What Causes Shortness Of Breath And Dizziness?

Hi, my husband has been suffering from shortness of breath and dizziness for a couple of years and no one has been able to diagnose why. He had 2 bypass open heart surgery late 2011 and was doing good for probably 6 months when the shortness of breath and dizziness started. He also has Type 2 diabetes which is being controlled by medication/metformin and is considered well controlled. He also suffers from anxiety disorder and mild depression which he takes medication for. He looks healthy until he tries to do anything strenuous and becomes short of breath, dizzy and sometimes nauseous. He has had every heart test and lung test available and has been told everything is within normal ranges with just a small amount of emphysema in his lungs but not enough to be causing his condition. He also has arthritis in his neck and hands pretty bad. Any ideas??? He is on numerous medications for high blood pressure, blood thinners, antidepressants, statins for cholesterol and something for his prostate.