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. Types Depending on the anatomical location of the defect which leads to a bundle branch block, the blocks are further classified into: Right bundle branch block, incomplete (IRBBB) or complete (CRBBB) Left bundle branch block, incomplete (iLBBB) or complete (cLBBB) The left bundle branch block can be further sub classified into: Left anterior fascicular block. In this case only the anterior half of the left bundle branch (fascicle) is involved Left posterior fascicular block. Only the posterior part of the left bundle branch is involved Other classifications of bundle branch blocks are; Bifascicular block. This is a combination of right bundle branch block (RBBB) and either left anterior fascicular block (LAFB) or left posterior fascicular block (LPFB) Trifascicular block. This is a combination of right bundle branch block with either left anterior fascicular block or left posterior fascicular block together with a first degree AV block. Tachycardia-dependent bundle branch block A simple way to quickly differentiate between the two types is to note the deflection of the QRS complex in the V1 lead. A (V1) QRS segment deflected DOWN, its LEFT, a deflection UP indicates RIGHT (RBBB). of course, in both types the QRS is long (>.12 seconds). Treatment Some people with bundle branch blocks are born with this condition. Many others acquire it as a consequence of heart disease. People with bundle branch blocks may still be quite active, and may have nothing more remarkable than an abnormal appearance to their ECG. However, when bundle blocks are complex and diffuse in the bundle systems, or are associated with additional and significant ventricular muscle damage, they may be a sign of serious underlying heart disease. In more severe cases, a pacemaker may be required to restore an optimal electrical supply to the heart muscle. See also Cardiac pacemaker Heart block First degree AV block Second degree AV block Third degree AV block References Cecil Textbook of Medicine. W.B. Sanders. 2004. Chapters 50; 58.