Epicardial activation in patients with left bundle branch block.

To elucidate the abnormalities in ventricular activation sequence in human left bundle branch block (LBBB), epicardial mapping was performed in five patients, ages 52-58 years, undergoing coronary bypass surgery, and the results were compared with similar data published from patients without conduction defect. Three patients had chronic and two patients intraoperative LBBB. ECGs during LBBB revealed a QRS duration of 130-160 msec and a mean QRS axis of -15° to +45. Epicardial mapping revealed 1) anterior right ventricular (RV) epicardial breakthrough (5-26 msec after QRS onset), normal in site in all patients, but abnormally early in timing relative to QRS onset in three patients with chronic LBBB, and earlier compared with preoperative maps in two patients with intraoperative LBBB; 2) normal location of latest RV epicardial activation in four of five patients, but abnormally late occurrence of this event 100, 108 and 110 msec after QRS onset in three of five patients; 3) absence of discrete left ventricular (LV) epicardial breakthroughs in all patients; 4) slow transseptal epicardial activation (crowded isochrones) from right to left, with anteroseptal crossing preceding inferoseptal crossing; 5) activation of the anterolateral left ventricle before the inferior LV epicardium; 6) more widely spaced isochrones, implying more rapid conduction, over the LV free wall epicardium; and 7) location and timing of latest LV epicardial activation in an abnormal site, and abnormally late relative to QRS onset (113-140 msec, mean 124 msec) in all patients. This event occurred a mean of 20 msec before the end of the QRS in the five patients. In conclusion, with normal axis, human LBBB is associated with initiation of ventricular activation closer to anterior RV recording sites than is normal activation, slow leftward transseptal activation, a generally anteroinferior orientation of LV activation, and probable engagement of the distal LV Purkinje system during the latter part of the QRS.