General pharmacology

Perioperative medications

This chapter is an overview of the drugs that may be needed prior to surgery. Sedatives, analgesics and potentially antibiotics are required for standing surgery. If we are going to use general anesthesia for recumbent surgery, we will add induction agents and maintenance agents, as well. See the later chapters for details on the drug types and for species specific recommendations.

Sedatives

Sedatives and calming agents are often necessary prior to procedures. Excited animals do not respond well to general anesthesia and it is important to induce sedation before attempting general anesthesia.

Sedatives also need time to work. If an animal is constantly stimulated (talked to, patted, poked), it may not sedate at all. Generally ask everyone to step back and stay calm and quiet for several minutes after administering a sedative or calming agent. Avoid pain or trauma while the sedative is working.

Most sedatives will work quickly.  IV (intravenous) medications will usually take effect within 2-3 minutes while intramuscular injections take 5-10 minutes if not longer.  Acepromazine is a calming agent (anti-anxiolytic agent) that can take at least 20 minutes to take effect regardless of administration route.

Analgesics

Analgesics (opioids, NSAIDs) should be given prior to any pain stimulus whenever possible. Once pain starts, the nerves become upregulated and hypersensitive (“wind-up”). To prevent wind-up, we provide “preemptive analgesia” through preoperative medication. We want the analgesics on board before we cause pain. Again, iv agents will work more quickly than im or sq agents. Orally administered drugs are generally cheaper but oral medications need 1-2 hours to take effect. Plan ahead!

NSAIDs have long durations of action (12-24 hours) while narcotics may need redosing during longer procedures.

Typically, surgical inflammation will continue for at least three days.  This means we need to control inflammation for three days so that healing can occur. 

We should always give analgesics preoperatively.  We know surgery is going to cause pain and inflammation.

Antibiotics

Antibiotics should be used only when necessary.  Sometimes you don’t know if it will be necessary. Preoperative antibiotics that are at high levels at the time of any “surprise” are more effective than trying to treat after the fact. This is considered prophylactic use. Prophylactic use of antibiotics is typically restricted to the preoperative +/- surgery period and does not include postoperative care. We typically give antibiotics preoperatively in most cases of abdominal, respiratory and urogenital surgery due to the risk of contamination.

Timing of drugs depends on the route. You want the peak levels to be when you are doing the stuff that might lead to contamination. This is often at least 10 minutes after you anesthetize your patient..

For prophylactic coverage (preventative), we try to have peak levels at the time of the incision. This varies by drug and route but generally

  • Oral: ~ Two hours after drug is administered
  • IV: ~ 15 minutes after injection/infusion
  • Intramuscular (IM): ~30 minutes after injection (does depend on the drug formulation with some taking much longer)
  • Subcutaneous administration – variable; avoid for preoperative medications if possible

Some antibiotics need to be re-administered during longer surgeries to maintain peak levels.

If the surgery is clean or clean-contaminated, further antibiotic therapy is generally not needed unless the animal is immunocompromised, infection would be life-threatening or an implant is used.

Timing of drugs is critical to ensure they are most effective at the time you need them.

When we don’t know what sort of surprises we will encounter, we err on the side of broad spectrum bactericidal drugs. We typically pick  first line drugs, rather than those saved for cases of resistance.

If antibiotic therapy is indicated postoperatively, it is often continued for 3-14 days. One common rule is to treat three days past the resolution of clinical signs (fever, signs of inflammation, etc). In hospitalized cases, a blood count is often checked prior to discontinuing antibiotics to ensure no further signs of infection.

Sample protocols for surgical procedures

Species Sedatives NSAIDs# Antibiotics
Horses Xylazine or detomidine Flunixin meglumine or phenylbutazone Penicillin + gentamicin or ceftiofur
Cattle Xylazine Flunixin meglumine or meloxicam Ceftiofur (lactating cattle)*, penicillin or ampicillin
Small ruminants Detomidine Flunixin meglumine or meloxicam Ampicillin
Camelids Butorphanol Meloxicam or ketoprofen (can go im) Ceftiofur or ampicillin
Swine Ketamine + midazolam or dexmedetomidine Flunixin meglumine or meloxicam Ceftiofur or ampicillin

*In lactating cattle, ceftiofur is typically our first choice due to the limited time required for milk.

#Narcotics are commonly added to all protocols

 

Resources

Perioperative analgesics

The Efficacy of Preemptive Analgesia for Acute Postoperative Pain Management: A Meta-Analysis,  Anesthesia and analgesia 100 3 (2005): 757-73- human work but nice overview

Equine NSAIDs and analgesics chapter

Food animal NSAIDs and analgesics chapter

Perioperative antibiotics

Principles of Antimicrobial Therapy: What Should We Be Using? Vet Clin Equine 22 (2006) 279–296- good review of surgical use of antibiotics

Randomized prospective trials to study effects of reduced antibiotic usage in abdominal surgery in cows. J. Dairy Sci. 101:8217–8223. With careful attention, maybe we can pick which cases get preop antibiotics.

Equine antibiotics chapter

Food animal antibiotics chapter

License

Icon for the Creative Commons Attribution-NonCommercial 4.0 International License

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.