Atrioventricular (AV) blocks are rhythms in which conduction through the AV node is altered (Box 9.1). In some cases, conduction is only minimally impacted, and thus clinical manifestations of the block may be mild or even absent. However, in other instances, AV conduction is markedly and adversely affected; significant clinical signs and symptoms will be evident. The AV blocks are described on the basis of the PR interval and the relationship between the P waves and the QRS complexes.

First‐Degree Atrioventricular Block

First‐degree AV block (Figure 9.1 and Box 9.2) is most often generated by an impulse from the sinoatrial (SA) node and produces a normal P wave. Conduction is simply delayed through the AV node. The rhythm is characterized by a prolonged PR interval greater than the normal 0.2 s. Although delayed, each impulse is conducted through the AV node to the ventricle, maintaining a 1 : 1 atrial–ventricular relationship. In other words, each P wave is followed by a QRS complex, and every QRS complex has a corresponding P wave. The PR interval is consistent from one beat to the next and yields consistent and equal P–P and R–R intervals. The axis is usually normal, and, except in cases of pre‐existing bundle branch blocks, the QRS complex is normal and narrow (<0.12 s).

While first‐degree AV blocks do not directly cause bradycardia, patients exhibiting this rhythm often have heart rates less than 60 bpm. This observation is particularly true in patients with other defects of impulse formation and conduction, as well as in patients who are taking medications that decrease heart rate. Furthermore, first‐degree AV block may be seen in conjunction with the higher degree blocks, described below, that often occur at slower rates. The altered AV conduction of first‐degree heart block rarely, if ever, leads directly to clinical compromise.

Second‐Degree Atrioventricular Block

Second‐degree AV block is further subcategorized into Mobitz type I (Wenckebach) and Mobitz type II (non‐Wenckebach) AV block. These subtypes are defined by the length of the PR interval and the progressive relationship of the P wave to the QRS complex.

Second‐degree AV block Mobitz type I (Figure 9.2 and Box 9.2), also known as the Wenckebach rhythm, shares some features of the first‐degree AV block and may demonstrate either a normal or a slow ventricular rate. Although this rhythm most often occurs in the setting of a prolonged PR interval, the initial PR interval may be normal. The hallmark of the Mobitz type I AV block is the progressive lengthening of the PR interval from one beat to the next. This progressive lengthening continues until there is a beat in which the impulse from the SA node generates a normal P wave, but is then completely blocked at the AV node. Therefore, there is no resulting QRS complex associated with this particular beat (often referred to as a dropped beat). This P wave is sometimes referred to as an orphan P wave, that is, one with no QRS complex following.