Page 34 - Manual of Equine Field Surgery

CHAPTER 5

Field Anesthesia

Keith R. Branson

Equine field surgery requires portable anesthesia. antiemetic and antihistaminic effects. The central

This factor makes it difficult to use inhalation sedation seen with the phenothiazine tranquiliz-

anesthesia. Fortunately; most of the patients un- ers is due to their antagonism of dopamine at cen-

dergoing surgical procedures in the field are rela- trally located receptors. In general, increasing the

tively healthy and require minimal supportive dose above that needed for sedation will only

care while under anesthesia. Adequate, safe anes- result in an increase in duration of tranquilization

thesia is still required, however, because regard- and increased undesirable side effects.1 Further

less of your surgical skills, a successful outcome increases in dose may even result i11 excitement,

also requires successful anesthesia. This chapter Acepromazine produces a decrease i11 arterial

includes a discussion of the physiologic effects of blood pressure caused by a peripheral a1-adreno-

the drugs commonly used for injectable anesthe- receptor antagonism.' Because of this a1 antago-

sia, how these drugs are used, and a brief discus- nism, a mixed adrenoreceptor agonist, such as

sion of supportive care measures and induction epinephrine, can have a more dramatic hypoten-

methods. In addition, a brief discussion of caudal sive effect since its vasodilatory ~ effects will be

epidural anesthesia and epidural catheter place- more pronounced. In addition, acepromazine has

ment is included. an antiarrhythmic effect 011 the heart, most likely

from a11 rz-adrenoreceptor antagonism i11 the

heart.?" Minimal respiratory depression/ other

TRANQUILIZERS AND exi-AGONISTS than a slight decrease in the respiratory rate, is

associated with the use of acepromazine alone, and

None of the injectable general anesthetics possess it does decrease the animal's ability to thermoregu-

all the properties needed to produce good anes- late. s-r Early reports of priapism and flaccid penile

thesia when used alone. For that reason other paralysis in stallions and geldings given acepro-

drugs are administered in conjunction with the mazine have led to hesitance to use this drug in stal-

general anesthetics. Tranquilizers and a2-agonists lions and geldings.i" Some practitioners think

are commonly used for their sedative, analgesic, that acepromazine is still a useful sedative for use

and muscle relaxant properties. iI1 male horses, and if used, it should be adminis-

tered intramuscularly (IM) and at the lowest effec-

tive dose.11 There is a dose-dependent decrease in
Acepromazine
the hematocrit attributed to sequestration of ery-

Acepromazine is the most commonly used rnem- throcytes in the splee11.10•12 Acepromazine is highly

ber of the phenothiazine family of tranquilizers. protein bound and has an elimination half-life of

Members of this group of drugs are known longer than 3 hours.'? It undergoes hepatic metabo-

for their sedative effects, but they also possess lism to form inactive metabolites.

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