Concordant and Discordant ST Changes in LBBB Associated with Acute Coronary Syndrome (ACS): Understanding Modified Sgarbossa Criteria.

 Dr. Zayed | Published : 07, May 2025.

Introduction

Diagnosing acute myocardial infarction (AMI) in the presence of left bundle branch block (LBBB) remains a major clinical challenge. LBBB alters both depolarization and repolarization, leading to secondary ST-T changes that can obscure or mimic signs of acute ischemia. This makes standard STEMI criteria unreliable, necessitating specialized tools like the Sgarbossa and Modified Sgarbossa Criteria.

What Is LBBB and Why It Matters

LBBB occurs when electrical conduction through the left bundle branch is delayed or blocked. As a result:

  • The left ventricle depolarizes later than the right

  • QRS complexes become wide (>120 ms)

  • ST segments and T waves are secondarily altered

These changes disrupt the typical ECG patterns of myocardial infarction, especially ST elevation, leading to potential under- or over-diagnosis.

Pathophysiological causes of LBBB include:

  • Myocardial infarction, especially involving the anterior wall.

  • Left ventricular hypertrophy, commonly due to chronic hypertension or aortic stenosis.

  • Dilated and infiltrative cardiomyopathies.

  • Degenerative conduction system disease (Lenègre and Lev diseases).

  • Iatrogenic injury during procedures such as transcatheter aortic valve replacement (TAVR) or septal myectomy.

Understanding Concordant vs. Discordant ST Changes

These terms describe the directional relationship between the QRS complex and the ST segment:

  • Concordant = ST segment and QRS go in the same direction

    • Positive QRS + ST elevation

    • Negative QRS + ST depression

  • Discordant = ST segment and QRS go in opposite directions

    • Positive QRS + ST depression

    • Negative QRS + ST elevation

In LBBB, discordant ST changes are generally expected due to altered ventricular depolarization and repolarization. However, when these changes become excessive, they may reflect acute transmural ischemia.


Original Sgarbossa Criteria (1996)

Sgarbossa et al. developed a scoring system to aid in the diagnosis of AMI in the presence of LBBB:

  1. Concordant ST elevation ≥1 mm in leads with a positive QRS complex (5 points)

  2. Concordant ST depression ≥1 mm in V1–V3 (3 points)

  3. Excessively discordant ST elevation ≥5 mm in leads with a negative QRS complex (2 points)

A score ≥3 was considered diagnostic of AMI with high specificity (~90%), though sensitivity remained modest.

Modified Sgarbossa Criteria (Smith et al., 2012)

Smith et al. proposed an improvement that replaces the fixed 5 mm threshold with a proportional rule to better account for baseline discordance:

  1. Concordant ST elevation ≥1 mm in any lead with a positive QRS

  2. Concordant ST depression ≥1 mm in V1–V3

  3. Excessively discordant ST elevation defined as ST/S ratio ≥0.25 in leads with a negative QRS

This adjustment significantly enhanced sensitivity while preserving specificity, allowing more timely identification of patients requiring reperfusion therapy.

Clinical Implications in ACS

In patients presenting with chest pain and LBBB:

  • Concordant ST elevation or depression is rare and almost always pathologic

  • Excessively discordant ST elevation (ST/S ≥0.25) is also abnormal and suggests transmural ischemia

  • Discordant ST depression, unless excessive or concordant in V1–V3, is usually not diagnostically useful

According to the ACC/AHA guidelines, a patient with new or presumably new LBBB and ischemic symptoms should be treated as having a STEMI equivalent.

Conclusion

Recognizing concordant and excessively discordant ST changes in LBBB is critical for accurate diagnosis of ACS. The Modified Sgarbossa Criteria offer a scientifically robust framework that enhances diagnostic accuracy and facilitates timely intervention in ambiguous ECG presentations.

References: 

  1. Smith SW et al. Diagnosis of ST Elevation Myocardial Infarction in the Presence of Left Bundle Branch Block using the ST Elevation to S-Wave Ratio in a Modified Sgarbossa Rule. Annals of Emergency Medicine 2012;60:766-776
  2. Meyers HP, Limkakeng AT Jr, Jaffa EJ, Patel A, Theiling BJ, Rezaie SR, Stewart T, Zhuang C, Pera VK, Smith SW. Validation of the modified Sgarbossa criteria for acute coronary occlusion in the setting of left bundle branch block: A retrospective case-control study. Am Heart J. 2015;170(6):1255-1264.

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