Page 35 - Manual of Equine Field Surgery
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Field Anesthesia 31
Whe11 used alone, the dose for acepromazine Lower doses are often used as an analgesic iI1
ranges from 0.02 to 0.09 mg/kg administered colic patients. The detomidine dose is 0.01 to
intravenously (IV) and 0.03 to 0.05 mg/kg IM. 0.04 mg/kg IV or IM when used alone, and, as
Acepromazine is rarely used alone when sedation with xylazine, lower doses of detomidine can be
and chemical restraint for standing surgical proce- used as an analgesic. Detomidine has a longer
dures are desired, since it has no analgesic proper- duration of sedation than xylazine (approxi-
ties. In addition, tune to onset can be highly rnately 45 and 30 minutes, respectively), and the
variable and the overall degree of sedation is hard sedation produced lasts longer than the analgesia.
to predict. Whe11 these agents are used with opioids, the a2-
agonist dose is reduced. These combinations are
discussed in a later section of this chapter.
exi-Agonists
Two other selective a2-adrenoreceptor agonists
Xylazine is 011e of the most widely used sedative- are used in horses=-romifidine and medeto-
analgesics in veterinary medicine. Unfortunately, midine. Both have physiologic effects similar
it also has significant undesirable cardiovascular to xylazine and detornidine.l=" A dose of romifi-
effects since it is a nonspecific a-adrenoreceptor dine 0.08 mg/kg IV is equivalent to approximately
agonist. It was thought that if agents that were 1 mg/kg of xylazine or 0.02 mg/kg of detomidine.
more specific for the centrally located a2- Detornidine, at least at higher doses, produces
adrenoreceptors could be developed, the periph- sedation of longer duration than romifidine
eral effects would be minimized. Detomidine is but romifidine appears to produce slightly less
more specific for a2-adrenoreceptors but its car- ataxia.25•26 Medetomidine is an a2-agonist ap-
diovascular effects are very similar to those of proved for use in dogs. Its use in horses has
xylazine when equipotent doses are compared. been limited, but it appears a dose of 0.0075
The initial response of the peripheral vasculature mg/kg will produce adequate sedation of a
is vasoconstriction. Although there may be differ- duration longer than that normally seen with
ences in the venous and arterial responses, an xylazine but shorter than that seen with detomi-
obvious increase in peripheral vascular resistance dine. 1s,27
and an accompanying increase in arterial blood One advantage of the a2-agonists is the avail-
pressuTe occur.13-15 This is especially evident if the ability of specific antagonists to reverse their
drug is given intravenously. These agents also have effects. The most commonly used antagonists are
significant central sympatholytic and parasympa- tolazoline, yohimbine, and atipamezole, with tola-
thomimetic effects, which result in a decrease in zoline being the least specific antagonist for the
cardiac output. A decrease in heart rate occurs a2-adre11oreceptor and atipamezole being tl1e
both as a result of the central effects and as a most specific.28 Because of tolazoline's relative
response to the initial vasoconstriction-induced lack of a2 specificity, its use is sometimes associ-
hypertension.":" In addition to bradycardia, atri- ated with significant clinical signs as a result of the
oventricular conduction disturbances increase antagonism of endogenous adrenergic substances.
following a2-ago11ist administration.Y" This group These signs can include diarrhea, abdominal pain,
of drugs routinely causes some decrease in the res- and hypotension caused by vasodilation. 28 The
piratory rate with little effect on Pacoz.; however, a use of the agonist atipamezole has been evaluated
decrease in Pao, is routinely observed at the doses as part of a lameness examination after light
needed to produce sedation. Ls,16 sedation with detomidine (0.01 mg/kg).29 Admin-
A transient increase in urine output is seen istration of atipamezole reversed most of the
after the administration of az-agorusts.17'18 Concern sedation-related stride changes, but some differ-
exists that a2-agorusts, especially xylazine, may ences were still evident. In general, the dose of
cause abortion in pregnant mares, but there is antagonist is determined by the agonist dose and
little evidence of this effect. However, intrauterine the specific agonist used. This relationship is a
pressure is increased after the administration of reflection of the relative affinity the agonist and
most a2-agonists.19 Both xylazine and detomidine antagonist have for the receptors. In general, 4 mg
undergo hepatic metabolism with rapid excretion of tolazoline is needed to adequately reverse 1 mg
of xylazine, and 10 mg of atipamezole is needed
of the metabolites in the urine.20-23
When used alone, the usual dose for xylazine to reverse 1 mg of detomidine.P'" The time inter-
is 0.3 to 1 mg/kg IV and 1 to 2 mg/kg IM. val since administration of the agonist should also
