ECG Blog #503 (14,57) — The Cause of the Pause

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NOTE: I’ve decided to update and republish several of my favorite cases from years past. (Today's post is an improved version of ECG Blogs #14,57 — initially published in 2011).

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QUESTIONS: Interpret the rhythm below in right-sided Lead MCL-1.

• There is group beating in Figure-1.  Is this Wenckebach?
• Extra Credit: Why is the PR interval before beat #7 shorter than the PR interval before beat #6 (and also shorter than the PR interval before the other sinus beats in this tracing?).

Figure-1: Is the group beating here due to Wenckebach?

MY Thoughts on the Rhythm in Figure-1:

The rhythm in Figure-1 is not regular, but does manifest a pattern of group beating (with 2 short pauses between beats #2-3 and #6-7). 

• The QRS complex for each of the 9 beats in this tracing is narrow (ie, not more than half a large box in duration = not more than 0.10 second in duration). 
• The underlying rhythm appears to be sinus, with similar-looking P waves showing a fixed PR interval preceding all beats except for beat #7.
• Despite the presence of group beating — there is no evidence of Wenckebach or other form of AV block on this tracing.  Instead, the "cause" of the pause lies partially hidden within the T waves of beats #2 and 6.

PEARL #1: It's important to remember that the most common Cause of a Pause is a blocked PAC. Although most premature supraventricular beats ( = PACs or PJCs) are conducted normally to the ventricles (ie, with a narrow QRS complex that looks like other sinus-conducted beats) — this is not always the case. Instead, PACs (or PJCs) may sometimes occur so early in the cycle as to be "blocked" (non-conducted) because the conduction system is still in an absolute refractory state.

• This is the situation for premature impulse A in Figure-2 — which shows impulse A occurring during the ARP (Absolute Refractory Period).
• At other times — premature (early beats may occur during the RRP (Relative Refractory Period) — in which case aberrant conduction (with a wide and different-looking QRS) occurs. This is the situation for premature impulse B in Figure-2.
• Because impulse B occurs during the RRP — part (but not all) of the ventricular conduction system has recovered. Most often PACs occurring at Point B will conduct with some form of bundle branch block and/or hemiblock (reflecting that part of the conduction system which has not yet recovered).
• Premature impulse C in Figure 2 occurs after the refractory period is over.  As a result — a PAC occurring at Point C will conduct normally (ie, with a narrow QRS that looks identical to other sinus beats on the tracing).

Figure-2: Absolute and Relative Refractory Periods (ARP & RRP) — explaining why beat A is blocked — and beat B is conducted with aberration.

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Returning to the Questions in Today's Case: 

Take another LOOK at Figure-1.

• Is the group beating in Figure-1 due to Wenckebach?
• Why is the PR interval before beat #7 so short?

Figure-1: Taking another look at Figure-1 ...

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ANSWERS:

There is no AV block in Figure-1. The PR interval as one moves from beat #3 to beats #4,5 and 6 is not increasing, as it would if Mobitz I (which is 2nd-degree AV block of the Wenckebach type) was present.

• Instead (as per PEARL #1) — The cause of the pause that we see between beats #6-7 in Figure-1 is a blocked PAC. We show this in Figure-3 — in which the RED arrow in the T wave of beat #6 highlights the "telltale notching" of a PAC buried in this T wave. 
• Note that a similar very early-occurring PAC (corresponding to a PAC occurring at point A in Figure 2) can be seen notching the T wave of beat #2.

PEARL #2: How do we know that the pointed deflection highlighted by the RED arrow in Figure-3 is truly a PAC — and not artifact? The KEY is that none of the normally conducted sinus P waves in this tracing manifest anything resembling an "extra" deflection (ie, The T waves of beats #1; 3,4,5; 7,8,9 are all smooth — and it is only the T waves of beats #2 and 6 that manifest this extra pointed deflection). 

• Thus, we need to first determine what the normal T wave for a sinus-conducted beat looks like — before we can determine if a PAC is partially hidden within the T waves of other beats.

Figure-3: Answer to Figure-1.

PEARL #3 — is related to PEARL #1: 

• Because the most common cause of a pause is a blocked PAC, it turns out that in clinical practice — blocked PACs are much more common than any form of AV block.
• That said — blocked PACs are often subtle and difficult to detect. As a result — they are often overlooked. But — blocked PACs will be found IF looked for (they'll often be hiding and/or notching a part of the preceding T wave — as seen above in Figure-3).

NOTE: The occurrence of a PAC depolarizes the rest of the atria — and therefore resets the SA Node. As a result — a brief pause usually follows after a premature P wave. These relationships are schematically illustrated in the laddergram shown in Figure-4:

• The rhythm in Figure-4 begins with 2 normally sinus-conducted beats (beats #1 and 2).
• The PINK circles in the Atrial Tier of the laddergram represent the 2 PACs that are not conducted to the ventricles, because they occur so early as to fall within the ARP (corresponding to impulse A, as was shown in Figure-2).
• Note the brief pause that follows each of these blocked PACs (ie, the pauses that occur between beats #2-3 and between #6-7).
• Unlike AV Wenckebach (in which the underlying P wave rhythm remains regular) — We can see that the sinus P wave before beat #3 ( = the 3rd RED circle in Figure-4) is delayed. When this 3rd RED arrow sinus P wave finally occurs — this P wave is conducted to the ventricles with a normal PR interval.
• There follow 3 more on-time sinus P waves (producing sinus-conducted beats #4,5,6) — until the next very early-appearing PAC occurs (the RED arrow highlighting this 2nd blocked PAC that notches the T wave of beat #6).
• Once again — a brief pause is seen after this 2nd blocked PAC. But note that when the next sinus P wave finally occurs — the PR interval before beat #7 is shorter than all other PR intervals on this tracing! (as per the open RED circle showing this very short PR interval before beat #7).

PEARL #4: The PR interval before beat #7 is too short to conduct. But since the QRS of beat #7 is narrow and virtually identical in morphlogy to all of the other sinus-conducted beats on this tracing — beat #7 must be a junctional escape beat! (schematically represented by the BLUE circle within the AV Nodal Tier).

• Normal sinus rhythm then resumes for the last 2 beats in Figure-4 ( = beats #8 and 9).
• To Emphasize: The occurrence of a junctional escape beat in Figure-4 is perfectly appropriate. The R-R interval preceding beat #7 is slightly more than 6 large boxes in duration, which corresponds to a junctional escape rate of slightly less than 50/minute — which means that the AV node is doing what it is "supposed to do" — namely, putting out an escape beat at the appropriate junctional escape rate of between 40-60/minute if and when the next sinus P wave is delayed.
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• Beyond-the-Core for a Very Advanced Point: Extra credit to any readers who used calipers, and on carefully measuring the R-R intervals of both pauses in Figure-4 — detected that the R-R interval for the 1st pause (between beats #2-3) — is actually slightly longer than the R-R interval for the 2nd pause. Since most of the time — the junctional escape rate is quite regular, I would have expected the junctional escape beat in this tracing ( = beat #7) to be preceded by a longer pause than the pause that precedes beat #3 which is sinus-conducted — but the opposite occurs. I attribute this unexpected finding to the slight variation in regularity that may occasionally be seen with escape rhythms.

Figure-4: Laddergram illustration of the rhythm from Figure-1. The cause of the 2 brief pauses (between beats #2-3 and #6-7) are blocked PACs. The PR interval preceding beat #7 is too short to conduct — which tells us that beat #7 is a junctional escape beat.  

Part 2 in Today's CASE: 

To emphasize the clinical importance of today's case — I present another challenging rhythm that I show in Figure-5.

QUESTIONS: 

Interpret the rhythm below in right-sided Lead MCL-1:

• What kind of AV block is present?
• Is a pacemaker likely to be needed?

Figure 5: Lead MCL-1 rhythm strip. Is this Mobitz I or Mobitz II? 

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ANSWER:

Hopefully you did not fall into the trap. There is no AV block in Figure-5. 

• As always — I find it easiest to be systematic. I favor the Ps,Qs,3Rs Approach (See ECG Blog #185).
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• The ventricular rhythm in Figure-5 is quite Regular at a Rate of ~50/minute for the first 6 beats. I'll defer attention to beat #7 for the moment.
• The QRS for these first 6 beats is narrow in this single lead rhythm strip. Assuming the other 11 leads in a 12-lead tracing confirm that the QRS is narrow — this would tells us that the rhythm is supraventricular.
• P waves are present! (ie, the colored arrows in Figure-6).
• The 5th parameter in the Ps,Qs,3R Approach addresses whether P waves are Related to neighboring QRS complexes — which they are for the first 6 beats, because the PR interval preceding beats #1-thru-6 is constant (RED arrow P waves in Figure-6). Thus, there is sinus conduction!
• But — every-other-P-wave is not conducted (No QRS follows the BLUE arrow P waves in Figure-6).

PEARL #5: The rhythm in Figure-6 is not a form of AV block! There are several reasons why we know this:

• The shape of the P waves highighted by the RED and BLUE arrows is different! (RED arrow P waves have an initial pointed positive deflection, followed by a wider, rounded negative deflection — vs — BLUE arrow P waves that have a triphasic negative-positive-negative morphology). This is consistent with atrial bigeminy (every-other-P-wave being a PAC) — because P wave morphology will be different when P waves arise from different atrial sites.
• The P-P interval is irregular! While true that 2nd- and 3rd-degree AV blocks often manifest slight P-P interval variation — the degree of P-P interval variation with this type of "ventriculophasic sinus arrhythmia" is generally not nearly as marked as the variation in P-P intervals seen in Figure-6.
• Beat #7 is a PAC that is conducted to the ventricles, here with a wider QRS complex due to aberrant conduction. P wave morphology of this last premature P wave is identical to the P wave morphology of each of the preceding BLUE arrow P waves — suggesting that all of these P waves are PACs.
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• CONCLUSION: The commonest cause of a pause is a blocked PAC, and not some form of AV block. There is no AV block in Figure-6. Instead — the rhythm is atrial bigeminy (every-other-P-wave is a PAC) — with the first 4 BLUE arrow P waves highlighting blocked PACs — and the last BLUE arrow representing a PAC that conducts with aberration (similar to impulse B in Figure-2).

Figure 6: Colored arrows highlight each P wave in Figure 5.