Have A Problem That When I Walk On The Treadmill

Have a problem that when I walk on the treadmill at a speed of 3 miles an hour I feel a pressure on my chest like someone has placed a weight on the chest. My Cardiologist has done angiogram and mentioned that one artery is blocked 50% and it is not bad. He has tried a few medicines like nitroglycerine with a light strength and some other medicines but did not help. I have gone to Pulmonary doctor and he has performed ct scan and found not any problem. He has asked my Cardiologist to check the arteries on the right side of heart otherwise he has no problem, I have taken Ranitidine and also Gaviscon to see if I have acid reflux but no relief.
What do you suggest