In case the distinction between second-degree AV block and third-degree AV block is difficult, the following rules may be helpful. Moreover, ventricular escape rhythms are slow, resulting in reduced cardiac output and risk of hypoperfusion. Junctional escape rhythm is more reliable (the risk of cardiac arrest is considerably lower). Ventricular escape rhythms are unreliable, such that they may cease and thus cause cardiac arrest. If the escape rhythm has wide QRS complexes and a frequency of 20–40 it is most likely a ventricular escape rhythm. The junctional escape rhythm is regular, with a frequency of roughly 40 beats per minute. This rhythm is often referred to as a junctional escape rhythm. 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. 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. Third-degree AV block causes cardiac arrest unless an escape rhythm emerges. This scenario, which is referred to as isoarrhythmic AV block, may even simulate sinus rhythm. 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. The atrial rate is typically faster than the ventricular rate. P-waves may occur on the ST-T segment ( Figure 1, upper panel). P-waves have constant PP interval and ride straight through the strip, without any relation to QRS complexes. On the ECG P-waves have no relation to the QRS complexes. Third-degree AV block (complete heart block, AV dissociation). However, second-degree AV block Mobitz type 2 frequently progresses to third-degree AV block.įigure 1. Progression from second-degree AV block Mobitz type 1 (Wenckebach block) is uncommon. Progression from first-degree AV block to third-degree AV block is rare. 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. 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). The atria and the ventricles are electrically dissociated from each other. In third-degree AV block, no atrial impulses are conducted to the ventricles. This chapter discusses third-degree AV block, which is synonymous with AV dissociation, complete AV block, AV block III and AV block 3. Third-degree (complete) AV block: ECG criteria, clinical characteristics and management
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