Third-degree atrioventricular block
In third-degree AV block (complete AV block, no AV conduction), no atrial impulses reach the ventricles, and ventricular rhythm is maintained by a subsidiary pacemaker. Since subsidiary pacemakers must be below the level of block, their location is in part determined by the site of block. In third-degree AV nodal block, the ventricular rhythm is usually maintained by pacemakers in the AV junction with resultant narrow QRS complexes. In third-degree AV block localized to the bundle branches, ventricular rhythm is maintained by a pacemaker in the Purkinje fibers, with resultant wide QRS complexes. The junctional pacemaker rate is usually faster (40–80 beats/min) compared with the peripheral Purkinje network (20–40 beats/min).
When third-degree AV block occurs at the AV node, it can result from increased parasympathetic tone associated with inferior infarction, from toxic drug effects or from damage to the AV node. Third-degree AV block with a junctional escape rhythm is usually transient and is associated with a favorable prognosis. When third-degree AV block occurs at the infranodal level, it is most often due to block involving both bundle branches. This indicates the presence of extensive infranodal conduction system disease. When it results from coronary atherosclerosis, it is usually associated with extensive anterior myocardial infarction. It usually does not result from increases in parasympathetic tone or from drug effects.
Treatment
The major interventions are atropine, transcutaneous pacing, catecholamine infusions (dopamine or epinephrine), and transvenous pacemaker. Isoproterenol is rarely indicated.
Summary of ECG criteria
- QRS: Generally normal looking. When block occurs at the AV node or bundle of His, the QRS complex will appear normal. When block occurs at bundle branch level, the QRS complex will be widened.
- P waves: Normal
- Rate: The atrial rate will be unaffected by third-degree AV block. The ventricular rate will be slower than the atrial rate. With intranodal third-degree AV block, the ventricular rate is usually 40 to 60 beats/min; with infranodal third-degree AV block, the ventricular rate is usually less than 40 beats/min.
- Rhythm: The atrial rhythm is usually regular, although sinus arrhythmia may be present. The ventricular rhythm will be regular.
- PR interval: Since the atria and ventricles are depolarized from different pacemakers, they are independent of each other, and the PR interval will vary.
Atrioventricular dissociation
AV dissociation is a rhythm in which atrial and ventricular activation occurs from different pacemakers. The atrial rhythm can be of sinus origin or from any of the atrial arrhythmias listed previously. Ventricular activation may be from either junctional or lower pacemakers. Third-degree AV block is one form of AV dissociation. The latter, however, is common in the presence of intact AV conduction. AV dissociation in the presence of intact AV conduction can occur when rates of subsidiary pacemakers, junctional or ventricular, exceed the atrial and there is associated AV block, as seen in ventricular tachycardia. This contrasts with AV dissociation during third-degree AV block, where atrial rates usually exceed ventricular rates.
This content is reviewed regularly. Last updated 12/5/08.
