A 52 y.o. male patient suffers from squeezing pain attacks in substernal area which irradiates to the left hand and occurs occasionally and on physical exercises. He has had it for 1 year. On examination: heart borders are enlargement to the left side, sounds are muffled, Ps- 76 bpm, rhythmic, AP-155/80 mm Hg, ECG: the left type, the rest signs are normal. What additional examination is necessary to confirm the diagnosis?