Left bundle branch block (LBBB) in acute myocardial infarction (AMI): clinical implications & Sgarbossa criteria

Contrary to right bundle branch block, left bundle branch block is always a pathological finding that affects cardiovascular and total mortality. Left bundle branch block is more common in individuals with structural and ischemic heart disease. Assessment of ischemia on ECG is difficult in the presence of left bundle branch block. This is because left bundle branch block causes substantial changes in left ventricular depolarization and repolarization, which result in secondary ST-T changes. Such ST-T changes may mimic (simulate) or mask ischemia.

Simulation of ischemia manifests as ST segment elevations in leads V1–V3, accompanied by ST segment depressions in leads V5, V6, I and aVL. Clinicians frequently confuse these elevations and depressions with those caused by STEMI (STE-ACS). Indeed, several studies have shown that the majority of patients inappropriately referred to the catheterization laboratory with suspicion of STEMI (STE-ACS) have left bundle branch block.

Masking of ischemia occurs because the ST-T changes induced by a left bundle branch block are more pronounced than those caused by ischemia. As a result, the ischemic changes are obscured and do not become apparent. However, there are exceptions to this rule, which will be discussed below. To address these challenges, researchers have developed ECG criteria designed to identify ischemia in the presence of left bundle branch block. The most widely recognized criteria to date were developed by Sgarbossa and colleagues and are therefore referred to as the Sgarbossa criteria. However, all such criteria remain, at best, suboptimal. Clinical judgment remains a more reliable tool for evaluating ischemia in these cases, a point emphasized in recent guidelines.

Implications of left bundle branch block in myocardial ischemia and infarction

A summary of the issues that arise when facing a patient with left bundle branch block (LBBB) and symptoms of ischemia follows:

• Left bundle branch block (LBBB) can mimic acute STEMI, as it often presents with similar ECG changes, including ST-segment elevations, ST-segment depressions, and T-wave inversions. These overlapping features frequently lead to confusion between LBBB and acute STEMI. In fact, studies have shown that LBBB is the most common cause of false activations of the catheterization laboratory.
• LBBB may mask (conceal) ongoing ischemia: LBBB causes severe disturbance of ventricular repolarization, which usually prevents other ST-T changes (such as those arising from ischemia) to come to expression on ECG. Therefore, ischemic ST-T changes (ST elevations, ST depressions, T-wave changes) are typically concealed in the setting of LBBB. A patient with acute STEMI may therefore display a normal LBBB pattern.
• LBBB may be caused by ischemia/infarction: There are numerous causes of LBBB, such as heart failure, structural heart disease, fibrosis of the conduction system and acute myocardial infarction (particularly anterior STEMI). Hence, an acute myocardial infarction may actually result in LBBB which then masks the ischemic ST-T changes on ECG.

In summary, left bundle branch block (LBBB) can result from, mimic, or obscure acute myocardial ischemia and infarction, creating significant diagnostic challenges. These complexities led researchers to study patients presenting with LBBB and suspected acute myocardial infarction (AMI) by referring them for urgent reperfusion therapy, which at the time was primarily fibrinolysis (Wilner et al.). Their findings revealed that a substantial number of these patients had complete coronary artery occlusions, and outcomes improved when they were treated as acute STEMI cases.

Management of left bundle branch block (LBBB) in patients with acute coronary syndromes (ACS)

For many years, European and North American guidelines recommended managing patients with symptoms of myocardial ischemia and new (or presumed new) LBBB as acute STEMI. However, subsequent studies found that this approach led to an unacceptably high rate of unnecessary catheterization laboratory activations. In response, the most recent North American guidelines (O’Gara et al.) advise that new (or presumed new) LBBB should not be considered diagnostic of AMI in isolation. Instead, patients with a high clinical suspicion of ongoing myocardial ischemia, regardless of ECG or biomarker findings, should be treated similarly to those with clear STEMI. Particularly, patients who remain symptomatic despite initial medical therapy, are hemodynamically unstable, or develop sustained ventricular arrhythmias. Similarly, the 2023 European Society of Cardiology (ESC) guidelines were updated to recommend that patients presenting with LBBB or RBBB and signs or symptoms strongly indicative of ongoing myocardial ischemia should be treated as having definitive STEMI, irrespective of whether the bundle branch block is previously documented (Byrne et al.).

Sgarbossa’s ECG criteria for detecting ischemia in the presence of left bundle branch block (LBBB)

It is evident why researchers have faced challenges establishing ECG criteria for diagnosing acute STEMI in the presence of left bundle branch block (LBBB). Among the most useful and well-validated criteria are those developed by Sgarbossa and colleagues (Neeland et al.). These criteria, known as the Sgarbossa criteria, are summarized in Figure 1A. Each criterion gives 2 to 5 points. Studies show that a cut-off of ≥3 points yields a sensitivity of 20–36% and a specificity of 90–98%. Thus, while the Sgarbossa criteria demonstrate high specificity for detecting acute myocardial infarction, the sensitivity is notably low, making the criteria unreliable for the identification of acute STEMI.

Modified Sgarbossa criteria

The modified Sgarbossa criteria, introduced by Smith et al (2012), replaces the third of the original Sgarbossa criteria (i.e the absolute 5 mm ST elevation) with an ST/S ratio less than -0.25. Measurement of the ST/S ratio is depicted in Figure 1B. Using this criterion improves the accuracy of the Sgarbossa criteria. Furthermore, the modified Sgarbossa criteria do not utilize a point system; instead, it only requires 1 of 3 criteria to be considered positive (i.e acute ischemia is strongly suggested).

Measuring ST/S ratio

Measure the amplitude of the R or S wave, whichever is most prominent, and ST segments (relative to the PR segment), to the nearest 0.5 mm. The ST/S ratio is calculated for each lead with a discordant ST deviation of ≥1 mm. Hence, whereas the original Sgarbossa criteria utilize an absolute ST-elevation measurement, the modified criteria suggest using a rule of proportionality; the amplitude of the ST deviation is compared to the amplitude of the R or S wave, which increases both sensitivity and specificity for acute myocardial infarction.

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Infarction criteria in patients with left bundle branch block (LBBB), left ventricular hypertrophy (LVH) and pacemaker rhythms

European and North American guidelines recommend against applying standard ECG criteria for ischemia or infarction in the presence of left bundle branch block (LBBB), left ventricular hypertrophy (LVH), or ventricular pacemaker rhythm. These conditions significantly alter the QRS complex and ST-T waveforms, potentially mimicking or concealing signs of ischemia. However, it is reasonable to assess for abnormal waveforms within the specific context of LBBB, LVH, or paced rhythms. Examples of such abnormalities are outlined in the Sgarbossa criteria. Furthermore, some general recommendations can be provided:

• Always compare the current ECG waveforms with previous recordings to identify any differences. Variations in the ST-T segment may indicate ischemia.
• Ensure the ECG waveforms align with the expected patterns for the underlying condition. For instance, T-wave inversions in leads V5, V6, aVL, and I are typical in left bundle branch block; their absence may suggest ischemia.
• Look for pseudonormalization of T-waves, as this can also be a marker of ischemia.