Heart disease. Electrocardiogram (ECG) of the heartbeat of a 76-year-old man with an atrial flutter and myocardial ischaemia. An ECG measures the pumping of the heart chambers using 12 external electrodes (I-III, V1-V6 and aVR, aVL and aVF), recording it as electrical waves (black). A flutter is an increased rate of contraction of the upper heart chambers (atria), seen as numerous shallow waves (for example in traces I and II). It may be caused by myocardial ischaemia, inadequate blood flow to the heart caused by narrowed vessels. This is revealed by the S-T wave depressions (pronounced troughs after tall peaks) in the traces for electrodes II, III and aVF.