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Six of the most common agents used for managing pain associated with oral surgery in dogs and cats will be discussed in this
third article of a series on pain management. They are the opioids (opiates), the Cox-2 selective non-steroidal anti-inflammatory
drugs (NSAIDS), the 5-Lox selective NSAIDs, the alpha-2 agonists, the NMDA-receptor antagonists and the serotonin norepinephrine
reuptake inhibitors.
Opioids
Table 1: Opiod-receptor subtypes
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The opioid group provides the most effective analgesia available for preoperative, perioperative and postoperative pain control
in dogs and cats. It is important to understand several characteristics of opioids to enable the clinician to make decisions
on which agents would best be suited for use in a particular patient. Drugs in this class produce variable clinical effects
based on stimulation of a particular opioid receptor subtype (Table 1). The characteristics of individual opioids determine
their effect at a given receptor (Table 2). Selective effects of common opioids on different receptors vary as well (Table
3). An individual opiate's ability to bind preferentially to a given receptor is termed receptor affinity. These different
classes of opiates are listed in decreasing order of receptor affinity: antagonist > partial agonist/mixed agonist antagonist
> agonist.
Table 2: Definitions of opioid-receptor activity with examples of common agents in each class.
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A full understanding of these concepts aids the clinician in choosing the correct opiate or combination of opiates based upon
the species, and the degree and duration of the anticipated pain. Opiate receptors are located throughout the body and are
concentrated in the central and peripheral nervous system, making the agents that bind to them invaluable analgesics.
Table 3: Selectivity for opiod-receptor subtypes
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It is widely understood that mu opioid-receptor stimulation results in the most significant analgesic effect beyond that of
any other receptor subtype. Research in this field has led to the ability to clone the mu, kappa and delta opiate receptors.
Knockout mice that lack specific opiate receptor genes give researchers the ability to study the effects of different opiates
on receptor subtypes.
The prototype agent in the opioid category is morphine. It is the drug of choice for severe acute pain in dogs. Cats have
been known to experience more of the undesirable effects at the sigma receptor (Table 1) with administration of morphine.
Studies suggest it is likely that the dysphoric effects were due to unusually high doses used in research settings. Evidence
now suggests that opiates actually convey euphoric effects in cats when used at lower doses.
In the author's extensive experience with pre-operative morphine in cats, dysphoria and excitation appear to be an issue mostly
post-operatively. In that respect, consideration must be given to the combination of agents administered in the preoperative,
induction and intraoperative periods combining to exacerbate the expected degree of recovery disillusionment.
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