Every time the heart beats, tiny electrical impulses are discharged. Using a process called electrocardiography, those electrical discharges can be recorded and used to measure the heart's condition. Several thin wires are attached to the body. The wires conduct the electrical charges into a machine that measures them and produces a readout.
Since most electrocardiograms show a healthy heart for patients at rest, doctors take the readout while the patient is under the stress of strenuous activity. Stress electrocardiography reveals a different picture of the heart's health. Usually, the patient walks on a treadmill machine while the heart readings are taken.