Heart disease diagnosis. Electrocardiogram (ECG) showing the heartbeat of a 63-year-old man with suspected myocardial ischaemia. An ECG uses external electrodes to measure electrical currents produced by the activity of heart muscle. Each of the wave-like traces seen is the output of one of 12 electrodes. Myocardial ischaemia occurs when heart muscle is damaged by poor blood supply caused by constricted or blocked blood vessels. The ischaemia is revealed by ST-T changes (shallow dips immediately after peaks), seen in the traces for electrodes V4-V6.