• Bundle branch block (BBB) arises when conduction via one of the 2 bundle branches is blocked resulting in delayed depolarization of the blocked branch.
• The final rhythm shows a fused complex of 2 QRS units with an early component from intraventricular septal and/or unaffected ventricular depolarization and a late component from the affected ventricular depolarization, which is seen together as one widened QRS complex.
• BBB might be further characterized as “complete” (if QRS duration is > 120 ms) or “incomplete” (if QRS duration is 110-120 ms).
• BBB could be either reversible (e.g. after resolution of STEMI), irreversible, stable, or intermittent (e.g. occurs only with increased heart rate).
• BBB might be also associated with axis deviation and discordant ST segment and T wave deflections.
• BBB is an example of interventricular conduction delay, which is different from intraventricular conduction delay IVCD, where conduction delay arises from within the myocardium rather than bundle branch. IVCD has Widened QRS's without typical BBB morphology.
• Left bundle branch block (LBBB) arises when the left bundle branch is blocked resulting in the left ventricular depolarization being delayed as compared to the right ventricular depolarization and depolarization wave spreads across the interventricular septum from right to left.
• In general, ECG shows a delayed R and delayed S (corresponding to delayed left ventricular depolarization) in left lead V5-6, I, aVL and right lead V1, respectively.
• QRS morphologies in leads V5-6, I, aVL could be either:
M-shaped RR’ (R from leftward septal depolatization, R‘ from left ventricular depolarization),
Broad monophasic R wave (from the overall leftward depolarization wave), or
Notched R wave (from the overall leftward depolarization wave)
• QRS morphologies in lead V1 could be either:
W-shaped QS (Q from septal depolarization away from the right lead, S from left ventricular depolarization),
rS complex (r from right ventricular depolatization, S from left ventricular depolarization), or
Dominant S wave (from the overall leftward depolarization wave)
• In addition, there is discordant ST-T deflection as compared to QRS deflection, for instance:
ST elevation and upright T wave in lead V1, following negatively deflected S waves
ST depression and T wave inversion in leads V5-6, I, aVL, following positively deflected R waves
• If ST-T deflection is concordant with QRS deflection, acute myocardial infarct should be considered (Sgarbossa criteria).
• Axis is either normal or deviated to the left.
• Right bundle branch block (RBBB) arises when the right bundle branch is blocked resulting in the right ventricular depolarization being delayed as compared to the left ventricular depolarization and depolarization wave spreads across the interventricular septum from left to right.
• In general, ECG shows a delayed R and delayed S (corresponding to delayed right ventricular depolarization) in right lead V1 and left leads V5-6, I, aVL, respectively.
• QRS morphologies in lead V1 could be either:
M-shaped RSR’ (R from rightward septal depolatization, S from left ventricular depolarization, and R’ from right ventricular depolarization),
If there is concurrent LAFB, S wave gets smaller due to weaker leftward depolarization
If there is concurrent LPFB, S wave becomes more prominent due to stronger leftward depolarization
Broad monophasic R wave (in right lead V1: )from the overall rightward depolarization wave), or
qR complex (q from left ventricular depolarization and R from right ventricular depolarization)
• QRS morphologies in leads V5-6, I, aVL could be either:
W shaped qRS (q from septal depolatization away from left leads, R from left ventricular depolarization, and S from right ventricular depolarization)
• In addition, there is discordant ST-T deflection as compared to QRS deflection, for instance:
ST depression and T wave inversion in lead V1, following positively deflected R’ waves
ST elevation and upright T wave in lead V5-6, I, aVL, following negatively deflected S wave
• Axis is often normal or sometimes deviated to the right.
