Analgesia after onychectomy in cats - Veterinary Medicine
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Analgesia after onychectomy in cats
Providing pain relief after onychectomies in cats is a must, and you have many options. Choosing which drug to use can be confusing, but, in general, a multimodal approach seems best.


VETERINARY MEDICINE


Continuing analgesia after a cat leaves the hospital is challenging because of difficulty with drug administration. Oral butorphanol has variable uptake and may be metabolized by the gastrointestinal tract and liver, reducing the chances of a therapeutic effect.27 Transdermal fentanyl patches must be replaced every three days, necessitating frequent trips to the veterinarian, and the patches have variable absorption. Buprenorphine is a long-acting opioid commonly given in cats.8,27,29 The pharmacokinetics of oral mucosal administration (i.e. placing drops in the side of the cat's mouth) of the injectable compound suggests that buprenorphine would provide excellent analgesia in a convenient dosing format.27 Although to our knowledge oral buprenorphine has not been evaluated for analgesic properties, the intramuscular route of administration has shown advantages in analgesia compared with butorphanol and morphine.27,29-31

Local anesthetics


Figure 1. A ring block for the dorsal paw. Arrows show where the local anesthetic should be infused subcutaneously on the dorsal carpus to block the branches of the radial nerve.
Local anesthetics such as lidocaine (up to 4 mg/kg perineurally) or bupivacaine (up to 1 mg/kg perineurally) are commonly used to complement other forms of analgesia or sedation.8,9 They can be applied topically or as a nerve block.8,9,33 The risk of seizures, one side effect of local anesthetics, in cats increases with greater total doses of either drug. In small kittens, saline solution can be added to increase the volume of administration without increasing risk.


Figure 2. A ring block for the palmar paw. Arrows show where the local anesthetic should be infused subcutaneously on the palmar surface to block the median nerve and palmar branch of the ulnar nerve medial to the accessory carpal pad and the dorsal branch of the ulnar nerve laterally.
Briefly, the mechanism of action of local anesthetics is to block the sodium channels imperative for nerve conduction.8,9 Efficacy likely varies with administration technique and location. Topical bupivacaine was less effective at preventing lameness when compared with opioids in one study.18 Similarly, topical bupivacaine did not improve pain scores when compared with placebo.33 Although randomized, prospective studies have not been performed, ring blocks around the carpus (radial, ulnar, and median nerves) have been advocated (Figures 1 & 2).8,9 The duration of action may be anywhere from three to 10 hours, depending on the local absorption.33 Lidocaine has a shorter duration of action, lasting only one or two hours.8,9 Optimally, we recommend combining regional or topical nerve blocks with other modes of analgesia.

NSAIDs

NSAIDs provide analgesia by decreasing inflammation. Moreover, some NSAIDs also decrease central pain perception.8,9 An additional advantage of NSAIDs is that they can be used in conjunction with opioids to provide analgesia.

In cats, the pharmacokinetics varies for different NSAIDs. Additionally, large variation from cat to cat and a potentially small safety margin make dosing difficult.8,9

Adverse effects, including vomiting and diarrhea, are typically self-limiting when an NSAID is discontinued; however, more serious adverse effects may include renal or hepatic necrosis or gastrointestinal ulceration. Drugs that preferentially block the cyclooxygenase 2 (COX-2) inflammatory pathway may have a larger safety margin when considering the probability of gastrointestinal side effects but provide no additional margin of safety for renal side effects.8,9 Because of these side effects, NSAIDs are generally contraindicated in animals that have compromised renal function or underlying gastrointestinal disease and in those that receive corticosteroids. Although directly monitoring renal blood flow is not routine, patients given NSAIDs preoperatively should be monitored closely to decrease the probability of interrupting blood flow to the kidneys.

Thus, drugs that are COX-2 selective are preferred to drugs that are COX-1 selective or nonselective. But even COX-2 selective NSAIDs should be used with caution.


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Source: VETERINARY MEDICINE,
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