Third-degree (complete) AV block: ECG criteria, clinical characteristics and management

This chapter discusses third-degree AV block, which is synonymous with AV dissociation, complete AV block, AV block III and AV block 3. In third-degree AV block, no atrial impulses are conducted to the ventricles. The atria and the ventricles are electrically dissociated from each other. This condition is referred to as atrioventricular (AV) dissociation. Importantly, for the ventricles to have any electrical (and thus pumping) activity at all, an escape rhythm must arise in an ectopic focus (located distal to the block). Third-degree AV block is a very serious condition because escape rhythms may (1) not occur, (2) occur transiently, or (3) occur but generate insufficient cardiac output. If no escape rhythm occurs, cardiac arrest will ensue.

Progression from first-degree AV block to third-degree AV block is rare. Progression from second-degree AV block Mobitz type 1 (Wenckebach block) is uncommon. However, second-degree AV block Mobitz type 2 frequently progresses to third-degree AV block.

ECG examples are given in Figure 1.

ECG features of third-degree AV block

On the ECG P-waves have no relation to the QRS complexes. The QRS complexes may be normal or wide. P-waves have constant PP interval and ride straight through the strip, without any relation to QRS complexes. P-waves may occur on the ST-T segment (Figure 1, upper panel). The atrial rate is typically faster than the ventricular rate.

It may be very difficult to establish a diagnosis of third-degree AV block if the atrial and ventricular rate is equal and the P-waves occur right before the QRS complexes. This scenario, which is referred to as isoarrhythmic AV block, may even simulate sinus rhythm.

Third-degree AV block causes cardiac arrest unless an escape rhythm emerges. The escape rhythm may have narrow or wide QRS complexes, depending on where the impulses are discharged and whether there is a concomitant bundle branch block. Escape rhythms with narrow QRS complexes indicate that the block and the ectopic focus (which generates the escape rhythm) are located proximal to the bifurcation of the His bundle. This rhythm is often referred to as a junctional escape rhythm. The junctional escape rhythm is regular, with a frequency of roughly 40 beats per minute. If the escape rhythm has wide QRS complexes and a frequency of 20–40 it is most likely a ventricular escape rhythm. Ventricular escape rhythms are unreliable, such that they may cease and thus cause cardiac arrest. Junctional escape rhythm is more reliable (the risk of cardiac arrest is considerably lower). Moreover, ventricular escape rhythms are slow, resulting in reduced cardiac output and risk of hypoperfusion.

Distinguishing AV block 2 and AV block 3

In case the distinction between second-degree AV block and third-degree AV block is difficult, the following rules may be helpful.

• Irregular ventricular rhythm suggests second-degree AV block because escape rhythms in third-degree AV block are regular.
• Regular ventricular rhythm with association between P and QRS and constant PR interval suggests second-degree AV block.
• Regular ventricular rhythm and varying PR interval suggest third-degree AV block because atrial and ventricular rates are most often not equal (which makes the PR interval appear as varying).

Management of third-degree AV block

Please refer to Management of AV block 1, 2 and 3.