Objective—To evaluate and compare effects of
epidurally administered morphine, alfentanil, butorphanol,
tramadol, and U50488H on avoidance threshold
to noxious electrical stimulation over the dermatomes
of the perineal, sacral, lumbar, and thoracic
regions in horses.
Animals—5 healthy adult horses.
Procedure—Using a Latin square complete repeatedmeasures
design, horses were randomly assigned to
receive 1 of 6 treatments (morphine, alfentanil, butorphanol,
tramadol, U50488H, or sterile water) at intervals
of at least 7 days. Agents were injected epidurally
at the first intercoccygeal epidural space, and electrical
stimulation was applied at repeated intervals for
24 hours to the dermatomes of the perineal, sacral,
lumbar, and thoracic regions. Avoidance threshold to
electrical stimulation was recorded.
Results—Administration of butorphanol, U50488H,
and sterile water did not induce change in avoidance
threshold. Alfentanil increased avoidance threshold
during the first 4 hours, but not significantly. Tramadol
and morphine significantly increased threshold and
analgesic effects. Complete analgesia (avoidance
threshold, > 40 V) in the perineal and sacral areas was
achieved 30 minutes after tramadol injection, compared
with 6 hours after morphine injection. Duration
of complete analgesia was 4 hours and 5 hours after
tramadol and morphine injections, respectively.
Conclusions and Clinical Relevance—Epidural
administration of tramadol and morphine induces
long-lasting analgesia in healthy adult horses. Epidural
administration of opioids may provide long-lasting
analgesia in horses without excitation of the CNS.
(Am J Vet Res 2000;61:1579–1586)