A simple broad complex tachycardia

A 74-year-old man was referred to our hospital with a 2-week history of progressive shortness of breath. The patient’s medical history reported hypothyroidism and depression, for which he was treated with levothyroxine and mianserine, respectively. Physical examination revealed that the patient was afebrile, blood pressure was 168/112 mmHg, pulse 150 beats/min and heart sounds were normal without murmurs. Pulmonary rales were present on auscultation. Laboratory tests were normal except for a raised creatinine of 123 μmol/l (normal <100 μmol/l). The chest X-ray showed marked cardiomegaly and signs of venous pulmonary congestion. The ECG on admission showed a broad complex tachycardia (Fig. 1). Adenosine was administered intravenously without any effect. Subsequently the arrhythmia was ended after cardioversion with 10 Watt. The ECG after cardioversion is shown in Fig. 2. What is your diagnosis with respect to Fig. 1?