Wednesday, December 22, 2010

Weakness due to Bradycardia due to Sick Sinus with Sinoatrial block, with Ashmann's phenomenon

A 68 yo female with a history of HTN on metoprolol and of paroxysmal atrial fib presented with weakness. This is her ECG.

There is sinus bradycardia at a rate of about 55, with a p-wave every 1.05 seconds. There are 2 dropped p-waves. The succeeding p-wave in each case is exactly 2.1 seconds later. This means that the sinus node continues to depolarize and do its pacemaking job every 1.05 seconds, but that occasionally that signal fails to leave the sinus node and get to the atrium where it would depolarize the atrium resulting in a p-wave.

Notice that the p-wave that comes directly after the 8th QRS conducts, but the one that comes after the 4th QRS does not.  This is Ashmann's phenomenon: the refractory period of a beat is longer when the preceding R-R interval is longer.  In this tracing, it is not obvious (you must measure it), but the R-R inverval between R-waves 3 and 4 is longer than the R-R between R-waves 7 and 8.

She was also found to be hypothyroid.

This is sino-atrial exit block. The sinus node fires but it does not escape. The combination of bradycardia (exacerbated by metoprolol), with paroxysmal atrial fib and sino-atrial block are typical of sick sinus syndrome. It will inevitably progress to the point where a pacemaker will be necessary.

16 comments:

  1. Thanks for this interesting ECG :why morpholoy of QRS with dropped p wave is the same as other one?

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  2. Good question. The QRS morphology is the same because the escape is nodal. So the node can be activated from above (sinus) or on its own (escape). Either way, it conducts down the Purkinje system identically.

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  3. Thanks Dr Smith but a more difficult question:What is your idea about the last beat? why the conducted p result in QRS while in the first group it doesn't?

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  4. Good observation. The 4th p-wave did not conduct, presumably because tissues were refractory at that point. The 8th p-wave does conduct. I suspect that the AV node and/or His bundle had simply repolarized a bit faster on the later one, thus allowing conduction. Does that help?

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  5. This is Ashmann's phenomenon: the refractory period of a beat is longer when the preceding R-R interval is longer. In this tracing, it is not obvious (you must measure it), but the R-R inverval between R-waves 3 and 4 is longer than the R-R between R-waves 7 and 8.

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  6. hello Dr. smith, is it T wave, just after 4th escape beat.

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    Replies
    1. Amita,
      No, it is a P-wave, then a T-wave. OK?
      Steve Smith

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  7. Wonderful ECG and discussion.
    Regarding the P wave following the 8th QRS complex: Given that the PR interval is so uncharacteristically long, isn't it possible that that P-wave is in fact not conducted and the following QRS is a PJC?

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  8. What is the explanation for the morphology of the 5th P wave (P wave of the 5th complex) being different than the rest?

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    Replies
    1. good question. not certain! (hadn't noticed it before!)

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    2. Couldn't it just be an ectopic beat?

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    3. I’m just seeing this interesting tracing now, that was posted years ago by Dr. Smith. As to MKJ’s question in 2018 — we KNOW that the 5th P wave is truly different in shape (and that this is not just artifact) — because we see a definite difference in P wave morphology BOTH in the long lead II, as well as in simultaneously-obtained lead V2. This 68-year old woman almost undoubtedly has SSS ( = Sick Sinus Syndrome). FIRST STEP = Stop the ß-blocker. I suspect this won’t solve the problem — and as per Dr. Smith, ultimately a pacemaker will probably be needed (ie, once off of ß-blockers there are pauses >3.0 second in duration). So — the sinus node is “sick”, therefore NOT functioning. Hard to say if the reason for the junctional escape beat ( = 4th beat in this tracing) after a pause of 1.6 second is a true 2:1 exit block vs some type of Wenckebach exit block vs simply marked sinus bradycardia and arrhythmia followed by a pause. SSS commonly also results in prolonged SA node "recovery" — so the 5th P wave is simply “rescue” occurring from another atrial site (which arose before the SA node could recover) — until finally with the 6th P wave there is return of sinus rhythm.

      So to answer the 9/21/2020 question by “medic” — the 4th QRS is not an “ectopic” beat — but rather an appropriate junctional escape beat that arises because of the 1.6 second pause. The problem with SSS is that in addition to a “sick sinus node” — there is also often a “sick AV node” — so eventually the junctional escape beats become slower — and then a pacemaker is needed.

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