Abnormalities of the Conduction System
The conduction system consists of the sinoatrial(SA) node, intraatrial
conduction fibers, atrioventricular (AV) node, His bundle, bundle branches, and the Purkinje system. Impulses from the SA node are conducted through this system to the ventricles. Interruption of the these impulses may occur in the SA node, within the atria, at the AV node, within the His bundle, or in the bundle branches.
The right bundle branch lies on the right side of the interventricular septum
and does not branch until it reaches the right ventricular apex where it
anastamoses with the Purkinje fibers to conduct the impulses to the right
ventricle. The left bundle branch lies on the left side of the septum and
almost immediately divides into two main divisions, the anterior superior and
the posterior inferior division. Each subdivides into a number of fascicles.
The anterior superior division supplies the anterior and superior wall, and
the posterior inferior division supplies the posterior interior wall of the
left ventricle . The two fascicles anastomose at the periphery and carry
impulses to the respective area supplied through the Purkinje fibers. There is probably a third septal division also, but it has not been
recognized electrocardiographically shows a simplified diagram
of the conduction system.
Interrupion of conduction in either division causes delayed depolarization of
the area of the ventricle supplied by those fascicles.
Left anterior hemiblock (left anterior fascicularblock)
Left Anterior Divisional Block
Left anterior hemiblock is caused by interruption of the anterior division of
the left bundle branch. This fascicle is fragile, easily exposed to
damage, and has a single blood supply (anterior descending of left coronary
artery).
In left anterior hemiblock, the inital impulses are transmitted through the
posterior inferior division inferiorly to the right, and then proceed throught
the Pirkinje fibers to the anterior lateral and superior walls of the left
ventricle. The inital QRS vector is , therefore, directed inferiory to the
right and terminal vector superiorly to the left resulting in q waves of not
more than 0.02 sec duration in Leads I and aVL, and rS complexes in II, III<
and aVF. The total duration of the QRS is increased by no more than 0.02 sec as
compared with a pre-block ECG since theimpulse is conducted rapidly through the
Purkinuje fibers and not through hear musclte.
Some electrocardiographers do not make the diagnosis of left anterior hemiblock
unless the axis is –450 or more . We have accepted an axis of –300, or more
superriorliy, providing the other criteria are also met. Left-axis deviation is
usually caused by left anterior hemiblock but is occasionally seen in
obeseindividuals with a horizontal heart and in inferior infraction, as will be described later.
Occasionally, small q waves appear in Leads V1 and V2 due to reflection of the
initial QRS forces on the negative axes of these leads, suggesting anteroseptal
myocardial infarction. However, if the electrodes are placed one interspace
lower. q waves are not present in uncomplecated left anterior hemiblock.
The QRS frontal plane loop of the VCG is counter-clockwise in rotation,
beginning inferiorly to the right then turning superiorly to the left. The
transverse plane QRS loop is not altered, but the rightt saggital QRS loop is
located superiorly .
LEFT POSTERIOR HEMIBLOCK (LEFT POSTERIOR FASCICULARBLOCK)
Left Posterior Divisional Block
Left posterior hemiblock is less common than left anterior hemiblock since the
bundle is much thicker and has a double blood supply (left and right coronary
arteries). This conduction disturbance is uncommonly seen alone, but is more
often associated with right bundle branch block.
When left posterior hemiblock occurs, the initial impulses are transmitted
superiorly to the left through the anterior division of the left bundle branch
supplying the anterior superior left ventricular wall, and then sweep around
inferiorly to the right through the Purkinje system to supply the posterior
inferior left ventricular wall.
The initial QRS vector is directed superiorly to the left and terminal
inferiorly to the right, causing qR complexes in Leads II, III, aVF, and rS
complexes in Leads I and aVL. The q waves in II and aVF do not measure more
than 0.02 sec. The mean QRS axis is greater than +1000 (Fig. 75).
ABNORMALITIES OF THE CONDUCTION SYSTEM
In the VCG of left posterior hemiblock the frontal plane QRS vector loop rotates
clockwise with the initial portion located superiorly to the left, turning
inferiorly to the right. The transverse plane loop is rotated counter-
clockwise with the terminal portion located more rightward than usual. The right
sagittal plane QRS loop is normal (Fig.76).
The diagnosis of left posterior hemiblock is difficult to make because a thin
individual with an anatomically vertical heart or a patient with COPD may have a
vertical axis without posterior hemiblock. However, if an individual of sthenic
build, who would be expected to have a more horizontal axis is fould to have an
axis of +1000 without other obvious causes, then the diagnosis of posterior
hemiblock should be considered, particularly if the initial forces are directed
superiorly to the left.
Anterior of posterior hemiblock may be caused chronically by any condition
producing fibrosis , such as coronary artery disease, sclerosis of the cardiac
skeleton, cardiomyopathy, scleroderma, etc., or, acutely, by myocardial
infarction. When caused by myocardial infarction, there are also associated ECG
evidences of infarction. Hyperkalemia may alter conduction, causing conduction
disturbances with patterns of anterior or posterior hemiblock (24.25)
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