Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CLINICAL PHARMACOLOGY OF INDIVIDUAL OPIOID AGONISTS
writing they do, however, retain some clinical use and
they retain the advantage over fentanyl of a shorter
duration of action, particularly following a prolonged
continuous infusion. Studies, both clinical and experimental,
have been undertaken using alfentanil in dogs
and cats. The results of these indicate that the actions
and side effects of alfentanil in these species are very
similar to those of fentanyl, in particular good analgesia,
MAC reduction of inhaled agents, bradycardia and
respiratory depression. Alfentanil is less potent than
fentanyl while sufentanil is 5–10 times more potent.
Both are full µ-agonists and more lipid soluble than
fentanyl.
Remifentanil
Clinical applications
Remifentanil is a 4-anilidopiperidine derivative of fentanyl
containing an ester linkage to propanoic acid,
making it susceptible to esterase metabolism. The
primary metabolite, remifentanil acid, has negligible
activity compared with remifentanil. It is ultra-short
acting and displays analgesic effects, consistent with its
agonist activity at the µ-receptor. The effect of remifentanil
on hemodynamics is typical of opioids (e.g.
decreased blood pressure and heart rate). The reduced
blood pressure is by virtue of the bradycardia and may
be prevented or reversed by use of an anticholinergic.
Remifentanil is proving to be a highly effective analgesic
when used as part of balanced anesthesia in the dog.
Infusion rates of between 0.05 and 0.6 µg/kg/min
produce profound analgesia and MAC sparing effects.
Where heart rate is supported, blood pressure is well
maintained due to the reduction of inhaled volatile
agent.
Remifentanil has also been shown to be effective
alongside infused propofol as part of a total intravenous
anesthetic technique. Because metabolism is independent
of hepatic function, remifentanil is useful as part
of anesthetic protocol for dogs with reduced hepatic
function. There are two main unwanted effects. Unlike
in humans, in whom ventilation is often well maintained,
dogs require assisted ventilation even at low
doses of infused remifentanil. Second, without the use
of an anticholinergic, bradycardia may be profound.
This is rapidly reversed on termination of the infusion.
In addition, it should be recognized that as the analgesic
effects wear off rapidly, analgesia should be preemptively
provided by a different opioid (e.g. morphine)
before terminating the remifentanil.
Mechanism of action
Remifentanil is a potent selective µ-opioid agonist.
Formulations and dose rates
Remifentanil is marketed as a sterile lyophilised powder for reconstitution
with sterile water.
DOGS AND CATS
Constant rate infusion – IV
• Loading dose: not needed
• Infusion rate: 0.05–0.6 µg/kg/min
Pharmacokinetics
Remifentanil has a rapid onset of action (