AV block: 3rd degree (complete heart block)

Definition

• In complete heart block, there is complete absence of AV conduction – none of the supraventricular impulses are conducted to the ventricles.
• Perfusing rhythm is maintained by a junctional or ventricular escape rhythm. Alternatively, the patient may suffer ventricular standstill leading to syncope (if self-terminating) or sudden cardiac death (if prolonged).
• Typically the patient will have severe bradycardia with independent atrial and ventricular rates, i.e. AV dissociation.

Example of complete heart block

• The atrial rate is approximately 100 bpm.
• The ventricular rate is approximately 40 bpm.
• The two rates are independent; there is no evidence that any of the atrial impulses are conducted to the ventricles.

Mechanism

• 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 (e.g. secondary to septal infarction in acute anterior MI).
• 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)

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”

Further Reading

• ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
• ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
• ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
• ECG Exam Template — a framework for the FACEM part 2 exam.
• ECG Reference Sites on the WEB — the best of the rest

Author Credits

References

• Hampton, JR. The ECG in Practice (5th edition), Churchill Livingstone 2008.
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
• Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.