Heart disease. Electrocardiogram (ECG) showing the heartbeat of a 78-year-old man in atrial fibrillation. An ECG measures the pumping of the heart's chambers using 12 external electrodes (I- III, V1-V6 and aVR, aVL and aVF) and records it as a series of electrical waves (red), one from each electrode. In atrial fibrillation, the heart muscle fibres contract asynchronously, producing a rapid, irregular heartbeat. Here it was caused by myocardial ischaemia, inadequate blood flow to the muscle caused by constricted or blocked blood vessels. The ischaemia is revealed by the S-T wave depressions (shallow dips immediately after deep troughs) in the traces for electrodes V4-V6.