Dexmedetomidine does not have direct cardiac action; however, it has several important indirect effects. A bolus dose of 1mcg/kg has a biphasic response – peripheral alpha 2 mediated vasoconstriction leading to transient increase in blood pressure, with a decrease in heart rate mediated by baroreceptor reflex. The first phase lasts for 5-10 min and is followed by a second phase in which there is decrease in BP and stabilization of heart rate (fall of 10 -20% below the baseline). These effects are caused by inhibition of central sympathetic outflow and activation of presynaptic alpha 2 adrenoceptors, leading to decreased release of norepinephrine and epinephrine.
The initial increase in BP can be attenuated by giving the bolus over 10 – 20 min. Continuous infusions of less than 0.4 mcg/kg without an initial bolus reduces the incidence of hypotension. Discontinuation of prolonged infusion does not lead to rebound effects.
Dexmedetomidine administration can lead to varying degrees of heart block and thus should be used with caution in patients with hypovolemia, hypotension, bradycardia, fixed stroke volume and advanced degrees of heart block.
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