Royalty free and rights managed images relating to electrocardiography (ECG). This technique uses electrodes to record the electrical activity of the heart muscle. The output is known as an electrocardiogram. This section includes illustrations of heartbeats and conceptual images showing the electrical output of the heart. Photographs and pictures show scientists using ECGs in sleep apnoea research and stress tests. Doctors are shown using ECGs to diagnose cardiac conditions and heart disorders such as an irregular heart beat (arrhythmia).