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

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:

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:

Example 2

Complete Heart Block:

Example 3

Complete Heart Block:

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

LITFL Further Reading