Heart disease. Conceptual artwork of a silhouette of a male figure, with an ECG (electrocardiogram) heart trace showing an abnormal heartbeat. An ECG heart trace measures the pumping of the heart as a series of waves. These waves show the electrical activity of heart muscle during contraction and relaxation. This measurement is a way of diagnosing heart diseases like coronary artery disease, thrombosis, arrhythmia and heart attack.