Page 38 - Manual of Equine Field Surgery

34 PRESURGTCAL PREPARATION AND ASSESSMENT

the inhibitory effect of this neurotransmitter by 2. After the drugs are administered, they should

decreasing the rate of its dissociation from its be allowed to have an effect. It is important to

receptor and directly increasing the duration of wait 5 to 15 minutes after IV administration

GABA-associated chloride channel opening. It has and 15 to 30 minutes after IM administration

no analgesic effects at subanesthetic doses. Its car- for the drugs to have their full effect.

diovascular effects are a mild peripheral vasodila- 3. If a tranquilizer or a2-agent alone is not effec-

tion (primarily venous) and a decrease in cardiac tive, it is usually best to add an opioid rather

contractility.46 A concurrent increase in heart rate than to give more of the initial drug.

often occurs. Although thiopental can be used

alone to produce equine anesthesia, the quality of For long procedures, it is sometimes easier to

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recovery is poor. For this reason, it is commonly administer a continuous infusion of detomidine

used with other agents. rather than to administer additional doses during

the procedure. This is especially useful during

Propofol laparoscopic procedures. After initial sedation

with detomidine and butorphanol or detomidine

Propofol is a phenolic compound that is chemi- and buprenorphine, an infusion of detomidine is

cally unrelated to thiopental, but the clinical and administered at the rate of 0.1 µg/kg/min (0.0001

physiologic effects are very similar. One signifi- mg/kg/min).44 A 450-kg horse would therefore

cant difference is the rapid hepatic metabolism require 27 mg of detomidine/hr. Alternatively, an

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of propofol. This combination of redistribution initial detomidine infusion rate of 0.6 µg/kg/min

and rapid metabolism results iI1 rapid recovery can be used. The detomidine infusion rate is then

even after long periods of administration. Cur- decreased by half every 15 minutes.59 Xylazine can

rently its cost precludes frequent use as an equine also be given by infusion at the rate of 0.55

anesthetic agent, although t1se may increase in the mg/kg/hr, 60 111 addition to the infusion, local

future. It is used after premedication with a tran- anesthetics should be used at the surgery site. Con-

quilizer. stant infusions are often superior to intermittent

administration in that the quality of sedation is

more uniform and there is usually less ataxia since

STANDING SEDATION AND RESTRAINT the peaks in plasma drug levels are eliminated.

COMBINATIONS

Many minor surgical and diagnostic procedures INJECTABLE GENERAL ANESTHESIA

are done without general anesthesia. In some

'
cases, a tranquilizer or a2-agonist alone is ade- There is no ideal general anesthetic; therefore,

quate. In most cases, however, the combination of combinations of drugs are used to produce

a tranquilizer or a2-agonist with an opioid pro- general anesthesia. When combining drugs, it is

vides superior sedation, analgesia, and restraint. important to select drugs tl1at have complemen-

Many combinations are used, and the doses tary effects. In addition, the duration of action of

within these combinations are variable. The the drugs must be considered as well as their

attending veterinarian should use his or her judg- undesirable side effects. When the combination of

ment as to the precise dose to use. One rule to xylazine and ketamine is analyzed, it is evident

remember is that you can always give more drug that ketamine provides the anesthesia as well

if needed but it is difficult to remove a drug from as some analgesia but is lacking in muscle relax-

the animal once it has been administered. Table ation a11d would cause rough inductions if used

5-1 lists the commonly used drugs and their alone. The addition of xylazine adds muscle relax-

doses, as well as some specific comments. ation as well as more analgesia and sedation to

It is important to remember that no matter smooth the induction. Their durations of action

which drugs are selected to sedate a horse, are complementary as well. Xylazine produces

several other factors are important for successful approximately 30 minutes of sedation so the

sedation. recovery from ketamine is generally smooth but

the sedation after anesthesia is not prolonged. The

1. The horse should be calm when the drugs are addition ofbutorphanol or a benzodiazepine adds

administered. more analgesia-sedation or muscle relaxation-