CRAT Bundle Branch Blocks and Fascicular Blocks 2 — Questions and Answers
Question 1: In left bundle branch block, the septal Q waves normally seen in leads I, V5, and V6 are typically:
- Deeper and wider than normal
- Absent or eliminated (Correct answer)
- Present but inverted
- Present but shorter in duration
Correct 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.
Question 2: 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?
- Right bundle branch block
- Left bundle branch block (Correct answer)
- Wolff-Parkinson-White syndrome
- Ventricular tachycardia
Correct 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.
Question 3: The Sgarbossa criteria are used to identify acute MI in the presence of which ECG pattern?
- Right bundle branch block
- Left bundle branch block (Correct answer)
- Wolff-Parkinson-White pattern
- Complete AV block
Correct 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.
Question 4: In LBBB, T-wave deflections that are discordant (opposite direction) to the QRS complex are considered:
- Pathological, indicating acute ischemia
- Normal secondary repolarization changes (Correct answer)
- A sign of hyperkalemia
- Indicative of pericarditis
Correct 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.
Question 5: New-onset LBBB in a patient presenting with chest pain should be managed as which of the following until proven otherwise?
- A benign finding requiring no urgent intervention
- A STEMI equivalent requiring emergent reperfusion evaluation (Correct answer)
- A sign of stable coronary artery disease
- An artifact requiring lead repositioning
Correct 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.
Question 6: Which underlying condition is the MOST common cause of left bundle branch block?
- Wolff-Parkinson-White syndrome
- Hypertensive heart disease or coronary artery disease (Correct answer)
- Electrolyte imbalance
- Acute pulmonary embolism
Correct 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.
Question 7: 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:
- Intermittent LBBB
- Rate-dependent (tachycardia-dependent) LBBB (Correct answer)
- Wolff-Parkinson-White syndrome
- Brugada pattern
Correct 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.
CRAT Bundle Branch Blocks and Fascicular Blocks 2
In left bundle branch block, the septal Q waves normally seen in leads I, V5, and V6 are typically: