3rd Degree Heart Block

3rd Degree Heart Block (Complete Heart Block) Overview

In complete heart block, there is complete absence of AV conduction – none of the supraventricular impulses are conducted to the ventricles. This one minute video shows what is happening really well.

A perfusing rhythm is maintained by a junctional or ventricular escape rhythm as Chris explained in the WhatsApp group. Failing this, the patient may suffer ventricular standstill leading to syncope or sudden cardiac death.

Complete Heart Block strip
  • The atrial rate (P waves) is approximately 100 bpm.
  • The ventricular rate (QRS) is approximately 40 bpm.
  • The two rates are independent; there is no evidence that any of the atrial impulses are conducted to the ventricles. (no relation between the P waves and the QRS complexes)

Causes of complete heart block

The causes are the same as for second degree heart block. The most important causes are:

  • Inferior myocardial infarction
  • AV-nodal blocking drugs (e.g. calcium-channel blockers, beta-blockers, digoxin)
  • Idiopathic degeneration of the conducting system

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.

Remember your blocks!


In our case study this 37 year old female presented with syncope but no chest pain. As you can see, the P waves are regular and the PR interval has no pattern. The P waves and QRS complexes are also at a different rate. The key to 3rd degree block is this disassociation between the two complexes.

Have a look at this post on LITFL (https://litfl.com/av-block-3rd-degree-complete-heart-block/) for some more ECG examples of 3rd degree block.

Source: https://litfl.com/av-block-3rd-degree-complete-heart-block/

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