A 57-year-old man complains of shortness of breath, swelling on shanks, irregularity in cardiac work, pain in the left chest half with irradiation to the left scapula. Treatment is uineffective. On physical exam: heart's sounds are diminished, soft systolic murmur on the apex. Ps - 100/min, arrhythmical, BP - 115/75 mm Hg. The liver is +2 cm, painful. Roentgenoscopy: enlargement of heart shadow to all sides, pulsation is weak. Electrocardiogram (ECG): leftventricled extrasystolia, decreased voltage. What method of investigation is necessary to do to determine the diagnosis?