A left bundle branch block occurs when electrical conduction ... while the other half are idiopathic or due to fibrosis of the cardiac conduction system which occurs during the aging process.

However, unlike in the presence of a left bundle branch block, myocardial ischemia and infarction can easily be detected on ECG when a RBBB is present (see anterior and inferior wall MI examples).

even with conservative treatment of prolonged PR intervals and bundle branch block. Relatively few patients with myotonic dystrophy appear to require implanted cardiac pacemakers.