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.
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.
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.