Update on feline anesthesia and pain management (Proceedings)

The emphasis on providing humane care and good pain management to small animal patients has primarily focused on dogs, and the pharmaceutical products available for use in that species. However, several analgesic and anesthetic products have recently been developed and approved by the FDA-CVM for use in cats. Given the increased public awareness, increased client expectations are impacting the care veterinarians are delivering to their feline patients. This lecture reviews some of the new products and clinical studies focused on feline pain management and anesthesia.

Pain Management for Cats

Practitioners should develop a comprehensive, individualized practice program for recognizing and managing pain. Many recent resources are available including journal articles and textbooks such as Lumb and Jones' Veterinary Anesthesia and Analgesia (1, 2). The management of pain in cats differs from that in the dog due to species differences in drug and technique effectiveness and toxicity of analgesic drugs including opioids and NSAIDs. Until recently, the only approved analgesic drug for cats was butorphanol. This lead to nearly universal extralabel drug use and increased practitioner liability. Some pharmaceutical manufacturers have responded to the need for approved feline products. Meloxicam (Metacam) and dexmedetomidine (Dexdomitor) have recently been approved for use as analgesics in cats.

Dexmedetomidine

Dexmedetomidine (Dexdomitor) was approved in 2007 for use in cats as a sedative/analgesic. While medetomidine was used in cats previously, this usage was extra-label. Building upon practitioner experience with Domitor, Dexdomitor is now approved for use in cats (at a relatively low dose) to provide sedation and analgesia and to facilitate restraint for minor procedures. While analgesia accompanies sedation, practitioners should not rely on alpha 2 agonists alone in any species for surgical anesthesia. Local anesthetic blockade is recommended if surgical trauma is anticipated to prevent sudden arousal and defensive behavior from the cat.

Cardiovascular effects of alpha 2 agonists are similar to those reported in dogs. Animals that cannot tolerate a decrease in cardiac output, increase in vagal tone, or increase in systemic vascular resistance should not receive alpha 2 agonists. Dexdomitor can be antagonized by intramuscular administration of atipamezole (Antisedan). Intravenous administration should probably be reserved for emergency situations since antagonist induced rapid vasodilation in the presence of reduced sympathetic tone can lead to precipitous hypotension. While Dexdomitor is now an approved label use, atipamezole reversal in cats is not approved, although quite common.

Meloxicam

Although meloxicam use in cats has been reported for several years, its use in the US was extra-label until 2004. Currently Metacam is approved for postoperative analgesia in cats as a single subcutaneous dose of 0.3 mg/kg. Repeated administration is not an approved label indication; however, several references discuss its use for multiple days (2). It should be remembered that repeated dosing, especially at the higher label dose has been associated with renal dysfunction in cats. It would seem prudent that repeated dosing be done with full informed consent of the owner with the goal of a careful dose titration to the lowest effective dose.

Carprofen

Carprofen is approved for treatment of perioperative pain and osteoarthritis in dogs. It is not approved for use in cats within the US although it has approved in a number of European countries for many years. Carprofen is effective in cats for control of pain and inflammation; however there is considerable individual variation in the elimination of the drug (3). This can result in accumulation of carprofen with repeated dosing in some cats resulting in toxicity including renal dysfunction and possibly death. If Rimadyl is used, a single dose is preferable. If repeated dosing is attempted a careful individualized safe dose interval should be determined.

Maropitant citrate

Maropitant (Cerenia) has recently been approved for use in dogs as an antiemetic. It is an NK-1 receptor antagonists and appears to interfere with emesis both peripherally and centrally. While not approved for use in cats, it has been used at the same dose as recommended for dogs for the treatment of signs associated with renal failure and other metabolic diseases. Maropitant can be useful for the prevention of emesis associated with opioid and anesthetic administration and should be considered when vomition is contraindicated (e.g., corneal ulcer repair) or when anesthetic recovery is accompanied by significant nausea or vomiting.

Studies

In addition to new FDA approved products, a number of feline clinical studies have been performed to evaluate the safety and effectiveness of currently available human and veterinary drugs. Buprenorphine is widely now used as an analgesic in cats and appears to have some advantages including good oral bioavailability, lower potential for dysphoria and hyperthermia, and reasonably good effectiveness for commonly performed procedures (4, 5). A number of clinical studies have further added to our knowledge of opioid therapy and indicate that cats are at risk for the development of hyperthermia following administration of many opioid agonists (6, 7). Anecdotal reports indicate hyperthermia can occur with most mu receptor agonists such as hydromorphone, morphine and fentanyl. The occurrence appears to vary with opioid agonist and coadministered drugs. While all feline patients should be observed for the onset of hyperthermia, the benefits of opioids for analgesia still likely outweigh the risks of hyperthermia and in most patients shouldn't preclude their general use.

Another interesting study published in 2007 reported on the mortality rates associated with anesthesia and sedation of cats in Europe. It is important for veterinarians to be familiar with this information because obtaining informed consent before anesthesia and surgery requires disclosure of the risks associated with the planned procedure. The overall mortality rate in this study was 0.24% (189 deaths in 79,178 anesthetics/sedations) (8). Factors associated with increased risk were poor health status, increased age, extremes of weight, emergent surgery, endotracheal intubation, and administration of intravenous fluids intraoperatively. Use of pulse monitoring and pulse oximetry were associated with a reduction in risk. While several increased risk factors were expected (e.g., age, weight, illness), the use of an endotracheal tube and fluid therapy were not expected by the authors to be factors associated with increased risk. While the authors concluded that intubation is still important, their data reinforced the need for careful intubation. Fluid therapy has been and still is recommended during anesthesia for restoration and maintenance or adequate circulating blood volume. However, it is increasingly emphasized from human studies that overloading of the cardiovascular system, especially with crystalloid solutions, can lead to adverse cardiovascular function and tissue edema (9). Carefully titrating fluid administration based on the individual patient's pathophysiology and monitored parameters is necessary to maximize the benefits of fluid therapy while reducing its risks. Intubation and fluid therapy may also have been disproportionately performed in higher risk patients within the study but this could not be statistically demonstrated.

References

1. AAHA/AAFP Pain Management Guidelines for Dogs and Cats. The American Animal Hospital Association and the American Association of Feline Practitioners. J Am Anim Hosp Assoc 2007; 43:235-248.

2. Lumb and Jones' Veterinary Anesthesia and Analgesia, 4th Edition. Tranquilli WJ, Thurmon JC, and Grimm KA eds. Blackwell Publishing 2007. Ames, Iowa.

3. The pharmacokinetics and effects of intravenously administered carprofen and salicylate on gastrointestinal mucosa and selected biochemical measurements in healthy cats. Parton K, Balmer TV, Boyle J, Whittem T, MacHon R. J Vet Pharmacol Ther. 2000 Apr;23(2):73-9.

4. Comparison of the effects of buprenorphine, oxymorphone hydrochloride, and ketoprofen for postoperative analgesia after onychectomy or onychectomy and sterilization in cats. Dobbins S, Brown NO, Shofer FS. J Am Anim Hosp Assoc. 2002 Nov-Dec;38 (6):507-14.

5. PK-PD modeling of buprenorphine in cats: intravenous and oral transmucosal administration. Robertson SA, Lascelles BD, Taylor PM, Sear JW. J Vet Pharmacol Ther. 2005 Oct;28(5):453-60.

6. Post-anesthetic hyperthermia in cats.Posner LP, Gleed RD, Erb HN, Ludders JW. Vet Anaesth Analg. 2007 Jan;34(1):40-7.

7. Postanesthetic hyperthermia in cats: a retrospective comparison between hydromorphone and buprenorphine. Niedfeldt RL, Robertson SA.Vet Anaesth Analg. 2006 Nov;33(6):381-9.

8. Risk factors for anaesthetic-related death in cats: results from the confidential enquiry into perioperative small animal fatalities (CEPSAF). Brodbelt DC, Pfeiffer DU, Young LE, Wood JL. Br J Anaesth. 2007 Nov;99(5):617-23.

9. Pathophysiology and clinical implications of perioperative fluid excess. Holte K, Sharrock NE, Kehlet H. Br J Anaesth. 2002 Oct;89(4):622-32.