Acute pain management for cats (Proceedings)

Cats experience acute pain related to surgical procedures or following trauma. In the first situation pre-emptive analgesia can be used and in the latter prompt initiation of analgesic treatment is important. In both situations careful and continued pain assessment is required to ensure patient comfort. The mainstay of acute analgesic therapy is the use of opioids; however non-steroidal anti-inflammatory agents can be used in many patients.

Many of the estimated 70 million cats in the United States undergo an elective surgical procedure early in their life. Most pet cats are spayed or castrated and many are also declawed at the same time. These so-called routine procedures account for a high percentage of all surgical procedures performed in cats in general practice. These patients are usually young and healthy but good anesthetic and analgesic management is important to ensure a quick recovery, to minimize post-operative discomfort and complications and to prevent development of maladaptive pain. Effective pain management involves considering both the type and duration of the anticipated pain the cat may experience. In addition, the choice of surgical technique may have an impact.

What type of pain?

When devising an analgesic plan the type of surgery should be considered; as an example we will use a female cat scheduled for ovariohysterectomy [OHE] and onychectomy. Performing an ovariohysterectomy will involve both somatic (peripheral) and visceral pain mechanisms; the body wall must be incised and sutured and the ovaries and uterus will be also be manipulated. This type of procedure involves trauma to soft tissues. Onychectomy involves removal of the distal phalanx and therefore involves bone pain in addition to dissection of the surrounding soft tissues. Regardless of the site of surgery an inflammatory response will be initiated. An OHE ± onychectomy is anticipated to cause acute pain lasting several days.

Pre-emptive analgesia

The term pre-emptive analgesia means institution of analgesic techniques before the painful stimulus occurs, in other words, pain may be easier to prevent than treat. To use a sporting analogy, defense is the best offence. The success and benefits of pre-emptive analgesia have been hotly debated in human medicine and many studies cannot demonstrate a positive effect[1]. However, several animal studied do show a beneficial effect. An additional benefit of using analgesics preoperatively is that opioids, local anesthetics, and alpha2-agonists can reduce the requirements for induction and maintenance agents resulting in lower doses of induction and maintenance drugs, less cardiopulmonary depression and faster recoveries. Clinically, there are two phases associated with surgery; the first is the sensory input arising directly from the surgery itself and the second is from the resultant, more prolonged inflammatory reaction. Pre-emptive use of analgesic agents may prevent the development of sensitization, but cannot completely eliminate post-operative pain; analgesic agents must be continued until the inflammatory response to injury abates.

Can the surgical technique have an impact on post-operative pain?

Lasers are being used more frequently in veterinary surgery laser onychectomy has been well described [2]. The proposed advantages of this technique are less bleeding, swelling and infection, which should increase post-operative patient comfort. In addition the use of a laser should make the use of tourniquets, gluing or suturing of wounds and bandaging unnecessary. Two studies have compared the incidence of post-operative discomfort and complications between laser, and blade onychectomy[3],[4]. Levy et al [3] reported that compared to the laser group (no bandages), the blade declaw (bandages applied) group were less willing to play and use their paws for two days after surgery. Mison et al [4] performed a laser declaw on one front paw and a blade declaw on the other and placed no post-operative bandages. These authors reported lower discomfort and complication scores with the laser declaw on the first post-operative day, but considered the differences between the techniques to be minor. In this study the laser provided superior surgical conditions with excellent intraoperative hemostasis.

The five major classes of analgesics employed for acute pain management are:

1. Opioids

2. Non-steroidal anti-inflammatory drugs

3. Local anesthetics

4. Alpha2 – agonists

5. NMDA antagonists (e.g. ketamine)

OPIOIDS- Buprenorphine has been used extensively in cats, particularly in Europe, Australia and South Africa. The transmucosal administration of this drug has proved to be both effective and acceptable in cats[5]. The buccal route seems to be more effective in cats than humans and may be a result of differences in ionization in the alkaline (pH of 8-9) environment of the cat's mouth. At a dose of 0.02 mg/kg (0.066 ml/kg) buprenorphine was as effective by the transmucosal route as it was by the intravenous route in an experimental model[6]. In a clinical study, buprenorphine outperformed morphine[7]. The advantages of this drug seem to be the small volume required, lack of undesirable side effects, long duration of action (4-6 hours) and ease of administration. Buprenorphine is a schedule III drug, and is an option for "at home" medication by owners. Hydromorphone is an excellent antinociceptive agent in cats [8, 9] but is often associated with hyperthermia[10, 11]. Butorphanol is short-acting (<90 minutes) and reaches a ceiling effect at low doses[12].

Epidural techniques

Epidural techniques can be used for abdominal or hind limb procedures. Opioids exert their major analgesic effect in the dorsal horn of the spinal cord and intrathecal or epidural administration provides long lasting analgesia with fewer systemic side effects. Morphine, fentanyl, pethidine (Meperidine, Demerol), methadone and buprenorphine have been given via the epidural route in cats[13-19]. Analgesia may be intensified and prolonged with the addition of the local anesthetic bupivacaine[18]. Epidural injection is technically more challenging in cats because of their small size and because the spinal cord ends more caudally, entering the subarachnoid space is more likely. If this occurs, half of the epidural dose may still be administered[20]. Duke et al [16] reported increased pain thresholds in cats for over 4 hours after medetomidine (10μg/kg) was given epidurally and found it to be superior to fentanyl (4μg/kg).

Local anesthetics

These agents can be employed successfully in cats and the two most commonly used agents are lidocaine and bupivacaine. An advantage of this group of drugs is their low cost and non-controlled status. A complete sensory block will prevent nerve transmission making use of these agents one of the most potentially useful pre-emptive techniques. A particularly useful technique is to implant a "soaker" catheter into a wound (for example a post amputation wound or after large tumor removal) to provide a method for maintaining continuous analgesia. After fibrosarcoma removal in cats the use of a wound infusion catheter significantly reduced the time the cat was hospitalized suggesting that this technique improves mobility and time to resume eating (the criteria used for discharge)[21, 22].

Non-steroidal anti-inflammatory drugs

NSAIDs are excellent agents for the alleviation of acute post-operative pain. As in other species there are concerns about the perioperative use of NSAIDs in cats. The main concerns relate to inhibition of prostaglandin synthesis, which may lead to gastrointestinal erosion, impaired renal function and bleeding. The limited ability for glucuronide conjugation in cats can prolong the duration of action of some NSAIDs, but with appropriate changes in dose and dosing intervals they can be used safely. The advantage of this category of drugs is their long duration of action and no DEA paperwork is required. In cats with no evidence of renal disease, this group of drugs is a good choice.

Carprofen, meloxicam, tolfenamic acid and ketoprofen have all been evaluated for their post-operative analgesic properties in cats[23], [24], [25], [26], [27]. Carprofen and meperidine have been compared when given SQ at the end of the procedure. Two-hours after ovariohysterectomy, meperidine (Demerol 10mg/kg IM) provided better analgesia than carprofen, but from 2-20 hours, carprofen was superior with those cats requiring less "rescue analgesia"[23]. Injection of carprofen before castration or ovariohysterectomy was found to be better than meperidine given at the end of surgery[24], and appeared to offer good analgesia for 24-hours. A single dose of ketoprofen (2mg/kg SQ) given at the end of anesthesia outperformed a single dose of buprenorphine or meperidine[25]. Comparison of four injectable NSAIDs given subcutaneously at extubation following ovariohysterectomy (carprofen 4 mg/kg, ketoprofen 2mg/kg, meloxicam 0.2 mg/kg and tolfenamic acid 4 mg/kg), resulted in 9 out of 10 cats in all groups had desirable overall clinical assessment scores for 18 hours[28]. Despite the cat's apparent comfort, none of the NSAIDs prevented postoperative wound tenderness. Currently the only NSAID approved for use in cats in the USA is the injectable formulation of meloxicam. NSAIDs should not be use in animals with preexisting renal disease, hypovolemia, and bleeding disorders or if severe surgical hemorrhage is anticipated.

Alpha2-agonists

The alpha2-agonists xylazine and medetomidine are commonly used in cats and possess analgesic, sedative and muscle relaxant properties. More recently dexmedetomidine (which is licensed for use in cats in the USA) has become available. These drugs are usually reserved for healthy animals because of the cardiopulmonary depression that accompanies their use. They are useful in cats that are difficult to handle because they produce profound sedation, and when combined with ketamine (IM) will result in complete immobilization. However, the dose required to produce analgesia may be higher than that for sedation as was recently demonstrated for dexmedetomidine[29]. Oral administration most likely results in transmucosal uptake and is a useful technique in fractious cats[30, 31]. A combination of xylazine and ketamine has been compared to acetylpromazine, butorphanol and thiopental as a total injectable protocol for anesthetizing cats for onychectomy[32]. Cats that received xylazine and ketamine had significantly lower post-operative pain scores and were more active and playful earlier, than butorphanol treated cats.

NMDA antagonists

The NMDA receptor plays an important role in central sensitization and there is a lot of interest in developing drugs that can inhibit this receptor. In veterinary medicine a commonly used NMDA antagonist is ketamine, which may be effective at preventing or at least lessening "wind-up" at sub-anesthetic doses. The post-operative analgesic effects in dogs have been reported [33] and ketamine does seem to be beneficial in cats undergoing surgery[34].

The cat need no longer be the "poor relative" of the dog when it comes to pain management. There are numerous analgesic drugs available, although few are licensed for use in the cat in the United States. Assessment of pain is pivotal in successful management as is choosing analgesic drugs that are safe, effective and easy to administer. Cat-specific studies suggest that for acute pain management buprenorphine is an ideal opioid. In healthy cats scheduled for elective surgery the addition of NSAIDs are beneficial.

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