General pharmacology

Analgesics

Analgesics need to be given prior to the pain stimulus to prevent hyperalgesia and wind-up phenomenom (basically when nerves become super sensitive and react even more strongly to pain). This preemptive analgesia decreases the amount of pain produced but does not prevent pain. Analgesics should also be used after the stimulus to control the resultant pain.

The most common analgesic classes are NSAIDs (nonsteroidal anti-inflammatory agents) and narcotics.

Non-steroidal anti-inflammatory drugs

NSAIDs are aspirin like drugs that block cyclooxygenase and inhibit the production of prostaglandins and leukotrienes. Prostaglandins and leukotrienes are inflammatory mediators and many cause heat, pain, swelling, redness and loss of use.  Most surgical pain is related to the release of these inflammatory mediators so NSAIDs are a good first choice for managing surgical pain.  Inflammation usually increases for about 3 days postoperatively before it starts to resolve (expect more swelling and incisional dehiscence about day 3!)

NSAIDs typically cause muscle necrosis so are administered iv, orally or topically. Ketoprofen can be given im but is not as effective as other NSAIDs.

NSAIDs have varying levels of toxicity. As they block prostaglandin release and prostaglandins do lots of things, side effects are widespread. Prostaglandins are important in most facets of physiology so NSAIDs have the potential to interfere with everything from bone healing to parturition. Most NSAIDs cause gastric irritation, ulceration and protein loss into the gastrointestinal tract. One of the earliest signs of NSAID toxicity is low total plasma protein. Other common concerns include renal damage and colitis. Neonates, geriatric animals, animals on other renally metabolized drugs, animals that are dehydrated or not drinking are at increased risk of NSAID toxicity.

Common LA NSAIDs include phenylbutazone, flunixin meglumine, meloxicam, firocoxib, and diclofenac sodium. Use of a particular NSAID varies by species.

Flunixin meglumine is the only approved NSAID for food animal species and is also used in horses. It is considered best for visceral pain but does work for musculoskeletal pain, as well. It is less toxic than phenylbutazone but more toxic than firocoxib. It is labeled for im use but should never be administered im except to pigs. Due to the muscle necrosis, it can lead to clostridial myositis. Give it iv or orally or topically, depending on the formulation.

Phenylbutazone is the most common NSAID used in horses – it is cheap and available in oral formulations. It is also the most toxic. It should not be used in food animals as it is also toxic for people.

Meloxicam is an oral NSAID used in food animals, including camelids. It is used off label so requires veterinarian-client-patient relationship. It works even though it has to pass through the rumen or C3.

Firocoxib is an oral NSAID used in horses that are at risk of NSAID toxicity. It is the safest NSAID. This is the same ingredient as in Previcox. However, it is illegal to use Previcox in horses since there is a horse formulation.

Most NSAIDs cannot be given intramuscularly due to the muscle irritation!

 

Narcotics

Narcotics are used to enhance the pain relief (especially before and during surgery) and/or when NSAIDs are not advisable. Narcotics are also needed when the pain isn’t caused by inflammation.

Narcotic use in LA species is expanding. Common narcotics include butorphanol, morphine, and buprenorphine. Use of a particular narcotic varies by species. Narcotics can cause excitement in many LA species so iv administration is avoided in most situations.

Narcotics are generally synergistic with alpha adrenergic agents (xylazine, detomidine) resulting in improved sedation and analgesia compared to the effects from individual agents.

Narcotic side effects include drowsiness, recumbency, and Gi stasis (colic, constipation). Side effects also include muscle twitches (annoying for head surgery) and excitement if a sedative is not on board first. Almost all  adult horses should be sedated prior to opioid administration. Animals also become less sensitive to the effects of narcotics over time due to downregulation of receptors.

Narcotics can be reversed by naloxone.

Butorphanol is a common additive to equine sedation protocols. We say “1.5/5” to mean 150 mg xylazine and 5 mg butorphanol. Or “5/5” to mean 1/2 ml detomidine and 1/2 ml (5 mg) butorphanol. Agreed – it is terribly inconsistent in terms of mgs and mls. The combination enhances and prolongs sedation as well as providing additional pain relief. Unfortunately, butorphanol does not last long so is not great for postoperative pain management.  Butorphanol is partial opioid agonist/antagonist so can actually reverse the effects of other narcotics. Do not combine it with other narcotics.

Morphine is a cheaper and longer acting opioid that is used in place of butorphanol in cattle.  Acepromazine is combined with either butorphanol or morphine to create standing sedation protocols in cattle, particularly for Csections. Morphine is used for pain control.

Buprenorphine is another narcotic that is used to manage postoperative pain in goats.

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Large Animal Surgery - Supplemental Notes by Erin Malone, DVM, PhD; Elaine Norton, DVM PhD; Erica Dobbs, DVM; and Ashley Ezzo, DVM is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.