Use of analgesics prior to surgery (preemptive analgesia) may also be beneficial. Non-steroidal anti-inflammatory drugs are
commonly being administered during the perioperative period. Debate exists as to when to administer the drugs (preoperatively
vs. postoperatively). Since intraoperative hypotension is always possible, the author prefers to administer NSAIDs during
or after surgery once adequate perfusion pressures have been assured during anesthesia. While data exists to support the safe
use of preoperative NSAIDs in healthy patients undergoing relatively short periods of anesthesia, if the approach of reducing
risk during anesthesia is taken, it is rational to wait until surgery is nearing completion before administering NSAIDs.
Chronic administration of NSAIDs is common practice in dogs for the symptomatic treatment of osteoarthritis, neoplastic conditions,
and a multitude of other ailments. Chronic NSAID prescribing in cats is done, however there are no currently FDA-CVM approved
NSAIDs for repeated dosing in cats and use in this manner is extra-label. Informed consent of the owners should be obtained
before initiating extra-label drug use in any species. Additionally, while the quality of life of many cats has likely been
improved using chronic NSAID dosing, the incidence of adverse events is not know with any degree of certainty therefore determining
the risk vs. benefit must be made on an individual basis. Anytime chronic NSAID administration is prescribed a baseline CBC,
serum chemistry, and urinalysis would seem prudent since subclinical disease may be present, especially in older animals or
animals with co-morbidities.
Analgesic Agents
Non-steroidal anti-inflammatory drugs continue to be the mainstay of acute and chronic pain management in both human and veterinary
patients. Traditionally, it has been believed that the analgesic effects of NSAIDs are related to their ability to inhibit
cyclooxygenase activity and prevent prostaglandin synthesis and peripheral nociceptor sensitization. However, there is considerable
evidence that at least some NSAIDs have a central spinal site of action, and may act synergistically with other analgesic
compounds.
Concurrent use of corticosteroids and other NSAIDs such as aspirin are generally not recommended due to the expected increase
rates of adverse events. When switching NSAIDs a washout period is also commonly recommended. The recommended interval is
usually on the order of 3-7 days, however; some have recommended washout periods of <1 day based on pharmacokinetic characteristics
of the NSAIDs approved for use in dogs. In the author's opinion, the interval should be established on a case-by-case basis
and consideration should be given to the animals co-morbidities, reason for switching (e.g., lack of effectiveness vs. toxicity),
the patient's quality of life without NSAIDs, availability of alternative analgesic classes, and the veterinarian/owner's
willingness to accept adverse events.
Adverse drug reactions to NSAIDs are common. There are significant species differences in the pharmacokinetics of most
NSAIDs and drugs approved for use in one species should not be used in another without careful verification of dosing information.
Human approved NSAIDs are usually not the drug of choice for veterinary species and clients should be educated of their dangers.
The most common adverse effect in dogs is gastrointestinal upset with the potential for erosion or ulceration. This is believed
to occur because of direct mucosal irritation related to the dosage formulation (unbuffered aspirin) or from altered mucosal
blood flow secondary to inhibition of prostaglandins. Gastrointestinal ulceration occurs mainly in the stomach but can occur
in the small intestine. Some clinicians will co-administer H-2 blockers, proton pump inhibitors, sucralafate, or synthetic
prostaglandins such as misoprostil to try to prevent, or reduce the signs of gastrointestinal side effects in patients at
risk of developing ulcers. NSAID administration to animals predisposed to gastrointestinal ulceration should not be done routinely.
Animals on concurrent steroid therapy, with gastrointestinal disease, animals that are not eating, and animals that are under
stress are predisposed to gastrointestinal ulceration.
Renal damage is a less common, but potentially life threatening side effect of NSAIDs in most species. The mechanism of renal
failure is thought to be impairment of renal blood flow associated with loss of vasodilatory prostaglandins in the kidney.
Usually the first sign of renal tissue damage is elevation of urine enzymes, but later serum BUN and creatinine increase.
Patients that have periods of poor renal perfusion, are on nephrotoxic drugs, or that have a history of renal disease are
not good candidates and are at potentially greater risk for NSAID-associated toxicity. NSAID administration to patients receiving
diuretics may inhibit diuretic action. The mechanism appears to be related to reduced glomerular blood flow.
Most NSAIDs will inhibit platelet function. The duration of the inhibited function varies with the drug. It is longest
for aspirin and shortest for "reversible inhibitors" of COX such as carprofen. Aspirin irreversibly binds to COX and will
require synthesis of new enzyme before platelet function returns. This can take several days and many clinicians are not comfortable
performing elective procedures sooner than 7-10 days after the last dose of aspirin. This is most troublesome for procedures
that are associated with difficult hemostasis such as neurosurgery. It is not a good idea to administer aspirin to a dog presenting
with intervertebral disk disease in case emergency surgery is required.
Idiosyncratic reactions to most NSAIDs have been reported. These reactions are usually due to a pharmacogenetic interaction
and are usually not predictable. The best-known reaction is hepatopathy associated with the administration of carprofen to
dogs. Others include hypoproteinemia and KCS associated with etodolac and bone marrow dyscrasias associated with phenylbutazone.
