Biology
Understanding Cardiac Conduction Anatomy Behind Bundle Branch Blocks
Quick fact
Bundle branch blocks occur when the electrical impulse is delayed or blocked in one of the two main branches of the His bundle, causing the ventricles to contract out of sync. Remarkably, a right bundle branch block can be a normal finding in healthy young people, while a left bundle branch block often signals underlying heart disease.
Why this is interesting
You know the heart has four chambers, but did you know it has a built-in electrical wiring system? When part of that wiring breaks, the heart still beats—but the pattern of its beat changes in a way you can see on an ECG.
Read the full explanation
Understanding Understanding Cardiac Conduction Anatomy Behind Bundle Branch Blocks
Think of the heart's electrical system as a well-orchestrated relay race. The signal starts at the sinoatrial (SA) node, the natural pacemaker, then spreads through the atria, causing them to contract and push blood into the ventricles. The signal then reaches the atrioventricular (AV) node, which acts as a 'gatekeeper'—it briefly delays the signal to allow the atria to finish filling the ventricles. From the AV node, the signal travels down the bundle of His, a kind of main cable, located in the septum between the ventricles. This cable then splits into two branches: the right bundle branch (RBB) and left bundle branch (LBB). These branches deliver the signal rapidly to the right and left ventricles, respectively, via tiny Purkinje fibers that spread through the ventricular muscle. Normally, both ventricles receive the signal almost simultaneously, so they contract together to pump blood efficiently. Now, imagine if one of these branches is partially or fully blocked. This is a bundle branch block. The ventricle on that side still receives the signal, but it has to spread more slowly through the muscle itself, rather than through the fast pathway. This causes that ventricle to depolarize (start contracting) a bit later than the other. The result is that the two ventricles are no longer perfectly synchronized. On an ECG, the QRS complex—which represents ventricular depolarization—becomes abnormally wide (more than 120 milliseconds) because it takes longer for the entire ventricular muscle to be activated.
A deeper explanation
The underlying principle is that the His-Purkinje system is designed for speed and synchronization. The bundle branches and Purkinje fibers are composed of specialized cardiac myocytes that conduct action potentials much faster than ordinary working muscle cells. They ensure that the depolarization wavefront reaches all parts of the ventricular muscle almost simultaneously, leading to a coordinated and efficient contraction. When a bundle branch is blocked, the electrical impulse must travel through the slower working myocardium to reach the distal portions of that ventricle. This slower conduction is what widens the QRS complex. The pattern of the QRS on the ECG depends on which bundle is blocked and the orientation of the electrodes. For a right bundle branch block (RBBB), the right ventricle depolarizes late, causing a prominent R' wave in the right precordial leads (V1-V2) and a wide, slurred S wave in the left lateral leads (I, aVL, V5-V6). This is because the depolarization wavefront moves from left to right, away from those leads. For a left bundle branch block (LBBB), the left ventricle depolarizes late, producing a wide, notched R wave in the lateral leads (I, aVL, V5-V6) and a deep S wave in the right precordial leads. The depolarization vector points to the left and posteriorly. Understanding this anatomy is clinically crucial. Bundle branch blocks can be a sign of underlying heart disease, such as coronary artery disease or cardiomyopathy. They can also occur after cardiac surgery or in the setting of hypertension. Because the normal sequence of ventricular activation is disrupted, bundle branch blocks can impair the efficiency of the heart's pumping, especially in patients with heart failure. In some cases, they may warrant therapy such as cardiac resynchronization therapy (a special pacemaker) to restore coordinated contraction.