On the second day after a total thyroi-dectomy, a 56-year-old woman developed severe agitation, fear of death, hand tremor nausea, vomiting, diarrhea , and fever, Objectively, the following is observed; hot and moist skin, profuse sweating, acute muscle weakness, adynamia, dry tongue and lips, body temperature - 40°C,Heart rate increases from 60-70/min to 130-220/min. Blood pressure - 210/70 mm Hg. Auscultation detects intensified and arrhythmic heart sounds, systolic murmur can be heard at the apex and in the Botkin-Erb's point. ECG shows a non-sinusoidal irregular rhythm and atrial fibrillation, the ventricular contraction rate is 140-160-220/min. What is the most likely diagnosis in this case?