Heart disease. Electrocardiogram (ECG) showing the heartbeat during sinus bradycardia in a 71-year- old man with myocardial ischaemia. An ECG records the pumping of the heart's chambers, tracing electrical waves (blue) that are the output from 12 external electrodes (I-III, V1-V6 and aVR, aVL and aVF). Sinus bradycardia is a slow heartbeat and may be normal or abnormal. Abnormalities seen in this ECG include changes in the R wave (main peak) in traces V2 and V4. There are also changes in some of the traces in the T wave that follows the R wave. These may be caused by myocardial ischaemia, inadequate blood flow to the heart muscle caused by blocked blood vessels.