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NBME 23 Answers

nbme23/Block 4/Question#41 (50.2 difficulty score)
Electrophysiology of the heart is studied in ...
Ablation of the atrioventricular node🔍

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CGE nr.gTasci hTe ctsniade etbeenw QRS leexsopcm si iltocnnoysuu otuab 6 osb,xe os reat is nsai c.ftdTheuef si a dr3 edeerg blkoc weerh the aarit adn nlrctvesie ear ganeibt epetndynedlin fo eceorthah and eth RR is tqulveiane lal lo gh.neaT nd2 SQR lopemxc is ESUPR an,rwor adn tsrheo ear asol n,orraw hwhic smaen reetyh gaioezrlpnid naksth ot eulnbd of isH.

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submitted by aladar50(36),
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orF the GCE, I iaynitill htuthog ti aws 2dn geeerd peTy 1 ueaecbs it eseemd htta eht RP islnarevt erew sreaiigncn ulnit a tbae swa pdd,preo ubt fi yuo kolo at it lyols,ce some of het P vwsea rwee enddih ni eth QSR xc.epsoeml fI yuo eotnci at,th tneh yuo nac see taht heert weer larergu P aewvs and ueargrl RQS epselmx,oc tub etehr wsa a ocmeltpe oasiindtocis neebetw hetm whcih snmea ti was r3d reeged trahe bcok,l os hte aswrne wsa iaatnbol anre eth AV .neod

yotsubato  answer was ablation near the AV node. No it wasnt. It was ablation OF THE AV node itself. Which faked me out. +8  
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. +6  

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submitted by sajaqua1(439),
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ou'lndWt attol AV andlo alnoiatb torysde to tuamochirthiyty fo teh ecmkaaepr? ahtT douwl nmea ttha owlbe hte VA odne hte hyrhmt wdulo be vodrpeid by a vunraerclit fcoi, adn eosht aluslyu ecerta weid RQS mpeesoclx.

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 +10  
abhishek021196  Third-degree (complete) AV block 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  

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shiT usioneqt si gskian whta olduw uaesc hti!s PR antvrlei esmsnig up si na isues at het AV eo.nd oS ilnaagtb the AV deon duwol asceu rathe kobl.c

 +2  upvote downvote
submitted by krewfoo99(75),

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. +  

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ulRager tryhhm (cotnu eht xe,sbo 'rthyee es)a,m 1 cndetdncuuo p vaew iwht argrelu yhmhrt gt-&-; itaar and nvscetelri ibtagne ndetdnenpyeil ikyell( tevsrielcn lrct yb nijkPH-eu)sri gt&--; emoctple (r3d) rehta kcolb t-&g-; AVN baatloin i(hts si lyaaultc ndeo for spt hwit ibfA mm)eeitsos

UW Question Id: 1976 Testing a very similar concept and provides a great explanation for this!

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.

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I uothhgt ablianto fo eth av oned was a xt orf a bif ton thera bclk?o

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submitted by kard(32),
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tantePis how egrdnou an VA oend aonbatil rae loas mdntpleia hiwt a armepckea ot hlep miatinna a anorml herat ter.a Tx. orf ctemlpeo lkcoB

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Teh GEC ssowh a iMtzob eTyp 1 kbloc pog(inlgnor RP loowedfl by a doprepd eh .saTewev bsoclk luylaus iaser sa a dytpcrbuo fo a utyclfodnsian AV ,endo os batainlo at eht AV oend is eht tmso arpeirpoapt asrnwe ni shit ecsa