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Atrioventricular Conduction Disturbances


AV block is a disturbance in conduction between the normal sinus impulse and the eventual ventricular response. The block is assigned to one of three classes, according to the severity of the conduction disturbance.

  • 1.

    First-degree AV block is a simple prolongation of the PR interval but all P waves are conducted to the ventricle.

  • 2.

    In second-degree AV block, some atrial impulses are not conducted into the ventricle.

  • 3.

    In third-degree AV block (or complete heart block), none of the atrial impulses is conducted into the ventricle ( Fig. 17.1 ).

    Fig. 17.1, Atrioventricular (AV) block.

I. First-Degree AV Block

  • 1.

    Description: There is a prolongation of the PR interval beyond the upper limits of normal (see Table 2.3) due to an abnormal delay in conduction through the AV node (see Fig. 17.1 ).

  • 2.

    Causes

    • a.

      In otherwise healthy children and young adults, particularly athletes, mediated through excessive parasympathetic tone.

    • b.

      CHDs (such as endocardial cushion defect, ASD, Ebstein anomaly).

    • c.

      Other causes, including infectious disease, inflammatory conditions (rheumatic fever), cardiac surgery, and certain drugs (such as digitalis, calcium channel blockers).

  • 3.

    Significance

    • a.

      Usually no hemodynamic disturbance results. Exercise, both recreational and during stress testing, induces parasympathetic withdrawal resulting in normalization of AV conduction and the PR interval.

    • b.

      Sometimes it may progress to a more advanced AV block.

  • 4.

    Treatment: No treatment is indicated except in digitalis toxicity.

II. Second-Degree AV Block

In second-degree AV block, some but not all P waves are followed by QRS complexes (dropped beats). There are three types: Mobitz type I (Wenckebach phenomenon), Mobitz type II, and high-grade (or advanced) second-degree AV block.

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