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An approach to heart blocks

As alluded to in a prior section the P wave starts the process, and the QRS should behave and follow it, like so: 
 

Whilst most ED clinicians would throw all semblance of order out of the window in order to spam call the cath lab, the author assumes that the reader has not yet joined the best speciality there is. 

Therefore, you will behave, and note that this is still a "normal sinus rhythm."


 

Again, note how the above ECG is also a normal sinus rhythm. 


This is because "normal sinus rhythm" =/= "no abnormality detected". 


I could 100% with a straight face sign the above ECG as being "normal sinus rhythm", and I would be correct.  Whilst it is true that there are other findings that need action, those actions are covered in part 1 of my lecture series

The reason is that "sinus rhythm" purely refers to two things: 

1) That it originates from the Sino-Atrial (SA) node. 
2) That is propogates down the expected pathways in sequence.  

(This diagram took way too much time to make from scratch and i hate myself for it)
 

Therefore...

Does every P wave have a QRS?
Does every QRS have a P wave?

Here are some examples: 
 

Looks good. P:QRS ratio is 1:1
The PR interval is >0.2s though. 
(This is a 1st degree Heart block, and as a general rule of thumb, 99.9% of the time these are benign. )

Wuh woh. 
Every QRS has a P. 
It seems not every P has a QRs...

This therefore might  be...

3rd degree heart block

I will tell you that there is logic to my madness (sometimes). 
3rd degree heart blocks are very straight forward to understand (electrographically) 
 



The moment you think "are the P's and Q's even talking to each other?" and it looks like theyre doing their own thing - 

That's 3rd degree heart block (AKA complete heart block, or CHB for short) in a nutshell. 

There is a disconnection between the atria and the ventricles somewhere, and the two are not talking to each other. 

This is bad, although not all causes require a pacemaker. (Again, for brevity we will not go through them here. Check out part 2 of my series if you want the deep dive!) 

 

Getting our ducks in a row

2nd degree heart block

I've chosen to loop back around to this because its slightly more complicated. 

It may come to a surprise to some of you, but inbetween 1 and 3 lies 2. 

Mind blowing information I know.

More importantly however, is that the reason 2nd degree heart blocks are categorised as such as because in these rhythms, the P:Q is not a nice 1:1 ratio. 
 

 However, there is a degree * of the P and Q talking to one another - they are not going around doing their own thing, such as in a 3rd degree. 

This category of "in between" heart blocks is therefore, surprise surprise, labelled as "type 2", because they're in between 1st and 3rd degrees. 

There are two types, named after  ̶V̶o̶l̶d̶e̶m̶o̶r̶t̶ ̶ Woldemar Mobitz. He was a Russian-German Physician kicking around in the first half of the 20th century. 

His German roots therefore saw the logical naming scheme of "Mobitz type 1" and "Mobitz type 2."

(Disappointingly, there is no absurdly long German word for this phenomenon. It is simply named "Mobitz Typ 1, Mobitz Typ 2")

 

2nd degree heart block, 
Mobitz type 1
 

In both types of heart block,
what IS a fixed ratio is the P-P interval. That is to say, the P waves, and thus atrial contractions, remain defined and constant. 

The difference is with respect to the response to the atrial contraction. 


Type 1 is the "going going gone" type. 

Every QRS will have a P wave, but in no particular fixed ratio. 

In Type 1, strictly speaking, it is the PR interval which progressively gets longer, until it gets too long and a "dropped beat" occurs 

This type is mostly considered benign, and is from the AV nodal cells being fatigued. By refusing to transmit a P wave, they get to recover, and the cycle continues. 

2nd degree heart block, 
Mobitz type 2
 

Type 2 is the more concerning type, and of note, has a fixed PR interval, and a fixed ratio of P wave - Q wave transmission. 

This is because it is usually due to structural damage, resulting in an "all or nothing" when it comes to transmitting a wave. 

This therefore leads us to the notion of "AV block" - that is to say, what ratio of "A(trial) impulses are transmitted to the V(entricles)"

The example above is 2:1. 

It can, however, come in higher ratios, like so: 

Again - the P-P Ratio is fixed. 

The common error here is to not realise that the P wave is hidden/augmenting the T wave. 

The above is therefore a 3:1 AV block. 

We are nearly at the end, so can basically summarise this page as follows: 

You should now be in a position where you can work out heart blocks with the following questions: 

Is the ratio of P:Q waves 1:1?
If yes:
if the PR <0.2, is a (probably**) a sinus rhythm. 
If it is >0.2, it is a 1st degree HB. 

If no, continue to the next question: 

Are the P and Q waves doing their own thing, and completely ignoring each other's presence?

If yes: This is a 3rd degree/complete heart block

If no: continue to the next question. 

Is the PR interval fixed?

If yes: This is a 2nd degree, Mobitz type 2

If no: This is a 2nd degree, Mobitz type 1. 

(NB: You can also determine type 1 vs 2 by seeing whether the P-P interval is fixed or not. Often, however, it is more obvious/faster to observe the PR interval than it is getting out our calipers. Which is definitely part of the EM physicians day to day tool kit.) 

We are just about ready to  ̶q̶u̶a̶c̶k̶  wrap things up...

 *See what I did there? No? I'll show myself out...

** - Assuming the P wave is actually from the SA node, and is not instead a junctional rhythm. Want to understand junctional rhythms? Covered in part 2 of my ECGlecture series!

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