CME: Left Bundle Branch Block and Painful Left Bundle Branch Block Syndrome Abstract. Left bundle branch block (LBBB) is the electrocardiographic correlate of a pathologic transmission of the electrical signals in the myocardium which can lead to a dyssynchronous left ventricular activation and thus to an inefficient contraction of the ventricles. It is usually the expression of an underlying cardiopathy and represents an independent risk factor of cardiovascular mortality, therefore further examination is indicated in each case. Besides the treatment of an underlying disease, a specific therapy has been available since the introduction of cardiac resynchronization therapy (CRT). A rarer phenomenon is the painful left bundle branch block in structurally healthy hearts.