Page 37 - Manual of Equine Field Surgery
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Field Anesthesia 33
phine in painful horses, minimal behavioral ef- analgesic effect. 48-51 Some evidence exists that sub-
fects were seen." Kappa agonists such as butor- anesthetic doses produce minimal analgesia.Y?'
phanol also produce some excitement but the The cardiovascular effects of ketamine must be
effect is somewhat less than that seen when mu separated into its indirect central effect and its
agonists such as morphine are administered. 41'42 direct peripheral effect. The central effect is an
Butorphanol has been administered as a continu- overall increase in sympathetic tone resulting in
ous infusion to maintain analgesia at a loading mild increases in heart rate, arterial blood pres-
dose of 0.018 mg/kg IV and then a continuous sure, and cardiac output. It does, however, have a
infusion at 0.0237 mg/kg/hr.43 Another opioid mild direct depressant effect on myocardial con-
that has seen limited use in horses, but has many tractility. 46•55 Little respiratory depression is seen
desirable characteristics, is buprenorphine. When at clinical doses although a slight increase in res-
combined with detomidine, buprenorphine has piratory rate and decrease in tidal volume are
provided good analgesia and sedation for stand- sometimes observed. Ketamine produces minimal
ing procedures.44 Transdermal delivery systems muscle relaxation. Because of its poor muscle
have provided an additional route of admin- relaxant effects and tonic-clonic limb spasms, it is
istration for opioids. Fentanyl is commonly not used alone in veterinary anesthesia.
administered in this manner (Duragesic; Janssen
Pharmaceutical Products) to a variety of species. Tiletamine
This delivery route has been studied in horses, and
it was determined that two 10- mg ( 100 ug/hr) Telazol is a proprietary mixture with equal con-
patches provided plasma levels of fentanyl that centrations of zolazepam, a benzodiazepam, and
should provide analgesia. Eight to 15 hours was tiletamine, a dissociative anesthetic, It is distrib-
required to reach peak plasma levels and patches uted as a dry powder that is reconstituted prior to
needed to be replaced at 48-hour intervals to use. Tiletamine is somewhat more potent than
maintain the desired plasma concentrations.45 No ketamine and has a slightly longer duration of
significant undesirable effects were noted in this action. Its cardiovascular, respiratory, and anes-
study. thetic effects are similar to those of ketamine. 56
Even though Telazol contains a tranquilizer as well
as a dissociative anesthetic, it is not commonly
INJECTABLE ANESTHETICS used alone in equine anesthesia.
None of the available injectable general anesthet- Thiopental
ics provide all of the actions of an ideal anesthetic.
Therefore, when used clinically they are almost Thiopental is an ultra-short-acting thiobarbitu-
always combined with other drugs. rate. It produces rapid unconsciousness after
intravenous injection. Recovery is a result of
redistribution of the drug from the brain to other
Ketamine
tissues in the body. Initially, the drug is redistrib-
Ketamine is a dissociative anesthetic. Patients uted to muscle and other nonfatty tissues with
receiving ketamine appear to be in a cataleptic moderate blood flow. The ultimate site of redis-
state while still maintaining many reflexes. The tribution is poorly perfused adipose tissue but, at
higher centers, the cerebral cortex, are dissociated the time of recovery from anesthesia, the major-
from somatic input. The site of action of keta- ity of the drug resides in moderately · perfused
mine is centrally located N-methyl-D-aspartate tissues. Ultimately, it undergoes hepatic metabo-
(NMDA) receptors. In addition, some of the lism. If thiopental is administered for prolonged
analgesia produced by ketamine may be due to periods and then discontinued, the primary
interaction with opioid receptors.46 Ketamine factors associated with the termination of anes-
undergoes hepatic metabolism with urinary ex- thesia are both redistribution into fat and hepatic
cretion of the metabolites. Some of the metabo- metabolism. These are slower than redistribution
lites have pharmacologic activity.47 The analgesic into moderately perfused tissues. The site of
properties of ketamine are somewhat controver- action for barbiturates has been shown to be
sial; however, many studies show a significant on the GABA receptor.46 Barbiturates enhance
