314 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



LBBB 



20 40 60 80 KIO 

 MSEC 



FIG. 38. Ventricular depolarization after left bundle branch block. Ihis figure should be com- 

 pared with fig. 30, which shows a normal depolarization pattern. Shaded area represents portion 

 of myocardium depolarized up to the instant indicated at bottom of column, and this is compared 

 with lead II electrocardiogram. Note that activity begins around right cavity, proceeds gradually 

 across septum as depolarization of right free wall is completed, and has reached approximately 

 center of septum at 2^ msec after beginning of QRS. Activity continues across septum through 40 

 msec, and even at 60 msec after beginning of QRS, lateral left ventricle is not completely excited. 

 Note that both septal activation and activation of left wall are altered by bundle branch block. 

 Increased time required to excite septum and left wall accounts for prolongation of depolarization 

 in complete left bundle branch blocks. This figure, like fig. 30, represents activity in the dog heart 

 in which the duration of normal QRS is 40 msec or less. [From Becker el al. (15).] 



was interrupted, the normal double envelopment of 

 the septum was replaced by one-way activation from 

 the unblocked side, and activation of the free wall 

 began at the sites first reached by spread of depolari- 

 zation across the septum. The wave of excitation 

 utilized the endocardial Purkinje fibers and traveled 

 along the endocardium on the side of the block at 

 about I m per sec (15, 44). In normal conduction, 

 simultaneous excitation of many endocardial points 

 by the branched Purkinje system leads to an appar- 

 ently infinite endocardial conduction velocity. 



Prolongation of the QRS complex in bundle 

 branch block results both from the increased period 

 of time required to activate the septum (5, 15, 44) 

 and from the greater time required to activate the 

 blocked free wall. The change in the activation of 

 the free wall during right bundle branch block in 

 the dog is shown in figure 39. Normally, it requires 

 about 18 msec to activate the right mural endo- 

 cardium, and a large central area is activated within 

 a few milliseconds by the branching Purkinje system. 

 After block, the impulse reaches the wall at the 

 inferior and posterior junctions of the wall and the 

 septum, and spreads anteriorly and superiorly. The 

 smooth progression of the wave is altered as it breaks 

 through the septum superiorly. The total time re- 

 quired to activate the free wall after block is 35 msec 

 (44). Similar changes in mural activation are seen 

 after left bundle branch block (29, 130). 



In the dog, complete right Inmdle block doubles 



the duration of QRS; complete left bundle branch 

 block increases it two and one-half times. Comparable 

 durations of the QRS in man would be 160 and 200 

 msec for complete right and complete left block, 

 respectively. A clinical diagnosis of complete block 

 is based on far less prolongation of the QRS complex, 

 i.e., 120 msec or more. In complete right bundle 

 branch block we would expect that the initial left- 

 to-right septal activation would be present and that 

 the loss of the early activity on the right septum 

 (right to left) would be partly counterbalanced by 

 the elimination of early inside-out activation of the 

 free wall (left to right). Such appears to be the case. 

 The free right wall is the last portion of the heart to 

 be activated, a situation which produces late positive 

 deflections in Vi and Vo, and aVR. Grant (49) 

 believed that these conditions are met only rarely in 

 clinical examinations and concluded that truly 

 complete right bundle branch block is extremely 

 uncommon. Complete left bundle branch block is 

 more common and is accompanied by clear signs of 

 right-to-left activation of the septum and left ven- 

 tricle. In bundle branch block, as in ventricular 

 ectopic beats, those portions of the ventricular myo- 

 cardium which depolarize first tend to repolarize 

 first, and, similarly, the last areas to fire recover 

 latest. Consequently, the T wave tends to become a 

 mirror image of the QRS complex; leads in which 

 the QRS is upright show a downward T wave, etc. 

 In conditions clinicalh" described as complete 



