Complete heart block on ECG: why is the QRS narrow (for nurses & nursing students)?
The short answer? It depends on where the escape rhythm originates.
Understand that a complete heart block involves an underlying escape mechanism/ rhythm since the lower chambers “think” there is no input coming from above.
What is an escape rhythm?
Think of an escape rhythm almost as a safety net. If something upstream fails to initiate an impulse then an area downstream needs to take over to prevent asystole essentially (or ventricular standstill in some cases).
Two cases in which you might expect an escape rhythm (or beat) to occur are:
Following a prolonged pause (think sinus arrest or exit blocks)
During a complete heart block
What does this mean?
In order for a complete heart block to be present you’ve got to have independent atrial and ventricular activity. So, what?
So… based on what we know about escape rhythms, think about this: what you basically have in a complete heart block is an underlying junctional rhythm OR an underlying idioventricular rhythm that’s superimposed with independent atrial activity.
If the AV junction picks up the escape responsibilities the QRS will be narrow
If an area in the ventricles picks up the escape the QRS will be wide
Why should I care?
The point I want to make here is that when we identify complete (third degree) heart blocks the focus should be on the relationship between atrial/ P waves and QRS complexes. Forget about the width of the QRS and instead look for a PR interval that’s inconsistent and variable with each beat.
Hope this helps!
-Tyler
