Heart disease. Abnormal electrocardiogram (ECG) of the heartbeat of a 45-year-old woman showing probable supraventricular tachycardia (high heart rate). 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 (yellow). The tachycardia is seen as QRS complexes (repeating peaks and troughs) that are closer together than in a healthy patient. The ECG also shows ST depression (slow return to the starting level after QRS complexes). This may have been caused by myocardial (heart muscle) injury or by ischaemia, which is an insufficient supply of oxygen to the heart muscle.