Heart blocks cheat sheet
I just read Student RN Tiffany’s woes regarding heart blocks and want to bless the blogosphere with my distilled knowledge on this subject, having pondered the differences at some length so I don’t look totally stupid at work.
- First-degree blocks are easy: if the P-R interval is longer than 0.20 seconds, it’s a first-degree AV block. Period. End of story. That’s longer than one large box on the EKG paper.
- There are two types of second-degree blocks, which is stupid because why didn’t they just make first-, second-, third-, and fourth-degree blocks? But I digress. Both involve dropped beats. You can tell them apart by whether the P-R interval is consistent. A type II second-degree block has consistent P-R intervals, but a type I (Wenkebach) block has longer and longer P-R intervals until the impulse just doesn’t quite reach the ventricles and the QRS is dropped. So the P-R interval is normal (therefore not a first-degree block) in a type II block—it’s just not always followed by a QRS complex—but progressively longer in Wenkebach.
- Third-degree (complete) heart blocks can be spotted in the wild pretty easily if you suspect them when you see a verrrrry slooooow ventricular rate. The ventricles have no access to the perky SA node and are left to their own devices to contract at 20 to 40 beats per minute. The atria, however, march along to their usual drummer at the normal sinus rate. To confirm your suspicions, march out the P waves (don’t be fooled by P waves hiding inside the QRS complexes) to see if they’re regular.
EKG classes and books tend to make this more difficult than it needs to be. Anything else, oh wise readers?
Are you including things like Junctional rhythms in 3rd degree blocks? What you’ve described as 3o blocks could be junctional, bundle branch or a few others. Just my two cents…
her 3rd degree HB explanation is almost correct. as far as Junctional or BBB, or even Ventricular Escape beats, you’re contrasting the speed of the actual ventricular contractions.
i would assume (some cardiologist help me here) because a 3rd degree HB is essentially a diseased AV node, then you would rarely see nodal (junctional) beats which are disassociated with the atrial beats.
therefore, every 3rd degree HB i’ve seen in the field has been Ventricular Escape rhythem coupled with P waves, neither of which have anything to do with each other…
MY 3rd degree HB indication: when the P waves are getting eaten by a QRS complex on a random basis, one should assume and investigate 3rd Degree HB.
Nice break down!
I’ve never understood them but then again never had too. That’s what we have junior docs for.
Yes, I think of junctional rhythms and bundle branch blocks as still having a (mostly) regular association of P waves with QRS complexes, whereas with a CHB they don’t if you trace them out (note the twice-mentioned caveat that the QRS complexes “eat” some of the P waves). If you have a junctional rhythm, you’ll have no P wave or an inverted or retrograde P wave, but it’ll still appear in the same spot relative to each QRS complex and not migrate all over the strip. Yes? I guess I should have stated the assumption that you’re starting out having figured you’ve got SOME type of a block and your issue is differentiating which type it is. Sometimes I can’t even tell that. We get some crazy rhythms at work. I watch the monitor going “are those P waves? Or T waves? Or are retrograde inverted P waves being eaten by the T waves?” (Or similar.) My favorite rhythm by far, though, is the wandering pacemaker. The mental image causes me to snort every time.
A fun way to learn this stuff is at Ambulance Driver’s blog:
http://ambulancedriverfiles.blogspot.com/2007/06/sex-relationships-and-cardiac.html
ACLS’s way of doing it is pretty straightforward. The only time it will ever be wrong is when a Second Degree Type I is so slow that it drops every other beat… in which case it might as well be a Type II anyway. Pure semantics.
Ask these 3 questions, in order, and go no farther than it takes to identify the rhythm:
1. Are there more P waves than QRS complexes? If no, it is not a 2nd or 3rd degree block. If yes, move on to Question 2:
2. Is the PR interval fixed? If yes, it is a Second Degree Type II block. If no, move on to Question 3:
3. Is the rhythm regular? If yes, the rhythm is a 3rd Degree Block, which may have either a junctional or ventricular escape pacemaker. If the rhythm is irregular, it is Second Degree Type I Block.
That will correctly identify the vast majority of heart blocks you will ever encounter.
Incidentally, did you know Wenckebach identified his phenomenon 3 years before Einthoven invented the EKG?
Thoroughly enjoyed this post. I love the bantering over how to discern and differentiate the differing rhythms. I guess that’s why EKG is always a sensitive area for most. It’s all in the interpretation.
Yes, we should know what we are looking at, but more importantly it’s what you have to do for them. What is your course of action or treatment for each of yoru findings.
I think this is the biggest factor when evaluating your EKG’s. Semantics will drive you buggy! It’s whether or not it’s a rhythm that needs treated.. and what is the treatment.
LOVED this post.
Thanks!
I remember spending hours in the Coronary Care Unit back in the day, discussing and arguing over exactly what a particular monitor strip showed. Had a cardiolgist say that if you can’t give at least three possibilities for each strip (including Sinus Rhythm…it could be PEA!), you didn’t know your stuff! : D
Of course, this is debating after the fact, not over the patient! : D
Remember 3rd degree (AV dissasociation) isn’t always 30-40 range. Had one a few weeks ago that was mid 60′s. Two completely different independent rhythms of the atria and ventricles. The cardiologist was upset when the nurse called him at 0300 for a “rhythm change.” And I got chewed out a little for it, but it was the right call, and he knew it :p
The problem was the previous shift had seen P waves moving through out the rhythm, and assumed a 2nd degree, even though they didn’t measure to see if they were “regular”, which indeed they were.
Anyways, my two cents.