Anterior infarction in a patient with a pre-existing left bundle branch block

ECG 1A: Sinus rhythm, normal PR interval; typical pattern of complete left bundle branch block: wide QRS (140 ms), delayed intrinsicoid deflection in V6, exclusive positive deflection in V6, QS pattern in V1, V2 without delayed intrinsicoid deflection, left axis; the repolarization disorders appear to be aspecific with weak elevation from V1 to V3 and depression in V6 with negative T-wave; negative Sgarbossa and Smith criteria (elevation in V2 < 5mm with negative QRS in V2);

ECG 1B: Emergency hospitalization a few weeks after this first tracing for typical retrosternal, uninterrupted chest pain since 3 hours; significant change in the ECG compared to the previous recording with a left bundle branch block pattern and significant elevation from V1 to V3, less pronounced in leads III and aVF; depression in V5-V6 and negative deep T-waves in leads I, aVL, inferior territory and from V4 to V6; Sgarbossa criterion at the limit of positivity in V2: elevation at 5 mm in V2; Smith criterion (modified Sgarbossa) at the limit of positivity in V2 (S wave: 20 mm, elevation 5 mm ; ratio: 0.25);

ECG 1C: This patient underwent emergency coronary angiography, which showed thrombosis of the median LAD treated by thromboaspiration, angioplasty and stent placement; this tracing was recorded upon return to the room following coronary angiography; a left bundle branch block pattern is found with a significant decrease in the amplitude of the elevation; negative Sgarbossa and Smith criteria;

Comments: Upon arrival at the emergency ward for the evaluation of chest complaints, the electrocardiogram usually allows classifying myocardial infarctions as either an acute coronary syndrome with hyperacute ST-segment elevation requiring immediate reperfusion or acute coronary syndrome without hyperacute ST-segment elevation whose management is more variable. In the presence of a left bundle branch block, the diagnosis of infarction becomes more difficult and rests on electrical abnormalities that are both inconsistent and weakly specific. Indeed, the complete left bundle branch block disrupts all the ventricular depolarization and repolarization vectors with a tendency of the ST-segment and T-waves to be deviated in a discordant direction relative to the QRS-complexes mimicking a conventional pattern of ischemia or lesion. Similarly, an evocative aspect of necrosis (poor R wave and/or Q-wave progression in the right inferior and precordial leads) can be observed in the absence of acute or chronic coronary artery disease. In the acute phase, the electrocardiogram can nevertheless highlight highly suggestive changes in ventricular repolarization, with a marked and evolving pattern of subepicardial ischemic lesion providing a near-certain diagnosis by analyzing different successive tracings. In light of an evocative clinical picture of acute coronary syndrome, a strong upward convex overestimation in right precordial leads and followed by a more or less ischemic-like negative T-wave is suggestive of an anteroseptal infarction; a similar pattern in inferior leads is suggestive of a posteroinferior infarction whereas the positive deflection of the T-wave in leads I and aVL is atypical in the left bundle branch block. In left precordial leads, it is more the ST-segment elevation than the inverted T-wave that suggests the lateral or anterolateral infarction.

The Sgarbossa criteria (published by Elena Sgarbossa in 1996) have long been used for diagnosing acute phase myocardial infarction and to guide the decision to perform emergency revascularization in a patient presenting a left bundle branch block. The 3 Sgarbossa criteria are not based on a dynamic analysis of the electrocardiographic changes (need for a rapid diagnosis and most often absence of a reference tracing) but based on the loss of appropriate discordance:

• Presence of a ST-segment elevation ≥ 1 mm when QRS-complexes are positive («positive concordance»)
• Presence of ST-segment depression ≥ 1 mm in V1, V2 or V3 when QRS-complexes are negative («negative concordance»)
• Presence of ST-segment elevation or depression ≥ 5 mm in presence of negative or positive QRS-complexes («excessive discordance », +/- or -/+)

The first two criteria have a high specificity but a lower sensitivity. The third criterion «excessive discordance » has a low specificity in that it can be observed in patients without ischemic heart disease when the S wave is of high amplitude. The value of 5 mm is arbitrary and probably not very specific.

To improve the diagnostic performance and in particular the specificity, the Smith criteria (modified Sgarbossa) were proposed in 2012. They are not solely based on the presence and amplitude of ST-segment elevation or depression but on the ratio between the amplitude of the S wave and of the ST-segment in septal precordial leads (V1-V3). Indeed, the amplitude of the elevation must be indexed to the amplitude of the S wave (probable infarction if major elevation with shallow S wave and inversely less probability of infarction if major elevation with high-amplitude S wave).

The Smith criteria are positive and the diagnosis of infarction is probable when there is:

• ST-segment elevation with an ST/S ratio > 0.25 (amplitude of the elevation greater than one quarter of the size of the S wave) in at least one lead
• ST-segment depression in the lateral or inferior leads with a ST/R ratio > 0.3 (amplitude of the depression greater than 30% of the preceding R wave)

The changes in QRS are even more difficult to interpret in defining the presence of a prior infarction ; they can be missed and an extensive necrosis can be masked by the presence of a left bundle branch block. In contrast, certain abnormalities are observed in the absence of coronary artery disease.

In a patient presenting an acute anteroseptal infarction, the diagnosis of a prior infraction cannot be based on the QS pattern in right precordial leads which is found in the absence of coronary artery disease in a patient with left bundle branch block. A persistent poor R wave progression up to V4 or V5 has a slightly higher diagnostic value.

The presence of a Cabrera’s sign (notching of at least 50 ms of the ascending branch of the S wave in leads V3 or V4) is inconsistent (low sensitivity) but relatively specific. A broad and early notching of the downward branch of the S wave on the right of the transition zone is also evocative.

In a patient presenting a posteroinferior acute infarction, the diagnosis of a prior infarction cannot be based on the QS aspect in lead III which is common in the left bundle branch block. The presence of a Q-wave in lead II or aVF is more evocative.

In a patient presenting a lateral infarction, the presence of deep S waves in left precordial leads and in leads I and aVL is evocative given its presence on several consecutive leads.

Take-home message: in a patient with a left bundle branch block, the diagnosis of acute myocardial infarction should be evoked in the presence of 1) a clinical situation indicative of infarction 2) a significant increase in cardiac enzymes 3) ST-segment abnormalities of «abnormal» magnitude for a left bundle branch block. The Sgarbossa criteria are based on the loss of appropriate discordance. They have a limited sensitivity and should be used with caution; the presence of a characteristic symptomatology should warrant performing a coronary angiography.