KETAMINE

Ketamine (100 mg/ml) was given at a dose of 10 mg/kg intramuscularly. Ketamine is a nonbarbiturate anesthetic widely used alone or in combination with other drugs (particularly tranquilizers, barbiturates, or benzodiazepines).¹ It produces a state in which the patient is dissociated from the environment, is immobile, and does not respond to pain; however, it is not a potent analgesic.² Ketamine is an appropriate anesthetic and sedative agent for short diagnostic and minor surgical procedures in cats, especially those that do not require skeletal muscle relaxation.³ Characteristics of sedation with ketamine include increased muscle tone and an apneustic breathing pattern.¹ Ketamine also preserves the swallowing and palpebral reflexes, and cats are typically hyperresponsive and ataxic during recovery.¹ Ketamine is eliminated almost exclusively by the kidneys, with fecal elimination accounting for less than 5% of drug metabolism.⁴

Studies have shown that intramuscular ketamine mixed with xylazine hydrochloride causes muscle necrosis and is not recommended for use in survival procedures involving laboratory rodents.^5,6 A similar situation may occur in cats.

Ketamine was our chemical restraint method of choice for blood collection for years, as it is fairly inexpensive, accessible, and easy to administer. Ketamine increases heart rate and blood pressure and produces minimal vasoconstriction. These cardiovascular effects permit efficient blood collection.

Disadvantages

Our donors became counterconditioned to being handled for subsequent donations. They appeared to associate the handling with pain. Ketamine also carries a risk of causing muscular necrosis.

It took 20 to 45 minutes for ketamine to induce an appropriate level of sedation when given intramuscularly. It was unknown why it took so long to reach satisfactory sedation. The cats were adequately sedated during collection, but they were hyperesthetic when handled. Muscle jerks and repeated swallowing sometimes made needle positioning difficult and caused some blood collections to be prematurely aborted.

Recovery times were prolonged, usually lasting six to eight hours and, in one case, 24 hours. Occasional violent emergence behavior was noted in several cats (e.g. growling, biting, scratching, lunging at the cage). The cats were occasionally hypothermic immediately after donation (never less than 98 F [36.7 C]). Tachyphylaxis occurred in one donor after years of receiving the same ketamine dose before blood collection (the cat had received ketamine no more frequently than every three weeks).

MIDAZOLAM AND KETAMINE

Midazolam (5 mg/ml) was given at a dose of 0.4 mg/kg intramuscularly in the same syringe with ketamine at a dose of 4 to 6 mg/kg intramuscularly. Only one cat required a second midazolam dose (0.4 mg/kg) because of inadequate muscle relaxation. Midazolam is a short-acting benzodiazepine.^1,7 It causes mild tranquilization by enhancing the effect of central nervous system (CNS) inhibitory neurotransmitters, opening chloride channels, and acting upon benzodiazepine receptors.¹ In people, benzodiazepines act at the limbic, thalamic, and hypothalamic levels of the CNS, producing anxiolytic, sedative, hypnotic, skeletal muscle relaxant, and anticonvulsant effects.⁸

Midazolam is often used in veterinary medicine as a water-soluble alternative to diazepam, particularly to provide skeletal muscle relaxation in combination with ketamine. Midazolam is primarily metabolized by the liver into two hydroxylated metabolites, which are then excreted by the kidneys.⁹ Midazolam was selected instead of diazepam based on its efficacy when given intramuscularly as well as the anesthesiologists' recommendations at The Ohio State University Veterinary Teaching Hospital.

Advantages

Midazolam is readily absorbed from muscle. The cats were less reactive to the injection of midazolam and ketamine when compared with ketamine alone. The quality of restraint was exceptional, as the muscle jerks and hyperesthesia seen when using only ketamine were alleviated or eliminated. The cardiovascular effects of ketamine were still present, allowing for quick and efficient blood collections.