A regular narrow QRS tachycardia at 176 bpm is present with RP shorter than PR (figure 1). The rapid remission of the tachycardia with carotid sinus massage points to a paroxysmal supraventricular tachycardia (option 2 incorrect). The P wave at the end of the tachycardia also rules out atrial tachycardia (option 1 incorrect). The third, fourth, and fifth beats observed after the maneuver, with broad QRS and short PR, are consistent with pre-excited beats, and this points to diagnosis of orthodromic tachycardia (option 4 correct, option 3 incorrect) via a hidden accessory pathway, that is, a pathway that can support anterograde conduction although not visible in the baseline ECG. This type of pathway is more usually left-sided (given the greater distance from the sinus node) and may become apparent with maneuvers that cause atrioventricular node block. Electrophysiological study confirmed the presence of a posterolateral accessory pathway, which was successfully ablated.

Figure 1

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