Chapter 12 Behavioral and Physiologic Responses to Pain Modified from Carroll GL: Small animal pain management, Lakewood, Colo., 1998, American Animal Hospital Association Press. Two behavior-based pain scales stand out in being easy to use and having less bias. The Glasgow Composite Measure Pain Score (GLCMPS)–Short Form was developed for use in dogs in acute pain (Reid et al, 2005). Numbers from 0 to 4 or 5 are assigned to differing behaviors and are tabulated to create a score on which analgesic therapy can be based (Fig. 12-1). The two parts consist of initial observation followed by a leash walk and a hands-on portion. Potential advantages that make it more accurate than many scales include limited bias from the observer, ease of use, and specific behaviors identified as being present or absent. One significant disadvantage is that it was developed for dogs, but not for cats. Additionally, it has limited value in the immediate postoperative period because postanesthetic sedation is not part of the evaluation (Mich and Hellyer, 2009). A modified form of the Glasgow Pain Scale has been studied and was found to be effective in assessing acute pain in dogs (Murrell et al, 2008). In this study, sedation was incorporated into the evaluation. The second behavior-based pain scale that deserves mention is the Colorado State University Veterinary Medical Center Acute Pain Scale (Fig. 12-2). It was designed to be a user-friendly scale with verbal and visual descriptions. Similar to the GLCMPS–Short Form, it assigns numbers from 0 to 4 and has both observational and hands-on sections. A section for evaluating body tension is also included. This is the first scale designed to address the feline population (Fig. 12-3). Midazolam is a water-soluble benzodiazepine available for IM or IV injection; it does not cause pain with either route. Additionally, it has a rapid onset of action and rapid metabolism. These qualities make it a very good choice, especially when used as a premedication with an opioid in very young, elderly, or sick dogs and cats. As with other benzodiazepines, midazolam is metabolized by the liver and excreted by the kidneys. Smaller doses may be warranted in older or debilitated animals or in those with liver or kidney disease. In humans, midazolam is the most widely used premedication, given alone or with fentanyl. In veterinary medicine, it is useful especially in small mammals such as ferrets and rabbits and in some birds (Lemke, 2007). It is well tolerated in many dogs, especially when combined with other sedatives (see Box 12-1). Its short duration of action frequently makes it a better choice for induction with propofol, etomidate, ketamine, or barbiturates than diazepam in both cats and dogs. In small animal veterinary medicine, the alpha-2 agonists most commonly used are medetomidine and dexmedetomidine. Historically, xylazine was used for many years, but it does not have selectivity for alpha-2 versus alpha-1 receptors, as do medetomidine and dexmedetomidine. Medetomidine is a racemic mixture, with dexmedetomidine being the active isomer. Therefore dexmedetomidine is twice as potent as medetomidine. Selective alpha-2 agonists may be used as premedication in healthy small animals for their sedative, analgesic, and muscle relaxant properties (Lemke and Creighton, 2010) (Table 12-2; see also Box 12-1). Even in low doses, they can augment the analgesic and anesthetic effects of other drugs. Both medetomidine and dexmedetomidine dosages are calculated using body surface area. This helps to reduce variations in sedative response from one type of body conformation to another. Premedication with 125 µg/m2 or 0.5-1 µg/kg has been shown to reduce the quantity of induction drugs required, as well as the inhalant agent needed for maintenance of anesthesia. Constant rate infusion (CRI) and microdoses of medetomidine and dexmedetomidine have been used with success as part of balanced anesthetic protocols to reduce isoflurane requirements (see Box 12-2). A comparison of 1, 2, and 3 µg/kg/hr determined that a CRI of 1 µg/kg/hr, after a 2.5-5-mg/kg loading dose, had the most stable hemodynamics (Uilenreef et al, 2008). Dexmedetomidine-Butorphanol-Ketamine Combination in Healthy Cats* *Based on a 4.5 kg (10 lb) cat. All drugs (except atipamezole) can be mixed in one syringe and administered as a single IM injection. If given IV, the drug doses in this combination should be halved; however, if a deeper plane of anesthesia is desired, the full IM dose can be given IV. ‡More invasive procedures should have a stronger opioid chosen for postoperative analgesia. From Ko JC, Knesl O, Weil AB, et al: FAQs: analgesia, sedation, and anesthesia: making the switch from medetomidine to dexmedetomidine, Compend Contin Educ Vet 31(Suppl 1A):1, 2009. The cardiovascular effects of dexmedetomidine and medetomidine are dose dependent and biphasic, with the initial phase of hypertension and reflex bradycardia lasting approximately 15 to 20 minutes after administration. This is followed by a decrease in sympathetic tone, resulting in vasodilation, hypotension, and bradycardia. Monitoring of blood pressure and heart rate is recommended. Before bradycardia is treated, blood pressure should be taken. If necessary, an anticholinergic (e.g., atropine, glycopyrrolate) may be used to treat the second phase of bradycardia when hypotension is also present. It has been shown that a marked hypertensive response occurs when atropine is given before dexmedetomidine (Alvaides et al, 2008). Because of this marked hypertension, anticholinergics are not recommended before alpha-2 agonists are administered or during the hypertensive phase (Ko et al, 2009). Because of blood pressure changes associated with these drugs, they are best used in healthy dogs and cats. Alpha-2 agonists are best avoided in hypotensive, hypertensive, hypovolemic, elderly, or critically ill patients. Respiratory function is maintained, but urine output and blood glucose are increased with both medetomidine and dexmedetomidine. The use of ketamine, not as an induction agent but as part of a multimodal approach, has increased in both veterinary and human medicine. It reversibly binds to NMDA receptors in the dorsal horn as an antagonist. Low doses of ketamine and CRI take advantage of this NMDA receptor binding. Ketamine helps with somatic more than visceral pain, but in a multimodal protocol with infusions that begin before surgery and last into the postoperative period, ketamine provides additional visceral analgesia over opioids alone (Himmelseher and Durieux, 2005). In veterinary patients, CRI has been shown to reduce isoflurane requirements while increasing heart rate and blood pressure under general anesthesia (Pascoe et al, 2007). Patients undergoing major procedures with significant surgical trauma or those patients with preexisting central sensitization may benefit from perioperative opioid-ketamine infusions (Lemke and Creighton, 2010).
Anesthesia and Perioperative Multimodal Therapy
Pain Assessment
TABLE 12-1
Premedications
Benzodiazepines
Midazolam
Alpha-2 Agonists
TABLE 12-2
Induction Medications
Ketamine
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