AV block: 3rd degree (complete heart block)

ECG features of complete heart block

In complete heart block, there is complete absence of AV conduction, with none of the supraventricular impulses conducted to the ventricles. The perfusing rhythm is maintained by junctional or ventricular escape rhythm. Alternatively, the patient may suffer ventricular standstill leading to syncope (if self-terminating) or sudden cardiac death (if prolonged).

Pathophysiology

• Complete heart block is essentially the end point of either Mobitz I or Mobitz II AV block
• It may be due to progressive fatigue of AV nodal cells as per Mobitz I (e.g. secondary to increased vagal tone in the acute phase of an inferior MI)
• Alternatively, it may be due to sudden onset of complete conduction failure throughout the His-Purkinje system, as per Mobitz II. This can be secondary to septal infarction in acute anterior MI, or as a result of progression of conducting system disease causing true trifascicular block
• The former is more likely to respond to atropine and has a better overall prognosis

Causes of complete heart block

The causes are the same as for Mobitz I and Mobitz II second degree heart block. The most important aetiologies are:

• Inferior myocardial infarction
• AV-nodal blocking drugs (e.g. calcium-channel blockers, beta-blockers, digoxin)
• Idiopathic degeneration of the conducting system (Lenegre’s or Lev’s disease), causing true trifascicular block

Clinical significance

• Patients with third degree heart block are at high risk of ventricular standstill and sudden cardiac death
• They require urgent admission for cardiac monitoring, backup temporary pacing and usually insertion of a permanent pacemaker

Differential diagnosis

Complete heart block should not be confused with:

• High grade AV block: A type of severe second degree heart block with a very slow ventricular rate but still some evidence of occasional AV conduction
• AV dissociation: This term indicates only the occurrence of independent atrial and ventricular contractions and may be caused by entities other than complete heart block (e.g. “interference-dissociation” due to the presence of a ventricular rhythm such as AIVR or VT)

ECG Examples

Example 1

Complete Heart Block:

• Atrial rate is ~ 85 bpm
• Ventricular rate is ~ 38 bpm
• None of the atrial impulses appear to be conducted to the ventricles
• Rhythm is maintained by a junctional escape rhythm
• Marked inferior ST elevation indicates that the cause is an inferior STEMI

Example 2

Complete Heart Block:

• Atrial rate is ~ 60 bpm
• Ventricular rate is ~ 27 bpm
• None of the atrial impulses appear to be conducted to the ventricles
• There is a slow ventricular escape rhythm

Example 3

Complete Heart Block:

• Atrial rate 100 bpm
• Ventricular rate only 15 bpm!
• This patient needs urgent treatment with atropine / isoprenaline and pacing!

Example 4

Complete Heart Block with Isorhythmic AV Dissociation (long rhythm strip):

• Atrial rate ~ 85 bpm
• Ventricular rate ~ 42bpm
• There is a junctional escape rhythm
• As the ventricular rate is approximately half the atrial rate, this rhythm at first glance appears to be second-degree AV block with 2:1 conduction
• However, on closer inspection, the PR interval varies with some of the P waves superimposed on the QRS complexes. The ventricular rate remains regular
• This confirms that the atrial impulses are not being conducted to the ventricles
• The apparent relationship between the P waves and QRS complexes occurs merely by chance (= isorhythmic AV dissociation)

Related Topics

• AV block: 1st degree
• AV block: 2nd degree, Mobitz I
• AV block: 2nd degree, Mobitz II
• AV block: 2nd degree, “fixed ratio blocks” (2:1, 3:1)
• AV block: 2nd degree, “high grade AV block”

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004
• Smith SW. The ECG in Acute MI. 2002 [PDF]

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest