Acute anterior myocardial infarction and appearance of a left bundle branch block

ECG 2A: Sinus rhythm, normal PR interval; complete left bundle branch block pattern with wide QRS, delayed intrinsicoid deflection in V6, exclusive positive deflection in V6, QS pattern in V1, V2 with no delay in intrinsicoid deflection, left axis; repolarization disorders are substantial with elevation from V1 to V4; positive Sgarbossa and Smith criteria in V2-V3 (ST elevation of 5 or 6 mm and S wave of 8 or 10 mm; ratio > 0.25);

ECG 2B: The combination of typical chest pain and ECG pattern led to an emergency coronary angiography which showed proximal LAD thrombosis treated with thromboaspiration, angioplasty and stent placement; an ultrasound performed the next day revealed a left ventricular ejection fraction of 30%; the ECG on lead II showed a regression of the elevation with T-wave inversion in the anteroseptoapical territory; negative Sgarbossa and Smith criteria;

Comments: Different conduction disorders can be demonstrated in an acute coronary syndrome. A left anterior fascicular block, isolated or associated with a right bundle branch block (common arterial supply from the LAD branches) is the most common conduction disorder complicating anterior myocardial infarction. A left anterior fascicular block can also be observed during an inferior infarction and is thus a marker of multiple coronary lesions affecting in particular the LAD and the right coronary artery. The appearance of a left posterior fascicular block is a rarer complication of anterolateral infarction yielding a QS or QR pattern in leads I and aVL.

The diagnosis of coronary syndrome with evidence of a left bundle branch block is relatively rare and corresponds to approximately 2% of all acute coronary syndromes. Management in this setting is problematic since it is often impossible to determine the age of the bundle branch block.

If the left bundle branch block is chronic, it is obvious that its presence in the setting of chest pain cannot be considered as an equivalent of STEMI justifying a coronary angiography or emergency thrombolysis in itself without supplementary analysis of the electrocardiographic pattern. It has long been commonplace to assert that the electrocardiogram becomes uninterpretable in the presence of a left bundle branch block or in paced patients. The previous clinical case highlights the possibility of seeking specific electrocardiographic changes (Sgarbossa criteria) over and above the traditional bundle branch block aspect. The recent American recommendations (2013) have changed the approach to be taken, with the positivity of the Sgarbossa criteria along with loss of appropriate discordance (concordant ST-segment elevation, the most predictive item of the score) being required to correspond to a STEMI equivalent and undertake an invasive approach or emergency thrombolysis.

The occurrence of a complete left bundle branch block at the end of the acute phase of a myocardial infarction reflects the presence of an extensive anterior infarction, which is a factor of poor prognosis and justifies an emergency reperfusion procedure. Given the frequent impossibility of establishing the age of the left bundle branch block, the original or modified Sgarbossa criteria are used. However, these criteria being of limited sensitivity, even if the criteria are negative, an emergency revascularization should be proposed in the presence of an evocative clinical picture and a rise in cardiac enzymes.

Take-home message: The management of a patient with chest pain and left bundle branch block is problematic given the frequent difficulty in determining the age of the conduction disorder.