Excerpt from Heart Block, Third Degree

Synonyms, Key Words, and Related Terms: atrioventricular block, AV block, third-degree atrioventricular block, third-degree AV block, complete heart block, AV node, cardiac conduction system

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Background: Complete heart block, also referred to as third-degree heart block, or third-degree atrioventricular (AV) block, is a disorder of the cardiac conduction system, where there is no conduction through the AV node. Therefore, complete disassociation of the atrial and ventricular activity exists. The ventricular escape mechanism can occur anywhere from the AV node to the bundle-branch Purkinje system. It is important to realize, however, that not all patients with AV dissociation have complete heart block. For example, patients with accelerated junctional rhythms have AV dissociation, but not complete heart block, if the escape rate is faster than the intrinsic sinus rate. Electrocardiographically, complete heart block is represented by QRS complexes being conducted at their own rate and totally independent of the P waves.

Pathophysiology: Complete heart block is caused by a conduction block at the level of the AV node, the bundle of His, or the bundle-branch Purkinje system. In most cases (approximately 61%), the block occurs below the His bundle. Block within the AV node accounts for approximately one fifth of all cases, while block within the His bundle accounts for slightly less than one fifth of all cases.

Duration of the escape QRS complex depends on the site of the block and the site of the escape rhythm pacemaker.

Pacemakers above the His bundle produce a narrow QRS complex escape rhythm, while those at or below the His bundle produce a wide QRS complex.

When the block is at the level of the AV node, the escape rhythm generally arises from a junctional pacemaker with a rate of 45-60 beats per minute. Patients with a junctional pacemaker frequently are hemodynamically stable and their heart rate increases in response to exercise and atropine.

When the block is below the AV node, the escape rhythm arises from the His bundle or the bundle-branch Purkinje system at rates less than 45 beats per minute. These patients generally are hemodynamically unstable and their heart rate is unresponsive to exercise and atropine.

Mortality/Morbidity: Patients with complete heart block are frequently hemodynamically unstable, and as a result, the patient may experience syncope, cardiovascular collapse, or death.