Third Degree Complete AV Block

In third-degree AV block, there is no atrioventricular conduction. The ventricles are paced by an escape pacemaker at a rate slower than the atrial rate ( Fig...24:30). Third-degree AV block can occur either at nodal or infranodal levels.

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FIG. 24-30. Third-degree AV block.

FIG. 24-30. Third-degree AV block.

When third-degree AV block occurs at the AV node, a junctional escape pacemaker takes over with a ventricular rate of 40 to 60, and—since the rhythm originates above the bifurcation of the bundle of His—the QRS complexes are narrow.

When third-degree AV block occurs at the infranodal level, the ventricles are driven by a ventricular escape rhythm at a rate of less than 40. In third-degree AV block located at the His bundle level, the escape rhythm has narrow QRS complexes about half of the time. Presumably in these cases, the escape pacemaker resides above the bifurcation of the conducting system into the separate bundle branches. Third-degree AV block located in the bundle branch or elsewhere in the Purkinje system invariably have escape rhythms with wide QRS complexes.

CLINICAL SIGNIFICANCE Nodal third-degree AV block may develop in up to 8 percent of acute inferior MIs, where it is usually transient, although it may last for several days.

Infranodal third-degree AV blocks indicate structural damage to the infranodal conducting system, as seen with an extensive acute anterior myocardial infarction (AMI). The ventricular escape pacemaker is usually inadequate to maintain cardiac output and is unstable, with periods of ventricular asystole. When third-degree block is seen in acute AMI, mortality is increased even when rhythm is controlled with pacing.

TREATMENT

1. Nodal third-degree AV blocks should be treated like second-degree Mobitz I AV blocks with atropine or ventricular demand pacemaker as required.

2. Infranodal third-degree AV blocks require a ventricular demand pacemaker. Isoproterenol can be used temporarily to accelerate the ventricular escape rhythm, or external cardiac pacing can be performed before transvenous pacemaker placement.

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