Bundle Branch Block
If there is a block in conduction through either the right or left bundle branches there will be a delay in the depolarisation of part of the ventricular muscle.
In sinus rhythm with bundle branch block, normal P waves are present with a constant PR interval. The extra time taken for depolarisation of the whole of the ventricular muscle causes a widening of the QRS complex to more than 0.12 seconds.
Bundle branch block may be permanent or intermittent. It may also be rate-dependent, occurring only when the heart rate exceeds a certain threshold.
The ECG trace below shows bundle branch block with a ventricular ectopic (second beat). The QRS complexes (shaded) are wide and the P waves (arrowed) are present with a constant PR interval.