10 key points for a well-balanced veterinary pain management program.
Prevailing research strongly supports incorporating pain management into all aspects of patient care from birth to the end of life. The pain pathways have their own unique version of memory, and inadequate pain management can have lasting consequences. For example, in people, if painful experiences are inadequately addressed early in life, patients presented for subsequent routine care may be fearful and may have enhanced pain responses as they age.1,2 If perioperative pain is not controlled through balanced analgesic methods and meticulous surgical technique, the risk of chronic postsurgical pain increases significantly.3 Anesthetic protocols devoid of analgesic and sedative influence, such as mask induction and maintenance, may minimize patient recovery time, but they can compromise patient comfort and safety.
Robert M. Stein, DVM, DAAPM, and Stephanie Ortel, LVT, BVS, CCRP
Medications alone cannot provide the best possible patient quality of life and maximum longevity. These optimal goals require integrated, multidisciplinary, multimodal pain management methods. We consider the 10 points below to be key aspects of any well-balanced veterinary pain management program.
While an anesthetized patient is not consciously aware of pain, nociceptive pathway activation and subsequent sensitization proceed unabated. Once the pain system has become sensitized, it is difficult to gain control of a patient's pain, requiring higher drug doses and more complex pain management strategies. To maximize pain control and minimize adverse effects, medications need to have taken effect before any painful stimulus occurs.
Stress influences a patient's pain experience and morbidity, making stress reduction a key—often overlooked—aspect of overall pain management. Highly stressed patients should receive sedatives when admitted and repeated doses as needed to maintain adequate anxiolysis before their procedures.
Effective pain management requires a combination of agents that not only target various aspects of the pain pathways but also target patient stress to provide a balanced, multimodal effect.
Opioids are the foundation of multimodal perioperative analgesia. They have analgesic activity peripherally and centrally. Mu agonists such as morphine, hydromorphone, and methadone and the partial mu agonist buprenorphine are our preferred presurgical opioids. The kappa agonists butorphanol and nalbuphine are best suited for sedation for nonpainful procedures.
Midazolam, a benzodiazepine, is also an excellent premedication component. Unlike diazepam, midazolam is well-absorbed intramuscularly. Midazolam provides additional sedative effects as well as analgesic benefit, and there is reason to believe that some patients given midazolam experience short-term amnesia, reducing their awareness of the hospital experience. In addition, most of our healthy patients require a third premedication component—acepromazine, medetomidine, or dexmedetomidine (Dexdomitor—Pfizer Animal Health).
Local anesthetics can almost always be included for enhanced patient comfort and safety. Nonsteroidal anti-inflammatory drugs (NSAIDs) are of marked patient benefit if the patient is NSAID-tolerant and blood pressures are effectively monitored and maintained. Low-dose intravenous infusions of the N-methyl-D-aspartate (NMDA) antagonist ketamine, which blocks glutamate receptors, inhibits central sensitization and completes a balanced perioperative pain management strategy.
Oral multimodal outpatient strategies often include several complementary analgesics. Tramadol is a generally well-tolerated oral opioid. NSAIDs are attractive if proper attention is paid to patient selection and monitoring. Amantadine, a once-daily oral NMDA antagonist, provides a convenient outpatient antihyperalgesic benefit. Gabapentin, a calcium-channel blocker, possesses broad analgesic benefits for many types of pain. Combinations of any or all of these medications can be customized to a patient's needs.
Local anesthetics provide a cost-effective preemptive analgesic benefit associated with few adverse effects. It is usually possible to incorporate local anesthetics into every surgical event. Local or regional anesthesia allows for a reduction in maintenance anesthetic requirements, predictably improving patient respiratory and cardiovascular dynamics. Applications include facial blocks during oral or facial surgeries, incisional line blocks, intra-articular injections, intratesticular blocks for castrations, and ring blocks before onychectomies. Advanced techniques include Bier blocks (intravenous regional anesthesia of a limb), nerve locater-assisted regional nerve blocks, and continuous soaker catheter infusions.
Lidocaine, the least expensive agent, has the shortest onset of all local anesthetic drugs, but it also has the shortest duration. Bupivacaine has slower block onset than lidocaine does, but it provides longer block duration. Using a combination of these two drugs provides the positive qualities of these two agents and often more than enough drug volume. Studies in people suggest that adding morphine or buprenorphine to the local anesthetic agent mix enhances the quality and duration of analgesia,4-6 and we use 0.075 mg/kg morphine or 0.005 mg/kg buprenorphine in this manner in dogs and cats.
Analgesics administered by intermittent bolus provide peaks and valleys in drug plasma concentrations. Intravenous constant-rate infusions (CRIs) establish more consistent plasma concentrations and better overall control over drug effects than intermittent bolus administration does. A mu-agonist opioid, lidocaine, and ketamine are most frequently combined in analgesic CRIs, providing an effective multimodal analgesic benefit with low potential for adverse events. Although delivery through a standard drip set is possible, you may prefer the fine control provided by an intravenous fluid pump or syringe pump. Calculators are available to eliminate the worry associated with drug delivery calculations (www.vasg.org). Analgesic CRIs can help contribute to a balanced anesthetic program, reducing maintenance agent needs and generally improving patient blood pressure and ventilation.
Epidural analgesia deserves your serious consideration. Epidurals concentrate analgesic medications at the nerve root or spinal cord level, gaining substantial patient benefit with minimal systemic effects. The procedure is not difficult to master, and the potential for serious adverse events is low if the proper technique is used.
Morphine is the most common opioid used for epidurals, but hydromorphone, oxymorphone, and buprenorphine are reasonable alternatives. The local anesthetics lidocaine and bupivacaine are commonly teamed with an epidural for superior overall analgesic effect. Ketamine and medetomidine or dexmedetomidine can also be included for additional analgesic effect.
NSAIDs provide important patient benefits including peripheral anti-inflammatory and central analgesic activities. Patient comfort is enhanced when NSAIDs are administered preoperatively. Safe administration of preoperative NSAIDs requires effective blood pressure monitoring. Blood pressure management is becoming a standard-of-care issue in veterinary medicine. By establishing a strong blood pressure management program, you open the door to expanded patient pain management options.7
We want to stay ahead of pain, not wait for signs of pain and then react to them. In fact, one saying about pain management is that if your patient shows signs of pain, you've waited too long to administer its next treatment. Our patients are good at hiding signs of pain. However, there are often pain indicators. The recently released American Animal Hospital Association and American Association of Feline Practitioners pain management guidelines include an excellent review of common indicators of pet pain. And the Glasgow pain assessment short form can help focus staff attention on patient pain after surgery. The best approach is to structure a broad-based pain management strategy and remain vigilant for signs of analgesic inadequacy.
Transmucosal buprenorphine is the preferred opioid for feline outpatient pain management. Research in cats has shown equal bioavailability between buprenorphine delivered transmucosally and intravenously.8 Early results from a recent canine study suggest that dogs may have similarly effective absorption of transmucosal buprenorphine. Dogs generally respond well to the weak mu agonist tramadol (5 mg/kg orally three to four times a day), whereas cats may have poor tolerance for tramadol's bitter taste.
It is generally recognized that butorphanol is not an adequate analgesic for perioperative pain management, even when given parenterally. Its use as an analgesic is further degraded when it is given orally. In people, the enterohepatic first-pass effect removes 80% to 90% of the drug before it has a chance to provide a benefit. The same is true when buprenorphine is mixed with a liquid and swallowed.
Fentanyl patch studies have shown inconsistent drug delivery, with one study showing that one-third of cats receiving a properly placed patch failed to develop any detectable drug plasma concentrations.9 Thus, fentanyl patches should not be relied on as the sole analgesic.
Taking the lead from the human medical community, physical rehabilitation therapy is now regarded as a critical aspect of veterinary pain management. Medications can help dampen pain signaling, but they do not address the associated underlying contributors, such as restricted joint range of motion, muscle tightness, reduced muscle strength, and coordination. By including therapy aimed at these additional factors, patient function, comfort, and resistance to injury improve substantially.
Pain management is arguably one of acupuncture's greatest strengths, and electroacupuncture is the most effective form of acupuncture for pain management. Natural pain relievers (endorphin, endomorphin, dynorphin, enkephalin) and biogenic amines (serotonin, catecholamines) are released during the acupuncture session, reducing patient pain.10,11 There may also be pain relief via the gate control theory12 similar to that obtained through transcutaneous electro-nerve stimulation, a common human physical therapy tool. Patient attitude and appetite can improve with acupuncture therapy, making it a particularly attractive approach for patients with renal disease suffering from pain.
The combination of physical rehabilitation therapy and acupuncture may be a good approach for patients with difficult-to-manage pain, especially for those patients that are intolerant of or poorly served by conventional medical therapy. In addition, by offering these services, you open the door to clients who find conventional medical therapies unattractive.
Robert M. Stein, DVM, DAAPM, and Stephanie Ortel, LVT, BVS, CCRP, Animal Pain Management Center, 2217 Kensington Ave., Snyder, NY 14226. Dr. Stein is the president of the International Veterinary Academy of Pain Management (www.ivapm.org).
1. Taddio A, Goldbach M, Ipp M, et al. Effect of neonatal circumcision on pain responses during vaccination in boys. Lancet 1995;345(8945):291-292.
2. Taddio A, Katz J, Ilersich AL, et al. Effect of neonatal circumcision on pain response during subsequent routine vaccination. Lancet 1997;349(9052):599-603.
3. Kehlet H, Jensen TS, Woolf CJ. Persistent postsurgical pain: risk factors and prevention. Lancet 2006;367(9522):1618-1625.
4. Candido KD, Winnie AP, Ghaleb AH, et al. Buprenorphine added to the local anesthetic for axillary brachial plexus block prolongs postoperative analgesia. Reg Anest Pain Med 2002:27(2):162-167.
5. Bazin JE, Massoni C, Bruelle P, et al. The addition of opioids to local anaesthetics in brachial plexus block: the comparative effects of morphine, buprenorphine and sufentanil. Anaesthesia 1997;52(9):858-862.
6. Lombardi AV Jr, Berend KR, Mallory TH, et al. Soft tissue and intra-articular injection of bupivacaine, epinephrine, and morphine has a beneficial effect after total knee arthroplasty. Clin Orthop Relat Res 2004;(428):125-130.
7. Blood pressure management. Available at www.vasg.org/blood_pressure_management.htm. Accessed May 7, 2008.
8. Robertson SA, Lascelles BD, Taylor PM, et al. PK-PD modeling of buprenorphine in cats: intravenous and oral transmucosal administration. J Vet Pharmacol Ther 2005;28(5):453-460.
9. Lee DD, Papich MG, Hardie EM. Comparison of pharmacokinetics of fentanyl after intravenous and transdermal administration in cats. Am J Vet Res 2000;61(6):672-677.
10. Mittleman E, Gaynor JS. A brief overview of the analgesic and immunologic effects of acupuncture in domestic animals. J Am Vet Med Assoc 2000;217(8):1201-1205.
11. Han JS. Acupuncture and endorphins. Neurosci Lett 2004:361(1-3):258-261.
12. Melzack R, Wall PD. Pain mechanisms: a new theory. Science 1965;150:971-979.