CONTENTS

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

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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.
  • 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, 🌊)

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[#1/4] axis is unaffected by LBBB

left axis deviation

• LBBB with LAD is often seen in dilated cardiomyopathy.
• 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.
• General discussion of RAD: 📖

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[#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%).

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[#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.

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[#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.

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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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ECG interpretation with a pacemaker

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[#1/4] rhythm?

• What is the underlying atrial rhythm?  (V1 might be a good place to look first).
  • Fibrillation waves?
  • 3rd degree AV block?
  • ⚠️ This is clinically relevant because AF/Flutter may indicate anticoagulation.
• What type of pacing?
  • Ventricular pacemaker only (e.g., VVI).
  • Atrial pacemaker only (e.g., AAI).
  • Dual chamber pacemaker (e.g., DDD).

[#2/4] how is the LV paced?

• RV pacing:
  • LBBB pattern in limb leads and anteroseptal precordial leads.
  • Unlike the LBBB pattern, QRS is almost always negative in V5-V6. (Berberian 2021)
• BiV pacing (CRTD) is suggested by:
  • [1] Q-waves in Lead I and aVL (qR or QS).
    • Monophasic negative in Lead I (QS) is the best criterion (90% sensitive and 90% specific).
    • RV pacing typically produces an R-wave in these leads.
    • Suggests left-to-right activation of the LV.
  • [2] R-wave in V1 (rS or R wave). (O'Keefe 2021)
    • Suggests posterior-to-anterior activation of the LV.

[#3/4] any evidence of pacemaker malfunction?

• ⚠️ Any pacemaker malfunction is clinically important because this is actionable (consult EP to revise pacemaker settings).
• Failure to capture:
  • ECG findings: Pacemaker spike is followed by nothing.
  • Causes include:
    • Electrode displacement.
    • Wire fracture.
    • Ischemia/infarct.
    • Electrolyte abnormalities (especially hyperkalemia).
• Failure to sense:
  • ECG findings: Pacemaker spikes occur right before, during, or after P/QRS complexes.
  • Causes include:
    • New bundle branch block.
    • Lead insulation break.
    • Electrolyte abnormalities.
    • Class IC antiarrhythmic agents. (Berberian 2021)
• Failure to pace:
  • ECG finding: Pacemaker fails to fire when it should be.
  • Causes include:
    • Oversensing.
    • Lead fracture.
    • Insulation defect.

[#4/4] diagnosis of ischemia

• Similar to LBBB, ischemia is diagnosed based on the Smith Modified Sgarbossa Criteria.  Concordance or discordance is based on comparison to the majority of the QRS complex.  Occlusive MI (OMI) is diagnosed if any of the following is found:
  • (1) Any lead with 1 mm of concordant STE
  • (2) Any of the leads V1-V3 with 1 mm of concordant STD.
  • (3) Any single lead with excessively discordant STE (STE >25% of the preceding S-wave)
• Changes compared to prior ECGs may also be instructive. 🌊

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questions & discussion

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