Feline anesthesia and pain management–what's the latest (Proceedings)

Anesthetic and pain related mortality would appear to be an easily quantifiable statistic that could be used to measure the outcome of the profession's current anesthetic practices. However, to rely solely on death rate as the measure of the quality of anesthetic care provided is inadequate. The anesthetist's goal should be to minimize the risks to the patient's health while reducing pain and stress.

Pain management in critical feline patients

All therapies have potential adverse effects. The key to clinical management of pain is to balance the potential risks to the potential benefits of proposed treatments. Generally adverse effects of analgesic drugs can be categorized as minor and major. Minor complications would include behavior changes (euphoria, dysphoria), vomiting and nausea, and other non-life threatening reactions. Major complications are potentially life-threatening and must have more weight when considering the risk-benefit of treatment. Major adverse effects would include severe cardiopulmonary depression, renal toxicity, and other severe physiological changes. Most young healthy patients undergoing elective procedures will have the ability to compensate for major adverse effects such as mild renal impairment of cardiopulmonary depression. However, critically ill animals and older patients may lack the ability to compensate and would be more likely to have catastrophic adverse events associated with some of the commonly recommended analgesic drugs.

Analgesic Agents

Opioids are a diverse group of natural and synthetic drugs used extensively in the management of post-operative pain in man and animals. Use in the surgical patient is becoming more widespread with the development of novel delivery systems such as the transdermal patch. The traditional view that a given drug always behaves as either an agonist or an antagonist at a particular receptor is a gross over-simplification, and recent studies have demonstrated that a number of variables appear to contribute to the effectiveness of various opioids in the clinical setting. Dosage, species, and stimulus intensity, character and duration can all alter the overall analgesic effect of an opioid. Opioids dampen peripheral and central afferent nociceptive transmission and thus, are extremely effective in treating acute inflammatory pain associated with recent surgical trauma. They are not, however, equally efficacious in managing all types of pain. Neuropathic pain syndromes are often characterized by a poor or short-lived response to opioid therapy.

Local anesthetics act either by blocking sodium channels that prevent nerve impulse transmission and nociceptor excitation, or by inhibiting modulatory nociceptive processing when administered centrally. In addition to their well-known topical, local and regional effects, recent studies have documented the efficacy of low dose intravenous lidocaine infusions in the management of hyperalgesia and neuropathic pain states induced by trauma or surgical procedure.

Non-steroidal anti-inflammatory drugs continue to be the mainstay of chronic pain management in both human and veterinary patients. Recent development of more selective agents has generated considerable interest in their use for intra- and postoperative pain as well. Traditionally, it has been believed that the analgesic effects of NSAIDs are related to their ability to inhibit cyclooxygenase activity and prevent prostaglandin synthesis and peripheral nociceptor sensitization. However, there is considerable evidence that at least some NSAIDs have a central spinal site of action, and may act synergistically with other analgesic compounds.

Alpha 2 adrenergic agonists bind to alpha 2 receptors located in the dorsal horn of the spinal cord, modulating the release of substance P, calcitonin gene-related peptide and various other neurotransmitters involved in rostral transmission of nociceptive information. Opioids likely exert their analgesic action through similar modulatory pathways and co-administration may result in additive or synergistic drug interactions. In humans alpha 2 agonists are often used as "rescue therapy" when opioid tolerance has developed. Use in surgical patients should be reserved for patients that have been stabilized with normal cardiopulmonary function.

A label indication for use in cats is a new feature of Dexdomitor when compared to Domitor. Although Domitor was used in cats however this was off-label. The dose of dexmedetomidine is approximately 20-40 mcg/kg. The IM route of administration is convenient when handling stressed patients, but intravenous administration will usually result in a faster onset of action. Dexmedetomidine is suitable for various short-term feline procedures, specifically for procedures requiring sedation and/or analgesia but not requiring intubation. The reversal agent atipamezole (Antisedan) is not approved for use in cats, however previous experience with atipamezole reversal of Domitor in cats suggests it is effective.

Summary

Pain assessment and management in critically ill feline patients is not easy. Careful observation of the patient's behaviour and a critical anthropomorphic evaluation of that behaviour should be the center piece of decision-making. Analgesic drugs should be selected based on knowledge of potential benefit and risks in individual disease processes and patient populations.

References

1. Marx G, Mateu C, and Orkin L. Computer analysis of postanesthetic deaths. Anesthesiology 1973; 39: 54-9.

2. Lunn J, Mushin W. Mortality associated with anesthesia. Anaesthesia 1982; 37: 856.

3. Jones RS. Comparative mortality in anaesthesia. British Journal of Anaesthesia 2001; 87(6):813-5.

4. Lumb W, Jones E. Veterinary Anaesthesia, 2nd Edn Philadelphia: Lea and Febiger 1973; 611-29.

5. Dodman N. Feline anesthetic survey. J Small Animal Practice 1977; 10: 653-8.

6. Clarke K, Hall L. A survey of anaesthesia in small animal practice. AVA/BSAVA report. J Ass Vet Anaesth 1990; 17: 4-10.

7. Dodman N, Lamb L. Survey of small animal anesthetic practice in Vermont. J Anim Hosp Ass 1992; 28: 439-45.