Shown here is the ECG from a 57 year old male who came to the emergency department because of palpitations and shortness of breath. Which of the following do you think is the correct diagnosis of this narrow QRS complex tachycardia? 1) sinus tachycardia 2) ectopic atrial tachycardia 3) AV junctional reentrant tachycardia 4) atrial flutter 5) atrial fibrillation