Bundle Branch and Fascicular Blocks



Right and left bundle branch block, and block in the left anterior and posterior fascicles of the left bundle branch, are commonly encountered.

   Complete bundle branch block prolongs QRS duration to 0.12 s or greater. With right bundle branch block, there will be a secondary R wave in lead V1, resulting in an M-shaped complex. With left bundle branch block, there is no M-shaped complex in V1; there will be a notched complex in left ventricular leads.

   Diagnosis of fascicular block requires an understanding of the hexaxial reference system. With left axis deviation, lead I is predominantly positive and both leads II and III are predominantly negative. The criteria for left anterior fascicular block are left axis deviation together with a small initial r wave in leads II and aVF: inferior infarction also leads to left axis deviation but there will be a Q rather than r wave in these leads.




The bundle of His divides into left and right bundle branches. These facilitate very rapid activation of the left and right ventricles. Block in conduction through one or other bundle branch results in delayed and disordered activation of ventricular myocardium, as evidenced on the ECG by a ventricular complex which is prolonged in duration and has an abnormal configuration.


Right bundle branch block


ECG appearance


In right bundle branch block there is delay in activation of the right ventricle, while activation of the interventricular septum and free wall of the left ventricle and hence the initial part of the QRS complex is normal (Figure 4.1). Delayed right ventricular activation results in:



1. an increase in duration of the QRS complex (≥ 0.12 s);

2. a secondary R wave in leads facing the right ventricle (V1 and V2) and hence an M-shaped complex in these leads; and

3. a broad S wave in left ventricular leads and lead I.


Figure 4.1 Right bundle branch block. There is an M-shaped complex in V1 and a deep slurred S wave in lead V6.

image

Partial right bundle branch block results in a similar ECG appearance but the QRS duration is 0.10 or 0.11 s.


Causes and significance


Right bundle branch block may be an isolated congenital lesion. It often occurs in congenital heart disease, in other causes of right ventricular hypertrophy or strain such as obstructive airways disease, and where there is myocardial damage. Right bundle branch block is common when there is disease of the specialised conducting tissues.


Based on limited data, neither pre-existing nor acquired right bundle branch block are of prognostic significance. However, a recent long-term survey has demonstrated a four-fold increased risk of developing AV block.


Extrasystoles and tachycardias of supraventricular origin may encounter a right bundle branch that is refractory to excitation and be conducted to the ventricles with a right bundle branch block pattern.


Left bundle branch block


ECG appearance


In left bundle branch block, activation of the interventricular septum is in the opposite direction to normal (i.e. from right to left), being initiated by an impulse arising from the right bundle branch. Thus:



1. The initial small, negative q wave normally seen in left ventricular leads (V5, V6, I and aVL) is replaced by a larger, positive R wave.

2. Activation of the left ventricle will be delayed, resulting in a broad and usually notched R wave in left ventricular leads, and prolongation of the duration of the QRS complex (≥ 0.12 s) (Figure 4.2).


Figure 4.2 Left bundle branch block. There is a broad positive complex in V6. There is no M-shaped complex in lead V1. The QS complex in V1 is also characteristic of left bundle branch block.

image

Partial left bundle branch block has a similar ECG appearance to complete left ­bundle branch block, but the QRS duration is 0.10 or 0.11 s.

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  2. Atrial Flutter
  3. Ambulatory ECG Monitoring
  4. Polymorphic Ventricular Tachycardia and Ventricular Fibrillation

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Jun 4, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Bundle Branch and Fascicular Blocks

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