Ventricular excitation in experimental bundle-branch block.

Multipoint 1 recording apparatus has been used to study the pattern of ventricular excitation in experimental bundle-branch block. After block, the wave of activation spreads uniformly from the contralateral ventricle across the septum to the free wall of the homolateral ventricle. Septal activation consumes more time than in the normal. Changes occur in the site and extent of early endocardia! activity in the blocked ventricle. Alterations in both septal and mural depolarization contribute to QRS changes. Endoeardial depolarization is described during recovery from transient bundle-branch block. E XPERIMENTAL bundle-branch block has been a means of studying both the normal role of the specialized conducting tissues and the pattern of ventricular excitation in clinical bundle-branch block. Lewis' thought that right ventricular activation in right bundle branch block is accomplished by a uniform wave moving through the myocardium from left to right. The classical study by Wilson and co-workers 2 in 1932 showed that the electro-cardiograms for right and left bundle-branch block had been previously confused with one another. Smith and associates, 3 who studied epicardial activation, postulated that altered conduction in the free wall and in the septum explain the prolongation of the QRS complex in bundle-branch block. Rodriguez and Sodi-Pallares 4 believe that in bundle-branch block there is an area of considerable delay at the junction of the septal muscle supplied by the right and by the left bundle. They consider this delay the cause of the prolonged QRS complexes , and believe that the homolateral (i.e., blocked) ventricle is depolarized as in normal excitation. Since many conclusions have been based on limited information from single uni-polar and bipolar leads, a need was felt for quantitative data based on multichannel recording. METHODS Experiments were conducted on 19 open-chested dogs under baibiturate anesthesia. To produce right bundle-branch block (RBBB) an iridectomy scalpel was inserted through the atrium into the right ventricle, and the septa! endocardium was incised immediate!}' basal to the light anterior papillary muscle. For left bundle-branch block (LBBB) a thin scalpel blade was introduced through the left ventricular wall and a horizontal incision made on the septum about 0.5 cm. below the aortic cusps. Block was considered successful when appropriate changes occurred in the lead II QRS, and when records across the septum indicated spread entirely from the unblocked side. The pattern of activation within the heart was traced with multipolar electrodes, which consist of 15 insulated wires with …