ECG Blog #174 (AV block – PACs – group beating)

The 2 lead II rhythm strips shown in Figure-1 were obtained from the same patient, just a few minutes apart. Unfortunately — no history is available.

• Note that there is group beating in both tracings. Is this Wenckebach? (ie, Is the rhythm in this figure 2nd-degree AV block, Mobitz Type I?).
• How certain are you of your answer?
• HINT: Find a pair of calipers — and USE them to figure out your answer!

Figure-1: Two lead II rhythm strips obtained minutes apart from the same patient. Is this 2nd-degree AV block of the Mobitz I type ( = AV Wenckebach)? 

COMMENT: The presence of group beating should be immediately noticed in Figure-1. That is, there are 2 groups of 3 beats (ie, in rhythm A — beats #3,4,5 and beats #6,7,8) — and multiple groups of 2 beats (ie, beats #11,12 in A — and in B, beats #2,3; 4,5; 6,7; 8,9, 10,11). Recognition of group beating can prove invaluable — because this finding instantly tells you: i) that some kind of repetitive pattern is present in the rhythm; and, ii) that you should strongly consider the possibility of some type of Wenckebach conduction.

• IF the rhythm in Figure-1 was AV Wenckebach — then at least several of the “FOOTPRINTS” of Wenckebach should be present. (This is a direct link to 32:20 in our ECG Video on the Basics of AV Block, and takes you to the part in which we discuss the “Footprints of Wenckebach”. If you click on SHOW MORE, below this video on the YouTube page — You’ll see a detailed linked Contents of all in this video!).
• Use of calipers instantly tells us that the rhythm in Figure-1 is not AV Wenckebach because: i) the P-P interval is definitely not regular (Usually the P-P interval is at least fairly regular when there is AV block); and, ii) the PR interval is not reliably increasing within groups of beats (We should see PR interval prolongation when there is AV Wenckebach) — SEE Figure-2!

Figure-2: We’ve labeled the P waves that were seen in Figure-1 (See text).

FIGURE-2: In Figure-2 — We’ve labeled sinus P waves with RED arrows. Additional P waves are highlighted by BLUE arrows. Note that the T waves under each of the BLUE arrows in Figure-2 are deformed (usually be a notch), compared to the T waves that are not hiding P waves within them (ie, NO P waves are hiding within the T waves of beats #3, 6, 9, 11 in A; and beats #2, 4, 6, 8 and 10 in B).

• Therefore — the BLUE arrows in tracings A and B in Figure-2 represent P waves. Compared to the P-P interval between 2 sinus P waves (ie, between 2 RED arrows) — each of the P waves highlighted by BLUE arrows occurs much earlier-than-expected, which means the P waves highlighted by BLUE arrows are PACs (Premature Atrial Contractions).
• Taking another look at the 2 rhythm strips shown in Figure-2 — we see that every 3rd P wave (ie, each of the BLUE arrows) — is a PAC. Therefore, the underlying rhythm in Figure-2 is Atrial Trigeminy (which is the term we use to describe an underlying sinus rhythm, in which every 3rd beat is a PAC).
• The PACs preceding beats #2, 5 and 8 are conducted. However, none of the other PACs in Figure-2 are conducted. Instead, the PACs following beats #10 and 12 in A — and following beats #1, 3, 5, 7 and 9 in B are all “blocked” (ie, non-conducted), and followed by a short pause.
• BOTTOM LINE: The underlying rhythm in Figure-1 is atrial trigeminy (ie, every 3rd P wave is a PAC). Some of these PACs are conducted. Others are “blocked”. All of the PACs in B are blocked — and this produces a form of group beating that simulates AV Wenckebach — BUT — there is no AV block in Figure-2, for the reasons we state above.
• P.S. — Use of calipers allows you to rule out AV block within seconds! It should take no more than a few seconds to determine that every 3rd P wave in Figure-2 occurs far too early to be a sinus rhythm (as would be expected when there is AV block).

BEYOND-the-CORE: As is often the case in “real life” — there is a final “twist” to the rhythm in Figure-2. Do you see it?

• HINT: Is P wave morphology for all of the P waves highlighted by RED arrows the same?
• NOTE: My answer below is complex. It is not meant to confuse — but rather to illustrate the challenges faced by the astute clinician when confronted with a truly complex “real life” tracing. Less experienced interpreters may want to ignore what I write below. My hope is that what I write below proves insightful, especially for more experienced interpreters.

ANSWER: Perhaps the greatest challenge the astute clinician faces when interpreting a complex arrhythmia that is less-than-perfect technically — is distinguishing between deflections that are due to artifact vs the slight variation in P wave, QRS and/or ST-T wave morphology that may normally occur in “real life”.

• Because rhythm strips A and B in Figure-2 were obtained just seconds apart — I would expect moment-to-moment fluctuations in P wave and/or QRS morphology to be minimal. Yet despite virtually no change in QRS morphology between tracings A and B — I thought the P waves under the RED arrows of beats #3,4; 6,7; 9,10; and 12 in A, were in general taller and more pointed than almost all of the other P waves. This made me wonder IF those smaller and rounder P waves in these 2 tracings might not reflect P waves arising from an alternate atrial site?
• That said — strongly against this possibility, is that fact that the R-R intervals between beats with successful RED-arrow P waves are virtually identical! I definitely would not expect this, if ectopic site atrial P waves were mixed in with sinus P wave activity.
• BOTTOM LINE: I suspect all P waves in tracings A and B of Figure-2, that are highlighted by RED arrows — are of sinus origin. I suspect there is just a lot of moment-to-moment variation in sinus P wave morphology. That said — I cannot prove this based only on these 2 tracings ...

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For MORE on PACs vs Wenckebach:

• Please see ECG Blog #66 and Blog #164 —