IntroductionThe heart has an intricate electrical system responsible for ensuring the most efficient sequence of cardiac contraction. Any change in this electrical conduction system can affect cardiac output. Show This article will focus on the physiology, pathophysiology, and clinical consequences of damage to the bundle branches. You might also be interested in our medical flashcard collection which contains over 1000 flashcards that cover key medical topics. The cardiac conduction systemThe electrical impulse travels through the heart via a specific conduction pathway. The sinoatrial node (SAN) acts as the initial pacemaker before the impulse spreads throughout the atria and towards the atrioventricular node (AVN). The depolarisation wave travels through the heart’s septum via the Bundle of His and Purkinje fibres. These are organised into the left and right bundle branches. The right bundle branch then depolarises the right ventricle and the left bundle branch depolarises the left ventricle simultaneously. The septum itself is depolarised by the left bundle branch, meaning the septum is depolarised from left to right. For more information, see the Geeky Medics guide to the heart’s conduction system. ECG basicsThe ECG is a graphical representation of the net direction of electrical depolarisation in the heart at any one time.3 Different leads look at the heart from different angles (most important to know for understanding bundle branch blocks is that V1 views the heart from the right and V6 from the left). An upwards spike means the net depolarisation is heading towards that lead. A downward spike means the net depolarisation is heading away from that lead. There is a greater muscle mass on the left side of the heart compared to the right, so depolarisation within the left ventricles has a greater impact on the ECG trace. The right and left ventricles should depolarise simultaneously, producing one uniform R wave. Ventricular depolarisation using normal pathways is complete within 120ms. When these pathways are disrupted or changed in any way, depolarisation takes longer, causing broad QRS complexes. A broad QRS complex always indicates abnormal ventricular depolarisation.3 For more information on ECG interpretation, see the Geeky Medics guides to reading an ECG and understanding an ECG. Table 1. Components of an ECG trace.
In normal cardiac conduction (Figure 1):
The main feature of bundle branch blocks is the broadening of QRS complexes. It is therefore important to ensure other causes of broad complexes are excluded. For more information see the Geeky Medics guide to atrioventricular blocks. As the problem is below the atria, the P waves and the PR intervals are normal. Right bundle branch block (RBBB)The diagnostic criteria for RBBB are:2
A quicker simpler way to recognise and interpret this in a clinical setting is to use the MarRoW mnemonic. Right bundle branch block: MaRroWComplexes in V1 resemble the letter M (the first letter of MaRroW) and complexes in V6 resemble a W (the sixth letter of MaRroW) In right bundle branch block (Figure 3):
RBBB can be either physiological or the result of damage to the right bundle branch. Causes of damage include underlying lung pathology (COPD, pulmonary emboli, cor pulmonale), primary heart muscle disease (ARVC), congenital heart disease (e.g. ASD), ischaemic heart disease and primary degeneration of the right bundle. Left bundle branch block (LBBB)When viewed from the right-hand side (V1), net depolarisation travels away (towards the left), resulting in negative ECG deflections. The first downward deflection represents the right ventricle, and the slightly delayed 2nd downward deflection corresponds to the depolarisation of the left ventricle. When viewed from the left-hand side (V6), where the net depolarisation is travelling towards the detector, deflections are positive on the ECG. Again, there will be two peaks (RR) due to the delay in left ventricular depolarisation.1 In left bundle branch block (Figure 4):
The diagnostic criteria for LBBB are:
A quicker simpler way to recognise and interpret this in a clinical setting is to use the WiLliaM mnemonic. Left bundle branch block: WiLliaMComplexes in V1 resemble the letter W (the first letter of WiLliaM) and complexes in V6 resemble the letter M (the sixth letter of WiLliaM). LBBB is always pathological. Left bundle branch block may be due to conduction system degeneration or a result of myocardial pathologies such as ischaemic heart disease, cardiomyopathy and valvular heart disease. LBBB may also occur after cardiac procedures which damage the left bundle branch or His bundle. A STEMI presenting as chest pain with LBBB is exceedingly rare. Branches of the left bundle branchDue to the relatively greater mass of the left ventricle, disruptions in the depolarisation of the left ventricular muscle can cause cardiac axis changes. The left bundle branch splits into anterior and posterior fascicles. LBBB = Left anterior fasicular block (LAFB) + Left posterior fasicular block (LPFB) Each branch of the left bundle branch may be damaged in isolation. Anterior fascicle block, which is much more common, causes left axis deviation. Posterior fascicle block may cause right axis deviation. However, the posterior fascicle does much less work than the anterior fascicle so can be blocked without any obvious ECG changes. The right ventricular muscle does not have a great enough mass to significantly deviate the cardiac axis. Other types of blockBifascicular block involves both right bundle branch block as well as blockade of one of the fascicles of the left bundle branch. Trifascicular block is present when a 3rd-degree heart block exists alongside bifascicular block. Key points
ReviewersDr Matt JacksonConsultant Cardiologist Dr Ben MarrowCardiology Registrar EditorDr Chris JefferiesReferences
|