Disappearance of right bundle branch block with left anterior hemiblock when associated with a type B pre-excitation syndrome

Abstract A patient with mitral valve disease showing RBBB plus LAH with an intermittent association of type B pre-excitation is presented. The bifascicular block is masked by the existence of pre-excitation. The RBBB image is entirely canceled and the LAH is altered because its inital forces are replaced by a delta wave. The electrical axis in the frontal plane is also modified. The mechanism of production of this ventricular complex is explained on the basis of the premature excitation of the right ventricle and the sum of forces produced by the activation through the posteroinferior division of the left bundle.