The left bundle may be blocked before its bifurcation into anterior and posterior divisions or within these divisions.
Depolarization then begins on the right side of the ventricular septum simultaneously with the endocardial surface of the right ventricle. Because of the size of septum and slow movement of impulses through this structure, the right ventricular forces are cancelled and the resultant initial forces are directed from right to left. The left ventricle is then depolarized possibly through muscle, or through Purkinje fibers, or both. The mid- and terminal forces are located leftward ahead.
The QRS is broad, often notched, and measures 0.12 sec or more. The ST-vectors are inscribed opposite to the QRS because of the secondary repolarization changes caused by abnormal sequence of depolarization forces. q waves are not seen in Lead I and rarely in aVL, and small or no r waves are inscribed in V1 and V2 because of the right to left direction of the initial forces.
The QRS axis may be normal or deviated to the left. If the QRS duration between 0.10 and 0.12 sec, associated with the other abnormalities of left bundle branch block, it is called incomplete left bundle branch block. Such changes may be seen in left ventricular hypertrophy and probably represent myocardial disease.
VCG in Left Bundle Branch Block In Fig.81, in the frontal plane, QRS vector has a counter-clockwise rotation with leftward initial forces and delay in the midportion as shown by the closenes of the dashes. The main body of the loop is usually beneath to the X axis, but occasionally lies above it.
The T loop is opposite to the QRS loop with an ST vector in the same direction as the T. The transverse plane loop shows an initial anterior counter-clockwise leftward inscription and later clockwise rotation with the main body of the loop in the left posterior quadrant. The efferent limb usually lies to the right of the afference limb. The T loop is displaced ahead and to the right with an obvious ST vector. Delay can be seen in the midportion of the loop.
The right sagittal QRS loop is written clockwise with anterior initial forces, and the main body of the loop posteriorly, with delay in the middle portion. The T loop lies ahead and up with an ST vector in the same direction.
In Fig.81, the frontal plane, initial forces are directed down to the left. There is the evidence of slowed conduction in the midportion of the QRS loops as indicated by the close proximity of the dashes. The T loop and ST vector are discordant to the QRS loop. In the transverse plane, the initial forces are directed ahead and to the left with counter-clockwise rotation. Then the loop goes back and clockwise with the efferent limb to the right of the afferent loop. The T loop and ST vector are discordant to the QRS loop. In the right sagittal plane, the initial forces are directed ahead and down, then back in a clockwise manner. The T loop and ST vector are discordant to the QRS loop.
The scalar lead X and Y in Fig. 81 are upright and often slurred with a duration of greater than 0.12 sec and S-T segment and T waves opposite to the QRS. If left axis deviation is associated there is a broad S wave in the Y lead.
Scalar Leads X and Y show no q wave followed by a broad slurred R wave with a negative S-T segment and T wave. In V1 there is a small or no r wave followed by a deep broad often slurred S wave with an upright S-T segment and T wave (Fig.61).
The Significance of Bundle Branch Block The electrocardiographic pattern of right bundle branch may be congential or due to a variety of acquired causes as previously discussed. It has been reported to occur in 0.05 to 29% of normal individuals under the age of 44 years. The incidence increases from 0.05% at age 16 to 19 years to 29% at age 40 to 44 years.
Most individuals with atrial septal defect will show a right bundle branch block pattern, incomplete or complete on the ECG. Acute corpulmonale due to pulmonary embolism may be also cause right bundle branch block probably because of the increased pressure in the right ventricle resulting in impairment of conduction in the right bundle branch. Chronic fibrosis due to idiopathic sclerosis of the cardiac tissues, coronary artery disease, cardiomyopathy, etc., may also cause right bundle branch block and may develop acutely during the couse of myocardial infarction associated with an increased mortality.
The prognosis of chronic right bundle branch block is that of the underlying disease, since the block does not itself cause any functional impairment of the heart.
Left bundle branch block is often associated with serios cardiac disease such as coronary anteriosclerosis, cardiomyopathy, etc. It may be due, however, to sclerosis of the cardiac skeleton without any desease in the functional capacity of the heart, and progress to complete block if the right bundle becomes involved in the same progress. This will be discussed further in the chapter on cardiac rhythms.
Left bundle branch block also occurs as a result of acute myocardal infarction carrying a serious prognosis. The pattern of incomplete left bundle branch block is frequently associated with left ventricular hypertrophy.
Bifascicular Block Right Bundle Branch Block and Left Anterior Hemiblock When interruption of the right bundle branch and the anterior division of the left bundle branch occurs, there is superior orientation of the unblocked portion of the QRS complex resulting in left axis deviation and right bundle branch block (Fig.82).
Figure 83A diagrammatically shows the ECG, which result from left anterior hemiblock and right bundle branch block as well as the vector analysis of the ECG. The VCG is also shown in Fig. 83B with the respective X, Y, and V1 scalar leads.
The frontal plane VCG shows the initial forces directed down to the right and the main body of the QRS loop directed up with slowing of the terminal portion. The transverse plane QRS loop is normal except for the changes produced by the right bundle branch block. The right sagittal QRS loop is located above and behind with the terminal blocked portion directed ahead. Blocks in the anterior division of the left bundle and in the right bundle are shown diagrammatically. Leads I, II, III and V1 are shown. The unblocked portion is oriented up to the left and the blocked portion is directed ahead to the right.
In Fig. 83A, the initial forces (I) are directed to the right down and the latter unblocked forces are located above and to the left. The terminal blocked forces (T) are directed ahead to the right.
In Fig. 83B, the QRS loop shows initial inferior rightward forces and the other unblocked forces directed leftward, up and back. The terminal-blocked portion as indicated by close proximity of the dashes is located to the right and before. Frontal transverse and right sagittal plane loops and scalar leads X, Y and V1 are shown.
Left anterior hemiblock and right bundle branch block may be caused by obstruction of the anterior descending branch of the left coronary artery since this supplies both the right bundle and anterior division of the left. Conduction is, therefore, solely through the posterior division of the left bundle.
Patients with this condition are suspectible to complete heart block though the incidence is less than previously thought. However, when there is a history of episodes of otherwise unexplained syncope, documented Mobitz type II block or intermittent complete block, pacemaker therapy is indicated.
Right bundle branch block may also be associated with left posterior hemiblock resulting in inferior rightward orientation of the unblocked forces. This phenomenos causes right axis deviation of the unblocked portion of the QRS accompanied by right bundle branch block.
Figures 85A and B show the ECG and VCG which are found in left posterior hemiblock and right bundle branch block with vector analysis of the ECG. The frontal plane VCG in Fig. 85B shows the initial forces to be upward directed to the left and the main body of the QRS loop located below to the right with slowing of the terminal forces due to the right bundle branch block. In the transverse plane, the initial forces are frequently directed ahead and to the left. The main body of the loop lies back to the right and the terminal portion ahead with slowing due to right bundle branch block. The right sagittal plane is normal except for the terminal portion showing right bundle branch block.
Right bundle branch block with left posterior hemiblock is less common and probably more serious than right bundle branch block with left anterior hemiblock because the posterior division is thicker and has a double blood supply. Its interruption indicates more diffuse disease.
There is also probably a higher incidence of complete heart block in individuals with this condition than in those with anterior hemiblock and right bundle branch block, although the same indications for pacemaker implantation obtain.
Figures 86A and B show, diagrammatically, the ECGs of left anterior hemiblock and left posterior hemiblock associated with right bundle branch block.
It has been shown by the left bundle electrogram in individuals with bifascicular block that conduction is impaired in the remaining fascicle in many cases.
This observation has not been particularly helpful in determining the need for pace, though there are studies which indicate that patients with increasing conduction time in the His Purkinje system are more likely to develop complete heart block i.e., trifascicular block.
When bifascicular block develop as a copmlication of acute myocardial infartion, the incidence of completel block is high and the prognosis is poor. Although pacemaker therapy indicated, the statistical evidence that mortality affected is meager if the patient has a pump failure too. In the absence of pump failure, pacing seems to be helpful.
The pattern of left anterior hemiblock associated with complete or incomplete right bundle branch block is also seen in the primum type of atrial septal defect and in AV canal congenital abnormalities.
Examples: