Human heart. Computer artwork of the outline of a human figure showing the heart and a healthy electrocardiogram (ECG) trace. The heart is a hollow muscle that pumps blood around the body. The main blood vessels of the heart can be seen on the upper part of the heart. ECG traces show the pumping of the heart as a series of waves of various sizes. These waves correspond to the electrical activity that occurs in the heart muscle immediately before it contracts. An ECG is an important heart monitoring technique and can be used to diagnose coronary heart disease, thrombosis and arrhythmias.