AV block: 3rd degree (complete heart block)

Complete Heart Block (CHB) Overview

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).

ECG Strip 3rd degree AV Block Complete heart block CHB

Typically the patient will have severe bradycardia with independent atrial and ventricular rates, i.e. AV dissociation.

3rd-degree-heart-block — Complete heart block strip

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

ECG Inferior STEMI with 3rd degree AV Block CHB

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).