Introduction

• there is a complete interruption of conduction between atria and ventricles, so they work independently

• The escape QRS rhythm is regular and originates in the junction or ventricles

• Junctional escape rhythm - with a narrow or wide QRS complex

• Ventricular escape rhythm with a wide QRS complex is more unreliable, slower (

• Third-degree AV block can occur during sinus rhythm or any other atrial rhythm, such as atrial fibrillation or flutter

• It may be persistent or transient

• Although third-degree AV block is a form of AV dissociation (a condition in which atrial activation is independent from ventricular activation), not all AV dissociation represents third-degree AV block

• Complete AV block with sudden onset can cause syncope with possible catastrophic consequences, or even sudden death, when there is no escape rhythm

ECG features

• The atrial (P wave) rate is faster than the ventricular (QRS complex) rate
• P waves bear no relationship to the ventricular QRS complexes
• If block occurs in the AV node - QRS complexes are usually narrow due to a subsidiary pacemaker arising in the bundle of His
• If block occurs below the AV node - QRS complexes are usually broad due to a subsidiary pacemaker arising in the left or right bundle branches 
• Any atrial rhythm can coexist with third-degree AV block - the P waves may be abnormal or even absent.

Picture 1 This rhythm strip from lead I shows AV dissociation of atrial and ventricular action typical for third-degree AV block with regular P-P and R-R intervals and variable PR intervals

ECG 1  Third-degree AV block with ventricular escape rhythm

• P waves have no relationship to the QRS complexes
• sinus tachycardia at a rate of 125 bpm is completely dissociated from the escape rhythm
• wide complex escape rhythm at a rate of 35 bpm with LBBB morphology
• a single ventricular premature beat – 2nd QRS complex in chest leads V1-V6 

ECG 2  Third degree AV block with RBBB morphology ventricular escape rhythm

• sinus rhythm at a rate of 70 bpm with P waves that have no relation to the QRS complexes
• ventricular escape rhythm at a rate of 40 bpm with RBBB morphology and QRS duration of 160ms
• deep Q waves in leads II, III, aVF – indicating previous inferior infarction

ECG 3  Third-degree AV block with atrial fibrillation and junctional escape rhythm 

• ECG shows a fibrillatory baseline with no discernible P wave that suggests atrial fibrillation
• R-R intervals are regular despite the atrial fibrillation – this is typical of atrioventricular (AV) dissociation consistent with complete heart block, with a narrow (supra-Hisian) escape rhythm
• junctional escape rhythm at a rate of 44 bpm, QRS complexes are narrow

A fibrillatory baseline and regular R-R intervals (arrows) indicate that none of the atrial impulses are conducted to the ventricles > complete AV block is present.

ECG 4  Complete AV block with atrial fibrillation and ventricular escape rhythm

• coarse atrial fibrillation – irregular baseline with atrial complexes at rate >400 bpm
• R-R intervals are regular despite the presence of atrial fibrillation > third-degree AV block
• QRS complexes are wide (150 ms) at a rate of 45 bpm > ventricular escape rhythm

Causes

• Complete heart block is most commonly acquired
• Congenital CHB is generally associated with a narrow QRS complex (junctional) escape rhythm
• In half or more of the cases, no specific reversible causes are identified, and the block is felt to be related to idiopathic progressive cardiac conduction disease with myocardial fibrosis and/or sclerosis that affects the conduction system

• potentially reversible causes:
  - Antiarrhythmic drugs - a class I antiarrhythmics or amiodarone, a calcium channel blockers, beta blockers, digoxin
  - Hyperkalemia 
• other pathologic causes include:
  - Myocardial infarction with ischemia of the conduction system
  - Idiopathic intrinsic degenerative diseases (Lenegre-Lev disease)
  - Myocarditis – Lyme disease
  - Infiltrative disease (sarcoidosis, amyloidosis)
  - post-catheter ablation – when in proximity to AV node
  - post-cardiac surgery - mitral and/or aortic valve repair or replacement, ventricular septal defect (VSD) repair

Symptoms

• Patients may be asymptomatic as long as the ventricular rate is more than 40 bpm.
• Acquired third-degree AV block is typically associated with symptoms such as:
  - syncope
  - near syncope
  - lightheadedness or weakness
  - fatigue
  - dyspnea or breathlessness
  - chest pain
  - sudden death

Management

• The initial management of the patient with third degree AV block depends on the presence and severity of any signs and symptoms related to the ventricular escape rhythm.
• Unstable patients require immediate pharmacologic therapy and, in most instances, should also receive temporary pacing to increase heart rate and cardiac output.
• Hemodynamically unstable patients – treat with atropine, isoprenaline, and/or temporary cardiac pacing (either with transcutaneous or, if immediately available, transvenous pacing)
• Hemodynamically stable patients - do not require urgent therapy with atropine or temporary cardiac pacing. However, many ventricular escape rhythms are unreliable and potentially unstable, so patients should be continuously monitored with transcutaneous pacing pads in place in the event of clinical deterioration

References

1. Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Chapter 2 - Bradyarrhythmias—Conduction System Abnormalities, Editor(s): Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Arrhythmia Essentials (Second Edition), Elsevier, 2017, Pages 28-86, ISBN 9780323399685, https://doi.org/10.1016/B978-0-323-39968-5.00002-0.
2. Strauss, David G., et al. Marriott's Practical Electrocardiography. Wolters Kluwer, 2021
3. William H Sauer, MD (2020). Third degree (complete) atrioventricular block. In Gordon M Saperia, MD (Ed.), UpToDate. Retrieved February 9, 2021 from https://www-uptodate-com.ezproxy.is.cuni.cz/contents/atrioventricular-nodal-reentrant-tachycardia?search=atrioventricular%20nodal&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1
4. 2018 Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay,  AM Coll Cardiol. Nov 2018; DOI: 10.1016/j.jacc.2018.10.044

‍