Atrioventricular Blocks
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Article at a Glance
- AV blocks represent varying degrees of impulse blockages from the atria to the ventricles.
- In a first-degree AV block, the PR interval is uniformly prolonged.
- A second-degree type 1 block presents an increasing PR interval until a QRS complex is dropped.
- In a second-degree type 2 AV block, P waves “march through” in a consistent manner.
- A third-degree block has a complete dissociation between the atria and ventricles. Each beats at an independent rate.
Atrioventricular blocks (AV blocks) are conduction delays which may cause a complete blockage of impulses from the atria to the ventricles.1 An increase in vagal tone during sleep, athletic training, pain, or stimulation of the carotid sinus may cause AV blocks.
A heart block occurs when the electrical signal traveling through the heart is impaired.
Ischemic heart disease causes 40% of AV blocks.2 Hereditary fibrosis or sclerosis of the cardiac skeleton known as idiopathic progressive cardiac conduction disease may damage the conduction system, causing AV blocks. Patients with cardiomyopathies, myocarditis, congenital heart disease, and familial disease may also develop AV blocks.
Clinicians induce AV blocks by increasing potassium concentration above 6.3 mEq/L via the use of medications such as verapamil, diltiazem, amiodarone and adenosine. It may also be iatrogenic in nature, such as a catheter ablation in patients with arrhythmias.
Related Video – What are Junctional Rhythms?
If the PR interval is prolonged (more than 0.20 seconds), it’s a first-degree AV block. Structural defects within the atrioventricular node, or an increase in vagal tone due to conduction-slowing medications such as digoxin, beta-blockers, and calcium channel blockers cause first-degree AV block. No actual “block” occurs in first-degree AV blocks. Most are harmless and transient. A P wave embedded within the T-wave and a PR-interval of about 0.28 seconds.First-Degree AV Block
Related Video – One Quick Question: What are the First-Degree Heart Block Criteria?
- RATE: depends on the core rhythm.
- RHYTHM: depends on the core rhythm.
- P WAVES: upright and uniform; each P wave is followed by a QRS complex.
- PR INTERVAL: more than 0.20 seconds, constant throughout the 6-second rhythm strip.
- QRS COMPLEX: less than 0.12 seconds.
Second-degree AV blocks have occasional non-conducted P waves with a prolongation of the PR interval. Mobitz type 1 AV block (also known as Wenckebach) is an intermittent conduction block within the AV node, causing conduction impulse failure from the atria to the ventricles. The impaired nodal conduction is progressive and results in a total block, causing an absence of impulse to the ventricles, as reflected in the QRS complex disappearing on the ECG tracing. There is a prolongation of the PR interval with each beat until a beat is dropped. This pattern repeats. An ECG tracing of a second-degree Mobitz type 1 AV block. The figure above shows a clustering of the first four QRS complexes. Clinicians should note the prolongation of the PR interval followed by the absence of a QRS complex. Mobitz type 1 AV block is benign. It rarely causes hemodynamic instability in patients, and most do not require any treatment. When a patient with Mobitz type 1 AV block has symptoms, they generally respond well to atropine. In rare cases, a pacemaker is required.Second-Degree (Mobitz Type 1) AV Block
Related Video – One Quick Question: What are the Second-Degree Heart Block Criteria? Video
Related Video – ECG Rhythm Review – Second Degree Type 1 Heart Block (Wenckebach)
- RATE: may be bradycardia.
- RHYTHM: regularly irregular with grouped beating pattern.
- P WAVES: upright and uniform; some P waves are not followed by a QRS complex.
- PR INTERVAL: progressively lengthens until one P wave is blocked and the QRS is dropped.
- QRS COMPLEX: less than 0.12 seconds.
Read: Junctional Rhythms
A second-degree Mobitz type 2 AV block involves the His-Purkinje system. Impulses are not conducted from the atria into the ventricles.3 A block occurs after the atrioventricular node, within the bundle of His or both bundle branches. The His-Purkinje system is an all-or-none conduction system. There are no changes in the PR interval, even after a non-conducted P-wave. The P waves “march through” at a constant rate. A Mobitz type-II AV-block contains a higher risk of advancing to complete heart block compared to a Mobitz type I AV block. This rhythm is a serious arrhythmia which may lead to death if untreated. Patients will likely require a permanent pacemaker. An ECG tracing of a constant PR- and R-R interval until a dropped beat (QRS complex) occurs.Second-Degree (Mobitz Type 2) AV Block
Related Video – ECG Rhythm Review – Second-Degree Heart Block (Type 2)
Third-degree AV block is a complete blockade of the atrioventricular node.4 Impulses from the atria cannot reach the ventricles. The perfusing rhythms are governed by junctional or ventricular escape rhythms. Third-degree AV block is secondary to atrioventricular nodal disease. It Involves the His-Purkinje system, and is caused by coronary artery disease, enhanced vagal tone, a congenital disorder, an underlying structural heart disease such as myocardial infarction, hypertrophy, inflammation, infiltration, Lyme disease, post-cardiac surgery, cardiomyopathies, rheumatologic diseases, autoimmune diseases, amyloidosis, sarcoidosis, or muscular dystrophy. Patients may have ventricular standstill, leading to sudden cardiac death. Patients with a third-degree AV block urgently need a pacemaker. An ECG strip shows a third-degree AV block. In the above strip, P waves occur every 0.92 seconds and the R-R interval is every 1.24 seconds, representing a dissociation in impulse rates from the atria and ventricles.Third-Degree AV Block
Related Video – One Quick Question: What are the Third-Degree Complete Heart Block Criteria?
Related Video – ECG Rhythm Review – Third-Degree Heart Block
AV blocks are caused by heightening degrees of impulse blockages from the atria to the ventricles. The different AV blocks are first-degree, second-degree type 1, second-degree type 2, and third-degree blocks. Each has its own heart rate, rhythm, P wave placement, PR interval, and QRS complex. First-degree isn’t very dangerous while third-degree is often fatal.Summary
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Editorial Sources
ACLS Certification Association (ACA) uses only high-quality medical resources and peer-reviewed studies to support the facts within our articles. Explore our editorial process to learn how our content reflects clinical accuracy and the latest best practices in medicine. As an ACA Authorized Training Center, all content is reviewed for medical accuracy by the ACA Medical Review Board.
1. Anthony H. Kashou; Amandeep Goyal; Tran Nguyen; Lovely Chhabra. Atrioventricular Block. National Library of Medicine. 2021.
2. Keri Wiginton. What Is Atrioventricular (AV) Block? WebMD. 2022.
3. Muhammad Asif Mangi; Wesley M. Jones; Mohamed K. Mansour; Laura Napier. Atrioventricular Block Second-Degree. National Library of Medicine. 2021.
4. Vinicius Knabben; Lovely Chhabra; Matthew Slane. Third-Degree Atrioventricular Block. National Library of Medicine. 2022.
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