Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS
Each heartbeat starts with the heart’s pacemaker cells in the sinoatrial node, or sometimes SA node, in the right atrium. The SA node sends out an electrical signal that propagates out and contracts both upper chambers, then moves through the atrioventricular (or AV) node down into the lower chambers, where it reaches the bundle of His and splits into the left and right bundle branches, serving the left and the right ventricles, respectively, the signal then goes on to each ventricles’ Purkinje fibers, leading 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 and heart attack, or myocarditis, inflammation of the heart tissue. Chronic conditions that might lead to fibrosis of the heart tissue include hypertension, coronary artery disease, and cardiomyopathies, because they can all cause slow and steady remodeling of the heart muscle.
So if the block happens on the right side, it’s referred to as a right bundle branch block. So 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. And so the left ventricle contracts first . The signal then spreads from the purkinje fibers of the left ventricle over to the right ventricle, which contracts, but this happens after the left contracts. So with right bundle branch block, the right ventricle contracts late. If the block happened to be on the left side instead, 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.
Now, on electrocardiogram or ECG, normally you have the characteristic P wave, which corresponds to the atria contracting, then the QRS complex, which corresponds to both ventricles contracting and is usually between 80 and 100 ms long. If there’s a delay in depolarization of one of the ventricles, the QRS complex gets wider because depolarization starts on time but ends later than usual. Since now one of the ventricles is contracting late, a QRS complex longer than 120 ms is common to see in a bundle branch block. Okay, so far we’ve been looking a relatively common view of the heart, called lead two, which is a type of limb lead, since we get it by attaching leads to the right arm and left leg, both of which are limbs.
Even though this lead can give some solid information, it’s often helpful to use other leads as well, especially when differentiating between left and right bundle branch block. The chest, or precordial, leads are really useful for doing just that. Chest leads look at the heart’s electrical activity when viewed from a slice parallel with the ground, so it’s a bit different than the limb leads which look at the heart in the frontal plane, which is perpendicular to the ground.
For chest leads, they go from V1 coming straight out the front of the chest, to V6, going out the left side of the chest. Each of these is like looking at the heart from a different perspective, right? V1 and V6 are particularly helpful since they’re the farthest apart from each other and therefore have the most contrast between views. Normally, the first thing to depolarize is the septum, and the wave of depolarization moves from the left bundle branch toward the right ventricle, so toward V1 but away from V6, meaning you’ll get a positive deflection on V1, and a negative deflection on V6. Then that wave moves to the ventricles themselves, and since the left ventricles is usually more massive, you’ll get a large depolarization directed away from V1 but toward V6, so negative on V1 and positive on V6. Now, let’s say there’s a block along the left bundle branch, so now the overall direction of depolarization comes from the right side and moves toward the left, so away from V1 and toward V6, meaning negative in V1 and positive in V6. V6 will also often be notched, from the left ventricle depolarizing late. So when it’s all said and done, V1 usually has a QS or “little r”-S complex, QS meaning an absent R wave, and “little r”-S meaning a small R wave, either of which give it a “W” shape. On V6, the large “notched” R wave gives it an “M” shape.
Alright, now let’s say there’s a block on the right bundle branch. The first part is actually similar to the normal situation, right, so the septum depolarizes from the left bundle branch toward the right ventricle, so toward and positive in V1 and away and negative in V6. Next the left ventricle depolarizes and you get a negative deflection in V1 and a positive deflection in V6. Finally, as the right ventricle depolarizes in a roundabout sort of way, you get a positive deflection in V1 and negative in V6. So V1 has a large terminal R wave, meaning it ends with large R wave, and this gives it an ‘M’ shape. On V6, there should be a slurred S wave, giving it a W shape.
Now, if we take a look at all these, you’ve got W-shape in V1 and M-shape in V6 for left bundle branch block, but then, an M-shape in V1 and a W-shape in V6 in Right bundle branch block...which sets us up for this awesome mnemonic WiLLiaM MaRRoW. Nailed it.
Some cases of bundle branch block are present since birth, and don’t cause symptoms and therefore probably don’t need treatment. In a lot of cases, though, the block is acquired through some other heart disease. In severe cases when the block is accompanied by heart failure, a special cardiac resynchronization pacemaker can be implanted that has electrical leads going to both the left and right ventricles, and can pace them at the same time, causing them both to contract at the same time, which helps improve cardiac function.
All right, as a quick recap… A bundle branch block is when the electrical signal from the AV node gets blocked or delayed along one of the bundle branches in the ventricles. It is usually caused by fibrosis of heart tissue from acute or chronic heart disease. It shows up as a wider QRS complex on an ECG, and using chest leads on the ECG can help confirm the bundle block’s location. YT Criticisms 20171221 27 upvotes lots of comments
Would the notch in V6 in the case of Left bundle branch block be due to late depolarization of the left ventricle instead of the right? 4:08
yes
Sources
edithttp://lifeinthefastlane.com/ecg-library/basics/left-bundle-branch-block/
http://lifeinthefastlane.com/ecg-library/basics/right-bundle-branch-block/