Written by Dr. Karen Xiao, MD and Dr. Eric Tang, MD
Last updated June 13, 2025
Introduction
Bundle branch blocks (BBBs) and intraventricular conduction delays (IVCD) alter ventricular depolarization, distorting the QRS complex. In emergency medicine, recognizing these patterns may help avoid false STEMI activations and uncover underlying structural or metabolic disease.
Left Bundle Branch Block (LBBB)
ECG Findings
- QRS duration > 120 ms
- Dominant S wave in V1-V2
- Broad, monophasic R wave in I, aVL, V5–V6 (“M-shaped”)
- R wave peak time > 60 ms in V5–V6
ECG Pearl : The presence of septal Q waves in I, aVL, V5–V6 is abnormal in LBBB — don’t ignore this. This may be due to MI (acute or old).
Classic LBBB
Pathophysiology
LBBB reflects delayed activation of the LV. The impulse travels RV → LV, distorting both depolarization and repolarization. This loss of early septal depolarization explains absent Q waves and the tall, late R waves in lateral leads.
ECG Pearl: The transition zone shifts rightward — in other words - R wave progression ( QRS complexes shifting from S wave to R wave) is delayed toward V4–V6. If you see early transition in V2–V3, question the diagnosis.
Associated Causes of LBBB
Category | Cause |
Structural Heart Disease | - Ischemic heart disease
- Dilated cardiomyopathy
- Hypertension-induced LVH |
Valvular Disease | - Aortic stenosis (chronic pressure overload) |
Degenerative Conduction Disease | - Lenègre–Lev disease (idiopathic fibrosis of the conduction system) |
Electrolyte/Drug | - Hyperkalemia
- Digoxin toxicity |
Other | - Post-TAVR or post-aortic valve surgery |
ECG Pearl: Don’t overdiagnose LBBB. Some wide-appearing QRS with lateral ST changes are just early repolarization or left ventricular hypertrophy — confirm QRS >120 ms and rule out competing causes.
STEMI Mimic? Look for Discordance
Abnormal depolarization leads to abnormal repolarization. Discordance is when the ST segments are in the opposite direction of the QRS complex. This leads to
- ST depression in leads with tall R waves
- ST elevation in leads with deep S waves (e.g., V1–V3) — if <25% the preceding S wave depth (Smith-modified Sgarbossa).
- A new LBBB alone is not a STEMI equivalent. This is supported by 2025 AHA guidelines.
Concordance is pathologic in LBBB and concerning for MI
Concordance is when the ST segment and qrs directions are the same. There will be STE with tall R waves and STD in leads with deep S waves. This is abnormal and concerning for ischemia.
ECG Pearl : New LBBB ≠ STEMI equivalent. That myth is outdated — most new LBBBs are not caused by acute MI. Use Smith-modified Sgarbossa (especially an ST/S ratio ≥ 25%) to identify true STEMI in the setting of LBBB. Don’t activate the cath lab unless you see concordant ST changes or appropriately discordant ST elevation.
Right Bundle Branch Block (RBBB)
ECG Findings
- QRS duration > 120 ms
- Dominant R / RSR’ pattern (M-shaped QRS) in V1–V2
- Wide, slurred S wave in lateral leads (I, aVL, V5–V6)
- ST Depression with TWI in V1-V3
ECG Pearl: RSR’ in V1 alone isn’t enough — it must be pathologic (QRS >120 ms) and accompanied by slurred S waves laterally. Up to 5% of healthy people may have benign RSR’!
Classic RBBB
Pathophysiology
The LV depolarizes normally, but the RV lags, creating a second R wave (R’) in right precordial leads. Lateral leads show wide S waves from delayed RV activation.
Associated Causes of RBBB
Category | Cause |
Normal Variant | - Seen in up to 5% of healthy individuals |
Right Heart Strain | - Pulmonary embolism
- Chronic pulmonary disease (cor pulmonale) |
Congenital | - Atrial septal defect (esp. ostium secundum) |
Ischemia | - RV infarct or proximal LAD occlusion |
Degenerative or Infiltrative Disease | - Sarcidosis, amyloidosis, fibrosis |
Electrolyte/Drug | - Hyperkalemia, class I antiarrhythmics |
Miscellaneous | - Myocarditis
- Brugada syndrome (RBBB pattern with ST elevation in V1–V3) |
ECG Pearl: Not all RSR’ is RBBB. If you see saddleback, coved, or convex ST elevation in V1–V2, think Brugada or anterior STEMI. RBBB shouldn’t distort ST segments that much.
Intraventricular Conduction Delay (IVCD)
ECG Findings
- QRS > 120 ms
- No clear LBBB or RBBB morphology
- Variable or nonspecific QRS shape
IVCD in TCA toxicity (from LITFL.com)
ECG Pearl: IVCD is a diagnosis of exclusion. If you identify it — you must explain why it’s not LBBB or RBBB. Lazy interpretation leads to missed diagnoses.
Pathophysiology
Global conduction delay from structural heart disease, metabolic derangements, or toxins. Unlike BBBs, depolarization is diffuse, not regional. IVCD is associated with ~2x increased risk of all-cause death given higher risk of arrhythmia.
Associated Causes of IVCD
Category | Cause |
Electrolyte | - Hyperkalemia (esp. with sine-wave QRS) |
Drug Toxicity | - Tricyclic antidepressants (TCA)
- Flecainide, Class IC antiarrhythmics |
Structural Disease | - Severe cardiomyopathy
- Extensive ischemia
- LV aneurysm |
Conduction System Disease | - Diffuse conduction fibrosis
- Post-cardiac surgery |
Pacemaker or Aberrant Conduction | - Malfunctioning ventricular pacer or fusion beats |
ECG Pearl: If IVCD is new or shows bizarre QRS shapes, assume danger — dig into potassium, pH, toxins, and ischemia. This isn’t just a wide QRS; it’s a clue.
ECG Examples: What’s the Conduction Pattern?
ECG 1
ECG 2
ECG 3
ECG 4
ECG 5
ECG 6
ECG 7
ECG 8
ECG 9
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Key Takeaways
- LBBB = Don’t confuse it with LVH or early repol — confirm QRS >120 ms.
- New LBBB ≠ STEMI equivalent. Use Smith-modified Sgarbossa to detect ischemia. Concordant ST changes are pathologic.
- RBBB = QRS >120 ms + RSR’ in V1–V3 + slurred S in I/V6. Isolated RSR’ in V1 can be normal in up to 5% of healthy people.
- IVCD = wide QRS without classic bundle features. Think hyperK, Na⁺ blockers, ischemia, or structural disease — not just “nonspecific.”
Resources
- Antzelevitch C, Brugada P, Borggrefe M, et al. Brugada syndrome: report of the second consensus conference. Circulation. 2005;111(5):659–670. doi:10.1161/01.CIR.0000152479.54298.51
- Burns E, Buttner R. Right and Left Bundle Branch Block. Life in the Fast Lane – ECG Library. October 2023. https://litfl.com/ecg-library/bundle-branch-blocks
- Chou TC, Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th ed. Elsevier Saunders; 2008.
- Davila D. The ECG. Self-published; 2024.
- Garcia TB. 12-Lead ECG: The Art of Interpretation. 2nd ed. Jones & Bartlett Learning; 2013.
- Houthuizen P, Van Garsse LA, Poels TT, et al. Left bundle-branch block induced by transcatheter aortic valve implantation increases risk of death. J Am Coll Cardiol. 2014;63(18):1808–1815. doi:10.1016/j.jacc.2014.01.071
- LITFL. Tricyclic Overdose (Sodium Channel Blocker Toxicity). Life in the Fast Lane Toxicology Library. Accessed June 2025. https://litfl.com/tricyclic-overdose-sodium-channel-blocker-toxicity/
- Mattu A, Brady WJ. ECGs for Emergency Physicians, Vols. 1 & 2. American College of Emergency Physicians (ACEP); 2003, 2008.
- Nathanson LA, McClennen S, Safran C, Goldberger AL. ECG Wave-Maven: Self-Assessment Program for Students and Clinicians. Beth Israel Deaconess Medical Center. http://ecg.bidmc.harvard.edu
- Smith SW, Dodd KW, Henry TD, Dvorak DM. Diagnosis of ST-Elevation Myocardial Infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule. Ann Emerg Med. 2012;60(6):766–776. doi:10.1016/j.annemergmed.2012.06.032
- Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th ed. Elsevier Saunders; 2008.