Dear Senthil, Sometimes ECG in rest is normal, as our heart needs less blood in rest, so even if there are some blockages in the arteries, at rest it may be sufficient, and rest ECG may be normal. During physical activity our organism needs more blood so the heart as well, and in case there are some blockages in the arteries, the patient may have chestpain and ECG changes. Maybe your father has chestpain or other typical symptoms, or positive stress test, abd your doctor wants to perform an angiography to check if there is any problems in his arteries. Take care