When symptoms are induced by physical effort there is a possibility that the coronary arteries may be blocked or narrowed and the heart is not being supplied with adequate blood. In this situation, the 12-lead ECG is carried out during exertion, normally on a treadmill or exercise bike, in a stress test.
As the exercise becomes progressively harder, the ECG is examined for ST segment depression and the prevalence of arrhythmia. Blood pressure is also taken at regular intervals. Studies vary, but the accuracy of a stress test to diagnose myocardial ischaemia is about 70%.
One drawback is that patients often reach their maximum not through chest pain or maximum heart rate but through leg fatigue, or shortness of breath as they are not often used to running or cycling. In such cases a thallium stress test may be advisable.