A right bundle branch block (RBBB) produces a distinctive pattern on an ECG: a wide QRS complex (120 milliseconds or longer) with a characteristic “M-shaped” or RSR’ pattern in leads V1 and V2, plus a broad, slurred S wave in leads I and V6. These features are recognizable at a glance once you know what to look for, and they reflect a specific delay in how electrical signals reach the right side of the heart.
The Key ECG Features
Three findings define a complete right bundle branch block on a 12-lead ECG:
- QRS duration of 120 ms or more. A normal QRS complex lasts 80 to 110 milliseconds. In RBBB, the complex widens because the right ventricle takes longer to activate.
- RSR’ pattern in V1 and V2. Instead of a simple up-down waveform, leads V1 and V2 show a double peak, sometimes described as “rabbit ears” or an M-shaped complex. The first small upward deflection (R) is followed by a dip (S), then a second, taller upward deflection (R’).
- Wide, slurred S wave in leads I and V6. On the left-sided leads, you’ll see a broad S wave that is either wider than 40 milliseconds or has a greater duration than the R wave in those same leads.
If the QRS is between 90 and 115 milliseconds but the RSR’ and S-wave patterns are still present, that’s classified as an incomplete RBBB. The waveform shapes are similar, just narrower and less pronounced.
Secondary Changes in the ST Segment and T Wave
RBBB also produces predictable changes beyond the QRS complex itself. In leads V1 through V3, you’ll typically see modest ST-segment depression and T-wave inversion. These repolarization changes point in the opposite direction from that tall R’ wave, a pattern called “discordant” ST-T changes. They result from the altered sequence of electrical activation, not from damage to the heart muscle. These secondary changes are usually stable over time and shouldn’t be mistaken for signs of a heart attack, though distinguishing the two can be tricky in an acute setting.
Why the ECG Looks This Way
Normally, both ventricles receive electrical impulses almost simultaneously through separate pathways: the right bundle branch activates the right ventricle while the left bundle branch activates the left. When the right bundle branch is blocked, the left ventricle still fires on schedule, but the right ventricle has to wait. It eventually depolarizes the slow way, with the electrical wave spreading cell by cell through the muscle wall from the left side over to the right.
That two-step process is exactly what creates the RSR’ pattern in V1. The first R wave represents normal activation of the thin wall between the ventricles. The S wave reflects the left ventricle powering up. Then the delayed R’ appears as the electrical signal finally reaches the right ventricle and moves toward the V1 electrode. Meanwhile, leads I and V6 (which face the left ventricle) pick up that late rightward electrical activity as a wide, trailing S wave.
RBBB vs. Left Bundle Branch Block
The easiest way to tell RBBB from a left bundle branch block (LBBB) is to look at lead V1. In RBBB, V1 shows that tall R’ (the second upward peak), because the delayed activity is heading toward the right side of the chest where V1 sits. In LBBB, V1 instead shows a deep, wide downward deflection because the delayed activity moves away from V1, heading leftward. Lead V6 mirrors this: RBBB gives a deep S wave there, while LBBB produces a tall, broad R wave.
Both conditions widen the QRS to 120 ms or more. The direction of the widening, rightward in RBBB and leftward in LBBB, is what separates them visually.
What Causes It
RBBB can appear in people with no underlying heart disease at all, and this becomes increasingly common with age. A landmark study of men tracked from age 50 to 80 found that RBBB prevalence rose from under 1% at age 50 to about 11% by age 80, with a cumulative incidence of nearly 13%. At age 75, right bundle branch blocks were more than four times as common as left bundle branch blocks. In younger, otherwise healthy people, an isolated RBBB on a routine ECG is generally considered benign.
When RBBB does have an underlying cause, the list includes heart attack (especially one affecting the front wall of the heart), inflammation of the heart muscle, chest trauma, pulmonary hypertension or other conditions that strain the right ventricle, congenital heart defects, and degenerative changes in the heart’s conduction system that accumulate with age. It can also appear after certain cardiac procedures like right heart catheterization.
Does It Always Need Treatment?
An isolated RBBB in someone with no symptoms and no other heart disease typically requires no treatment. The pattern may stay on your ECG for life without causing problems. The clinical significance depends on context: RBBB that appears suddenly during a heart attack, accompanies symptoms like fainting, or shows up alongside other conduction abnormalities warrants closer evaluation. In those situations, the block itself isn’t usually treated directly. Instead, the focus shifts to the underlying condition causing it.