Pain issues and management in veterinary dentistry and oral surgery: Part 2 (Proceedings)
Part II discusses some of the common agents used for managing pain associated with oral surgery in dogs and cats. There are six classes of agents discussed and they are the opiods (opiates), the cox-2 selective NSAID's, the 5-Lox selective NSAID's, the alpha-2 agonists, the NMDA receptor antagonists and the serotonin norepinephrine reuptake inhibitors.
The most effective analgesia available for pre, peri and postoperative pain control in dogs and cats is provided by the opiod group. It is important to understand several characteristic of opiods 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 upon stimulation of a particular opiod receptor subtype. Selective effects of common opiods on different receptors vary as well. An individual opiate's ability to bind preferentially to a given receptor is termed receptor affinity. The characteristics of individual opiods determine their effect at a given receptor. (Table 1). These different classes of opiates are listed in decreasing order of receptor affinity: antagonist > partial agonist/mixed agonist antagonist > agonist.Table 1: Definitions of Opiod Receptor Activity with Examples of Common Agents in Each Class
Pure Mu Agonist = Produces an optimal effect by binding to a given receptor.
Morphine, hydromorphone, fentanyl.
Partial Mu Agonist = Binds to the opiate receptor and produces a less profound effect than a pure agonist. Buprenorphine
Mu Antagonist = Causes no effect at the opiate receptor Naloxone, naltrexone, butorphanol
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.
It is widely understood that mu opiod 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 opiod category is morphinea . 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 morphinea 1 . Studies now suggest that 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.2.3 In the author's extensive experience with preoperative morphinea in cats, dysphoria and excitation appear to be an issue mostly postoperatively. 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.
Hyperthermia in cats with the use of opiates is predicatble and therefore monitoring is essential.4 Patients should be monitored during the anesthetic periodand up to five hours during post procedure. If hyperthermia becomes significant, reversal agents that are mu antagonists can be administered. Butorphanolb and nalbuphinec are kappa receptor agonists. That property provides some residual analgesia when they are used as mu antagonists to reverse morphine.a Naloxoned is an antagonist at the mu, kappa and gamma receptors so administration of this reversal agent provides no residual analgesia.