A 53-year-old woman complained of cardiac pain and rhythm intermissions. She had experienced these presentations since childhood. The patient's father had a history of cardiac arrhythmias. Objectively: the patient was in grave condition, Ps - 220 bpm, AP - 80/60 mm Hg. ECG: heart rate - 215/min, widening and deformation of QRS complex accompanied by atrioventricular dissociation; positive P wave. Some time later heart rate reduced down to 45/min, there was a complete dissociation of P wave and QRST complex. Which of the following will be the most effective treatment?