Atrioventricular and Bifascicular Heart Block

Prevalence: First degree (0.65–1.6%); second degree (0.003% in young adults; higher in organic heart disease); third degree (overall 0.02%; congenital 1:20,000 live births); increases with age presumably because of small vessel disease

Inferior MI: Carries low mortality even if associated with high-degree AV block

Anterior MI: If high-degree AV block results, then mortality approaches 80%

Progression of benign heart block to second degree type II or third degree

Heart failure, myocardial and global ischemia, shock, and pacemaker failure

Autonomic changes influencing the degree of blockade

Pacemaker failure or electrocautery interference

Intracardiac wire or PA catheter placement leading to third-degree block

β-blockers, calcium channel blockers, digoxin, and anticholinergics influencing the degree of heart block

AV blocks: First degree (PR interval >0.20 sec). Block site =AV node. Usually benign. Associated with anterior MI, digitalis, and certain neuromuscular diseases.

Second-degree type I (Mobitz I or Wenckebach): Increasingly prolonged PR interval until QRS has dropped. Block site =AV node (normal QRS). Usually benign. Usually does not progress over time to second-degree type II or third degree. May progress acutely with anesthesia, autonomic influences, or intracardiac catheters/wires.

Second-degree type II (Mobitz II): Fixed PR interval with occasional dropped QRS. Block site = usually infranodal (wide QRS) and permanent. The larger infranodal block site yields a slower ventricular rate and symptoms. It commonly progresses to third degree. High mortality is associated.

Bifascicular block: Three “fascicles”/bundles” of nerves conduct via the ventricles: Right bundle branch, left anterior fascicle, and left posterior fascicle. When two of three are blocked, it is termed bifascicular. When third fascicle is blocked, pt is in third-degree heart block.

Third degree: Atria and ventricles have separate pacemakers. Any atrial rhythm (e.g., AFIB/flutter) could be present. Ventricular rate/rhythm depends on the site of the blockade. The more infranodal block yields a slower ventricular rate. If only upper AV node is blocked, the patient may have junctional rhythm (normal QRS) and be more stable. If entire AV node is blocked, then the ventricular rate will be 20 to 40 bpm, and perfusion is compromised.