Chapter 27
Post-Operative Analgesia – Approaches and Options
So many choices!
Erin Wendt-Hornickle
Department of Veterinary Clinical Sciences, College of Veterinary Medicine, University of Minnesota, USA
- Q. What are the benefits of providing analgesia to my post-operative patients?
- A. Any injurious event (i.e., surgery) will not only cause changes in the sensory nervous system and produce pain, but will also cause changes in the autonomic nervous system, endocrine and immune systems. In addition, inflammatory substances such as cytokines, prostaglandins, and bradykinins that are released locally can have an effect on subsequent neural processing of pain if not treated promptly and appropriately. When analgesia is provided to patients peri-operatively, these deleterious interactions between body systems are avoided and patients recover faster with a lower rate of complications.
- Q. What is the best approach in the treatment of post-operative pain?
- A. In the last several decades, there have been major advances in the knowledge and understanding of acute pain physiology. For instance, we know that there are several biological processes that occur prior to a patient perceiving pain. This is called nociception and consists of the transduction, transmission, modulation, and perception of pain. Each of these processes can be broken down further and targeted separately with analgesics of different classes. This is termed multimodal analgesia and is the most complete approach to pain management.
Though controversial, the timing of analgesic administration may also be important. Most evidence suggests that analgesia given at some point prior to recovery and return of consciousness is best. Frequently assessing patients for the presence of pain and continuing treatment when necessary provides more effective pain management.
- Q. Which drugs should I choose to treat post-operative pain?
- A. There are four major groups of analgesics: nonsteroidal anti-inflammatory drugs (NSAIDs), local anesthetics, opioids, and other adjuncts. Recognizing that pain is a fairly complex physiologic process, a combination of these analgesics is usually best.
- Q. Why and how should I choose a NSAID?
- A. NSAIDs are extremely useful in the treatment of post-operative pain that is inflammatory in origin, and should be included unless there is a contraindication. Since each of the well-known COX enzymes (COX 1 and COX 2) also serves very important positive physiologic functions such as gastric mucosal defense and healing, modulation of vascular tone in the kidney and proper functioning of platelets, one must choose NSAIDs carefully. It is advantageous to choose an NSAID that has limited inhibition of COX-1 to decrease the gastrointestinal side effects related to administration of these drugs. NSAID approval for use in certain species varies by country. For more information about NSAID use in veterinary medicine, see Chapter 6.
- Q. Should I include a local anesthetic as part of my protocol?
- A. Yes! Local anesthetics reversibly block impulse conduction through sodium channels, thus preventing membrane depolarization and action potential formation of sensory afferent neurons. These drugs vary in their potency, onset, and duration. Many types of post-operative pain can be managed, in part, by performing local or regional anesthetic techniques prior to surgery. Topical, infiltrative, and regional techniques are valuable in cats and dogs with acute pain. For more information about specific local anesthetic drugs and techniques, see Chapters 24 and 28.
Lidocaine can also be used as an intravenous constant rate infusion (CRI) to provide peri-operative analgesia. If used intra-operatively, it can be continued into the post-operative period at rates of 25–50 mcg/kg/min in dogs [1,2]. If the lidocaine infusion is not started until the post-operative period, a loading dose of 1–2 mg/kg in dogs should precede infusion [1]. It is imperative to use a fluid pump or syringe pump for accurate lidocaine delivery and avoidance of toxicity. For more details regarding lidocaine CRI, see Chapter 22.
A transdermal lidocaine patch is available (LIDODERM, Endo Pharmaceuticals; LidoPAIN SP, EpiCept Corporation) as another option in the treatment of post-operative pain. Originally developed for the treatment of neuropathic pain induced by shingles in humans, lidocaine patches can be safely used on skin incisions for 3–5 days in dogs [3]. They can be cut to fit the size of the patient or shape of the incision without affecting the integrity of the patch [3]. Plasma concentrations of lidocaine are low for the duration of adherence, suggesting very local effects rather than systemic uptake [4,5]. Clinical studies proving their effectiveness are lacking.
- Q. Can I give opioids to my patient post-operatively?
- A. Yes and you definitely should give them post-operatively in all patients with moderate to severe pain! Opioids, specifically μ agonists, remain the most effective analgesics in controlling moderate to severe acute pain, especially as part of a multimodal regime [6]. There are many different potential routes of administration, including injectable, oral transmucosal, transdermal/topical, and oral.
- Q. What opioids can I give via injection?
- A. Several full and partial μ agonists are available in an injectable formulation given as bolus injections and also as constant rate infusions. Morphine, hydromorphone, oxymorphone, fentanyl, meperidine, methadone, and buprenorphine are the most commonly used injectable opioids in veterinary medicine. Though these drugs are similar in action, their pharmacokinetic profiles differ considerably. For more detailed information on specific opioids and their analgesic uses, see Chapter 6.
- Q. Is there something injectable that lasts longer than 8 h?
- A. Simbadol™ (buprenorphine injection; Abbott Laboratories, IL, USA) is the first FDA approved long-lasting injectable analgesic for use in cats. It is administered subcutaneously every 24 h for a maximum of 3 days at a dosage of 0.24 mg/kg. Analgesic effects of Simbadol™ begin 1 h from injection; it is recommended to administer it 1 h prior to surgery. This formulation showed analgesic efficacy in both orthopedic (onychectomy) and soft tissue surgeries, though 39 and 29% of cats, respectively, required additional analgesia [7].
There is also a sustained release formulation of buprenorphine (ZooPharm, CO, USA) that provides analgesia for up to 72 h when given subcutaneously. Limited studies suggest that it provides similar analgesia and has a similar side effect profile to OTM buprenorphine when used prior to ovariohysterectomy in dogs and cats [8]. Clinical experience has found that some patients develop a local skin reaction at the site of injection. Since some patients often object to medication, these longer-acting options have a definite advantage for treating mild to moderate post-operative pain.
- Q. What if I have a patient that will not tolerate injections?
- A. Some opioids can be given via the oral transmucosal route (OTM). This route of opioid administration is most commonly associated with cats due to their small dosage requirements and basic (high pH) mouths. There are two opioids that have been administered most often via this route: methadone and buprenorphine.
Methadone can be administered OTM in cats and has adequate bioavailability and analgesic qualities when given by this route. The antinociceptive effects of OTM methadone may last longer than intravenous administration; though plasma levels are lower [9]. Euphoric effects (kneading, purring, rubbing against objects) are commonly seen along with mydriasis; however, dysphoric effects (excitement, anxiety, restlessness) are not common [9–12].
Buprenorphine, a partial μ agonist, is almost 100% bioavailable when given to cats via the OTM route at doses of 0.02 mg/kg [13]. It is very well tolerated by feline patients. Studies of its efficacy at providing analgesia by this route are contradictory, but may be related to the doses used (i.e., 0.02 mg/kg is more effective than 0.01 mg/kg) [14–16]. Because of buprenorphine’s partial agonist actions, it is best suited for treating mild to moderate pain.
- Q. Can I give an oral opioid for post-operative pain?
- A. Despite limited evidence-based information of the analgesic efficacy of opioids given orally in veterinary patients, this route is used for the treatment of post-operative analgesia.
Codeine alone, or in combination with acetaminophen, has been used in dogs for the management of mild to moderate pain on an outpatient basis. Similar to other oral opioids, this drug has limited bioavailability (4%) and a short half-life when given orally. Plasma levels of one of its metabolites, codeine-6-glucuronide, remain high and may provide analgesia [17]. The acetaminophen may also contribute to the apparent analgesic effects.
The pharmacokinetics of hydrocodone with and without acetaminophen in dogs has also been described [18,19]. At clinically relevant doses, plasma concentrations of hydrocodone and its active metabolite, hydromorphone, are above the level thought to provide analgesia for dogs [20].
Stay updated, free articles. Join our Telegram channel
- Q. Which drugs should I choose to treat post-operative pain?
Full access? Get Clinical Tree
