An 80-year-old male was admitted to our hospital with complaints of palpitations. He was diagnosed with atrial fibrillation that stopped spontaneously one hour after admission (not shown). Before he was discharged the following ECG was recorded. How would you judge this ECG (Fig. 1) and, in particular, how would you explain the occurrence of narrow QRS complexes intervening with the wide QRS complexes? Fig. 1 ECG before discharge