Atrioventricular block (AV block): definition, causes, diagnosis & management
This article covers the fundamental principles of atrioventricular (AV) blocks, which are classified into three types: first-degree, second-degree, and third-degree AV blocks. Readers already familiar with the basics of AV blocks can proceed directly to the subsequent chapters, which provide detailed discussions on each type.
The atrioventricular (AV) conduction system and AV blocks
The atrioventricular (AV) system is composed of the AV node and the His-Purkinje system, which transmit electrical impulses from the atria to the ventricles. Conduction through the AV node is intentionally slow (due to the low concentration of gap junctions in AV nodal cells), which allows for ventricular filling to complete prior to ventricular contraction. In contrast, contractile cells–and Purkinje fibers in particular–are rich in gap junctions, enabling rapid impulse conduction through the ventricles.
After passing the AV node, the electrical impulse travels through the His bundle, which bifurcates into the left and right bundle branches. The left bundle branch further subdivides into two fascicles. From these branches and fascicles, Purkinje fibers extend into the myocardium. Conduction through the Purkinje system is very rapid, due to the high density of gap junctions. This rapid transmission ensures that the majority of the ventricular myocardium depolarizes nearly simultaneously, which is crucial for maximizing the efficiency of ventricular contraction (Figure 1).

The atrioventricular (AV) node is densely innervated by both sympathetic and parasympathetic nerve fibers. Sympathetic stimulation enhances impulse conduction through the AV node, a phenomenon known as the bathmotropic effect. In contrast, parasympathetic stimulation increases resistance within the AV node, further slowing the transmission of impulses. Intense parasympathetic activity can result in a complete blockage of impulse conduction. If such blockage persists for 6 seconds or more, syncope occurs unless an escape rhythm arises.
Overview of AV blocks
Impulse conduction from the atria to the ventricles may be delayed or 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 slightly misleading 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 disconnected. This condition is referred to as AV dissociation. Importantly, for the ventricles to exhibit any electrical—and consequently mechanical—activity, an escape rhythm must originate from an ectopic focus located distal to the block. Third-degree AV block is a critical condition; as escape rhythms may fail to develop, be transient, or produce an insufficient heart rate. In the absence of an escape rhythm, cardiac arrest will ensue.
Symptoms caused by AV blocks
First-degree AV block is virtually always asymptomatic. It may cause symptoms if the delay is very long, because atrial and ventricular activity may become severely desynchronized.
Second-degree AV block is usually asymptomatic unless there is a high-degree block (i.e. many atrial impulses are blocked). The patient may experience irregular heart rate, palpitations, pre-syncope or even syncope. Syncope is more common in Mobitz type 2 blocks.
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 phenytoin may all cause AV block.
- Hypothermia.
- Borreliosis (Lyme disease, caused by Borrelia spp.).
Beta-blockers are not contraindicated in patients with first-degree or second-degree Mobitz type 1 AV block; however, these conditions necessitate ECG monitoring after initiating beta-blocker therapy to ensure the block does not worsen. In contrast, second-degree Mobitz type 2 and third-degree AV blocks are contraindications for beta-blocker use unless a permanent pacemaker has been implanted.
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). 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).