This Heart is Blocked Not

People still make Borat references, right?

Anyway, I’ve got a quick one for you today. This patient was being evaluated for possible pericarditis. What’s the rhythm?

Click to enlarge.


Here’s another look at lead II from a different 12-lead. Click to enlarge.

And here’s a third strip of lead II for your consideration. Click to enlarge.

Let’s first discuss what this rhythm is not:

  • Sinus arrest or sinus pause (as it was read by one physician)
  • Type II AV Block (as diagnosed by another physician and the computer)

So why were these providers mistaken?

  • The first was too hasty in his reading. He saw only irregular pauses in the sinus rhythm and wrongfully assumed they were due to inactivity of the SA-node. Looking closely, however, you can appreciate that there is a “blip” in the T-wave preceding each break in the rhythm. This blip is a buried P-wave. Although it’s not followed by a QRS-complex, it resets the SA-node before the next scheduled sinus beat and results in a compensatory pause while the sinus node repolarizes and prepares to resume normal pacemaking.

Red arrows denote buried P-waves. Click to enlarge.

  • The second physician managed to avoid that pitfall and picked-up the hidden waves, but then made another common error. If you march out the P-waves in these strips, you’ll notice that the buried ones arrive early in the cardiac cycle and are thus premature atrial complexes (PAC’s). In fact, they arrive so early that the AV-node is still in its absolute refractory period and cannot conduct to depolarize the ventricles. This results in a P-wave that is not technically “blocked,” but instead is what we term “non-conducted.”
    In this case semantics matter. It is the job of the AV-node to keep the ventricles from being overwhelmed if the atria fire too rapidly (i.e. atrial fibrillation), so refusing to conduct these excessively early beats demonstrates that the AV-node is in-fact behaving normally. Saying these P-waves were “blocked” would give the impression that there was pathology involved and carry a much different prognosis.

It’s easy, right? This rhythm is very simply: Normal Sinus Rhythm with Multiple Non-conducted PAC’s.

During your career the number of ECG’s you’ll see with non-conducted PAC’s will far exceed the cases of Type II AV Block. Whenever you encounter a pause in a rhythm strip, remember these two things:

  • The most common cause of a pause is a non-conducted PAC,
  • The most common cause of blocked P-waves is not.

9 thoughts on “This Heart is Blocked Not

  1. ekgpress says:

    Nice illustration and discussion of the phenomenon of non-conducted PACs! I would add to this excellent discussion the clinical point that that blocked PACs are commonly mistaken not only for Mobitz II – but exceedingly frequently for 2nd degree AV block, Mobitz Type I (Wenckebach) – because (as in this example) – there is group beating and a narrow QRS (as well as the additional “footprint” of Wenckebach that the pause containing the dropped beat is less than twice the shortest R-R interval). As Vince astutely points out – the reason this tracing is not AV block is that the ‘blip’ (buried PAC) in the T wave at the beginning of each pause occurs early – whereas with Mobitz I or Mobitz II the P-P interval should be regular (or at least almost regular if there is underlying sinus arrhythmia).

    Finally – I’ll utter a preference to allow continuation of the terminology initiated by the esteemed Dr. Marriott (a linguist himself) – who popularized the wonderful pearl of wisdom contained in the phrase, “The commonest cause of a pause is a blocked PAC”. I’ve quoted that phrase literally hundreds of times in national talks and local rounds over the past 3 decades.

    While fully extolling the notion against calling Atrial Flutter with 2:1 conduction a “2-to-1 block” (because conduction of every-other-impulse in the setting of 300/minute flutter is truly physiologic) – I never got the impression that saying a non-conducted PAC is “blocked” connotes pathology. And after decades of use by myriad Marriott followers – substituting “non-conducted PAC” for “blocked PAC” in the phrase that begins with “commonest cause of a pause” just doesn’t have the same ring. If that phrase had been started by anyone other than Barney Marriott (who was the most astute cardiology master of language use I’ve ever encountered) – I might consider otherwise …

    THANKS again Vince for an EXCELLENT post on PACs that just don’t make it to the ventricles ….

    • Vince D says:

      Thanks Dr. Grauer!
      I actually picked up the blocked-vs.-non-conducted distinction from the eighth edition of Marriott’s “Practical Electrocardiography.” On page 149 he states:

      “It is better to refer to such beats as “nonconducted” rather than “blocked” because, by definition, block implies pathology and many such beats fail to be conducted only because they arise so early in the cycle that the A-V tissues are still normally refractory. We should always be at pains to differentiate pathological from physiological nonconduction, especially since failure to do this has led to widespread overdiagnosis and overtreatment of heart block.”

      Then at the bottom of the page:


      But here’s the clincher… if you’re saying that in actual discussion most people use the term “blocked,” I’m totally on-board. I know you recognize the distinction from, as you stated, your atrial flutter terminology. Most of what I’ve learned about electrocardiography has been from textbooks since, outside of our online groups, I don’t get to “talk ECG’s” with many folks around here. I’ll probably still keep using the term “non-conducted” because I like it, but you’ve been doing this on a much higher level for far longer than me, so I’m completely with you if the vernacular is to call them “blocked-PAC’s.”

      • ekgpress says:

        THANKS Vince for the update (I no longer have the much earlier Marriott edition that I studied so much from in years past). I think what we are both agreeing on that “block” (non-conduction) of a PAC (early atrial beat) does not truly “strike fear” in the heart of the listener – whereas Atrial Flutter with 2:1 Block (in which half of the beats are not conducted) sounds more ominous …. ergo my preference to emphasize AFlutter with 2:1 AV CONDUCTION (not block) – while still employing the fluid saying, “Commonest Cause of a Pause = a Blocked PAC”.

        Again – THANKS for your excellent post on your blog! – : )

  2. Thea says:

    blocked premature atrial extra

  3. Igor says:

    A beautiful ECG. Even though, the diagnosis is quite simple. A good ecg reader would never assume this as an AVB, wouldn’t mistake that P wave 400 ms after the preceding P wave with a sinus activity, especially with this heart rate and without junctional cycles after the pauses, all things that can make the task a little bit harder.

  4. Floyd says:

    What about a patient with sinus p waves and slightly varying r-r intervals, with a dropped and or non conducted p waves without being able to see a buried p wave?

    • ekgpress says:

      @Floyd – Got a particular tracing in mind? I’m sure VinceD would LOVE to post it! The KEY (as you suggest in your question) is being able to identify that “buried” P wave. That can at times be VERY challenging – esp. if there is some baseline artifact on the tracing (often difficult in such cases to know what is “real” vs artifact). BUT – if you have a relatively CLEAN tracing – I’ve found that by VERY careful attention to ALL “normal” T waves on the tracing – and comparing them to the T waves at the beginning of the pause – that I can usually find some “telltale” notching or roundness or other subtle deformity to make the diagnosis. In contrasting – the slight variation characteristic of sinus arrhythmia is usually not something you would confuse with blocked PACs … Again – I’m SURE VinceD would love it if you had a particular tracing to illustrate your question – : )

  5. marty braff says:

    why would it be important to know whether the ecg shows blocked pacs or a sinus pause.
    is there any treatment for either condition?

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