Heart disease. Electrocardiogram (ECG) of the heartbeat of an 18-year-old man showing supra- ventricular 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 (black). The tachycardia is seen as close QRS complexes (large repeating peaks and troughs) in the trace across bottom, and may be caused by coronary artery disease. The ECG also shows ST depression (slow return to the starting level after QRS complexes), for example in traces II and aVF. This is commonly caused by ischaemia, insufficient oxygen supply to the heart muscle.