(See also Overview of Arrhythmias Overview of Arrhythmias The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more 
.)
Conduction blocks (see figure ) can be caused by many heart disorders, including intrinsic degeneration without another associated heart disorder.
Electrical pathway through the heart
The sinoatrial (sinus) node (1) initiates an electrical impulse that flows through the right and left atria (2), making them contract. When the electrical impulse reaches the atrioventricular node (3), it is delayed slightly. The impulse then travels down the bundle of His (4), which divides into the right bundle branch for the right ventricle (5) and the left bundle branch for the left ventricle (5). The impulse then spreads through the ventricles, making them contract.  |
Right bundle branch block (RBBB—see figure Right bundle branch block Right bundle branch block 
) can occur in people with no evidence of heart disease. It may also occur with anterior myocardial infarction Acute Myocardial Infarction (MI) Acute myocardial infarction is myocardial necrosis resulting from acute obstruction of a coronary artery. Symptoms include chest discomfort with or without dyspnea, nausea, and/or diaphoresis... read more 
, indicating substantial myocardial injury. New appearance of RBBB should prompt a search for underlying cardiac pathology, but often, none is found. Transient RBBB may occur after pulmonary embolism Pulmonary Embolism (PE) Pulmonary embolism (PE) is the occlusion of pulmonary arteries by thrombi that originate elsewhere, typically in the large veins of the legs or pelvis. Risk factors for pulmonary embolism are... read more 
. Although RBBB distorts the QRS complex, it does not significantly interfere with ECG diagnosis of myocardial infarction.
Right bundle branch block
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Left bundle branch block (LBBB—see figure Left bundle branch block Left bundle branch block 
) is associated with a structural heart disorder more often than is RBBB. LBBB usually precludes use of ECG for diagnosis of myocardial infarction.
Left bundle branch block
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Fascicular block involves the anterior or posterior fascicle of the left bundle branch. Interruption of the left anterior fascicle causes left anterior hemiblock characterized by modest QRS prolongation (< 120 millisecond) and a frontal plane QRS axis more negative than −30° (left axis deviation). Left posterior hemiblock is associated with a frontal plane QRS axis more positive than +120°. The associations between hemiblocks and a structural heart disorder are the same as for LBBB.
Hemiblocks may coexist with other conduction disturbances: RBBB and left anterior or posterior hemiblock (bifascicular block); and left anterior or posterior hemiblock, RBBB, and 1st-degree atrioventricular (AV) block (incorrectly called trifascicular block; 1st-degree block is usually AV nodal in origin).
Trifascicular block refers to RBBB with alternating left anterior and left posterior hemiblock or alternating LBBB and RBBB. Presence of bifascicular or trifascicular block after myocardial infarction implies extensive cardiac damage.
Bifascicular blocks require no direct treatment unless intermittent 2nd- or third-degree AV block is present. True trifascicular blocks require immediate, then permanent pacing.
Nonspecific intraventricular conduction defects are diagnosed when the QRS complex is prolonged (> 120 millisecond), but the QRS pattern is not typical of LBBB or RBBB. The conduction delay may occur beyond the Purkinje fibers and result from slow cell-to-cell myocyte conduction. No specific treatment is indicated.
