Opioid receptors are distributed ubiquitously throughout the body and can be found in most central and peripheral tissues.
Several opioid receptor types and subtypes have been isolated, each with a variant effect; activation of an opioid receptor
inhibits presynaptic release and postsynaptic response to excitatory neurotransmitters. The resulting effect is greatly impeded
Several opioid drugs are available that vary in their relative potency and receptor affinity. Of the pure mu agonists, morphine
remains the prototype in widest use; it has no ceiling effect on analgesia or respiratory depression, elicits histamine release
and causes vomiting at low doses. (Higher doses, intravenous administration and chronic use don't elicit vomiting, presumably
by interaction with mu receptors in the antiemetic center.15 )
Cats lack glucuronate metabolism, resulting in minimal production of the analgesic M6G metabolite16 ; therefore, morphine may not be the ideal opioid for use in this species. Oxymorphone and hydromorphone cause minimal histamine
release (and, therefore, may be wiser choices in cases of hypovolemia, such as trauma or dehydration), and nausea may be less
pronounced. But they have a shorter duration of action than morphine. Also, hydromorphone in particular is implicated in episodes
of hyperthermia in cats.17
Fentanyl is a short-acting intravenous opioid commonly administered as a constant-rate infusion. A fentanyl transdermal patch
remains useful in veterinary medicine, though studies have demonstrated wide kinetic variability in veterinary patients due
to such factors as species, body condition score, body temperature, surgical procedure and where and how well the patch is
Buprenorphine is a partial agonist on the mu receptor, though it has greater affinity for it than morphine (and will displace
it if given together). A great benefit of the drug in veterinary medicine is that its pKa (8.4) closely matches the pH of
the feline oral mucosa (9.0). This allows for nearly complete absorption when given buccally in that species,20 with kinetics nearly identical to intravenous and intramuscular administration,21 and eliciting very little sedation.
Butorphanol is a partial mu antagonist and a kappa agonist; its short duration of action in dogs (30 to 40 minutes22 ) and minimal analgesia23 make it a poor choice for an analgesic in this species, though when used parenterally, it has utility as an adjunct with
other medications such as alpha-2 agonists.
For all their effectiveness, opioids may create clinical challenges. In the acute setting, opioid-induced dysphoria, hyperalgesia
and respiratory depression (though this appears to be a much greater hazard in people than in dogs and cats) may be encountered.
Recognizing and having strategies for counteracting these effects will minimize the complications they present.24 However, the most effective way to avoid opioid-induced adverse effects is to develop opioid-sparing strategies. This is
precisely the goal of a multimodal approach: using several modalities so the use of any one can be minimized. Opioids customarily
are administered preoperatively in combination with anxiolytics as described earlier. They can be followed postoperatively
with subsequent doses or constant-rate intravenous infusions, if indicated.
Local anesthetics were once a mainstay of pain management in veterinary medicine and may now be one of the most underused
modalities. The effect of properly administered local anesthetics can be complete anesthesia to a site rather than mere analgesia
or hypoesthesia. Further, local anesthetics have also been shown to have anti-inflammatory25 and antimicrobial26 activity.
Local anesthetics have variable onsets and durations of action, and they may be combined for a rapid and extended effect.
The locality of administration often is limited only by a clinician's ability to learn various utilities and anatomic landmarks;
few are outside the scope of any clinician to master. Consider some of the following types of locoregional anesthesia:
- Local line, paraincisional27 and subcutaneous infiltrative blocks
- Carpal radial, ulnar, median (RUM or Ring) nerve block28
- Dental (orofacial) nerve blocks
- Intracavitary (abdominal, thoracic) infusions
- Intercostal nerve blocks29
- Testicular blocks30
- Intra-articular blocks31
- Retrobulbar blocks32
- Brachial plexus blocks33
- Intravenous regional anesthesia (Bier blocks)34
Use of subcutaneous diffusion catheters is a locoregional technique now well-established in people to reduce both static and
dynamic postoperative pain, and it is opioid- and hospitalization-sparing.35 This technique is finding increasing traction in veterinary medicine; commercial diffusion catheters are available (http://recathco.com/,
http://milaint.com/) or can be fashioned out of 5-F red rubber catheters.