What are the three types of acquired AV block?

There are three degrees of AV block: first, second, and third (complete). This classification is based on the electrocardiogram.

First-degree block refers to prolongation of the PR interval to 200 msec and represents delay in conduction in the AV node. There are no accepted indications for pacing in isolated first-degree block.

Second-degree block is divided into two types. Type I (Wenckebach) exhibits progressive prolongation of the PR interval before an impulse fails to stimulate the ventricle. Anatomically this form of block occurs above the bundle of His in the AV node. Type II exhibits no prolongation of the PR interval before a dropped beat and anatomically occurs at the level of the bundle of His. This rhythm may be associated with a wide QRS complex. Advanced second-degree block refers to the block of two or more consecutive P waves.

Third-degree or complete block refers to complete dissociation of the atrial and ventricular rhythms, with a ventricular rate less than the atrial rate. The width and rate of the ventricular rhythm help to infer an anatomic location for the block: narrow QRS results in minimal slowing of the rate, generally at the AV node, and wide QRS in considerable slowing of rate at or below the bundle of His.

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