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.
- 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
- 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.