Objective-To determine the analgesic, sedative, and cardiopulmonary ef
fects of epidural ketamine in the horse. Animal Population-Six healthy
horses (three males and three females) weighing between 350 and 450 k
g. Methods-Three doses of ketamine were selected (0.5, 1, and 2 mg/hg)
. Two months before the beginning of experiments, the carotid artery w
as exteriorized, and 1 week before experiments began, an epidural cath
eter was placed percutaneously in all animals with the tip located 12
cm cranially in the midsacrum. One week later, either saline (control)
or one of three doses of ketamine was injected epidurally. Each anima
l received each ketamine dose and saline in random order at 1-week int
ervals. Ketamine was diluted in saline 0.9% before the experiment, and
the volume used was adjusted to horse size and correlated to clinical
ly used volumes. All the animals received a standard noxious stimulus
consisting of needle insertion into the skin and deep muscle using a 3
-point scale for scoring the response. A second scale was used to scor
e the degree of sedation. The response to a noxious stimulus, the degr
ee of sedation, and arterial blood pressure were assessed at previousl
y determined intervals: before drug and 2, 5, 10, and 15 minutes and e
very 15 minutes to 210 minutes after ketamine or saline administration
. Arterial blood samples were drawn for blood gas analysis before drug
and at 15, 30, 60, and 90 minutes. Results-All the tested doses of ke
tamine were effective in producing analgesia of the tail, perineum, an
d upper hindlimb. Total tail and perineal analgesia times were similar
depending on dosage (30 minutes for 0.5 and 1.0 mg/kg and 75 minutes
for 2.0 mg/kg). A sedative effect of ketamine was also observed in a d
ose-response manner with a peak effect between 15 and 30 minutes posta
dministration. No cardiopulmonary effects were observed with any dose
of ketamine. Conclusions-Results indicate that epidurally administered
ketamine in the horse produces local spinal and central nervous syste
m effects with analgesia and sedation but minimal cardiopulmonary effe
cts. Clinical Relevance-Further studies are required to determine whet
her the analgesia is sufficient for surgery. (C) 1998 by The American
College of Veterinary Surgeons.