine on the diaphragm of an animal which is recumbent for a prolonged period.
A thorough clinical examination should be carried out prior to anesthetic administration; an accurate weight should be determined if possible.
Equine anesthesia is frequently carried out in the field, sometimes under adverse conditions.This means that precautions must be taken to protect both the patient and the operatives from environmental effects and from injury, particularly during the induction and recovery phases.
Anesthesia in the horse is more complex than in many other species and it is important to prevent anesthetic emergencies and predict potential anesthetic problems with each individual case.

Print off the Owner factsheet All about anesthesia All about anaesthesia to give to your clients.

The stages of anesthesia

Premedication

The aim of premedication Anesthesia: premedication - overview is to provide good patient co-operation, reduced anxiety, smooth induction, provide analgesia and muscle relaxation, without excessive adverse cardiopulmonary effects. Many of the drugs used for premedication also reduce the requirements for intravenous and inhalational anesthetics.
A combination of sedatives, tranquilizers and analgesics Anesthesia: analgesia - overview may be used alone or in combination.

Induction

Induction Anesthesia: induction - overview is usually by administration of an intravenous agent. Drugs used for induction have a rapid onset and bypass the excitement phases of anesthesia.
Historically, foals were given inhalational inductions but this method may be less favorable than intravenous inductions Anesthesia: neonate. However, in very sick foals it is the technique that is often employed, particularly since the introduction of volatile agents with low blood solubilities, eg sevoflurane Sevoflurane.
The aim of induction is a safe, smooth descent into unconsciousness with minimum movement by the horse.
The placement of an intravenous catheter Intravenous catheterization into the jugular vein will make administration of an intravenous drug easier and safer, especially in a restless patient, and also facilitates the administration of further anesthetics, analgesics and supportive drugs.

Sites for induction

Clinic or purpose-built induction box

Padded walls (at least 2 m high) and floor with some method of restraining the horse are the essentials.
Other design features will depend on the availability of separate rooms for surgery and for recovery Anesthesia: recovery - overview.

Indoor arena/school

Usually a good choice, because the ground is soft and even and there should be no sharp objects, plenty of space and environmental effects are reduced.
A disadvantage can be the dustiness of many indoor arenas from which the horse's eyes will need to be protected and can make a poor surgical environment.

Stable

Far from ideal.
In addition to removing protruding objects such as mangers etc, from the area, hazards such as walls which do not reach high enough or gaps under stable doors must also be noted and dealt with.
Clean straw bedding (left long enough for any dust to settle) is usually the best choice.
The horse should be stood against a wall with its hindquarters in a corner for induction. The side to be operated on should be closest to the wall and only two handlers should be present, because of the confined space.

Ensure that the stable is strong enough for a horse to recover from anesthetic in it.

Outdoors

A sheltered area, protected from the prevailing wind and shaded from bright sunlight in summer should be sought.
A layer of straw on the ground should help to insulate the patient from the cold and a clean blanket may be used to cover the animal during the surgery.
Care should be taken to clear the surrounding area of any sharp or hard objects and any noise should be kept to a minimum.
A strong, secure headcollar and soft, thick, cotton lead-rope will help the handler to control the horse more effectively. Extra handlers are also of use, as they may be able to help control the place where the horse lands and which side it lands on. For example, turning the horse's head to the left just before it begins to go down will encourage the horse into right lateral recumbency.

Maintenance

Maintenance Anesthesia: maintenance - overview of anesthesia may be by giving increments or variable rate infusions of intravenous anesthetic drugs, but more commonly by use of gaseous agents.

Hypoxemia

This is a major problem in equine anesthesia Anesthesia: monitoring - respiratory management.
Pressure of abdominal organs on the diaphragm → decreased lung volume compared with that of the conscious animal. Pre-operative starvation to decrease the volume of gastrointestinal contents is routine.
Anesthetic-induced depression of respiration Anesthesia: monitoring - cardiac output and blood pressure → decreased arterial oxygen tension.
Obtundation of hypoxic pulmonary vasoconstriction (HPV) which serves to shunt blood away from underventilated areas of the lung in the conscious horse, occurs when volatile anesthetics are administered.
HPV is somewhat preserved during IV anesthesia and, as such, oxygen levels may be improved. However, the use of inhaled anesthetics require an anesthetic machine and an oxygen supply, and therefore a higher inspired fraction of oxygen can be provided.
Horses undergoing total intravenous anesthesia (TIVA) are often breathing room air.

Monitoring anesthesia

This takes on a great importance in equine anesthesia Anesthesia: monitoring - overview .
Quality of respiration (rate, depth and rhythm) should be continually monitored Anesthesia: monitoring - respiratory management .
Digital palpation of the pulse should be regularly carried out, note being taken of both the rate and quality Anesthesia: monitoring - heart .
The capillary refill time and color of mucus membranes can give a good guide to the state of peripheral tissue perfusion.
Ocular signs, such as the position of the eye, palpebral reflex, sponteneous blinking, and the presence or absence of nystagmus can all be useful, but some variation is seen with different anesthetic agents and at different stages of the procedure.

The palpebral reflex can fatigue if too frequently tested, eg when the horse is in lateral recumbency the upper eye may have reduced response to stimulation, whereas the lower (unstimulated) eye will respond vigorously.

Anal reflex can be monitored, but is somewhat unreliable as an indicator of depth.
Response to stimulation can be observed.
Blood pressure may be monitored either directly (by catheterizing the facial, transverse facial or greater metatarsal arteries and using an aneroid manometer and pressure veil, or more sophisticated transducer/amplifier/recorder systems), or indirectly (using an inflatable cuff and Doppler ultrasound, or oscillometric monitors, eg Critikon-Dinamap) Anesthesia: monitoring - cardiac output and blood pressure .
Electrocardiography (ECG) Cardiovascular: ECG (electrocardiography) .
Blood gas analysis Blood: gas analysis .
End-tidal carbon dioxide levels (capnography).
Pulse oximetry.

Recovery

For further information on recovery see the overview Anesthesia: recovery - overview.
A well-protected area should be provided, similar to those described under sites for induction.
Oxygen supplementation during the recovery period may be of some value, especially in those individuals suffering from nasal edema and partial upper airway obstruction, where a nasotracheal or nasopharyngeal tube may be used to administer oxygen. It can be taped in place for the period of recovery.
Alternatively, the endotracheal tube can be secured to the interdental space and left in situ until the horse is standing, and oxygen can be insufflated through it. However, the insufflation of oxygen at the rates possible from flowmeters, fails to match peak inspiratory flow in an adult horse; the benefits are therefore contentious.

Provide a minimum of 15 l/min for an adult horse by insufflation.

Recovery from anesthesia can take 20-60 min on average (but this is very dependent on the duration of anesthesia) and can occasionally involve some violence if the animal tries to stand while it is still severely ataxic.
The nature of the recovery varies according to the drugs administered, the duration of anesthesia and the temperament and breed of the horse, and many other factors not yet fully understood.