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NBME 23 Answers

nbme23/Block 4/Question#41

Electrophysiology of the heart is studied in an ...

Ablation of the atrioventricular node

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 +5  upvote downvote
submitted by sajaqua1(347),

Wouldn't total AV nodal ablation destroy to autorhythmicity of the pacemaker? That would mean that below the AV node the rhythm would be provided by a ventricular foci, and those usually create wide QRS complexes.

haliburton  that was my reasoning as well. guess not. +  
yotsubato  Shitty NBME grammar strikes again. +1  
charcot_bouchard  No. No guys. Bundle of his located below AV node and it can generate impulse. it calls junction escape rhythm and narrow complex. Below this is purkinje, bundle branch & ventricular muscle. those are wide complex +3  
abhishek021196  Third-degree (complete) AV┬áblock The atria and ventricles beat independently of each other. P waves and QRS complexes not rhythmically associated. Atrial rate > ventricular rate. Usually treated with pacemaker. Can be caused by Lym3 disease +1  

 +3  upvote downvote
submitted by aladar50(30),

For the ECG, I initially thought it was 2nd degree Type 1 because it seemed that the PR intervals were increasing until a beat was dropped, but if you look at it closely, some of the P waves were hidden in the QRS complexes. If you notice that, then you can see that there were regular P waves and regular QRS complexes, but there was a complete dissociation between them which means it was 3rd degree heart block, so the answer was ablation near the AV node.

yotsubato  answer was ablation near the AV node. No it wasnt. It was ablation OF THE AV node itself. Which faked me out. +5  
makinallkindzofgainz  The tangent by user "brbwhat" says that there is "pr lengthening progressively" but there is not. This is 3rd degree AV block. The P waves march out consistently at their own rate, and the QRS complexes march out at their own rate. There is complete dissociation between the P waves and QRS complexes. They have no relationship. This is exactly what you would see if you ablated the AV node. The SA node would continue to to create P waves. The bundle of His would continue to generate junctional (normal looking) QRS complexes. +2  

ECG Tracings. The distance between QRS complexes is continuously about 6 boxes, so rate is unaffected. This is a 3rd degree block where teh atria and ventricles are beating independently of eachother and the RR is equivalent all along. The 2nd QRS complex is SUPER narrow, and others are also narrow, which means theyre depolarizing thanks to bundle of His.

This question is asking what would cause this! PR interval messing up is an issue at the AV node. So ablating the AV node would cause heart block.

 +1  upvote downvote
submitted by krewfoo99(40),

In boards and beyond, It is said that third degree heart block is due to block in the HIS Purkinjee system. So why would ablation of AV node cause this disease?

Wouldnt destruction of part of left ventricle be a better answer ?

brbwhat  Had the same doubt, Read the part again and found this. Type 2 Is caused when purkinje is hanging by a thread and therefore some impulses conducted, some not. Chb is caused by purkinje not conducting impulses from san, some lower pacemaker ie purkinje or his is depolarising by itself hence venrticles beat independently. There is BLOCK in purkinje for conduction from san. Among options the only thing that establishes this block is avn ablation. +  

Regular rhythm (count the boxes, they're same), 1 unconducted p wave with regular rhythm --> atria and ventricles beating independently (likely ventricles ctrl by His-Purkinje) --> complete (3rd) heart block --> AVN ablation (this is actually done for pts with Afib sometimes)

I thought ablation of the av node was a tx for a fib not heart block?

I think the question refers to a scenario caused by ablation of a particular region of the heart.

If you look carefully there are some pre-mature ventricular beats (or rather just 1). Pre-mature ventricular beats can be caused by ablation of the AV node since the AV node is responsible for rhythm control by controlling the number of SA node impulses that are transmitted to the ventricular electrical network.

 +0  upvote downvote
submitted by kard(15),

Patients who undergo an AV node ablation are also implanted with a pacemaker to help maintain a normal heart rate. Tx. for complete Block

This is complete heart block. You can find the p wave and QRS complex showed up in different rate

The ECG shows a Mobitz Type 1 block (prolonging PR followed by a dropped wave. These blocks usually arise as a byproduct of a dysfunctional AV node, so ablation at the AV node is the most appropriate answer in this case