The electrocardiogram might be confused with a sinus rhythm with a long PR interval (240ms); however, on closer inspection, a P wave should theoretically be present but this is negative in aVL. This is a supraventricular tachycardia with atrial cycle length of 250ms and 3:1 atrioventricular conduction (80 bpm) after the patient was taking β-blockers. Atrial activity is integrated in the QRS complexes and T wave, and so it may go unnoticed. It can mainly be observed in leads II and aVF (Figure 1). Carotid sinus massage (Figure 2) as the first measure confirms the diagnosis (therefore, response 2 is correct) as it induces transient atrioventricular block that allows the atrial activity to be clearly seen. The electrophysiological study will then enable classification and treatment of the specific arrhythmia (response 3, incorrect). The electrocardiographic monitoring tests would have no diagnostic value in such cases (responses 1 and 4, incorrect).

Figure 1

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Figure 2

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