Why does dexmedetomidine cause bradycardia?

OBJECTIVE: This study aims to investigate the potential mechanism by which dexmedetomidine (Dex) causes bradycardia. METHODS: Twenty-four healthy rabbits were divided into 4 groups (n = 6 each): Dex 10 µg/kg group (group D1), Dex 20 µg/kg group (group D2), Dex 40 µg/kg group (group D3) and sham group (group C). Heart rate and frequency of vagal efferent discharge were recorded before and at 0, 0.5, 1, 2, and 10 min after intravenous injection. Then, the action potentials of the sinoatrial (SA) node pacemaker cells were measured after the SA node was exposed to Dex at concentrations of 0.06, 0.6, 6, and 12ng/ml. RESULTS: The heart rate obviously decreased, and the vagal efferent discharge increased significantly at 0, 0.5, 1, and 2 min. In addition, these changes showed a dose-dependent effect. Dex exerted a negative chronotropic action on SA node pacemaker cells and could obviously reduce the amplitude of action potential (APA), velocity of diastolic depolarization in phase 4 (VDD), rate of pacemaker firing (RPF), as well as prolong action potential duration (APD) at 90% repolarization (APD90) in a concentration-dependent manner. CONCLUSIONS: Dex may induce bradycardia through central and peripheral mechanisms.