ECG Blog #503 — 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.
• 
  
• 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).
• 
  
• 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.
• 
  
• 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.

ECG Blog #500 — Can You Solve this CASE?

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NOTE: I started my ECG Blog in 2010 — and this is my 500th ECG Blog case! The reason I saved this case for #500 — is that it is challenging — but in the spirit of the great fictional detective Sherlock Holmes — logical deduction (which is what we often need to apply when solving a complex arrhythmia) allows us to arrive at the most plausible answer. Are YOU up for the challenge?

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The ECG in Figure-1 is from an older patient who reports 2 syncopal episodes, but no chest pain. He is on a ß-blocker and a calcium-channel blocking agent.

QUESTIONS:

• What is the rhythm in Figure-1?
• What is the cause of this rhythm?
• What is the recommended treatment?

• Extra Credit: Can you explain each of the 10 beats?

Figure-1: The initial ECG in today's case — from an older patient with syncope, but no chest pain. (To improve visualization — I've digitized the original ECG using PMcardio).

My Initial Thoughts:

The history — and a 2-second look at this tracing gets us started!

• The patient is "older" — he/she presents with an obviously slow and not completely regular rhythm (overall heart rate under 50/minute) — and, is on rate-slowing medication ( = the ß-blocker — and perhaps also verapamil or diltiazem, which are the main rate-slowing calcium blocker medications).
• 
  
• PEARL #1: Given this history — if the very slow heart rate is not the result of rate-slowing medication — and, acute ischemia/infarction, hypothyroidism and sleep apnea are not factors — then a component of SSS (Sick Sinus Syndrome) is probably operative (See ECG Video below in the ADDENDUM for review of the features of SSS). 

As to the Rhythm ...

The reason this case is so challenging — is that the P waves are tiny!

Take Another LOOK at the ECG in Figure-1:

• Focus on lead II — because this is the best lead to use when searching for sinus P waves (ie, If we see an upright P wave in lead II with similar P wave morphology in a number of beats — this probably reflects an underlying sinus rhythm).
• Are there any of the 10 beats in this tracing that we know are preceded by upright P waves in this lead II?
• Are there any P waves that we think may be conducting?
• Are there any P waves that we know are not conducting?
• 
  
• PEARL #2: The Sherlock Holmes principle that we apply for complex arrhythmia interpretation is simple: Start with what you know to be true. After this is established — we can work our way toward assessing those aspects of this complex tracing that we are not yet certain about.

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What I Immediately Knew to be True:

Although tiny — I was quickly able in Figure-1 to identify a number of P waves. I have labeled what I quickly saw in Figure-2:

• The last 4 RED arrows in lead II are clearly highlighting sinus P waves (ie, Despite being of extremely low amplitude — all 4 of these P waves are upright and manifest the same P wave morphology).
• The PR interval preceding beats #7,8,9 is decreasing and different for each of these beats. We know the PR interval preceding beat #9 is too short to conduct.
• In addition — it is clear that the last RED arrow P wave in lead II can not be conducting, because it occurs after beat #10.
• Given that the PR interval preceding beats #7 and 8 is different (ie, The PR interval before beat #8 being a little bit shorter than the PR interval before beat #7) — this means that at most — only one of these P waves can be conducting (depending on what the “normal” PR interval for conduction is for this patient).

Armed with the knowledge that today’s ECG ends with 4 fairly regular sinus P waves ( = the last 4 RED arrows) — it seems logical to suspect that underlying sinus P waves might be present throughout this tracing. This puts us to the task of testing this hypothesis, keeping in mind how small sinus P waves are in this tracing.

• KEY Point: There is virtually no artifact on this tracing. As a result — even minor differences in morphology are most probably "real" — and likely to represent hidden atrial activity.
• With this in mind, as we look at the beginning of ECG #1 — it should be clear that the 1st RED arrow in lead II highlights a sinus P wave, albeit with a PR interval too short to conduct.
• 
  
• PEARL #3: Knowing what the P-P interval is from the last 4 RED arrow P waves in lead II — tells us approximately where to look for additional sinus P waves in the beginning of the lead II rhythm strip.
• For this reason — I thought the tiny distortion in the baseline seen immediately after beat #2 in lead II (ie, between the 2 RED arrows right after beat #2) most probably represents the 2nd sinus P wave in this tracing (albeit this P wave is partially hidden within the last part of the QRS complex before it).
• 
  
• PEARL #4: This is where the use of simultaneously-recorded leads is so useful for confirming our suspicion of additional atrial activity. Use of this concept allows me to confirm that the small upright deflection seen right after the QRS of beat #3 in lead II ( = the 3rd RED arrow in this lead) is real — because the vertical BLUE timeline below it highlights comparable small deflections at the same point in the cycle just after beat #3 in simultaneously-recorded leads V4,V5,V6.
• 
  
• An especially subtle distortion then appears near the beginning of the T wave of beat #4 in lead II (ie, between the 2 light BLUE arrows in this lead). Referral to the 2nd vertical BLUE timeline confirms that this subtle distortion of the T wave of beat #4 in lead II is indeed the 4th sinus P wave (because a comparable subtle distortion of the T wave of beat #4 occurs at the same point in lead V4).
• All that remains for us to do at this point — is to confirm where the 5th sinus P wave in lead II occurs (and the vertical RED timeline does this by highlighting a similar T wave distortion at the same point after beat #5 in lead V3).

Figure-2: I have labeled the sinus P waves that we have identified with colored arrows in lead II.

Which Beat in Figure-2 Occurs Earlier than Expected?

Now STEP BACK for a moment. Take a look at what we've established in Figure-2?

• We know that the rhythm is supraventricular (because the QRS is narrow in all leads throughout this tracing).
• There is a fairly regular atrial rhythm ( = the colored P waves in the lead II rhythm strip).
• Most of the 10 beats in this rhythm are not sinus-conducted. They can't be — because the PR intervals before beats #1 and #9 are too short to conduct — and the P waves closest to beats #2,3,4,5 and #10 all occur after the QRS. 
• This tells us: i) That there is AV dissociation for at least part of this tracing — because the P waves nearest to beats #1,2,3,4,5 and #9,10 are not related to their neighboring QRS complex; — and, ii) That these 7 beats (#1,2,3,4,5; and #9,10) — are all junctional escape beats occurring at an appropriate junctional escape rate of between 40-50/minute.
• Finally (as we step back a bit from this tracing) — We can see that the ventricular rhythm in Figure-2 is almost regular — with the exception of one beat.

QUESTION:

• Which beat in Figure-2 occurs earlier-than-expected?
• Why does this beat occur early?

ANSWER:

• Beat #6 in lead II clearly occurs earlier-than-expected. 
• 
  
• PEARL #5: When there is an underlying regular (or at least fairly regular) sinus rhythm, such that all sinus P waves are "on time" (as shown by the colored P wave arrows in Figure-2) — the finding of a beat that occurs earlier-than-expected strongly suggests that this beat is conducted. This tells us that beat #6 in Figure-2 is a "capture" beat that is being conducted by the "on time" sinus P wave in front of it!

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Let's Magnify the Lead II Rhythm Strip:

At this point in our analysis — I'm going to magnify the lead II rhythm strip that we have been focusing on, as this will greatly facilitate our observations.

• I have done this in Figure-3 — in which I break up the 10-beat tracing from Figure-2 into 2 parts.

Figure-3: I've magnified the lead II rhythm strip from Figure-2.

Orient yourself to the rhythm in Figure-3:

• RED arrows highlight the underlying sinus bradycardia, with a component of sinus arrhythmia.
• As described earlier — beats #1,2,3,4,5 are all junctional escape beats at a rate in the 40s — and, beat #6 represents a sinus-capture beat.
• The rhythm strip ends with 2 additional junctional escape beats ( = beats #9,10).
• This leaves us with beats #7,8 that we have not yet defined.

PEARL #6: If your goal is to confidently interpret complex arrhythmias — then the use of calipers is essential!

• Escape rhythms are usually regular (or at least almost regular). Awareness of the wisdom in this statement holds the KEY for determining which of the 2 remaining beats (#7 or #8) is sinus-conducted.

I illustrate the above concept in Figure-4 — in which I show my measurements for each of the R-R intervals in today's tracing.

• QUESTION: What do these R-R interval measurements tell you about beats #7 and 8?

Figure-4: I've measured R-R intervals from Figure-3.

ANSWER:

• Note that the R-R interval preceding each of the junctional escape beats in Figure-4 is constant at 1480 milliseconds, with the exception of the slight variation (to 1460 msec.) preceding junctional beat #9.
• KEY Point: The R-R interval preceding beat #7 is shorter-than-expected ( = 1430 msec. — instead of 1480 msec.). As per PEARL #5, this tells us that beat #7 is sinus-conducted — whereas beat #8 (which manifests a slightly shorter PR interval) must be another junctional escape beat.

I illustrate the above findings in Figure-5 — in which the RED arrow P waves in lead II indicate the 2 sinus-conducted beats.

• YELLOW arrow P waves highlight "on-time" P waves that are not conducting.
• Note in Figure-5 that the PR interval preceding beat #7 is slightly more than 1 large box in duration — which tells us that there is 1st-degree AV block for this one "on-time" sinus P wave that is conducted normally to the ventricles.

Figure-5: RED arrows indicate sinus-conducted beats. YELLOW arrows highlight "on-time" P waves that are not conducting.

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Laddergram Illustration:

To clarify of the above relationships — I add in Figure-6 my proposed laddergram for today's tracing:

• As noted above — it is the RED arrow P waves that are sinus-conducted. All other beats on this tracing are junctional escape beats. The reason junctional beats are able to occur — is that for most of this tracing, the junctional escape rate is slightly faster than the rate of the sinus bradycardia.
• Junctional impulses in Figure-6 are seen to conduct retrograde for a short distance (dotted butt ends within the AV Nodal Tier).
• The reason the "capture" beat ( = beat #6) is preceded by a longer PR interval than the PR interval for sinus-conducted beat #7 — is that retrograde conduction from junctional beat #5 delays conduction of the next sinus P wave.
• The 2nd RED arrow P wave is right "on-time" — and able to conduct to the ventricles, albeit with 1st-degree AV block. Thereafter, the rate of sinus P waves slows — with the result being that the slightly faster junctional escape rate once again takes over the rhythm to produce junctional beats #8,9,10.

Figure-6: My proposed laddergrams for today's case.

Putting It All Together:

• The underlying rhythm in today's case is sinus bradycardia and arrhythmia. This probably is being exacerbated by use of rate-slowing medication (ie, the ß-blocker and the calcium blocker, if the specific drug used is verapamil or diltiazem).
• There is a tendency to interpret today's rhythm as junctional escape. That said — this is not an optimal interpretation of today's rhythm. Instead, it would be better to describe today's rhythm as underlying sinus bradycardia with sinus arrhythmia — that results in AV dissociation and an escape junctional rhythm with occasional "capture" beats.
• Given that today's patient is an older adult with 2 syncopal episodes — he/she may have SSS (Sick Sinus Syndrome). In this case, even after stopping cardioactive medications and ruling out ischemia/infarction — hypothyroidism — medication effect — and sleep apnea — IF the rhythm in Figure-6 persists — the patient will need a pacemaker as treatment for SSS.
• 
  
• PEARL #7: Perhaps the most concise way to describe today's rhythm — is by saying this is an "escape-capture" rhythm as a result of sinus bradycardia with junctional escape (See LINKS below for other examples of "escape-capture" rhythms).
• 
  
• PEARL #8: AV dissociation is not a diagnosis. Instead, it is merely a description of the lack of relationship between "on-time" sinus P waves and neighboring QRS complexes. AV dissociation may be transient (lasting for as little as a single beat) — or — it may persist throughout the entire rhythm strip. But even though there is AV dissociation for most of the beats in today's ECG — neither 2nd-degree nor 3rd-degree AV block is present. Instead — there is AV dissociation by "default" of the sinus node pacemaker that slows below the rate of the junctional escape rhythm (See the 3rd ECG Video in the ADDENDUM below for more on the 3 Causes of AV Dissociation).

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Acknowledgment: My appreciation to Abdallah Sbai Sassi (from Rabat, Morocco) for the case and this tracing.

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Additional Relevant ECG Blog Posts to Today’s Case:

• ECG Blog #185 — Review of the Ps, Qs, 3R Approach for systematic rhythm interpretation.
• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 100 laddergram cases — many with step-by-step sequential illustration) — See the quick access LINK in the upper Menu on top of every page in this Blog!
• 
  
• ECG Blog #256 — Escape-Capture Bigeminy (with junctional escape and "capture" from retrograde conduction — with AUDIO Pearls on "Escape-Capture" and on "Sick Sinus Syndrome" plus Step-by-Step Laddergram).

Other Post with "Escape-Capture" Rhythms: 

• ECG Blog #349 — another example of Escape-Capture with Step-by-Step Laddergrams.

• ECG Blog #163 — Escape-Capture Bigeminy (with sinus bradycardia and resultant junctional escape — and possibly also with SA block).
• ECG Blog #315 — Escape-Capture Bigeminy (from marked sinus bradycardia).
• ECG Blog #144 — Escape-Capture Bigeminy (from 2nd-degree AV block of uncertain severity).

ADDENDUM:

• These 2 ECG Videos cover KEY concepts in today's case:

—  —

ECG Media PEARL #68 (6:15 minutes Audio) — Reviews the meaning of the term, "Escape-Capture" (this being a special form of bigeminal rhyhm).

—  —

ECG Media PEARL #69 (2:45 minutes Audio) — Reviews the ECG findings of SSS = Sick Sinus Syndrome (excerpted from the Audio Pearl presented in Blog #252).

ECG Media PEARL #9 (4:45 minutes) — reviews the 3 Causes of AV Dissociation — and emphasizes why AV Dissociation is not the same thing as Complete AV Block.