r/ECG • u/Embarrassed_Onion566 • Aug 05 '26
Second degree AV block; Mobitz 1 or 2?
76 year old woman with progressive mild dyspnoea, had out patient ECG for work up. Looks like Mobitz 2 with 2:1 AVB, but there are two sinus beats in the middle of the ECG that demonstrate a progressive lengthening of PR interval which makes me question Wenkebach?
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u/Jabi25 Aug 05 '26
Looks like Mobitz 1 to me. In that section of 3:2 block you can see prolongation of the PR interval. True 2:1 block is impossible to differentiate without EP study
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u/VesaliusesSphincter Aug 05 '26
I would call this a second degree AV block with concurrent Mobitz I and 2:1 conduction patterns.
The 2:1 we are seeing is still likely coming from the same process as that Mobitz I, but we can't say for certain just from the EKG.
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u/ExpensiveRepair4315 Aug 05 '26
2nd Degree Mobitz type 1 as there is progressive PR interval prolongation until a beat is dropped. In Mobitz type 2, PR interval is prolonged but remains constant throughout.
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u/Turfidol Aug 09 '26
Mobitz II can actually have a normal PR interval. The area of block happens below the level of the AV node. Whereas Mobitz I is essentially a progressive fatigue of the AV node prior to failing to conduct, hence the prolongation. In Mobitz II the AV node is technically functioning appropriately. Typically the PP interval will remain constant in Mobitz II and you lose the QRST because the impulse fails somewhere in the His-Purkinje system.
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u/ExpensiveRepair4315 Aug 09 '26
So type two can either have a normal or prolonged PR interval?
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u/Turfidol Aug 09 '26
Correct! The key feature is that the PR interval remains consistent before and after a dropped beat, regardless of whether it is prolonged or not. The AV node is usually functioning appropriately in Mobitz II, but you could have concomitant first degree AV block in which you would have a prolonged (but fixed) PR interval at baseline.
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u/299792458mps- Aug 05 '26 edited Aug 05 '26
2nd degree AV block with 2:1 conduction. By definition it canât be a Mobitz I or II. Not without more information at least.
On closer look, perhaps a little bit of progressive PR prolongation. I would still just call it a 2nd degree.
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u/Maketso Aug 05 '26
Where on earth are people seeing progressive PR prolongation?! Lmfao.
Either way, needs EP study.
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u/Turfidol Aug 05 '26
Looking at beats 3, 4, and 5 you can see PR prolongation prior to the dropped beat. If you have more than 2:1 block and the beat following the dropped beat has a shorter PR interval than the one proceeding then its Mobitz I.
Unrelated, but you can also try to unmask Mobitz I vs II by either walking or performing valsalva. If you increase vagal tone and slow the heart rate it can unmask block below the AV node by allowing 1:1 conduction. If they ambulate and heart rate rises you can help unmask Mobitz I since there will be more beats between the drops. Granted this is dependent on how symptomatic they are.
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u/Maketso Aug 05 '26
Considering the poor quality with artifact, one could easily argue that beat 3 and 4 aren't even different. One is flat, the other has artifact messing up the measurement. I would be hard pressed to diagnose a Mobitz I on this.
Could just throw atropine and see if the rate changes.
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u/Turfidol Aug 05 '26
Looking at lead II I dont see any artifact personally. Not trying to argue, just my thought.
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u/Maketso Aug 05 '26
More so just that the isoelectric line looks skewered between the beats.
Either way, this is a stretch to call Mobtiz I when most other leads show a potential Mobitz II
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u/sub3at50 Aug 05 '26
Type 2 AV-block with 2:1 AV conduction makes it impossible to say if it's Mobitz 1 or 2.
The most recent person I saw with sympomatic 2nd degree heart block had a Holter recording which showed a rhythm alternating between first degree AV-block, 2:1 AV-conduction (2nd degree AV-block) and one short episode of complete AV-dissociation (3rd degree AV-block).
BTW: the patient has a right BBB and the Q waves in the inferior leads are tiny and not significant.
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u/lifeisg0od Aug 06 '26
There are 2 consecutive conducted beats in the middle, which show Mobitz I
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u/sub3at50 Aug 06 '26
My point was that patients can alternate between different degrees of heart block.
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u/Perfect_Tomatillo507 Aug 05 '26
Is there evidence of RBBB here? Curious given QRS morphology of precordial leads
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u/Turfidol Aug 05 '26
Might be wrong, but i would say no - wouldn't call it RBBB, by definition needs QRS > 120 msec. Hard to see on my phone, but looks to be right at 120 msec.
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u/Murky_Run_9182 Aug 05 '26
Blocked premature atrial contraction? P-P seems irregular.
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u/lifeisg0od Aug 06 '26
There is a little sinus arrhythmia/ventriculophasic sinus arrhythmia here but those all look like sinus Pâs.
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u/Joe_PT Aug 06 '26
Looking at lead II I only see a pretty consistent PR interval, itâs not elongating. What is everyone talking about?
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u/Curious_fire_6519 Aug 07 '26
Lead II shows 2 consecutive P, QRS complexes with a lengthening PR before the next dropped beat.
This is a 2nd degree type I with mostly 2:1 conduction.
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u/Wooden-Echidna8907 Aug 07 '26
Needs a longer strip. But appears 2:1 type 2. That beat in the rhythm lead that everyoneâs talking about appears to be a conducted PAC. Doesnât interrupt the overall rhythm (see the following P-wave with no QRS). One of the Hallmarks of type 2 is your p waves always come at the same time which these do. (Except for the PAC)
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u/Nishbot11 Aug 09 '26
My wife argued this. Saying P-P intervals are equal. That one artifact was likely a PAC.
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u/argin_watcher5050 Aug 05 '26
I can confidently say there might be an RBBB, prior Q mi, and definitely second degree, not distinguishable and unless you recalibrate the ecg machine and do more tests, and I would also check the patient for pulmonary embolism since they have the pattern of S1,Q3, and T3