Ecg

Type

First degree Second-degree type I

1 Second-degree type II 6 High-grade—2:1, 3:1, etc.

Third-degree (complete)

PR prolonged

Gradual PR increase before dropped beat

Dropped beat with no PR increase Every other P not conducted, 2 consec P waves not conducted, etc.

No P waves conducted

Level of block AVN or HPS

Usually AVN, but can be in

HPS if QRS is wide HPS

Narrow QRS usually at AVN; Wide QRS usually in HPS

Narrow QRS usually at AVN; wide QRS usually in HPS

Therapy None

QRS is wide PPM if no reversible cause is found PPM if sx; otherwise PPM if HBE documents HPS block

PPM if sx; otherwise PPM if HBE documents HPS block

AVN = atrioventricular node, HPS = His-Purkinje system, PPM = permanent pacemaker, sx = symptoms, HBE = His bundle electrogram.

Atrial Fibrillation Pacemaker

Fig. 2. A 74-yr-oid man with a history of right bundle-branch block (RBBB) and left anterior fasicular block was admitted to the hospital after a syncopal episode. Complete heart block with a wide-QRS-complex escape rhythm was noted on his ECG, and a temporary pacemaker and later a permanent pacemaker were inserted. Approximately two-thirds of patients presenting with complete heart block and a wide-complex escape rhythm had antecedent RBBB with left anterior fasicular block.

Fig. 2. A 74-yr-oid man with a history of right bundle-branch block (RBBB) and left anterior fasicular block was admitted to the hospital after a syncopal episode. Complete heart block with a wide-QRS-complex escape rhythm was noted on his ECG, and a temporary pacemaker and later a permanent pacemaker were inserted. Approximately two-thirds of patients presenting with complete heart block and a wide-complex escape rhythm had antecedent RBBB with left anterior fasicular block.

specialized conduction system. The traditional electrocardiographic categorization of AV block is presented in Table 2. Cells of the conduction system possess a property called automaticity; i.e. the ability to depolarize spontaneously. In general, the rate of depolarization is highest in the SA node and decreases progressively with descent through the conduction system. Thus, from a functional and therapeutic standpoint, the level at which the block is located is more important than the electrocardiographic classification. Block at the level of the AV node is usually associated with a junctional escape rhythm (and a narrow QRS complex). The escape rhythm tends to be reliable, the rate is characteristically adequate (e.g., 50-60 BPM) and responds to autonomic interventions (such as atropine) because the node has extensive sympathetic and para-sympathetic innervation. In contrast, block within the His-Purkinje system is associated with an idioventricular escape rhythm characterized by a wide QRS complex (Fig. 2) that is morphologically different from the conducted beats. It tends to be considerably slower and less reliable than a junctional escape rhythm, and is relatively unaffected by autonomic maneuvers.

From a therapeutic standpoint, chronic high-grade block at the level of the AV node is a rare disorder that tends to be relatively benign. The incidence of sudden cardiac death is low in individuals without accompanying severe organic heart disease, and asymptomatic patients can be followed without specific therapy. Individuals who have minimal symptoms can be given conduction-enhancing medications such as theophyl-line, although these are of questionable efficacy. One possible exception to this approach is the patient with congenital AV block. Although block in this situation characteristically occurs at the level of the AV node, the clinical course may be unpredictable, and sudden cardiac death has been reported (although admittedly, high-grade block has not been documented as the cause), prompting some authorities to routinely recommend permanent pacing for adult patients with congenital second- or third-degree block (37,38). The official ACC/AHA recommendations for permanent pacing in acquired AV block are as follows (1):

Class I: 1. Third-degree AV block at any anatomic level associated with any of the following:

a. symptomatic bradycardia, either spontaneous or as a result of drug therapy that is required for treatment of other conditions.

b. Documented periods of asystole >3 s or any escape rate <40 BPM.

c. Following catheter ablation of the AV junction.

d. Postoperative block that is not expected to resolve.

e. Neuromuscular diseases with complete AV block such as Kearns-Sayre syndrome.

2. Second-degree AV block associated with symptomatic tachycardia, regardless of site or type of block. Class Ila: 1. Asymptomatic complete AV block with an average rate of >40 BPM while awake.

2. Asymptomatic type II second-degree AV block.

3. Asymptomatic type I second-degree AV block regardless of site.

4. First-degree AV block with symptoms suggestive of pacemaker syndrome and documented improvement with temporary dual-chamber pacing.

Class lib: 1. PR interval >0.30 s in symptomatic patients with left ventricular dysfunction in whom it has been demonstrated that shorter AV delays improve hemo-dynamics.

Class III: 1. Asymptomatic first-degree or type I second-degree AV block.

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