Epicardial activation in patients with left bundle branch block.

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Abstract
To elucidate the abnormalities in ventricular activation sequence in human left bundle branch block (LBBB), epicardial mapping was performed in 5 patients aged 52-58 yr undergoing coronary bypass surgery; the results were compared to with similar data published from patients without conduction defect. Three patients had chronic and 2 patients intraoperative LBBB. ECG during LBBB revealed a QRS duration of 130-160 ms and a mean QRS axis of -15.degree. to +45.degree.. Epicardial mapping revealed anterior right ventricular (RV) epicardial breakthrough (5-26 ms after QRS onset), normal in site in all patients, but abnormally early in timing relative to QRS onset in 3 patients with chronic LBBB, and earlier compared with preoperative maps in 2 patients with intraoperative LBBB; normal location of latest RV epicardial activation in 4 of 5 patients, but abnormally late occurrence of this event 100, 108 and 110 ms after QRS onset in 3 of 5 patients; absence of discrete left ventricular (LV) epicardial breakthroughs in all patients; slow transseptal epicardial activation (crowded isochrones) from right to left with anteroseptal crossing proceding inferoseptal crossing; activation of the anterolateral left ventricle before the inferior LV epicardium; more widely spaced isochrones, implying more rapid conduction, over the LV free wall epicardium; and location and timing of latest LV epicardial activation in an abnormal site and abnormally late relative to QRS onset (113-140 ms, mean 124 ms) in all patients. This event occurred a mean of 20 ms before the end of the QRS in the 5 patients. 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.