Bundle branch block

Each heartbeat starts with the heart’s pacemaker cells in the sinoatrial node, sometimes called the SA node, in the right atrium. The SA node sends out an electrical signal that propagates out and contracts both upper chambers. The signal then moves through the atrioventricular node, or AV node, down into the lower chambers. Here it reaches the bundle of His and splits into the left and right bundle branches, which serve the left and the right ventricles. The signal then goes on to each ventricles’ Purkinje fibers, which leads to ventricular contraction.

Now, a “bundle branch block” describes when that electrical signal gets completely blocked or held up along one of the bundle branches. In most cases, this block, or delay, is caused by fibrosis, or scarring, that either occurs acutely or chronically. Acute causes can be things like ischemia, heart attack, or myocarditis, the inflammation of the heart tissue. Chronic conditions might lead to fibrosis of the heart tissue, because they all can cause slow and steady remodeling of the heart muscle; these include: hypertension, coronary artery disease, and cardiomyopathies.
If the block happens on the right side, it’s referred to as a right bundle branch block. With this type, the electrical signal starts at the SA node, contracts the atria, moves through the AV node, splits at the bundle of His, and then moves down the left bundle branch, but is blocked on the right bundle branch. This causes the left ventricle to contract first. The signal then spreads from the purkinje fibers of the left ventricle over to the right ventricle, which causes the right ventricle to contract after the left has contracted. So, with right bundle branch block, the right ventricle contracts late. If the block happened to be on the left side instead, which is called a left bundle branch block, the signal would be delayed on that side, and so the right ventricle would contract first, and then the left ventricle would contract late.