Best of the rest
Many, if not most, practices use one or more of the above modalities for surgical patients. But a practice that uses all four
of these modalities for each surgical procedure is providing superior pain management to patients. These four modalities form
the foundation of any transoperative protocol. Building on this foundation, various additional tools can further enhance patient
comfort, reduce hyperalgesia and perhaps minimize the likelihood of chronic pain states. The following are among the easiest
Topical prilocaine-lidocaine: Commercial transdermal products are useful in facilitating catheter placement and for minor procedures involving the dermis
and epidermis. And 5% lidocaine patches (Lidoderm [Endo Pharmaceuticals], manufactured and labeled for post-herpetic neuralgia
such as shingles) can provide postoperative wound paraincisional analgesia.36
Tramadol: This is a synthetic, nonscheduled (for now) opioid whose active M1 metabolite in humans has 1/100th of the affinity for the mu receptor as morphine but a much better analgesic effect than this would predict.37 This is likely due to the combined effect of serotonin and norepinephrine (NE) (both inhibitory neurotransmitters) agonism.
However, recent work demonstrates that dogs make little of the M1 metabolite,38 so in dogs the primary pain-modifying potential may be found in the serotoninergic and noradrenergic effect. Furthermore,
it appears to have a short half-life (1.7 hours) in dogs,39 so for 24-hour effectiveness, it might need to be given as often as every six hours. Only in the past year have studies
appeared to confirm an antinociceptive and analgesic effect in dogs.40,41 Tramadol should not be used with other serotoninergic medications such as tricyclic antidepressants.
Alpha-2 agonist: Medetomidine and dexmedetomidine bind opioid-like receptors on C- and A-delta fibers, especially in the central nervous system.
When binding presynaptically, NE production is reduced and sedation occurs; binding postsynaptically, analgesia is produced
and is profoundly synergistic with opioids. It also blocks NE receptors on blood vessels, resulting in vasoconstriction. The
resulting hypertension parasympathetically induces bradycardia, which is extended by a subsequent direct decrease in sympathetic
tone. However, central perfusion is maintained, and I have found a wide use for these alpha-2 agonists in acute and perioperative
settings, though only in combination with opioids and at doses much lower than those suggested by the manufacturer. Anecdotally,
one particularly novel and user-friendly utility is intravenous microdoses (0.25 to 1 µg/kg) given intraoperatively and postoperatively.
This may result in intravenous volumes of only 0.01 to 0.03 ml in even the largest of dogs and approximates a "bolus" dose
of the constant-infusion rate reported for dogs.42
Ketamine: This is a dissociative anesthetic, and the evidence is strong for its pain-preventive effects when given at subanesthetic
doses in an intravenous constant-rate infusion. Ketamine binds to a phencyclidine receptor inside the NMDA receptor (note,
the calcium channel would have to already be open and active for ketamine to exert its effect). But once bound, it decreases
the channel's opening time and frequency, thus reducing calcium ion influx and dampening secondary intracellular signaling
cascades. Hence, it's unlikely (and has not been shown) to be truly analgesic in nature. Rather, it appears to be protective
against hyperalgesia and central hypersensitization postoperatively,43 including in dogs.44
Gabapentin: Labeled for use as an anticonvulsant drug, gabapentin is widely used in people for its analgesic properties. Its primary
pain-modifying effect appears to be through its interaction with the alpha-2-delta subunit of the voltage-gated calcium channel.45 Although in veterinary medicine it's most often considered to treat chronic pain, the human literature is replete with data
regarding its utility in the perioperative setting (often just a single dose before and after surgery). In a study of women
undergoing hysterectomy, only the patients receiving both an NSAID and gabapentin were completely satisfied with their postoperative
pain management when compared with women receiving either an NSAID or gabapentin alone.46 And in a meta-analysis of 896 patients undergoing various surgical procedures, gabapentin significantly reduced pain both
four and 24 hours postoperatively when compared with placebo.47 The preoperative and postoperative oral administration of gabapentin has begun to gain traction among some veterinarians.
However, a study of gabapentin in dogs failed to demonstrate a positive postoperative pain-modifying effect,48 although the design may have prevented detecting such an effect. Pharmacokinetic studies in dogs show a half-life of three
to four hours,49 suggesting a two- or three-times-daily administration schedule if dispensed postoperatively. The primary adverse effect
in dogs appears to be somnolence (as in people).