Background Recent reports suggest that adenosine, in addition to terminating supraventricular tachycardia involving the atrioventricular (AV) node, may have antiarrhythmic effects on atrial tachycardia. The electrophysiological effects of adenosine on supraventricular tissue include shortening of action potential duration in atrial myocytes mediated by the potassium current, IKACh,Ado; shortening of action potential duration and hyperpolarization in sinus node cells; and antiadrenergic electrophysiological effects resulting from inhibition of adenylyl cyclase. We therefore hypothesized that the response of atrial tachycardia to adenosine would be mechanism specific, with termination of atrial tachycardia due to sinus node reentry or cAMP-mediated triggered activity, transient suppression of automatic atrial tachycardia, and an absence of antiarrhythmic effect on tachycardia due to intra-atrial reentry.Methods and Results Adenosine (mean+-SD, 143+-54 micrograms/kg IV) was administered to 27 patients (55+-19 years) in atrial tachycardia whose mechanism was confirmed by electrophysiological study. Adenosine terminated sinus node reentrant tachycardia in 6 of 6 patients and terminated atrial tachycardia due to triggered activity in the 1 patient in whom it was identified. Adenosine transiently suppressed automatic atrial tachycardia in 7 of 7 patients and had no effect in 13 patients with intra-atrial reentrant tachycardia, including 8 patients with atrial flutter.Conclusions These findings demonstrate that adenosine's effects on atrial tachycardia are mechanism specific and can be used to differentiate between reentrant tachycardia confined to the region of the sinus node or atria and between nonreentrant atrial tachycardia due to either triggered activity or automaticity. (Circulation. 1994;89:2645-2654).