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).

 

 

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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).

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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 either: i) That these negative P waves originate from lower down in the atria (or from the AV node); or, ii) 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).

 

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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 b, e, h, k 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.

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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.

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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.

 

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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.

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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 drawing) Laddergrams, with LINKS to numerous Laddergrams I’ve drawn and discussed in detail in other blog posts.