Overview of atrioventricular (AV) blocks

 

Atrioventricular block (AV block): definition, causes, diagnosis & management

This article discusses the basic principles of atrioventricular (AV) blocks. There are three types of AV blocks, referred to as 1st degree AV block, 2nd degree AV block and 3rd degree AV block. Readers who are already familiar with AV blocks may skip to the subsequent chapters discussing each type of AV block in detail:

• 1st degree AV block
• 2nd degree AV block
• 3rd degree AV block

The atrioventricular (AV) conduction system and AV blocks

The AV system consists of the atrioventricular node (AV node) and the His-Purkinje system. These structures conduct the atrial impulse to the ventricles. Impulse conduction through the atrioventricular node is slow. This is explained by the scarcity of gap junctions in the cells of the atrioventricular node. Contractile cells and, in particular, Purkinje fibers, have an abundance of gap junctions that enables rapid impulse conduction. Nevertheless, the slow impulse conduction through the atrioventricular node has a physiological purpose. It causes a delay which gives the atria enough time to empty their blood into the ventricles before ventricular contraction starts.

After leaving the atrioventricular node, the impulse continues through the His bundle which branches into the left and right bundle branches. The left bundle branch is further divided into two fascicles. From these bundles and fascicles, the Purkinje fibers sprout out into the myocardium. Impulse conduction through the Purkinje system is very rapid due to the high abundance of gap junctions. The rapid impulse transmission enables the majority of ventricular myocardium to be depolarized (more or less) simultaneously. This is important since it optimizes the efficiency of the contraction. Refer to Figure 1

Figure 1. Components of the ventricular conduction system and the temporal association between the ECG waveforms and impulse transmission through the heart. Atrioventricular (AV) blocks occur due to dysfunction in the conduction system.

The atrioventricular (AV) node is richly innervated with sympathetic and parasympathetic fibers. Sympathetic input causes increased impulse conduction (bathmotropic effect), whereas parasympathetic input causes increased resistance in the atrioventricular node (additional slowing of the impulse). Very strong parasympathetic input may lead to a complete block of impulses.

An overview of AV blocks

Impulse conduction from the atria to the ventricles may be abnormally delayed or even blocked. These conditions are referred to as atrioventricular (AV) blocks, subdivided according to the degree of block. First-, second- and third-degree AV block may all be diagnosed using the ECG.

First-degree AV block

Synonyms: AV block 1, AV block I

The term block is somewhat misleading in this case because first-degree AV block only implies that the conduction is abnormally slow. By definition, the PR interval is >0.22 s. However, all impulses are conducted to the ventricles. First-degree AV block is rarely serious and may be left untreated in the vast majority of cases (exceptions are discussed later).

Second-degree AV block

Synonyms: AV block 2, AV block II

In second-degree AV block, some impulses are completely blocked, such that not all P-waves are followed by QRS complexes. Second-degree AV block is subdivided into the following variants:

• Second-degree AV block Mobitz type 1. May also be referred to as Wenckebach block.
• Second-degree AV block Mobitz type 2.

Second-degree AV block (particularly Mobitz type 2) requires treatment.

Third-degree AV block

Synonyms: complete heart block, AV dissociation, AV block III, 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 not arise, exist transiently, or generate insufficient heart rate. Cardiac arrest occurs if no escape rhythm arises.

Each of these AV blocks will be discussed in detail in separate articles.

Symptoms caused by AV blocks

First-degree AV block is virtually always asymptomatic. It may cause symptoms if the delay is extremely long, because atrial and ventricular activity may become severely desynchronized.

Second-degree AV block is usually asymptomatic unless there is high-degree block (many atrial impulses blocked). The patient may experience irregular heart rate, palpitations, pre-syncope or even syncope. However, this is uncommon (particularly syncope).

Third-degree AV block is mostly symptomatic because it causes a reduction of cardiac output due to bradycardia. Lightheadedness, dyspnea, angina, dizziness, pre-syncope, or syncope may occur. Cardiac arrest occurs if an escape rhythm is not established.

Causes of AV blocks

AV blocks occur due to functional or anatomical blocks in the AV system. The block may be located in the atrioventricular node, His bundle, bundle branches or fascicles. A wide range of conditions may cause AV blocks:

• Idiopathic fibrosis of the conduction system: Roughly half of all AV blocks are due to fibrosis. This correlates strongly with age.
• Ischemic heart disease: 35% of all AV blocks are due to acute or chronic ischemic heart disease (coronary artery disease). All types of AV block may occur due to ischemia/infarction. Note that inferior myocardial infarction usually causes transient AV blocks (which resolve within 7 days), whereas anterior wall infarction generally causes permanent AV blocks. AV block due to myocardial ischemia and infarction has been discussed in the chapter Supraventricular and Intraventricular Conduction Defects in Myocardial Infarction and Ischemia.
• Vagal stimulation: The Vagus nerve slows the heart rate and conduction through the AV node. Vagal activity increases in the following situations: carotid sinus massage (intentional or not), Valsalva maneuver, acute pain, and hypersensitive carotid sinus reflex. Vagal fibers unload acetylcholine on AV nodal cells which slows conduction and may even block conduction with ensuing asystole. In the vast majority of cases, the asystole is transient.
• Structural heart disease: aortic stenosis, aortic regurgitation, mitral valve stenosis, mitral valve regurgitation, myocarditis, perimyocarditis, myocardial infarction, heart surgery and cardiomyopathy may all bring about damage to the conduction system and cause AV blocks.
• Congenital: Any degree of AV block may occur at birth.
• Hyperkalemia, hypokalemia.
• Digoxin: Recall that digoxin may cause all arrhythmias and conduction defects, including all degrees of AV block.
• Verapamil, amiodarone, beta blockers, and fenytoin may all cause AV block.
• Hypothermia.
• Borreliosis (Lyme disease, caused by Borrelia spp.).

Localization of the level of the block

Localizing the level of the block is relevant as it has implications for the prognosis and treatment. The more distal (from the atrioventricular node) the block, the greater the risk of development of complete heart block (third-degree AV block). This is because automaticity diminishes gradually with the distance from the AV node. It is often difficult to localize the level of the block on the 12-lead ECG. There are, fortunately, some rules of thumb that should be used. The block in first-degree AV block is mostly located in the atrioventricular node. The block in second-degree AV block Mobitz type 1 is also mostly located in the atrioventricular node. These types of AV block are the most benign. The block in second-degree AV block Mobitz type 2 is mostly located in the bundle of His or distal to it. The block in third-degree AV block is mostly located in the atrioventricular node or the bundle of His.

QRS duration may be used to differentiate between blocks located in the AV node and the bundle of His (i.e. proximal to the bifurcation of the bundle of His). In order for the QRS duration to be normal (QRS duration <0.12 s) the impulse must pass through the bundle of His and be delivered to both bundle branches. Thus, normal QRS duration implies that the block is located proximal to the bifurcation of the bundle of His. Prolonged QRS duration (QRS duration ≥0.12 s) is less helpful, because it may be due to either (1) block located distal to the bifurcation, or (2) block located proximal to the bifurcation but with concomitant (separate) bundle branch block.

To conclude, if the QRS duration is <0.12 the block is most likely located in the AV node or bundle of His, which indicates a better prognosis than broad QRS complexes, which are much more likely to be due to blocks distal to the bifurcation of the bundle of His. An electrophysiological study is necessary to firmly establish the level of the block, but this is only rarely needed (because management is based primarily on the degree of the AV block).

Figure 2 shows the principles of location of the block and the QRS complex.

Figure 2. Principles of AV blocks and the emergence of escape rhythms / beats.