Sunday, July 4, 2021

ECG Blog #239 (54) — Why 2 Kinds of P Waves?

Today's case is a fascinating ECG, albeit without clinical information (Figure-1).

  • How would you interpret the ECG in Figure-1?
  • And — WHY are there 2 kinds of P waves? 


Figure-1: How would you interpret this 2-lead rhythm strip? (See text).




NOTE: Some readers may prefer at this point to listen to the 5:00-minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to refer to My Thoughts on this tracing (that appear below ECG MP-54).


Today's ECG Media PEARL #54 (5:00 minutes Audio) — Reviews what Echo Beats are — and clinical applications of this ECG finding.



My THOUGHTS on this Case:

As usual — I approach rhythm interpretation in systematic fashion by addresing the Ps, Qs & 3Rs (See ECG Blog #185)NOTE — It is not essential to perform this assessment in order (ie, I'll often look at QRS width or regularity of the rhythm before assessing atrial activity — depending on whatever is easiest to assess for the tracing at hand). For the rhythm in Figure-1 — My sequential thought process was as follows:

  • The QRS complex is narrow — so the rhythm is supraventricular.
  • Although at 1st glance, the rhythm in Figure-1 may appear regular — this rhythm is not regular! Instead, there is Group Beating, in the form of alternating longer and slightly shorter R-R intervals.
  • NOTE: To prove that the rhythm in Figure-1 truly represents group beating — Measure all R-R intervals! This is easiest to do with calipers. Doing so will establish that all of the "shorter" R-R intervals are the same duration ( = 800 msec) — and — all of the "longer" R-R intervals are the same duration ( = 830 msec), albeit slightly longer (ie, 830 vs 800 msec — as in Figure-2).

Figure-2: Measuring duration of each R-R interval in Figure-1 establishes group beating (See text).

PEARL #1: As we have often emphasized (See ECG Blog #186) — the finding of Group Beating provides an excellent clue to the likelihood that some form of Wenckebach conduction may be present!

  • NOTE: Not all group beating is the result of Wenckebach conduction. This is because other conditions (ie, atrial or ventricular bigeminy or trigeminy) may also produce a "regular irregularity" to the rhythm. That said — Recognition of group beating is helpful — because it immediately tells us to look closely for some form of Wenckebach conduction


Continuing with the Ps, Qs & 3Rs Approach:

  • P waves are present in Figure-2. However, there are 2 different P wave shapes! 


For clarity — we have labeled all of the P waves in today's tracing in Figure-3.

  • QUESTION: Why are there 2 kinds of P waves?


Figure-3: I've labeled all P waves in today's tracing with letters.


My THOUGHTS on Figure-3:

The 2 types of P waves seen in Figure-3 are an upright, m-shaped notched P wave in lead II ( = a, c, d, f, g, i, j, l, m) — and a shallow, negative P wave in lead II that occurs just after the QRS complex of beats #1, 3, 5, 7 and 9 ( = b, e, h, k, n).

  • Sinus P waves should be upright in lead II. This suggests that the upright, m-shaped notched P waves that we see in Figure-3 are sinus P waves.
  • When P waves are negative in lead II — this suggests eitheri) That these negative P waves originate from lower down in the atria (or from the AV node); orii) That there is retrograde conduction.

PEARL #2: The sinus P waves that we see in Figure-3 appear to be conducting. We say this because there are 2 sets of repeating PR intervals!

  • The PR intervals preceding beats #2, 4, 6 and 8 are equal ( 0.17 second).
  • The PR intervals preceding beats #1, 3, 5, 7 and 9 are also equal ( = 0.47 second).
  • It is highly unlikely that PR intervals that are precisely equal in so many places on the tracing are occurring by chance. Instead — it must be that these repeating PR intervals indicate that all of these beats are conducted!


We are left with determining the etiology of these negative P waves in Figure-3 (ie, b, e, h, k and n).

  • Against these P waves being retrograde from a junctional rhythm — is the finding of a fixed (albeit long) PR interval preceding beats #1, 3, 5, 7 and 9 (ie, beats #1, 3, 5, 7 and 9 are sinus-conducted and not junctional escape beats).
  • Against P waves b, e, h, k and n being blocked PACs — is the expectation that blocked PACs would "reset" the SA node, and produce a longer delay after beats #1, 3, 5, 7 and 9. Instead — the shortest R-R intervals in this tracing follow these negative P waves.
  • By the process of elimination — these negative P waves are most likely to represent Echo beats!



WHAT are Echo Beats?

On occasion, an atrial impulse that has already passed through the atria — may be conducted backward (ie, retrograde) — to return and reactivate the atria. This process produces an Echo beat. If the timing of the "echo" beat is just right — the impulse may also propagate downward to activate the ventricles.

  • PEARL #3: The principal factor that predisposes to development of Echo beats is delay in conduction through the AV node — as commonly occurs with Mobitz I 2nd-degree AV block (ie, AV Wenckebach). The reason for this is simple — the longer it takes for a P wave to get through the AV node — the more opportunity there is for retrograde conduction to occur.
  • Because an Echo beat is conducted backward (retrograde) to the atria — the P wave produced by this retrograde conduction will be negative in lead II.

PEARL #4: At times, it may be difficult to distinguish Echo beats from PACs — because both entities may produce a negative P wave in lead II.

  • BUT — IF negative P waves are only seen after lengthening of the preceding PR interval — this strongly suggests that these negative P waves are Echo beats (by the concept expressed in Pearl #3).



A picture is worth 1,000 words!

  • Consider my proposed Laddergram in Figure-4.

Figure-4: My proposed laddergram to explain the mechanism of the rhythm in today's case (See text).

My THOUGHTS on Figure-4:

Confession: I was not comfortable with my assessment of today's rhythm until I was able to draw out a plausible explanation on a laddergram.

  • I strongly suspected some form of AV Wenckebach was operative in Figure-3 — because of group beating and the 2 repetitive PR intervals. But the fact that sinus P waves were not regular throughout the rhythm meant that something other than "simple" Mobitz I was at play.
  • IF the rhythm in Figure-3 was AV Wenckebach — then the PR interval increment (ie, the amount that the PR interval increases from 1 beat to the next) was much larger than usual (ie, the jump from a PR interval of 0.17 second to 0.47 second means that the PR increment = 0.30 second, which is extremely long!). Although not common — the presence of dual AV nodal pathways could account for this unexpectedly large PR interval increment.
  • The laddergram that I drew in Figure-4 is based on presuming that P waves behk and n are Echo beats. This is a logical assumption — because these negative P waves in lead II only occur after PR interval lengthening of the preceding sinus-conducted beats (as per Pearls #3 and #4)
  • In Figure-4 — The retrograde conduction back to the atria of these atrial echos (to produce negative P waves b, e, h, k and n in lead II) — is represented by dotted lines in the laddergram.


Final Interpretation of Figure-4:

My final interpretation of today's rhythm (as schematically represented in the Figure-4 laddergram) — is 2nd-Degree AV Block, Mobitz Type I, with Wenckebach cycles terminated by atrial Echo beats. Presumably, the marked PR interval increment is a result of conduction alternating between dual AV nodal pathways.



PEARL #5: Uncomplicated 2nd-degree AV block of the Mobitz I type is characterized by a regular (or almost regular) atrial rhythm — with progressive increase in the PR interval until a beat is dropped. That said, on occasion — AV Wenckebach cycles may be terminated by other ECG events that occur before the next on-time P wave can be discharged from the SA node. Examples of other phenomena that may terminate Wenckebach cycles include junctional escape beats PACs and Echo beats.

  • This is precisely what happens in Figure-4! The atrial echo beats (ie, P waves b, e, h, k and n) arrive at the atria just before the next sinus P wave would occur. As a result — these atrial echos suppress the next sinus impulse — and in so doing, terminate each of the 2-beat Wenckebach cycles seen in this laddergram.

Why Else should we Care about Echo Beats?

Echo beats are not common in practice. That said, when they do occur — they may have important clinical implications. As a result — it is useful to be aware of this advanced arrhythmia concept. Consider Figure-5.

  • QUESTION: Can you explain what happens in Figure-5? 

Figure-5: Please NOTE that Figure-5 is not a real ECG. I took the first 5 beats from Figure-4 — and I made up the next 5 beats (ie, beats #6-thru-10), so as to simulate a potential complicating arrhythmia that might arise from the presence of Echo beats. Can you explain what happens after beat #5?

My THOUGHTS on Figure-5:

The first 5 beats in Figure-5 are identical to what we saw in the Figure-4 laddergram — namely Mobitz I (with a large PR interval increment) — that is then terminated by atrial echo beats b and d.

  • The 2-beat Wenckebach cycles continue with beats #4 and #5 — but this time, atrial echo h is able to conduct down to the ventricles! This initiates a reentrant SVT rhythm that continues at least until the end of the rhythm strip.
  • This theoretical laddergram in Figure-5 — illustrates how not only PACs may precipitate reentrant SVT rhythms (such as AVNRT, AVRT) — but, if conditions are "just right" — reentrant SVTs may also be initiated by Echo beats.



Acknowledgment: My appreciation Victor Svensson (from Växjö, Sweden) and Peter Hammarlund (from Helsingborg, Sweden) for making me aware of this case and allowing me to use this tracing.



Related ECG Blog Posts to Today’s Case: 

  • ECG Blog #185 — Reviews the Ps, Qs & 3Rs Approach to systematic rhythm interpretation.
  • ECG Blog #186 — Highlights the importance of Group Beating — and reviews when to suspect the Mobitz I form of 2nd-Degree AV Block ( = AV Wenckebach). 
  • ECG Blog #188 — Reviews the essentials for reading (and/or drawingLaddergrams, with LINKS to numerous Laddergrams I’ve drawn and discussed in detail in other blog posts.

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