ECG rhythms Conduction, ST and QT
Left bundle branch block ECG
Left bundle branch block. Wide QRS (≥120 ms) with a deep QS or rS in V1–V3, and broad notched or slurred R waves without a septal q wave in I, V5 and V6; the ST segments and T waves point opposite to the main QRS deflection.
A 30-second recording of the heart model, played in real time. Run live starts the engine itself with this rhythm.
Computed by the TracingsMD heart model (Junkers engine): simulated for education, not a patient recording and not for diagnosis.
Measured on this recording
| Ventricular rate | 72/min |
|---|---|
| PR | 157 ms |
| QRS | 163 ms |
| QT | 443 ms |
| QTc (Bazett) | 486 ms |
| QTc (Fridericia) | 471 ms |
What it is
The left bundle branch does not conduct, so activation spreads from the right ventricle across the septum to the left. The QRS is 120 ms or more, with broad notched R waves in I, aVL, V5 and V6, a deep S wave in V1, and ST–T changes opposite to the main QRS.
How to read this strip
| Rate | 60–100/min. |
|---|---|
| Rhythm | Regular sinus. |
| P waves | Normal. |
| PR | Normal. |
| QRS | Wide (≥120 ms); deep QS or rS in V1–V3; broad notched or slurred R waves in I, aVL, V5 and V6 without a septal q wave. |
| ST–T | Discordant (secondary) change: ST elevation with upright T waves in V1–V3, ST depression with inverted T waves in I, aVL, V5 and V6 — opposite to the main QRS deflection. |
Causes
- Hypertension and left ventricular hypertrophy.
- Coronary artery disease.
- Dilated cardiomyopathy and heart failure.
- Aortic valve disease and transcatheter aortic valve replacement.
- Degenerative conduction disease; rarely in a healthy heart.
What to consider
- New LBBB deserves evaluation for structural heart disease, usually with echocardiography.
- LBBB hides ischemic changes: in suspected myocardial infarction, use specific criteria (such as modified Sgarbossa) with the clinical picture. New LBBB alone is no longer treated as a STEMI equivalent, but LBBB with ongoing ischemic symptoms, hemodynamic instability or positive Sgarbossa-type criteria still needs emergency reperfusion assessment.
- In heart failure with an ejection fraction of 35% or less and symptoms despite guideline-directed medical therapy, sinus rhythm with LBBB and a QRS of 150 ms or more is the strongest indication for cardiac resynchronization therapy (CRT).
- The wide QRS lengthens the QT, so the QTc overstates repolarization time in bundle branch block.
Key point
Read a wide QRS in V1 and V6 first: a mainly negative V1 with a broad notched R in V6 and I is LBBB; a terminal R′ in V1 with a wide S in I and V6 is RBBB. In LBBB, ST elevation opposite a deep S wave is expected; ischemia needs specific criteria such as Sgarbossa.
Not to be confused with
- Right bundle branch block — RBBB ends with a tall R′ in V1 and a wide S wave in I and V6. Here V1 is negative and I and V6 show broad, notched R waves.
- Anterior ST-elevation myocardial infarction — A wide QRS does not rule out STEMI, but the ST elevation in V1–V3 here is discordant: it sits opposite a deep S wave and is small compared with it, the expected secondary pattern of the bundle branch block, and there is no chest pain. In LBBB, acute MI is judged with specific criteria such as the (modified) Sgarbossa criteria, not the usual ST rules.
- Ventricular tachycardia — The rate is normal and a sinus P wave with a fixed PR precedes every QRS.