Bifascicular block (RBBB with LAFB) on a 12-lead ECG

Right bundle branch block (wide QRS, rSR' in V1, slurred S in I and V6) together with left axis deviation from a left anterior fascicular block (rS in II, III and aVF).

What to look for

Two of the three fascicles are down: the right bundle, and the anterior fascicle of the left. So you read both. V1 has the RBBB rSR', I and V6 the slurred S, and the limb leads the LAFB pattern: left axis deviation of -45° or more, rS in II, III and aVF. Only the posterior fascicle is left carrying the beat. With syncope, the risk of sudden complete heart block is high, so monitor and have pacing ready. New bifascicular block with chest pain can be the only sign of a proximal LAD occlusion. Add a long PR and it's often called trifascicular, though the PR usually lengthens in the AV node.

Examples

Each is a standard 12-lead: 2.5 seconds of each lead in four columns, with 10 seconds of lead II along the bottom, drawn to scale (25 mm a second, 10 mm a millivolt). The measurements are under each one.

Don't confuse it with

• Sinus rhythm with right bundle branch block

  A QRS of 0.12 s or more with rSR' (an 'M') in V1-V3 and a wide, slurred S in I, aVL, V5 and V6.

• Left anterior fascicular block

  Left axis deviation (-45° to -90°) with qR in I and aVL, rS in II, III and aVF, and a QRS under 0.12 s.

• Sinus rhythm with left bundle branch block

  A QRS of 0.12 s or more with a deep, broad S (QS or rS) in V1 and a broad R with no Q wave in I, aVL, V5 and V6.

• Left axis deviation

  A QRS axis more negative than -30°: lead I positive, lead II and aVF mostly negative.

Where the criteria come from

• LITFL: Bifascicular block
• LITFL: Trifascicular block
• LITFL: Right bundle branch block (RBBB)

Others in the normal, axis and fascicles group

• Normal 12-lead ECG
• Left axis deviation
• Right axis deviation
• Extreme axis deviation
• Left anterior fascicular block
• Left posterior fascicular block
• All 12-lead practice