A 52-year-old woman with hypertension and no underlying structural heart disease. Past medical history of repeated presyncope associated with emotional stress and one sudden syncope. The first ECG (Figure 1) shows blocked sinus P waves alternating with conducted P waves with a narrow QRS and others with a long PR interval and wide QRS. In the second ECG, performed a few minutes later (Figure 2), in association with increased sinus tachycardia, atrioventricular conduction becomes consistently 1:1 with a long PR and wide QRS. What is causing these changes? (Clinical case courtesy of Dr. Enrique Rodríguez-Font.)

Figure 1

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Figure 2

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