Response to Adenosine in Atrial Tachycardia
Abstract & Commentary
Synopsis: Adenosine is useful for distinguishing focal from macro-reentrant atrial tachycardia.
Source: Iwai S, et al. Circulation. 2002;106:2793-2799.
Iwai and colleagues evaluated the effects of adenosine in 42 patients who underwent an electrophysiologic study and radiofrequency ablation for recurrent atrial tachycardias. During electrophysiologic study, the patients underwent 3-dimensional electro-anatomic mapping (CARTO, Biosense-Webster) to help characterize the arrhythmias. Attempts to induce the arrhythmias were made using standard atrial stimulation protocols. Isoproterenol or dobutamine infusions were used where necessary. Focal atrial tachycardia was noted to be present if several conditions were met: there was a centrifugal atrial activation pattern, the site of origin could be dissociated from the rest of the atrium with atrial extrastimuli, local atrial activation was early relative to the surface P wave, and the tachycardia could not be entrained. Focal atrial tachycardia was considered to either be due to enhanced automaticity or to triggered activity. The latter tachycardias demonstrated reproducible initiation and termination with stimulation, as well as sensitivity to adenosine and verapamil. Macro-reentrant tachycardia was considered to be present when concealed or manifest entrainment could be demonstrated and activation mapping using CARTO identified macro-reentry. Adenosine was administered during sustained tachycardia and the effects of the drug monitored.
There were 43 tachycardias in the 42 patients included in this report. In 35 patients, the arrhythmia was thought to be a focal atrial tachycardia. With 1 exception, all of these tachycardias were sensitive to adenosines (terminated arrhythmia). Verapamil was only administered to 6 patients. In all these, verapamil terminated the tachycardia. Macro-reentrant atrial tachycardia was seen in 8 patients. Adenosine failed to terminate macro-reentrant atrial tachycardia in all 8 patients.
Macro-reentrant atrial tachycardia was more likely to be associated with structural heart disease than was focal atrial tachycardia (88% vs 47%, P = 0.05).
Iwai et al conclude that adenosine is useful for distinguishing focal from macro-reentrant atrial tachycardia.
Comment by John P. DiMarco, MD, PhD
Adenosine is an agent that is uniquely valuable to electrophysiologists. Most electrophysiologic studies in patients with supraventricular tachycardias are now performed as a prelude to ablation. Use of any pharmacologic intervention, which would produce more than very brief effects on the tachycardia, are therefore inadvisable. That is why only 6 patients in this paper received verapamil. Since adenosine has a very brief duration of action, it can be administered numerous times during a study without affecting subsequent findings necessary to proceed with ablation. This paper shows some nice illustrations of the effects of adenosine in various types of atrial tachycardia.
Adenosine has both direct and indirect effects on atrial tissue. The direct effects are mediated by an enhanced outward potassium current (IKAdo). It also has indirect effects through inhibition of cyclic AMP generation in response to adrenergic stimulation. These effects should result in suppression of atrial automaticity and inhibition of triggered activity. Macroreentrant atrial tachycardia circuits involve conduction through main atrial tissue. Conduction through the atrium is usually not affected by adenosine, as shown in this series. It is still impossible to completely differentiate between triggered activity and micro-reentry through nodal tissue since conduction in nodal tissue would be expected to be adenosine-sensitive.
Dr. DiMarco is Professor of Medicine, Division of Cardiology University of Virginia, Charlottesville.