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Monday, 14 January 2013

Mobitz Type I / Wenckebach. (Second degree Heart Block).


Welcome back to ECG Class and Happy New Year to everyone!
I hope you are all well rested and ready for another term.

For the benefit of newcomers:
This ECG educational blog runs alongside Twitter.
A new ECG "quizz" is launched most Monday evenings, in term time. 
Cases are generally aimed at Primary Care. 

Please use the Hashtag #ECGclass if you want to ask the patient any questions, or request any further investigations. 

Remember there are no right or wrong answers!  

ECG interpretation is often open to debate, and will usually evolve as new information becomes available. Everyone's opinion is valid, and useful for others, as this evolution process takes place and we form an interpretation together. 

Please feel free to join in, but most important of all, have fun! :-)

Case 20

I'm throwing you in the deep end a bit tonight, no New Year gradual warm up!  So don't be disheartened. It may look daunting at first glance, but bear with me, and you'll be surprised how easy it is to get on the right track, once you break the ECG down methodically!

It's Friday Morning and you are the duty GP. 
The CHD nurse has just done a 'routine' annual CHD review on a well 81yr old lady. As a matter of routine, she has done an ECG, and wanted to check you are happy with it before she lets the lady go home.

She puts the ECG below on your desk. 
(ECG1.)


Q1 Is there any thing else you'd like to know?

Update 1
Before making any decisions, or reviewing the lady, you decide to look back through her electronic notes. You see that she also had routine ECG's done 1 year ago, and 3 yrs ago. 
These are the traces respectively:

1 year earlier (ECG2):



3 years earlier (ECG3):



Q2 You now feel in a position to go an see the lady. What things might you want to know from her?

Update 2
You go to see the patient. She looks clinically well.
She denies any symptoms including chest pain, shortness of breath, palpitations, dizzy spells or blackouts.  
Examination is completely normal, other than an irregular pulse.
For her well controlled IHD, she takes Amlodipine, but is not on a beta-blocker, nor is she on an other rate limiting medication. She does take Losartan, but has never tolerated aspirin or statins well so she chooses not to take these. For her age, she enjoys a good quality of life, and is very independent. She thinks you are fussing by coming to see her and just wants to go home.

Lets concentrate on the presenting (first) ECG above.

Q3  Is it regular, or irregular? Can you see P waves?


Update 3
Lets look at ECG1 to start with. 

Sometimes it can be difficult to know which waveform is the P wave and which is a T wave! 
Often the easiest way to establish this is to pick any lead with a nice clear P-QRS complex and look out for matching morphology in other leads.

In ECG 1 we can see that the very first complex in leads I, II and II, appears to be a normal P-QRS complex, with a larger T wave following. The T wave is closely followed by a ventricular ectopic beat.

So, concentrating on the Rhythm strip (Lead II), we can recognise several P waves which match that morphology seen in the first waveform: a small rounded, subtly peaked, dome. 

If you map out the P waves of a separate piece of paper, they are in fact regular. 
This may not appear so at first, but where you might expect one to fall, the complex seen is distorted by the hidden P-wave. 
This is perhaps most clearly seen if you study the last 4 complexes of the Rhythm strip: 
Each of the last 4 QRS complexes are preceded by a P wave. Map them out. The wave form which follows the penultimate QRS is distorted by a 'hidden' P wave.

But what about the P-R interval? Those astute ones amongst you might have already noticed that this is inconsistent. The P-R interval in lengthening. 

Look at the 3rd QRS complex on the lead II rhythm strip. (Don't count the Ventricular ectopic beat between the 1st and 2nd normal QRS complexes). 
This 3rd complex has a P-R interval just to say within normal limits (3-5 small squares). 
The 4th complex has a much more prolonged PR interval of about 8 small squares.
The 5th complex has a PR interval the same as the 3rd, BUT - note that the RR interval between the 4th and 5th QRS complexes has lengthened....This should make you suspicious of a missing QRS complex.  (If this occurred post-ectopic beat, you might put it down to a compensatory pause, but this doesn't apply here). 
If you map out the P-waves, you will see that the P falls within the T-wave and distorts the T wave. The expected QRS following this P-wave is missing. i.e the impulse doesn't conduct through to the ventricles
The whole cycle then restarts with a normal PR interval in the 5th complex. 

This is typical of Wenckebach (or Mobitz type1) SECOND degree heart block.

Features of Wenckeback, Mobitz Type I:
  • P-R Interval Progressively lengthens until the P wave fails to conduct
  • The P-R Interval then re-sets to normal
  • Cycle repeats
  • The R-R interval progressively shortens  (This is not seen in the ECG above as the Wenckebach cycle repeats after only three P waves. You would need a longer Wenckebach cycle to see this shortening of the R-R interval). 
So Her presenting ECG today, shows second degree heart block with ventricular ectopics thrown in just to make analysis more tricky on the eye. 
Don't let the ectopics beat you ( :) sorry! ).  When faced with multiple ectopics, always try and find a section on any ECG which has a run of complexes unaffected by the ectopics. It helps! 

You might have been forgiven, on this ECG, for thinking there was a mixture of Ventricular and Atrial ectopic beats (having noticed some QRS complexes without a preceding P wave). This demonstrates the importance of mapping out the P waves, and measuring the consistency of, or variation in, the P-R interval. 

Q3. So, what now?

This lady is 81yrs old. She's well. She's asymtomatic. 
Is Mobitz Type I, Second degree heart block a benign condition? 
Do we need to do anything? 

Update 4
Mobitz Type I has always tended to be regarded as benign, especially in young otherwise healthy individuals. 
However, even in the young, there is some evolving evidence that daytime Wenckebach (as opposed to purely nocturnal Wenckebach) may not be so innocent.  For this reason, more and more cardiologists would often advocate a 24hr tape. 
For all patients it is wise to review their medications.

In this 81yrs lady, the Wenckebach is possibly more significant. 

Looking back at her old ECG's I think we can see the block slowly evolving. 

In ECG 3  (taken 3 yrs ago) it is interesting that the computer analysis reports at 1st degree AV block.  I think this is dubious, but certainly borderline. There are a few atrial ectopics there as well, but for a routine ECG, in an asymptomatic patient, it is quite appropriate that this ECG was filed without action.

In ECG 2 (taken 12months ago) the computer makes nothing of the PR interval, but even with the poor quality baseline, I think we can see that the PR interval is consistent but prolonged, in keeping with 1st degree Heart Block.  Again, at this stage, in a well patient, the only action necessary may be a medication review to make sure she is not on any rate-limiting medication. An alert on her electronic records may be wise, to remind clinicians not to initiate beta-blockers, or rate-limiting calcium channel blockers.

In conjunction with the new onset of ventricular ectopics, this lady's new onset Wenckebach may represent serious underlying conductive tissue disease.
An appropriate plan might be:

  • Medication review : any beta-blockers should be stopped.
  • Electrolytes should be checked
  • Other medications such as Digoxin/Tricyclics should be reviewed and considered.
  • Refer for a 24hr ECG tape - with advice NOT to drive until reviewed with results. 

Well done All!
Thank you, once again, for joining in. 
H. :)



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