Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 14 OPIOID ANALGESICS
Small dogs and cats may be dosed with half a patch,
but the patch should not be cut in half. Cover half the
gel membrane with tape. ‘Half-patch dosing’ is suggested
for pediatric, geriatric and systemically ill cats
and small dogs.
The patch may be placed either on the dorsal
or lateral cervical area or the lateral thorax. If the neck
is used, collars/leashes cannot be placed over the patch.
The thorax is easily used and contact maximized
(especially in cats), but can be difficult to bandage. The
site must be clean and dry at the time of application.
The patch should not be placed where a heating
pad may come into contact as this may increase release
of drug from the patch. All patients wearing patches
should have heart and respiratory rates monitored
regularly.
CLINICAL PHARMACOLOGY OF
INDIVIDUAL OPIOID AGONISTS
Choice of the most appropriate opioid analgesic, as well
as its dose and route of administration in a given clinical
situation, is dictated by the pharmacodynamic and
pharmacokinetic factors discussed above. Table 14.2
summarizes important pharmacodynamic features of
the available opioids discussed below and Table 14.3
lists appropriate doses and routes of administration in
different species.
Morphine
Morphine is the opioid analgesic against which others
are compared.
Table 14.2 Relative activities of opioid agonists and antagonists at µ- and κ-receptors
Drug Receptor activity Mu (m) Kappa (k) Analgesic efficacy
Morphine Agonist ++ 0 Strong
Pethidine (meperidine) Agonist + + Moderate-strong
Methadone Agonist ++ 0 Strong
Oxymorphone Agonist ++ 0 Very strong
Fentanyl Agonist ++ + Very strong
Alfentanil Agonist ++ + Very strong
Codeine* Agonist + 0 Weak
Tramadol** Agonist + 0 Moderate
Buprenorphine Partial agonist P+ P+? Moderate
Butorphanol Agonist–antagonist P+ + Moderate-weak
Pentazocine Agonist–antagonist P+ ++ Moderate-weak
Nalbuphine Agonist–antagonist P+? + Moderate-weak
Naloxone Antagonist – - N/A
Nalorphine Antagonist – P+ N/A
Nalmefene Antagonist – - N/A
++, Strong agonist activity; +, agonist activity; P+, partial agonist activity; 0, no activity; -, weak antagonist activity; – –, strong antagonist activity;
N/A, not applicable,?, the action is very weak or absent.
* Opioid actions of codeine are due exclusively to its slow metabolic conversion to morphine.
** Tramadol is moderately analgesic because it is a dual µ-receptor agonist (its metabolites are more potent) and monoamine (noradrenaline
and serotonin) transport inhibitor.
Table 14.3 Opioid dosages and duration of action
Drug Route of administration Dose rate (mg/kg) † Duration of analgesic
action (h)
Dog Cat ‡
Agonists
Morphine Intravenous 0.05–0.1 0.05 1–4 §
Intraoperative bolus 0.1 0.05
Intramuscular or subcutaneous 0.1–0.5 0.1–0.3 4–6
Extradural 0.1–0.2 0.1–0.2 12–24
Interpleural 0.5–1.0 8–12
Intra-articular 0.1–1.0 8–12
Oral 0.1–3.0 0.1–1.0 4–8
Sustained release 1.5–3.0 N/A 8–12
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