ECG trace. Computer artwork of an ECG (electrocardiogram) trace from a healthy heart, superimposed over a normal chest X-ray. An ECG uses electrodes to measure the electrical activity of the heart during the pumping of its chambers, and records it as an electrical wave (yellow). The tall peak seen here represents the contraction of the heart's lower chambers (ventricles). Electrocardiography is a useful tool for the diagnosis of heart disease. The waves of an ECG trace can reveal the nature and location of many cardiac disorders. The chest X-ray shows a healthy heart (light area at lower right) and lungs (dark cavities).