Noninvasive testing may clarify the diagnosis (Table 10.1). In contrast to reentrant forms of SVT, Holter monitoring may demonstrate a characteristic gradual acceleration of the heart rate in AET, the so-called warm-up period. The first beat of the tachycardia typically occurs late in the cycle, and the initial P-wave will have the same morphology as subsequent P-waves. Participation of the ventricle in the tachycardia is not required, excluding an accessory pathway in these patients. During periods of reduced adrenergic tone and heightened parasympathetic tone (sleep, carotid sinus massage), the P-waves of an AET may not conduct to the ventricles, resulting in varying degrees of atrioventricular block. On occasion, this block can be induced with adenosine (Fig. 10.2); on the other hand, some AETs may be transiently suppressed with adenosine, limiting the usefulness of this test.

AET also has distinguishing features observed at electrophysiology study. First, it cannot be terminated by overdrive pacing. If ventricular participation can be excluded during either spontaneous or adenosine-induced AV node Wenckebach, the remaining diagnostic possibility for a narrow QRS tachycardia with P-waves preceding each QRS complex is atrial flutter (intra-atrial reentrant tachycardia). While macroreentrant tachycardia, including atrial flutter, can be interrupted with appropriate atrial overdrive pacing, an automatic tachycardia is only momentarily suppressed before the ectopic focus gradually warms up and returns. Another diagnostic finding is the sequence of atrial-atrial-ventricular electrograms (A-A-V) following cessation of ventricular burst pacing, elicited in almost 80 % of patients with a paroxysmal form of this arrhythmia. Use of catecholamine to induce AET may make it difficult to identify the mechanism of the tachycardia (Fig. 10.3).