It is known that the pharmacologic effects of midazolam can be seen clinically within five to 20 minutes,8 but in our experience maximal effects did not occur until 30 to 60 minutes after an intramuscular injection. The elimination
half-life of midazolam alone in people is about two hours.10 Clinically, we noted that cats continued to be ataxic, unresponsive, and recumbent for as long as four to six hours after
phlebotomy when we used the ketamine-midazolam combination. The combination also costs more than ketamine alone (Table 2).
MEDETOMIDINE AND ATIPAMEZOLE REVERSAL
Medetomidine (1 mg/ml) was given at a dose of 40 to 50 μg/kg intramuscularly. Medetomidine is an alpha2-adrenergic agonist that produces sedation by stimulating alpha2 receptors, thereby decreasing norepinephrine release.1 A sleeplike state that can be readily reversed with an alpha2 antagonist is produced. Atipamezole is an alpha2-adrenergic antagonist that directly inhibits the actions of medetomidine. Immediately after blood collection, subcutaneous
fluids were given, and then atipamezole (5 mg/ml) was administered at a dose of 0.1 to 0.15 mg/kg intramuscularly.
Cats were minimally reactive when the drug was injected intramuscularly. It took 10 to 20 minutes for medetomidine to produce
adequate sedation. Drug reversal with atipamezole facilitated quick recoveries.
Medetomidine was initially selected for evaluation in our donor program because it provides appropriate levels of sedation,
is reversible, and allows quick recoveries. However, we found medetomidine unsatisfactory for use in cats during blood collection.
Medetomidine results in marked bradycardia and increased peripheral vascular resistance, and it decreases cardiac output by
30% to 50%.1 This is undesirable during phlebotomy; several cats became pale and bradycardic (heart rates of 40 to 60 beats/min) with
this protocol. Moreover, vasoconstriction made venipuncture and blood collection difficult. Several procedures had to be aborted
because of inadequate blood flow into the collection system. In addition, medetomidine invariably caused emesis within five
minutes of intramuscular administration. Medetomidine with or without atipamezole has a high cost per average dose (Table 2).
Atropine sulfate (0.4 mg/ml), an anticholinergic, was added to this protocol later as a presedative (0.02 mg/kg intramuscularly)
to improve cardiac output. When using this combination, we noted that the quality of sedation was insufficient and unpredictable,
although we failed to find an explanation for this phenomenon. Possible risks associated with using an anticholinergic with
medetomidine include arrhythmias, hypertension, and increased myocardial workload. We found little benefit with adding atropine
and do not recommend its use in cases such as this.
We added ketamine (4 to 6 mg/kg given intramuscularly in the same syringe as medetomidine) to this protocol11-16 in an attempt to improve cardiac output, but this resulted in prolonged recovery (a few hours in some cases), even with reversal
of the alpha2 agonist, and most cats reacted painfully to the injection. The combination of ketamine and medetomidine potentially increases
the risk of hypertension and increased systemic vascular resistance.