CONTENTS

• Diagnosis of LBBB
• Interpretation of ECG in the context of LBBB
  • Axis
  • Chambers
  • Signs of prior infarction
  • Signs of active ischemia

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LBBB

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diagnosis of LBBB

⚠️ About a third of people who meet the conventional diagnosis of LBBB may actually have LVH +/- LAHB.  (21376930) If the QRS interval is close to 120 ms, it may be difficult to differentiate between the two.

LBBB criteria (#1-2 help differentiate from LVH)

• [1] QRS >120 ms (more convincing if >130♀ or >140♂).
  • Greater QRS duration is more specific for LBBB as opposed to LVH (QRS >130 ms in women or >140 ms in men). (21376930)
• [2] V5, V6, I, aVL: Broad and notched/slurred R waves.
  • LBBB is supported by mid-QRS notching/slurring in at least two contiguous leads (especially V5, V6, I, and aVL). (21376930) The most specific finding for LBBB is time-to-QRS-notch in lead I is >75 ms (figure below). (38536171)
  • R-wave peak time may be prolonged >60 ms.
• [3] V1 is predominantly negative (QS or rS) with a positive T-wave. (de Luna 2022)
• [4] No septal Q-waves in left-sided precordial leads (unless prior Q-wave MI).
• [5] No evidence of LBBB mimics:
  • Short PR interval (suggests WPW).
  • QRS >~180 ms. This suggests hyperkalemia, Na blocker, or ventricular complexes. (3190044, 🌊)

[#1/4] axis is unaffected by LBBB

left axis deviation

• LBBB with LAD is often seen in dilated cardiomyopathy.
• LAHB isn't the explanation (all LBBB have LAHB).
• General discussion of LAD: 📖

right axis deviation (uncommon)

• Causes include:
  • Extreme verticalization of the heart.
  • Acute right ventricular overload (e.g., PE).
  • Right ventricular hypertrophy.
• LPFB isn't the explanation (all LBBB have LPFB).
• General discussion of RAD: 📖

[#2/4] chambers

atria are unaffected.

RVH is suggested if:

• RAA.
• Right axis deviation, with an RS in lead I that, isn't explained by other etiologies. 📖
• V1 has an r-wave that is conspicuous (in the absence of prior myocardial infarction). (de Luna 2022)

LVH:

• ~80% of patients with LBBB have increased left ventricular mass. (O'Keefe 2021)
• ECG features suggestive of LVH: (39348743, 38204852)
  • LAA (sensitivity ~40%, specificity ~88%).
  • R-aVL >11 mm (sensitivity ~10%, specificity ~95%).
  • Sokolow-Lyon (sensitivity ~20%, specificity ~90%).

[#3/4] signs of prior infarction in LBBB

Q-waves

• Q-wave >30 ms in III suggests for prior IMI.
• Pathological Q's in lateral leads.
• (QS is expected in V1-V4.)

other signs of prior infarct/scar

• Cabrera's sign: V3, V4, or V5 shows notching of the ascending limb of the S-wave.
• Chapman's sign: V6, Lead I, or aVL shows notching of the upstroke of the R-wave.

[#4/4] signs of acute ischemia in LBBB

• [1] ST deviation (Smith-Modified Sgarbossa criteria):  Any of the following indicates ischemia (even if only in a single lead):
  • Any lead with 1 mm of concordant STE.
  • Any single lead with excessively discordant STE (STE >25% of the preceding S-wave).
  • Any of the leads V1-V3 with 1 mm of concordant STD.
  • Potential confounders that may generate false-positive results are tachycardia, respiratory failure, hyperkalemia, and diastolic Bp > 120 mm.  
• [2] Convexity
  • Marked convexity may support ischemia, especially in the precordial leads (example below).
• [3] Concordant T-waves
  • Normally, T-waves should be discordant.
  • TWI in leads with negative QRS (i.e., right precordial leads) is relatively specific (albeit insensitive) for ischemia.
• [4] Changes from baseline
  • Especially qualitative changes.

clinical significance of LBBB

• Causes of LBBB include:
  • LVH.
  • Myocardial infarction.
  • Congenital heart disease.
  • Degenerative conduction system disease.
  • (Very rarely seen as a normal finding.)
• Patients with LBBB and systolic heart failure (EF <35%) may benefit from cardiac resynchronization pacemakers (CRT), especially if the QRS is >130-140 ms.

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questions & discussion

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