AV (atrio-ventricular) block occurs when an atrial impulse is conducted with delay or fails to conduct to the ventricle at a time when the AV node should not be refractory.

1. First degree AV block usually results from conduction delay within the AV node but rarely from intra-atrial delay, or delay within the His-Purkinje system (often seen in conjunction with bundle branch block) may be responsible. The ECG reveals a PR interval greater than 200 milliseconds.

Asymptomatic patients require no therapy. In symptomatic patients, therapy is directed toward the underlying etiology, and treatment includes atropine, or cardiac pacing.

Trifascicular heart block

Trifascicular heart block is the triad of first degree heart block, right bundle branch block, and either left anterior or left posterior heart block seen on an electrocardiogram (EKG).

The relevance of this entity is that it's impossible to tell how serious the disease of the electrical conduction system of the heart is by examination of the EKG.

2. Second-degree AV block occurs when some of the atrial impulses are not conducted to the ventricle at times when the AV node should not be refractory. Two types of second-degree AV block are recognized, and dis�tinctions between type I and type II are important, as they carry differ�ent prognostic implications.

a. Mobitz type I (Wenckebach's) block has progressive delay in AV con�duction prior to block. The site of conduction block almost always is within the AV node. The EGG reveals progressive PR prolongation prior to a non-conducted P wave resulting in QRS complexes in regular groupings (grouped beating). The RR interval progressively shortens prior to a blocked P wave. Symptomatic type I AV block is managed initially with atropine, 0.~2.0 mg IV For persistent symptoms, therapy should be instituted (as in Sec. lilA).

b. Mobitz type II block is characterized by AV conduction block without preceding conduction delay. The site of block is localized most often to the His-Purkinje system.

Type II block, especially in the setting of a bundle branch block, often antedates the development of complete heart block.

The EGG reveals no change in PR interval preceding a non-conducted P wave.

Response to pharmacologic therapy is poor and often temporary Consequently, permanent pacemaker Insertion frequently is recommended.

3. Third degree (complete) AV block is the absence of conduction of atrial impulses to the ventricles. The site of block may be the AV node (as occurs in congenital heart block), or within the His-Purkinje system (typical for acquired heart block). In complete heart block, the atrial rate typically exceeds the ventricular rate, with no fixed relation between atrial and ventricular activity (AV dissociation), and the ventricular rate usually is regular, owing to the regularity of the escape rhythm focus. Complete heart block should be distinguished from competitive AV dis�sociation. The latter results from an AV nodal or ventricular focus exceed�ing the sinus or atrial rate (i.e. the ventricular rate exceeds the atrial rate) and usually is benign. Response to pharmacologic therapy is poor and often temporary. Persistent symptoms, transient episodes with an under-lying etiology likely to recur, or the failure to identify a reversible cause, necessitates pacemaker therapy.