In 3rd Degree Heart Block (Complete heart block), the atria and the ventricles are disconnected from eachother. As a result, the sinus node is still firing and you will see P waves marching throughout. However, these P waves are not being conducted to the ventricle. As a result of this, the PR intervals will be of variable lengths.

As the top half of the heart is firing on it’s own, there must be some electrical source to take over the bottom half, or else you would end up with asystole. This is an escape rhythm, and it can originate in two places: (1) The AV Node or (2) The Ventricle.

ECG Findings:
1. P waves present
2. Variable PR intervals
3. Lack of a 1:1 atrial ventricular relationship
4. More atrial beats than ventricular beats



Picture 5

*The Red Arrows point to the P waves marching out
*The black lines show variable PR intervals (this distinguishes it from Mobitz II)