<P> The heart's electrical activity begins in the sinoatrial node (the heart's natural pacemaker), which is situated on the upper right atrium . The impulse travels next through the left and right atria and summates at the atrioventricular node . From the AV node the electrical impulse travels down the bundle of His and divides into the right and left bundle branches . The right bundle branch contains one fascicle . The left bundle branch subdivides into two fascicles: the left anterior fascicle and the left posterior fascicle . Other sources divide the left bundle branch into three fascicles: the left anterior, the left posterior, and the left septal fascicle . The thicker left posterior fascicle bifurcates, with one fascicle being in the septal aspect . Ultimately, the fascicles divide into millions of Purkinje fibres, which in turn interdigitise with individual cardiac myocytes, allowing for rapid, coordinated, and synchronous physiologic depolarization of the ventricles . </P> <P> When a bundle branch or fascicle becomes injured (by underlying heart disease, myocardial infarction, or cardiac surgery), it may cease to conduct electrical impulses appropriately . This results in altered pathways for ventricular depolarization . Since the electrical impulse can no longer use the preferred pathway across the bundle branch, it may move instead through muscle fibers in a way that both slows the electrical movement and changes the directional propagation of the impulses . As a result, there is a loss of ventricular synchrony, ventricular depolarization is prolonged, and there may be a corresponding drop in cardiac output . When heart failure is present, a specialized pacemaker may be used to resynchronize the ventricles . In theory a pacemaker like this will shorten the QRS interval, thus bringing the timing of contraction of the left and right ventricles closer together and slightly increasing the ejection fraction . </P> <P> A bundle branch block can be diagnosed when the duration of the QRS complex on the ECG exceeds 120 ms . A right bundle branch block typically causes prolongation of the last part of the QRS complex, and may shift the heart's electrical axis slightly to the right . The ECG will show a terminal R wave in lead V1 and a slurred S wave in lead I . Left bundle branch block widens the entire QRS, and in most cases shifts the heart's electrical axis to the left . The ECG will show a QS or rS complex in lead V1 and a monophasic R wave in lead I. Another normal finding with bundle branch block is appropriate T wave discordance . In other words, the T wave will be deflected opposite the terminal deflection of the QRS complex . Bundle branch block, especially left bundle branch block, can lead to cardiac dyssynchrony . The simultaneous occurrence of left and right bundle branch block leads to total AV block . </P> <P> Depending on the anatomical location of the defect which leads to a bundle branch block, the blocks are further classified into: </P>

Can you have a right and left bundle branch block