AV block: 3rd degree (complete heart block)

third degree heart block


  • 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

3rd degree 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.


  • 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:

  • 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

Further Reading

Author Credits


  • 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.

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  1. Hassan Gargoum says

    In example #3 (ECG shows 3rd degree AV block with sinus rate of 100/min and ventricular rate of 17 beats per minute). It is suggested in the comments that “This patient needs urgent treatment with atropine / isoprenaline and pacing!”. What do you think would happen if you would give atropine or isoprenaline to this patient? I do not think you will achieve more than increasing his sinus rate without any effect on the ventricular rate. This patient should be paced immediately. You may consider giving intravenous calcium gluconate if hyperkalemia is suspected. I do not think that beta blocker toxicity/overdose is an issue her given the baseline sinus tachycardia, otherwise a beta stimulant agent such as ispretrenol or dobutamine may reverse this condition. Thanks

  2. says

    Thanks for the comments guys. Perhaps I should revise my comments re ECG 3.
    While most cases of 3rd degree AV block are unlikely to respond to atropine, there are a small group of atropine-responsive patients whose 3rd degree AV block occurs due to increased vagal tone (e.g. due to digoxin toxicity, Bezold-Jarisch reflex in inferior wall MI). These patients are more likely to have a narrow QRS, with the level of block occurring at the AV node or Bundle of His. ECG 3 has a broad QRS and very slow ventricular rate, indicating a more distal block (e.g. Purkinje fibres) and so would be unlikely to respond to atropine.
    My practice in a haemodynamically unstable patient with 3rd degree AV block is usually to fire off a quick VBG (to check the K+) and try a test dose of atropine (usually readily available at the bedside in resus) while the nursing staff are applying the pacing pads. I’m not necessarily expecting the atropine to work, but if it does then that is a bonus -- we have bought ourselves some time. This approach is endorsed by the American Heart Association 2010 Bradycardia guideline (http://circ.ahajournals.org/content/122/18_suppl_3/S729/F3.expansion.html). However, it would be perfectly reasonable to omit the atropine if the QRS complexes are broad. Giving drugs such as atropine / isoprenaline should not delay the institution of pacing.

  3. Jessie says

    Quick question. But first, I wanted to thank you for the information, its very useful for studying!

    My question is, if the patient’s atria and ventricles are contracting at different rates (ex above: atria contracting 100 times/min, and ventricles at 40 times per minute) how would determine the heart rate of the individual? If you knew of an equation that would be great.

    Thanks in advance!

  4. says

    Hi Jessie,

    The heart only ejects blood during ventricular systole, and so the heart rate (the “pulse”) is equal to the ventricular rate on the ECG. If you were asked to comment on the overall heart rate, you would use the ventricular rate. However, it would be more informative to comment on both atrial and ventricular rates, to draw attention to the fact that AV block is present.


    • Jessie says

      Thank you so much!! Yeah that makes complete sense, I think I just got confused a little because of the different rates of depolarization.

  5. Katy says

    Hi there, in example 4 Im wondering why you do not see wide QRS complexes as you would expect since the ventricular beat is initiated below the AV node? Thx!