In this video we review a not infrequently identified change on the electrocardiogram. This test looks at the electrical patterns of the heart from electrodes placed on the skin around the body. Sometimes, you may be told that this test has shown a 'bundle branch block'. Often, people confuse this with a 'blockage' in the arteries around the heart. This is not the case and here Professor Barlis reviews what the bundle electrical pathways in our heart are and the two major types of changes seen causing a right and left bundle branch block.