Bundle Branch Blocks and Fascicular Blocks Flashcards

In left bundle branch block, the septal Q waves normally seen in leads I, V5, and V6 are typically:

Answer: Absent or eliminated

LBBB eliminates normal septal Q waves in leads I, V5, and V6 because septal depolarization now travels right to left rather than the normal left to right direction.

An ECG shows a QRS of 0.14 sec with broad notched R waves in I, aVL, V5–V6 and a broad QS in V1. What is the most likely diagnosis?

Answer: Left bundle branch block

Broad notched R waves in lateral leads (I, aVL, V5–V6) combined with a QS pattern in V1 and a QRS ≥0.12 seconds are the hallmark features of left bundle branch block.

The Sgarbossa criteria are used to identify acute MI in the presence of which ECG pattern?

Answer: Left bundle branch block

Sgarbossa criteria help identify acute MI in the setting of LBBB because LBBB causes secondary ST-T changes that can mask ischemic ST changes.

In LBBB, T-wave deflections that are discordant (opposite direction) to the QRS complex are considered:

Answer: Normal secondary repolarization changes

Discordant ST-T changes are expected secondary repolarization abnormalities in LBBB resulting from the abnormal depolarization sequence and are not independently pathological.

New-onset LBBB in a patient presenting with chest pain should be managed as which of the following until proven otherwise?

Answer: A STEMI equivalent requiring emergent reperfusion evaluation

New or presumably new LBBB with chest pain is treated as a STEMI equivalent per ACC/AHA guidelines because it may conceal ST elevation and indicate an acute coronary occlusion.

Which underlying condition is the MOST common cause of left bundle branch block?

Answer: Hypertensive heart disease or coronary artery disease

LBBB is most commonly caused by hypertensive heart disease, coronary artery disease, or cardiomyopathy — chronic conditions that damage the left-sided conduction system.

A patient has a normal narrow QRS at 70 bpm but develops LBBB morphology when the rate increases to 110 bpm. This phenomenon is called:

Answer: Rate-dependent (tachycardia-dependent) LBBB

Rate-dependent (phase 3) LBBB occurs when an increased heart rate causes the cycle length to fall within the refractory period of the left bundle branch, producing block only at faster rates.