Page 41 - Manual of Equine Field Surgery

Field Anesthesia 37

thesia time with this mixture should be limited to eyes with a towel will protect the eyes somewhat

30 minutes or less. After short anesthetic periods, and eliminate visual stimulation of the patient.

recovery from this mixture is usually quiet and The surface the horse is placed on should be

uneventful. smooth and can be padded if padding is available.

It is important to minimize the time spent

positioning the patient for short procedures

MONITORING HORSES WHILE UNDER since limiting anesthesia time usually results in

INJECTABLE GENERAL ANESTHESIA better recovery. Proper positioning of the patient

will limit the potential for myopathies and

In general, sophisticated monitoring equipment is neuropathies.

not used during field anesthesia. Efforts should be When in lateral recumbency, the lower front

made to ensure adequate circulatory and respira- leg should be pulled as far rostral as possible to

79
tory function. This can be as simple as palpating eliminate pressure on the brachia! plexus and

the pulse, observing chest wall movement, and associated blood vessels. The halter should also be

observing mucous membrane color. Normal removed to prevent facial nerve damage on the

horses will have a pulse rate between 25 and 50 down side. Positioning of the lower hind limb is

beats per minute and a respiratory rate of 6 to 12 not as critical, but it is commonly pulled forward

breaths per minute. as well to decrease pressure on the medial mus-

Anesthetic depth is determined by assessing culature. Both the front and hind upper limbs

the palpebral and corneal reflexes and watching should be supported in a position parallel with

for the presence of nystagmus. Horses under the ground. Horses positioned in dorsal recum-

injectable general anesthesia will appear to be at a bency should have their head and neck positioned

lighter plane of anesthesia than those under in a natural position to ensure a patent airway.

inhalation anesthesia. The corneal and palpebral The legs should be allowed to assume a natural,

reflexes should be present. Also, the character of semi-flexed position. Unless necessary for the pro-

breathing will often change as the depth of anes- cedure, the hind limbs should not be kept in an

thesia decreases. Commonly, deep "sighs": and extended position.

intermittent breath holding will occur at a light Fluid therapy is not commonly administered

level of anesthesia. Usually, if nystagmus is to equine patients undergoing short field proce-

present, the depth of anesthesia is inadequate dures but is appropriate if indicated. AI1y balanced

unless the proced11re is almost complete. If anes- crystalloid solution such as 0.9% saline or lactated

thesia is being maintained by an infusion, the rate Ringer's solution can be administered at a rate

of the infusion can be increased if deeper anes- of 5 mL/kg/hr to maintain vascular volume and

thesia is needed. Whe11 the horse is very light, a promote tissue perfusion. Any significant blood

small bolus (0.1 to O.SmL/kg) of the infusion can loss can be treated by administering 3 mL of crys-

be rapidly administered. During anesthesia with talloid fluid for each milliliter of estimated blood

an a2-agonist-dissociative combination, an addi- loss. In healthy animals, this is usually adequate

tional dose of a2-agonist-dissociative or dissocia- therapy for blood losses of up to 100/o of the

tive alone can be administered. The usual dose is blood volume. Ideally any preexisting dehydration

half of the induction dose. should be corrected prior to anesthesia. If this is

not possible, the fluid deficit can be replaced while

under anesthesia. The fluid deficit is commonly

SUPPORTIVE CARE FOR HORSES WHILE estimated by multiplying the perceived amount of

UNDER INJECTABLE GENERAL ANESTHESIA dehydration (in percent) and the animal's weight

(in kilograms) to determine the deficit (in liters).

Because patients undergoing surgical procedures A 450-kg animal that is 5°/o dehydrated would

in the field are usually relatively healthy and need 22.5 L to replace its deficit (0 .. 05 x 450kg =

support facilities are limited, intensive supportive 22.5 L).

care is usually not administered. AI1 ocular lubri- Under field conditions, intubation is not essen-

cant or ocular antibiotic ointment ( without tial for the equine patient. It does protect the

steroids) should be placed in both eyes to prevent airway from occlusion and allow mechanical ven-

corneal drying and there should be nothing near tilation if needed. It would certainly be beneficial

the eye that could rub on the cornea. Covering the to at least have an endotracheal tube available.