An interesting electrocardiogram

A 37-year-old man was admitted to our cardiology department with sustained palpitations for 36 hours. He had suffered from paroxysmal palpitations for five years. This had happened one to three times per month, especially in the six months before admission and syncope occurred occasionally. The most useful way to terminate the acute episode of palpitations was a loading dose of propafenone (up to 210 mg). The patient had been diagnosed with dilated cardiomyopathy seven years ago by an ultrasound examination. The onset electrocardiogram (ECG) was a fast and regular rhythm with wide QRS complexes (174 beats/min). During the administration of propafenone, the following ECG was documented (figure 1). What is your diagnosis?