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

Contrary to right bundle branch blockleft 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 repolarizationwhich 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–V3accompanied by ST segment depressions in leads V5V6I and aVL. Clinicians frequently confuse these elevations and depressions with those caused by STEMI (STE-ACS). Indeedseveral 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 resultthe ischemic changes are obscured and do not become apparent. Howeverthere are exceptions to this rulewhich will be discussed below. To address these challengesresearchers 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. Howeverall such criteria remainat bestsuboptimal. Clinical judgment remains a more reliable tool for evaluating ischemia in these casesa 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 STEMIas it often presents with similar ECG changesincluding ST-segment elevationsST-segment depressionsand T-wave inversions. These overlapping features frequently lead to confusion between LBBB and acute STEMI. In factstudies 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 repolarizationwhich usually prevents other ST-T changes (such as those arising from ischemia) to come to expression on ECG. Thereforeischemic ST-T changes (ST elevationsST depressionsT-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 LBBBsuch as heart failurestructural heart diseasefibrosis of the conduction system and acute myocardial infarction (particularly anterior STEMI). Hencean acute myocardial infarction may actually result in LBBB which then masks the ischemic ST-T changes on ECG.

In summaryleft bundle branch block (LBBB) can result frommimicor obscure acute myocardial ischemia and infarctioncreating 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 therapywhich at the time was primarily fibrinolysis (Wilner et al.). Their findings revealed that a substantial number of these patients had complete coronary artery occlusionsand 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 yearsEuropean and North American guidelines recommended managing patients with symptoms of myocardial ischemia and new (or presumed new) LBBB as acute STEMI. Howeversubsequent studies found that this approach led to an unacceptably high rate of unnecessary catheterization laboratory activations. In responsethe 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. Insteadpatients with a high clinical suspicion of ongoing myocardial ischemiaregardless of ECG or biomarker findingsshould be treated similarly to those with clear STEMI. Particularlypatients who remain symptomatic despite initial medical therapyare hemodynamically unstableor develop sustained ventricular arrhythmias. Similarlythe 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 STEMIirrespective 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 criteriaknown as the Sgarbossa criteriaare 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%. Thuswhile the Sgarbossa criteria demonstrate high specificity for detecting acute myocardial infarctionthe sensitivity is notably lowmaking the criteria unreliable for the identification of acute STEMI.

Modified Sgarbossa criteria

The modified Sgarbossa criteriaintroduced 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. Furthermorethe modified Sgarbossa criteria do not utilize a point system; insteadit 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 wavewhichever is most prominentand 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. Hencewhereas the original Sgarbossa criteria utilize an absolute ST-elevation measurementthe modified criteria suggest using a rule of proportionality; the amplitude of the ST deviation is compared to the amplitude of the R or S wavewhich increases both sensitivity and specificity for acute myocardial infarction.

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 waveformspotentially mimicking or concealing signs of ischemia. Howeverit is reasonable to assess for abnormal waveforms within the specific context of LBBBLVHor paced rhythms. Examples of such abnormalities are outlined in the Sgarbossa criteria. Furthermoresome 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 instanceT-wave inversions in leads V5V6aVLand I are typical in left bundle branch block; their absence may suggest ischemia.
• Look for pseudonormalization of T-wavesas this can also be a marker of ischemia.