Background: The antiarrhythmic effects of sodium channel and calcium channel blockers are known to be rate dependent. Little is known about the rate-dependent effect of adenosine on human atrioventricular (AV) nodal conduction. The purpose of this study was to determine whether the negative dromotropic effect of adenosine is dependent on heart rate. Methods and Results: Atrial pacing at 20-millisecond increments decreasing stepwise was performed, and the curves that relate the AH interval to the atrial pacing cycle length were analyzed. The change in AV nodal function was evaluated in three protocols: (1) In 8 group 1A and 6 group 1B patients, an intravenous infusion of adenosine at a dose of 140 and 320 μg · kg-1 · min-1 was given, respectively; (2) a bolus injection of a fixed dose of adenosine was given to 12 group 2A patients without and 6 group 2B patients with propranolol (0.1 mg/kg) treatment; and (3) in 12 group 3 patients, the AV nodal function was evaluated after intravenous propranolol (0.05 mg/kg) and after subsequent intravenous aminophylline (loading dose, 5 mg/kg; maintenance dose, 0.9 mg · kg-1 · h-1). No significant depression of AV nodal function could be demonstrated during intravenous infusion of adenosine. The bolus injection of adenosine could prolong the AH interval, which was dependent on heart rate and more significant at a shorter pacing cycle length. Intravenous propranolol significantly depressed the AV nodal conduction and shifted the curves of the AH interval versus the pacing cycle length to the right. Subsequent intravenous aminophylline shortened the AV nodal conduction time, however, in a rate-independent manner. Conclusions: The negative dromotropic effects induced by intravenous bolus injection of adenosine became more pronounced at fast atrial pacing rates. These results indicate that adenosine causes rate-dependent prolongation of AV nodal conduction in humans.
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