Update on feline anesthesia and analgesia (Proceedings)

Some random thoughts about cats:  We all know they are not small dogs, but I think a certain discomfort with feline anesthesia persists with many veterinarians.  Why is that?  This is what I know for sure: cats do not respond in quite the same fashion as dogs to many similar drug and anesthetic protocols.  Their sedation requirements are different than dogs (they often need more!).  They need and deserve superb analgesia, despite our inability to accurately assess pain in these patients 100% of the time.  They don't metabolize drugs in the same fashion as dogs.

Veterinarians have several choices when it comes to anesthetizing cats.  Injectable protocols remain a mainstay of anesthetic practice in the feline, inhalants are also frequently used, and a combination of both types of anesthetics should be considered.

Injectable anesthesia

There are a variety of injectable anesthetic protocols available in cats, including combinations like “kitty magic” and TTDex.  I have included some doses and combinations in Tables 1 and 2.  Most injectable combinations are based on the use of an alpha-2 agonist, a dissociative anesthetic, and an opioid. In terms of alpha-2 agonists, Dexmedetomidine has recently replaced the use of medetomidine in these combinations.  Dexmedetomidine is the dextro-isomer of medetomidine, and is thought to provide most of its anesthetic and analgesic effect.  It is marketed in a formulation that is 0.5 mg/ml, instead of the 1 mg/ml of medetomidine.  The volume administered should be the same.  Atipamezole is the antagonist best suited for reversal of this drug.  Cats usually require higher doses of alpha-2 agonists than a dog to achieve a desired level of sedation---they are definitely not “a cheap date!”  It is noteworthy that higher doses of alpha-2 agonists do not result in as much bradycardia in the cat as they do in the dog.

Opioids most commonly used in these injectable combinations include butorphanol and buprenorphine.  Butorphanol is a kappa agonist with a shorter duration of action.  Buprenorphine, a partial mu agonist, has been the focus of much work recently.  It has a long onset and duration of action.  It also has substantial bioavailability when administered bucally or transmucosally, achieving similar plasma drug levels when given in this manner as when administered intravenously.  For cats having needs for more substantial analgesia, full mu agonists like hydromorphone or fentanyl can be given.  Fentanyl has a very short duration, so is most commonly administered as an infusion or in a continous release transdermal patch application.  Care must be taken not to mix butorphanol, a kappa agonist and mu antagonist, with mu agonists.  Recent work at Purdue University has compared different opioids in combination with dexmedetomidine and ketamine for use in cat castrations.1  We found that both butorphanol and hydromorphone were suitable opioids for this procedure, but additional time was needed to wait before commencing surgery if buprenorphine was used.

Dissociative Anesthetics such as ketamine or Telazol provide the backbone of anesthesia in these combinations, while alpha-2s and opioids supplement analgesia and sedation.  Variable dosing regimens are used, depending on the level of anesthesia required.

Regardless of the protocol used, we recommend the administration of 100% oxygen.  All anesthetics contribute to hypoventilation and respiratory depression which may lead to hypoxemia when the patient is breathing room air.  Simple surgeries such as castrations may be performed with these drugs administered IM and not intubated, whereas it is simple to intubate these cats and transition to inhalant for more complicated procedures.

Some information to be aware of

Propofol:   A new formulation of propofol has been released recently that has a 28 day shelf-life.  It contains benzyl alcohol and should not be used in cats.  No formulation of propofol is labeled for cats, although propofol is commonly used in cats.

NSAIDs:  The addition of a boxed warning to meloxicam labels was announced by the FDA last October.  Meloxicam is labeled for one time use only in the cat.  Robenacoxib has very recently been approved in this country for the control of postoperative pain and inflammation.

Post Anesthetic Hyperthermia can occur with a variety of anesthetic protocols. Hydromorphone and ketamine have been associated with the syndrome.

Suggested readings

Ko JC, Austin BR, Barletta M, Payton ME, Weil AB, Inoue T. Evaluation of dexmedetomidine-ketamine and opioids as injectable anesthetic combinations for orchidectomy in cats. J Am Vet Med Assoc. In press

Ko JC, Knesl O, Weil AB, et al. FAQs: Analgesia, sedation, and anesthesia-Making the switch from medetomidine to dexmedetomidine. Compend Contin Educ Vets. 2009;31(suppl 1A):1-24.

Table 1. “Kitty Magic” Combination in Cats Level of Sedation/Procedure

Dexmedetomidine

(0.5 mg/ml)

Butorphanol

(10 mg/ml)

Ketamine

(100 mg/ml)

Atimpamezole

(5 mg/ml) Profound sedation-analgesia

0.1 ml

(11.1 µg/kg)

0.1 ml

(0.22mg/kg)

0.1 ml

(2.2 mg/kg)

0.1 ml

(111.1 µg/kg) Castration or laceration repair

0.2 ml

(22.2 µg/kg)

0.2 ml

(0.44mg/kg)

0.2 ml

(4.4 mg/kg)

0.2 ml

(222.2 µg/kg)

*Based on a 4.5 kig cat. All drugs mixed and administered as a single IM injection.

Table 2. Tiletamine-Zolazepam-Butorphanol-Dexmedetomidine (TTDex) Combination in Cats Desired Purpose IM Dosages of Individual Components Volume (ml/kg IM) of Reconstitued TTDex Premedication (mild to moderate sedation)

Tiletamine-zolazepam, 1 mg/kg

Butorphanol, 0.05 mg/kg

Dexmedetomidine, 2.5 µg/kg 0.01 Chemical Restraint (profound sedation)

Tiletamine-zolazepam, 2 mg/kg

Butorphanol, 0.1 mg/kg

Dexmedetomidine, 5 µg/kg 0.02 Surgical plane of anesthesia

Tiletamine-zolazepam, 3 mg/kg

Butorphanol, 0.15 mg/kg

Dexmedetomidine, 7.5 µg/kg 0.03

*Each agent may be drawn up and given separately and given IM. Alternatively, Telazol powder can be reconstituted using 2.5 ml of butorphanol (10 mg/ml) and 2.5 ml of dexmedetomidine as dilutents. If given IV, drug dosages should be halved.