A 65-year-old patient with a history of arterial hypertension complains of dizziness and palpitations that occurred throughout the last hour. Objectively, the following is observed: blood pressure - 80/40 mm Hg, heart rate-150 min, pulse-106/min. ECG revealed missing P wave and varying RR intervals, ventricular contraction rate is 136-148/min. What aid must be provided to the patient first?