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Tachycardias: One or Two?

A 71-year-old patient presented with a 3-year history of paroxysmal, rapid regular palpitations despite trials of sotalol and diltiazem. A previous Holter study had documented a narrow QRS tachycardia (Fig. 1). The patient underwent an electrophysiology study (EPS) during which a tachycardia was inducible with programmed atrial stimulation (Fig. 2). What is the diagnosis? The Holter rhythm strips A and B (Fig. 1) document a narrow QRS tachycardia. The tachycardia is initiated by an atrial premature beat (APB) (third beat in Panel I) without a P wave to R wave interval (PR) jump. After the first four cycles, atrial but not ventricular activity becomes regular. The irregularity in RR cycle length (CL) persists until the end of Panel II, at which time it too becomes regular. The CL of the tachycardia in Panel III is 340 msec (shorter than in Panel II by 20 msec). The relationship between the ventricular and atrial excitation is not constant in Panels I and II. In Panel III, there is a constant and short RP relationship favoring atrioventricular nodal reentrant tachycardia (AVNRT). The irregularity in the RP relationship in Panels I and II could potentially be explained by AVNRT with variable block to the ventricle. An alternative explanation would be an atrial