Hi, Welcome to HCM, Your father is having Coronary artery disease and a CABG was done 4 years back and , he also has Lipid abnormality for which he is taking Tonact TG,. Now as he is having fatigue and dyspnea on exertion it is like angina /angina equivalent. Even after CABG basic disease atherosclerosis continues so sorbitrate is nitrate which dilates coronary artery and increases blood supply to heart, but you need to take more doses or you can take sustained release formulation which can be taken once a day.and he can use it in emergency sublingually. He should be evaluated by ECG and Echocardiography and Stress Thallium if ischemia is there or LV function is poor then treatment may be adjusted accordingly. I am sure he will improve with treatment changes. Take care Good Luck.