Diagnosing a 2:1 AV block using Vagal manuevers?

[CuriousGeorge2]

Hey all,

I'm learning about 2nd degree AV block right now and am using Rapid Interpretation of EKGs by Dubin as my primary EKG learning resource. It's been fairly helpful so far, but I am reading about how to differentiate between 2:1 AV block as being a Wenckebach or Mobitz block and it just doesn't make any sense to me.

Dubin says that employing a vagal manuever will convert a 2:1 block to:

- A 3:2 block if it is a Wenckebach block.

or

- Normal 1:1 conduction if it is a Mobitz block.

Why would increasing parasympathetic stimulation to the heart improve a Wenckebach block? If the problem with Wenckebach is in the AV node and the parasympathetic nervous system inhibits conduction in the AV node, wouldn't you just make things worse? I would think that a 2:1 Wenckebach might result in a complete block (only P waves, no QRS responses).

I don't understand why a Mobitz block would be improved by parasympathetic stimulation either.

Took a look at a couple of other books and one said vagal maneuvers worsen the Wenckebach while another said that vagal manuevers are not reliable at all 👎

Any EKG pros out there who have a better understanding of this?

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[CuriousGeorge2]

-

 

[medicsb]

Hey all,

I'm learning about 2nd degree AV block right now and am using Rapid Interpretation of EKGs by Dubin as my primary EKG learning resource. It's been fairly helpful so far, but I am reading about how to differentiate between 2:1 AV block as being a Wenckebach or Mobitz block and it just doesn't make any sense to me.

Dubin says that employing a vagal manuever will convert a 2:1 block to:

- A 3:2 block if it is a Wenckebach block.

or

- Normal 1:1 conduction if it is a Mobitz block.

Why would increasing parasympathetic stimulation to the heart improve a Wenckebach block? If the problem with Wenckebach is in the AV node and the parasympathetic nervous system inhibits conduction in the AV node, wouldn't you just make things worse? I would think that a 2:1 Wenckebach might result in a complete block (only P waves, no QRS responses).

I don't understand why a Mobitz block would be improved by parasympathetic stimulation either.

Took a look at a couple of other books and one said vagal maneuvers worsen the Wenckebach while another said that vagal manuevers are not reliable at all 👎

Any EKG pros out there who have a better understanding of this?

I suppose" improve" is subjective in this sense. The SA and AV nodes are both innervated by the vagus so increased parasympathetic stimulation could cause the SA nodal action potentials to decrease in frequency and "match" the conduction velocity of the AV node allowing the conduction to seemingly improve. However, I would also suspect that simultaneous increased parasympathetic stimulation on the AV node would nullify any changes on the SA node. (Anyone feel free to point any flaw to my logic.)

As a medic I employed vagal maneuvers ~15 times for PSVT and only had it work once, so I'm suspicious of using the vagal maneuver to differentiate an AV block. And, having had patients in 2nd AVBs, it is usually possible to differentiate with careful analysis of the ECG (12 lead preferred), so I don't really see it as useful. However, assuming that the block can't be differentiated, I am doubtful that differentiation would be clinically relevant as treatment would essentially be the same as far as I know.

 

[Doktorp]

I'm wondering the exact same thing, also stuck at the same page in Rapid interpretation of EKGs. So counterintuitive. I'd be very grateful for an explanation.

 

[Rendar5]

Don't trust Dubin on that. The truth is, you need to consider a 2:1 AV Block to be a Mobitz II until proven otherwise in an EP lab.

 

[JGimpel]

Dubin is a horrible book to learn EKGs from.

Vagal maneuvers can be a carotid massage which may help with converting a tachycardia to a more acceptable RRR condition or having the patient bear down to increase venous return back to the heart. Both of these can cause a vagally-mediated response, either to the increased baroreceptor firing or to that sense of increased preload. The AV node is told to stop firing as fast so that the heart beats in a slower, but still around normal, more controlled fashion. This is how you improve a tachycardic patient and possibly stabilize the rhythm at the same time. Remember that last point.

I know, I know; we're talking about blocks.

If they have a block AT the AV node, that is more likely to be a Wenckebach, which is a Mobitz 1 and you can see it improve since this is primarily an AV problem and a vagal response will primarily serve to slow the AV node down. As the AV node slows down and stabilizes, it's speed becomes more in line with that of the ventricle and the Mobitz 1 block may resolve completely. Since both the atria and ventricles are still firing and the chambers of the heart are still responding, this is not an absolute emergency and people can live a long time with this if it stays stable this way.

Mobitz 2 blocks often occur lower than the AV node, thus there is no varrying in the PR; you're going into ventricular territory. If you're somewhere in the HIS bundle, your AV node has passed it's impulse down to the ventricle, but your ventricle has an impaired ability to follow along with it, whether it wants to or not, whether it receives the full impulse or non at all. This is why a Mobitz 2 indicates a higher level of acuity and will need much closer management.

Wenckebachs/Mobitz 1s can be annoying, but Mobitz 2 can be lethal if they progress on to a 3rd degree block where there is erratic communication and subsequent action of the two worlds which can ultimately lead to a fatal arrhythmia. The communication may become so bad that something called an "escape rhythm" is created by the ventricles since the communication from the AV node onward might not be reaching through the HIS bundle at all. Thus, you have a complete block due to complete dissociation of the atria from the ventricles and circle the drain quickly from there. Definitely something better differentiated in an EP lab ASAP so you don't miss the latter assuming the former.

I hope this helps! Dubin sucks!

 

[Lanolin]

Just get an EKG and see if the PR interval is increasing and then dropping a beat.

 

[duckie99]

Just get an EKG and see if the PR interval is increasing and then dropping a beat.

the point is that in 2:1 block you cannot see the change in a PR interval since every other ORS is dropped. Apparently there are some "hints" on an EKG that can help you in deducing whether it is type 1 or type 2 such as also having a bundle branch block indicating that it may be type 2 (don't hold me to that though...). Regardless they have to go to EP lab for diagnosis.

 

[Doktorp]

Thanks for the replies everyone, if I see a 2:1 block I'll definitely send the patient to the EP lab!

Other than the 2:1 block explanation, I've found Dr Dubin's book to be great, especially considering I knew very little about ECGs initially. So I would recommend it to ECG-noobs like myself. I find it's good way to learn the basics 🙂

Suggestions on other great books on ECGs are much appreciated!

 

[Rendar5]

"The Only EKG Book You Will Ever Need" is much better than Dubin. Garcia is good once you get past that book as it has a graded level of instruction.

 

[JGimpel]

Great resource

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[Bluesclera]

Hey all,

I'm learning about 2nd degree AV block right now and am using Rapid Interpretation of EKGs by Dubin as my primary EKG learning resource. It's been fairly helpful so far, but I am reading about how to differentiate between 2:1 AV block as being a Wenckebach or Mobitz block and it just doesn't make any sense to me.

Dubin says that employing a vagal manuever will convert a 2:1 block to:

- A 3:2 block if it is a Wenckebach block.

or

- Normal 1:1 conduction if it is a Mobitz block.

Why would increasing parasympathetic stimulation to the heart improve a Wenckebach block? If the problem with Wenckebach is in the AV node and the parasympathetic nervous system inhibits conduction in the AV node, wouldn't you just make things worse? I would think that a 2:1 Wenckebach might result in a complete block (only P waves, no QRS responses).

I don't understand why a Mobitz block would be improved by parasympathetic stimulation either.

Took a look at a couple of other books and one said vagal maneuvers worsen the Wenckebach while another said that vagal manuevers are not reliable at all 👎

Any EKG pros out there who have a better understanding of this?

Instead of Dubin I recommend Clinical Electrocardiography: A Simplified Approach by Ary L. Goldberger MD

It really takes your ECG interpretation to the next step and its probably in your medical library. there are some other resources if you are interested but I suggest this book first.

 

[JGimpel]

I definitely agree that "The Last EKG Book etc." and the Goldberger book, in that order, are much better than the Dubin book. If you're really trying to impress a Cardiologist or CT Surgeon, you could ask if you could borrow one of his (Edward) "Chung's" but that's way overboard. Have an established way you read an EKG every time, just as you have a way to read a CXR every time, and you'll be fine.