A 59-year-old man underwent ablation for cavotricuspid (CTI)-dependent right atrial flutter. Post ablation programmed stimulation reproducibly initiated supraventricular tachycardia with a cycle length 300–310 ms. The rhythm strip showed short RP tachycardia with a prolonged P wave duration (175 ms), which was much longer compared with during sinus rhythm (110 ms) (top panel, left).
The tachycardia was determined to be atrial tachycardia (AT) with first-degree atrioventricular (AV) block. Typically, this mechanism involves decremental AV nodal conduction during tachycardia. However, in this case, first-degree AV block was caused by prolonged intra-atrial conduction as a result of the focal AT arising lateral to the line of conduction block from CTI ablation.
The AH interval during AT was 75 ms and the HA interval was 235 ms, resulting in a short septal AH/HA ratio (<1) (top panel, right). Also, the intra-atrial activation time is prolonged (240 ms). In contrast, AT with first-degree AV block due to prolonged AV nodal conduction will have a relatively long AH interval and a septal AH/HA ratio ≥1 (bottom panel).
This case highlights the often under-appreciated contribution of prolonged intra-atrial conduction, manifested by a prolonged P wave duration, to the spectrum short RP tachycardia.
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