Most individuals who have a cardiac problem will be identified with an ECG and an echocardiogram. You're probably aware that the ECG shows electrical problems of the heart and an echocardiogram reveals structural problems of the heart and gives us an idea of the heart size. In some situations, the echocardiogram is not as good as the MRI scan of the heart to show various parts of the heart. For example, the right heart or the right ventricular chamber is much better assessed with an MRI scan. Secondly, the arteries of the heart or the origins of the coronary arteries are much better assessed with an MRI scan than they are with an echocardiogram.
There are some people who have abnormalities that will only be picked up when the heart is under stress. For example, if someone has a blocked artery then the heart will only complain when it is being challenged. So someone with a blocked artery will have a normal ECG when they're lying down resting but when they exercise, the ECG may become abnormal. So if we suspect blocked arteries in someone, that is someone who is complaining of chest pain when they rush about, then we would perform an exercise stress test.
A 24 hour ECG is very useful at identifying individuals who have intermittent palpitation. A resting ECG at the time that the individual doesn't have any symptoms will be normal but a 24 hour ECG may pick up individuals who have intermittent palpitations and identify the rhythm disturbance in that individual.