ECG criteria
- Baseline LBBB morphology: QRS ≥ 120 ms, wide notched R in I, aVL, V5–V6, deep S in V1–V2, secondary ST-T discordance (ST directed opposite to the main QRS wave)
- Concordant ST-segment elevation at the J-point ≥ 1 mm relative to the isoelectric line in a lead with a predominantly positive (upward) QRS complex — 5 points according to Sgarbossa
- Concordant ST-segment depression ≥ 1 mm in V1–V3 — 3 points according to Sgarbossa
- Discordant ST-segment elevation ≥ 5 mm in a lead with a negative (downward) QRS — 2 points according to Sgarbossa (sum ≥ 3 points is highly specific for acute coronary occlusion)
- Modified Smith criterion (replacing the third component): in the lead with discordant ST elevation, measure the J-point deviation from the isoelectric line with its sign and divide by the S-wave amplitude (also with its sign) — an ST/S ratio ≤ -0.25 is more sensitive than the absolute 5 mm threshold
- Dynamic assessment: comparison with previous ECGs in patients with known old LBBB helps to identify new ST-T changes
- Clinical context: new or unknown-onset LBBB with high clinical probability of ongoing ischemia requires immediate evaluation for potential acute coronary occlusion; LBBB alone is not an indication for reperfusion regardless of ECG signs
What it looks like on ECG
In LBBB, left ventricular depolarization occurs via the right ventricle, resulting in a wide QRS (≥ 120 ms), wide notched or slurred R waves in I, aVL, V5–V6 without Q waves, and deep S or QS waves in V1–V2. The ST segment and T wave in LBBB are normally discordant to the QRS complex: where the main R wave is, the ST segment is usually depressed and the T wave is negative; where the deep S wave is, the ST segment is elevated. These are secondary ST-T changes related to conduction disturbance rather than ischemia.
In MI with LBBB, look for deviations from this "expected" discordance across all 12 leads:
| Sign | Sgarbossa points | Where |
|---|---|---|
| Concordant ST elevation ≥ 1 mm at the J-point | 5 | any lead with a positive QRS |
| Concordant ST depression ≥ 1 mm | 3 | V1–V3 |
| Discordant ST elevation ≥ 5 mm | 2 | lead with a negative QRS (deep S wave) |
A total score ≥ 3 points increases the likelihood of acute coronary occlusion with high specificity, but low sensitivity. Therefore, the modified Smith criterion is additionally applied: the ratio of the magnitude of ST elevation to the depth of the preceding S wave (ST/S ≤ -0.25) — it captures cases with moderate but disproportionate discordance.
Mechanism
LBBB itself alters the direction of ventricular depolarization and, consequently, repolarization — resulting in secondary ST-T discordance (associated with conduction delay rather than ischemia). In acute ischemia/myocardial ischemic injury, frequently due to coronary occlusion, a current of injury in the affected area is added to the altered sequence of depolarization. If the resulting ST deviation matches the direction of the main QRS wave, concordance occurs, which is not physiologically expected in LBBB. Concordance and excessive discordance increase the likelihood of acute ischemia/coronary occlusion; definitive diagnosis of MI is based on troponin dynamics combined with clinical presentation and other signs of ischemia, although in clear cases of occlusion, reperfusion is not delayed while awaiting troponin results.
Causes
- Acute coronary artery occlusion of any location — a single ECG in the setting of LBBB usually cannot reliably identify the culprit artery
- New-onset LBBB in the setting of acute coronary syndrome
- Decompensation of coronary artery disease in a patient with chronic LBBB (due to dilated cardiomyopathy, Lenègre–Lev disease, post-infarction cardiosclerosis)
- Concomitant marked left ventricular hypertrophy, which enhances baseline discordance and complicates evaluation
Differential diagnosis
- Isolated LBBB without ischemia: ST-T discordance is proportional to QRS, there is no concordant elevation or depression, and no excessive discordant elevation (ST/S > -0.25)
- Hyperkalemia with QRS widening: accompanied by tall peaked T waves, P-wave flattening, absence of typical LBBB morphology
- Ventricular pacing (pacemaker ECG): similar secondary ST-T discordance, evaluated using the same Sgarbossa/Smith criteria (adapted for paced rhythm)
- LV hypertrophy with systolic strain: ST-T discordance is present in lateral leads, but in isolated hypertrophy QRS is usually < 120 ms; a duration ≥ 120 ms prompts searching for concomitant LBBB or another intraventricular conduction delay
- Old MI with LBBB without acute dynamics: ST-T changes are stable compared to old tracings
Clinical significance and management
LBBB masks classic STEMI signs; therefore, in a patient with ischemic chest pain and LBBB (new or unknown onset), Sgarbossa criteria and modified Smith criteria are applied alongside assessment of clinical probability of ongoing ischemia, hemodynamics, and serial ECGs. A Sgarbossa score ≥ 3 or a positive Smith criterion increases the likelihood of acute coronary occlusion — immediate activation of the emergency invasive pathway is indicated, similar to STEMI management. New or unknown-onset LBBB alone is not a STEMI equivalent and does not serve as an independent basis for immediate reperfusion: decisions are made based on the clinical picture, hemodynamics, ECG dynamics, and Sgarbossa/Smith criteria.
In the presence of persistent typical ischemic symptoms, especially with hemodynamic or electrical instability or high clinical probability of occlusion, immediate expert evaluation and emergency coronary angiography are indicated regardless of negative Sgarbossa/Smith criteria and troponin results — these criteria lack full sensitivity and do not rule out acute occlusion. Serial ECGs and dynamic troponin assessment are acceptable in a stable patient without ongoing ischemia. The final diagnosis of MI is confirmed by troponin dynamics combined with clinical data. Old, unchanged LBBB without ST-T dynamics and without clinical symptoms is not an indication for emergency revascularization.
Common interpretation errors
- Automated interpretation of "LBBB + chest pain = STEMI" without assessing clinical probability of occlusion and Sgarbossa/Smith criteria — leads to unjustified emergency reperfusion
- Ignoring LBBB with overt concordant ST elevation or depression as "just a block" — leads to delayed reperfusion
- Using only the absolute 5 mm threshold without calculating the ST/S ratio — missing probable occlusion with moderate discordance and a small S wave
- Failure to compare with previous ECGs in patients with known chronic LBBB
- Confusing LBBB criteria with pacemaker criteria without accounting for differences in morphology
1. How many points on the Sgarbossa scale are assigned to concordant ST elevation ≥ 1 mm in a lead with a positive QRS?
Concordant ST elevation is the most specific sign of probable acute coronary occlusion in LBBB, assigned the maximum weight of 5 points.
2. What ST/S ratio by the modified Smith criterion indicates probable acute coronary occlusion in LBBB?
The ratio of ST elevation magnitude to S-wave depth ≤ -0.25 in the lead with discordant ST is more sensitive than the fixed 5 mm threshold.
3. What determines the emergency invasive strategy in new-onset LBBB and ischemic chest pain?
New LBBB alone is not a STEMI equivalent; the decision for emergency coronary angiography is made based on the combination of clinical presentation, hemodynamics, and ECG criteria for probable occlusion.
Frequently asked questions
How to distinguish MI from uncomplicated LBBB on ECG?
In isolated LBBB, the ST segment and T wave are discordant with the QRS complex in proportion to its amplitude. In suspected MI, concordant ST elevation, concordant ST depression in V1–V3, or excessive discordant elevation not meeting Smith's ST/S ratio ≤ -0.25 criterion appears.
Can STEMI be diagnosed in newly detected LBBB?
New LBBB with a high probability of ongoing ischemia requires immediate expert evaluation and, in the presence of clinical and ECG signs of acute occlusion (Sgarbossa/Smith criteria, hemodynamic instability), emergency coronary angiography. The sheer novelty of LBBB does not dictate management or serve as an independent diagnostic criterion for STEMI.
Which is more sensitive — the Sgarbossa criteria or the modified Smith criteria?
The modified Sgarbossa rule replaces the third component (absolute discordant elevation threshold of 5 mm) with a proportional ST/S ratio threshold of ≤ -0.25, generally increasing sensitivity without a significant loss of specificity. The result is evaluated in conjunction with the two concordant criteria and the clinical picture, rather than as a standalone rule.
Is LBBB dangerous on its own without ischemia?
Chronic isolated LBBB does not always pose an immediate acute threat, but it warrants clinical evaluation and investigation for structural heart disease (CAD, cardiomyopathy, Lenègre–Lev disease), as it masks the ECG signs of acute ischemia. Urgent evaluation is necessary in the event of new symptoms, instability, or suspected ischemia.
Are the Sgarbossa criteria applicable in ventricular pacing?
Yes, using the same principles of concordance and ST/S ratio, but studies on their validity in paced rhythms are less extensive than in LBBB.
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Related terms
More in the section "Ischemia and Myocardial Infarction"
Sources
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice, 6th ed.
- Goldberger A.L. Clinical Electrocardiography: A Simplified Approach
- 2023 ESC Guidelines for the management of acute coronary syndromes
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the ECG, 2009
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.