Sandra Z Perkowski, VMD, PhD, DACVAA 

Pre-emptive and multimodal use of analgesics in the perioperative period is becoming increasingly common in veterinary practice, as awareness of the complexity of the pain pathways and the potential for interaction among those pathways has also increased. Improved comfort with the use of continuous rate infusions, new transdermal applications, increased use of locoregional analgesic techniques and better understanding of the pharmacokinetic and pharmacodynamic differences between our many species and with humans has led to new and multiple combinations of many drug types in the perioperative period, including opioids, local anesthetics, NSAIDs analgesics targeting different portions of the nociceptive pathways. The net analgesic effect is synergistic, allowing for decreased doses and side effects.

Understanding mechanisms of pain transmission and antinociceptive mechanisms allows a logical choice in prescribing analgesics for our patients. Analgesia may be directed at minimizing inflammatory changes at the site of injury (e.g. by using an NSAID) at inhibiting transduction or transmission of the nociceptive signal (both at peripheral and spinal endings, e.g. by using a local anesthetic), or at increasing the activity of descending inhibitory pathways acting at the CNS (e.g. by using an opioids: hydromorphone or morphine or synthetic opioid : fentanyl).

Opioids 

Opioids are a diverse group of agents that produce analgesia by their actions on specific opioid receptors (mu, kappa, delta). These receptors vary in their pharmacological effects and their distribution throughout the body. Pure opioid agonists, including morphine, oxymorphone, hydromorphone, fentanyl and methadone bind to all of these receptors and provide the most profound analgesia. In addition methadone is a racemic mixture, having both opioid and NMDA receptor antagonist properties (ie ketamine) that may improve the analgesia achieved from using an opioid agonist alone. Several studies in both rats and humans have demonstrated the development of both opioid tolerance and an opioid- induced hyperalgesia (OIH) with relatively short term use of opioids. These may be minimized by the concomitant use of an NMDA receptor antagonist (e.g. ketamine). In addition, addition of an NMDA receptor antagonist can be helpful in patients with a high potential for the development of neuropathic pain (disc herniation, limb amputation, lateral thoracotomy). Methadone has a relatively short half-life (1.75 h) after 1 mg/kg i.v.

Pure opioid agonists (fentanyl, morphine, remifentanil) are frequently used as continuous rate infusions during the anesthetic period. At lower doses, these infusions may be continued into the post-operative period and provide continued analgesia while minimizing respiratory depression (e.g. fentanyl: analgesic dose: 0.1 - 0.2 microgram/kg/min). More recently, combination infusions of morphine or fentanyl with lidocaine, and ketamine have been recommended (see below). Use of the NMDA receptor antagonist ketamine has been shown experimentally to attenuate the development of opioid tolerance and OIH in rodents,thereby providing an opioid sparing effect. Butorphanol may be preferred as a CRI in some cases. A loading dose equivalent to the low end of the CRI range is generally given for any of the above.

Opioid administration can produce clinically significant side effects, including sedation, dysphoria or excitement, respiratory depression, vomiting, and decreased GI motility. Therefore, opioid agonist-antagonists and partial agonists (butorphanol, buprenorphine) may be preferred in some cases. These drugs generally provide less analgesia than pure opioid agonists, but the side effects also tend to be less severe. Butophanol, a kappa agonist and mu antagonist, is generally considered a mild to moderate relatively short-lived analgesic, most effective in models of visceral pain (0.1 - 0.4 mg/kg IM, IV q 1 - 4h). Butorphanol also may be useful for its antiemetic
properties in patients that are nauseated. Buprenorphine, a partial mu agonist, provides effective analgesia for many types of procedures and has a relatively long duration of action (6 -20 microgram/kg IM,IV q 6 - 8h; peak effect at 2h). It is readily absorbed across the oral mucosa in cats. However, recent studies in cats suggest that IV or IM routes of administration is much more effective in providing analgesia than either SC or transmucosal routes.

Tramadol
The poor oral bioavailability of the above opioid compounds has led to an increased interest in and use of tramadol. The parent compound, tramadol, acts as a very weak mu-agonist and acts as an analgesic by inhibiting norepinephrine (NE) and serotonin reuptake. In dogs, it is very rapidly and extensively metabolized to several metabolites, including the active metabolite, Odesmethyltramadol (M1) that is a more potent analgesic than the parent compound. M1 acts as a weak mu agonist with 200X the binding affinity for the receptor than the parent compound. However, the affinity is still only 10% that of morphine. Tramadol (11 mg/kg PO) was 65% bioavailable with a short half life of 1.7 hours. For M1, however, the half life is also relatively short at 1.7 hours. Tramadol is metabolized through the cytochrome p450 enzyme system to a number of metabolites in both dogs and cats. Recent studies have shown that in many dogs, the M1 metabolite is a relatively minor metabolite, and suggest that efficacy as an analgesic may in part be dependent on rate of metabolism. Tramadol should not be combined with other psychotropic drugs (e.g. amitryptiline) due to the possibility of serotonin syndrome. In addition, decreased NE and serotonin reuptake may potentially lead to cardiovascular changes such as hypertension and increased platelet activation, potentially increasing the risk for GI bleeding.

Local Anesthetics

Lidocaine and bupivacaine are local amide anesthetics and act by blocking the sodium channel in the nerve membrane, inhibiting action potential generation and transmission of the nociceptive signal. Both lidocaine and bupivacaine may be used for local injection at the nerve. With the advent of nerve locators, both of these agents are being used with increased frequency for brachial plexus blockade in patients undergoing forelimb procedures and femoral and sciatic nerve blockade for hindlimb procedures. Generally a maximum dose of 4 mg/kg lidocaine or 1.5 mg/kg bupivacaine is used. When used epidurally, the site for injection in small animals is usually the lumbosacral space. A combination of morphine and bupivacaine is recommended to take advantage of the synergistic effect of using analgesic agents from two different classes: 0.1 mg/kg of a 1 mg/ml preservative-free solution (e.g. DuraMorphâ) and 0.1 ml/kg of 0.5% bupivacaine. With this combination, the bupivacaine works within the first 30 minutes and lasts about 8 hours, while the morphine's peak effect occurs about 4 - 8 hours after administration and lasts 24 hours. Oxymorphone, fentanyl, butorphanol, and buprenorphine have also been proven effective when given epidurally.

Lidocaine may be given as a low dose infusion (25 - 100 ug/kg/min) to enhance the effect of other analgesic drugs. It is frequently used in combination with either morphine or fentanyl given as a CRI +/- ketamine (see below). CRIs of lidocaine should be used cautiously, if at all, in cats due to pharmacokinetics which are very different from the dog, leading to relatively high 536 peak levels even at low doses. In addition, cardiopulmonary depression may be pronounced. Bupivacaine should NEVER be given intravenously in any species.   

Recently, lidocaine transdermal patches (Lidoderm ®) have been introduced in human medicine and are being used more frequently in veterinary medicine. Lidoderm is a 5% lidocaine patch, containing a total dose of 700 mg of lidocaine suspended in gel on a felt backing. Penetration of the lidocaine into the skin produces an analgesic effect in the area of the patch, with minimal effect on normal sensation. Overall, little systemic absorption occurs (in contrast to a transdermal fentanyl patch). In human studies, the patch has been shown to be effective in treating chronic, neuropathic, or osteoarthritic pain. It has been used acutely for back pain or after cruciate repair. Care must be taken when using local anesthetics drugs by any route, due to their relatively high systemic toxicity. Toxic cardiovascular and neurologic effects (i.e. convulsions) may be seen at doses relatively close to the effective dose. Arrhythmias and myocardial depression from local anesthetics, particularly bupivacaine, can be extremely difficult to treat. The IV seizure dose for lidocaine in dogs has been reported as 11 mg/kg, while that for bupivacaine is 3 mg/kg. Cardiotoxic doses are slightly higher than the seizure dose. These agents rely on hepatic metabolism; therefore, adjustments should be made in patients with liver disease. In addition, half-lives are longer in cats and toxic doses are lower than for dogs. There are no available reversal agents.

N-Methyl-D-Aspartate (NMDA)
Antagonists While traditionally considered a dissociative anesthetic, ketamine is also recognized as an NMDA receptor antagonist. The NMDA receptor appears to play a central role in the development of central hypersensitization and "wind up" of the dorsal horn neurons. Given as a low-dose infusion at a rate of 0.1 - 0.6 mg/kg/h, ketamine may be used as an adjunct to other analgesic therapy such as continuous rate infusion morphine (at a rate of 0.1 mg/kg/hr +/- lidocaine) or fentanyl without causing anesthesia or pronounced sedation. It may help to prevent the development of opioid- induced hyperalgesia and opioid tolerance. Amantadine is an oral NMDA receptor antagonist which has been recommended to help decrease wind- up in patients ith chronic pain (e.g. 3 - 5 mg/kg PO sid for 1-2 weeks).

Non-Steroidal Antiinflammatory Drugs (NSAIDs) 

Non-steroidal antiinflammatory drugs (NSAIDs) inhibit the cyclooxygenase pathway of arachidonic acid metabolism. At least two related, but distinct, isoforms of the COX enzyme, COX-1 and COX-2, are present. COX-1 is constitutively expressed in most tissues, including the gastric mucosa, liver, kidneys and platelets, and is involved in normal "housekeeping" functions. These include gastric mucosal barrier protection, maintenance of liver and renal blood flow, particularly in low perfusion conditions such as hypovolemia and/or hypotension, and normal platelet aggregation. COX-2 is primarily an inducible enzyme found within a more limited subset of cell types, predominantly inflammatory cells, endothelial cells, articular chondrocytes and synovial fibroblasts, peripheral nerves and the central nervous system. COX-2 has also been found constitutively within the central nervous system and renal vasculature, as well as synovial chondrocytes in some species. Development of newer NSAIDs that target the COX-2 enzyme and "spare" the COX-1 enzyme, result in selective antiinflammatory effects with decreased gastric side effects and minimal effects on coagulation. However, COX-2 derived eicosanoids are increasingly recognized as physiologically important mediators of cardiovascular and renal homeostasis and play an important role in angiogenesis and wound healing ie slowing wound healing. Therefore, care must still be taken when using these newer drugs as GI and renal side effects still occur.

New information and the availability of new drugs has changed the way that NSAIDs are viewed as part of the multimodal approach to pain management. While inhibition of peripheral COX activity and the resulting anti- inflammatory effect is part of the mechanism by which NSAIDs provided analgesia, it is now recognized that much of their analgesic effect is due to inhibition of COX activity, more specifically COX-2 activity, centrally. It may take up to two hours to achieve effective inhibitory levels within the dorsal horn of the spinal cord and this should be taken into account perioperatively, when measuring the risk/benefit ratio of using these drugs at a time when anesthetic agents may adversely affect blood pressure and/or renal blood flow and oxygen delivery. Idiosyncratic side effects may also occur. Newer COX-2 drugs which have a rapid distribution to inflamed target tissues and a short plasma half-life (e.g. 1 mg/kg robenacoxib PO (Onsior) has a plasma half-life less than 2 hours) may prove to have fewer systemic side effects while still providing effective pain relief.

When selecting a NSAID for perioperative use, decisions should be made based on both safety and efficacy. In addition, thought should be given to the physical status of the patient, procedure to be performed, and anesthetic agents being used. A COX-2 preferential or selective agent is generally preferred to minimize intraoperative bleeding. Due to the potential for clinically significant changes in renal blood flow and oxygen delivery, intravenous fluid support and blood pressure monitoring during the anesthetic period are highly recommended. Reassess and reevaluate your patient regularly. Do not combine NSAIDs or NSAIDs and steroids. If preemptive analgesia is the goal, additional analgesics from other drug classes such as the opioids may be used and may be preferred in many instances. Avoid NSAIDs in patients which are hypovolemic, hypotensive or with underlying renal, hepatic or GI disease.

Veterinary patients frequently require analgesics for a period of time after acute trauma or surgery. Close attention should be paid as to whether the initial treatment provides the desired effect (i.e. analgesia!) and how long the analgesic effect lasts. Reevaluate the effectiveness of treatment regularly! Before administering any drug, carefully observe the animal and consider the underlying disease process. If any expected side effects are undesirable, alter your analgesic technique!

Click here for table of perioperative analgesics.