A 50-year-old woman complains about dull cardiac pain, asphyxia, body temperature rise up to 38°C. She had influenza a week ago. Objectively: Ps -100 bpm, dropped-beat pulse during inspiration. AP - 100/70 mm Hg, heart sounds are muffled. ECG: reduced voltage, ST segment is above the isoline in all leads. X-ray picture shows extensively enlarged cardiac silhouette. Palmus is of small amplitude. What is the most probable diagnosis?