A bundle branch block shows up on an ECG as a widened QRS complex, the tall spike that represents each heartbeat. In a complete block, that spike stretches to 120 milliseconds or longer, noticeably wider than the normal 80 to 100 ms. The specific shape of that widened complex tells you whether the right or left bundle branch is affected, and the two types look distinctly different from each other.
To understand why the ECG changes, it helps to know what’s happening electrically. Your heart has two bundle branches, left and right, that carry electrical signals down into the lower chambers. When one branch is blocked, the signal still reaches both sides of the heart, but it has to take a detour through regular muscle tissue instead of the fast-track wiring. That detour is slower, which is why the QRS complex gets wider. The side of the heart with the blocked branch contracts slightly late, and that delayed activation creates the distinctive patterns on the tracing.
Right Bundle Branch Block on ECG
A right bundle branch block (RBBB) is easiest to spot in leads V1 and V2, the ECG leads that sit over the right side of the chest. In those leads, you’ll see a characteristic pattern sometimes called “rabbit ears” or described as an rsR’ complex. That means there’s a small initial upward spike, a brief dip downward, and then a second, taller upward spike. The second peak represents the delayed activation of the right ventricle catching up after the left side has already fired.
In the lateral leads (V5, V6, and lead I), the pattern looks different. You’ll typically see a broad, slurred S wave at the end of the QRS complex. This is the mirror image of what’s happening in V1: those leads are looking at the heart from the opposite angle, so the late rightward electrical activity shows up as a downward deflection instead of an upward one.
For a complete RBBB, the QRS duration is 120 ms or greater. When the QRS falls between 100 and 119 ms with the same general shape in V1 and V2, it’s classified as an incomplete RBBB. The pattern is the same, just less exaggerated.
Left Bundle Branch Block on ECG
A left bundle branch block (LBBB) produces a very different picture. The hallmark findings include a QRS duration over 120 ms, a deep downward deflection (QS or rS pattern) in lead V1, and a tall, broad, monophasic R wave with no Q wave in leads V6 and lead I. “Monophasic” just means the spike goes up and stays up without dipping below the baseline first. It often looks like a wide, notched tower, sometimes described as an “M” shape because of a small notch near the peak.
The reason V1 looks so different from V6 in LBBB is directional. The electrical signal is heading away from the right-sided chest leads (producing that deep downward deflection in V1) and toward the left-sided leads (producing the tall upward wave in V6). The notching or slurring you see reflects the slower, less organized path the signal takes through the left ventricle.
ST and T-Wave Changes
Bundle branch blocks don’t just widen the QRS. They also change the ST segment and T wave, the parts of the tracing that come right after the main spike. In both RBBB and LBBB, the T wave normally points in the opposite direction from the main QRS deflection. So if the QRS is predominantly upward in a given lead, the T wave should dip downward, and vice versa. This is called “appropriate discordance,” and it’s expected.
This matters because ST segments and T waves are exactly what doctors look at to diagnose heart attacks. A bundle branch block, especially LBBB, can mask or mimic heart attack patterns. Specialized criteria exist to help sort this out. The Barcelona algorithm, for instance, flags a possible heart attack during LBBB if the ST segment shifts in the same direction as the QRS complex by at least 1 mm, or if there’s excessive ST deviation relative to the size of the QRS. These are the kinds of subtle distinctions that make reading an ECG with a bundle branch block more challenging than reading a normal one.
Complete vs. Incomplete Blocks
The dividing line between complete and incomplete is straightforward. A complete bundle branch block has a QRS duration of 120 ms or more. An incomplete block has a QRS between 100 and 119 ms with the same general morphology but less pronounced. The shapes and patterns are the same in both cases, just narrower and less dramatic in an incomplete block. An incomplete block is generally considered less clinically significant, though it still indicates some degree of conduction delay.
What Causes the Block
The two types tend to have different underlying causes. Left bundle branch block is more commonly associated with heart disease. Typical causes include heart attack, high blood pressure, inflammation of the heart muscle, and cardiomyopathy (a condition where the heart muscle becomes thickened, stiffened, or weakened). Finding a new LBBB on an ECG usually prompts further investigation because it rarely occurs in a completely healthy heart.
Right bundle branch block has a broader range of causes and can sometimes appear in people with no heart problems at all. When it does have an underlying cause, the list includes blood clots in the lungs, heart attack, congenital heart defects like a hole between the upper chambers, high blood pressure in the lung arteries, and heart muscle inflammation. RBBB can also show up as a normal variant, particularly in younger, otherwise healthy people.
What It Feels Like
Most people with a bundle branch block feel nothing at all. The block itself doesn’t typically cause symptoms. When symptoms are present, they usually come from whatever underlying condition caused the block in the first place, not from the conduction delay itself. Some people with bundle branch blocks report feeling lightheaded or fainting, but these symptoms point to a broader electrical or structural heart problem rather than the block in isolation.
A bundle branch block is often discovered incidentally during a routine ECG or a preoperative workup. If you’ve been told you have one, the ECG pattern is the main finding. Whether it needs any follow-up depends almost entirely on which branch is blocked, whether it’s new or longstanding, and whether there are signs of underlying heart disease.