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 exertion. He has had it for 1 year. On examination: heart boarders are dilated to the left side, sounds are muffled, pulse- 76 bmp, rhythmic, AP- 155/80 mm Hg, ECG: the left type, the rest of signs are normal. What additional examination is necessary to confirm the diagnosis?