A 7 y.o. boy was admitted to the hospital. He complains of unpleasant sensations in the heart region, pain in the epigastrium, dizziness, vomiting. Objectively: evident paleness of skin, dyspnea, jugular pulse. Heart borders are within the normal range. Heart sounds are clear, HR- 170/min, small pulse. AP- 90/50 mm Hg. EKG showed: paroxysm of ventricular tachycardia. The paroxysm can be suppressed by: