Pathophysiology
Electrical impulses in the heart are generated in the right atrium by the sinoatrial (SA) node, stimulate the contraction of atrial myocardium, then propagate through the intranodal tracts to the AV node in the interatrial septum. From here, the impulses travel down the Bundle of His – a collection of specialised cardiac conduction cells that connect the AV node to the ventricles. The Bundle of His is divided into left and right bundles, and the left bundle is further divided into anterior and posterior fascicles (see figure below). The left anterior fascicle (LAF) conducts the impulses towards the anterior and upper part of the left ventricle, while the left posterior fascicle (LPF) conducts impulses to the posterior and inferior areas. Electrical conduction continues through a network of filaments called Purkinje fibres resulting in the coordinated contraction of the ventricles.
Chronic bifascicular blocks are usually associated with structural heart disease (50-80%), ischaemic heart disease (40-60%) or congenital heart disease and fibrosis of the cardiac conduction system. blocks are usually associated with structural heart disease (50-80%), ischaemic heart disease (40-60%) or congenital heart disease and fibrosis of the cardiac conduction system1.
New onset bifascicular block with chest pain can be a manifestation of acute occlusion of the proximal left anterior descending artery (LAD), causing anterior wall myocardial infarction. In up to 30% of cases, the ECG will not show ST elevation and acute bifascicular block will be the only ECG change seen6.
Medications including digoxin, beta blockers and calcium channel blockers should always be considered as a cause. Electrolyte imbalance, particularly hyperkalemia, can also lead to bifascicular block.
Learning bite
Chest pain with a new bifascicular block on ECG suggests acute myocardial infarction due to proximal LAD occlusion. These patients need admission, cardiac monitoring and discussion with cardiology.