ECG Blog #349 — Is there Capture?

The Lead II rhythm strip shown in Figure-1 — was obtained from a 60-year old man who was admitted to the hospital for an orthopedic injury. No chest pain. Hemodynamically stable with this rhythm. No acute injury was seen on his 12-lead tracing. The patient does give a history of "one or two episodes of fainting".

QUESTIONS:

• How would YOU interpret the rhythm in Figure-1?
• Is there capture?

Figure-1: Lead II rhythm strip obtained from a 60-year old man admitted for an orthopedic injury. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Approach to the ECG in Figure-1:

As always — My approach to rhythm diagnosis is encompassed by the memory aid, "Watch Your Ps, Qs and the 3Rs" (See ECG Blog #185):

• The overall Rate of today's rhythm is slow (ie, ~40-50/minute). Although the rhythm is not Regular — there is a "pattern" in the form of a bigeminal rhythm (ie, There are coupled beats that are spaced at a similar distance to the preceding QRS complex).

 

In Figure-2 — I schematically show that there are alternating longer-with-shorter R-R intervals (with BLUE and RED lines in Figure-2 showing very similar R-R intervals that alternate in "bigeminal" fashion).

• NOTE: The relationship between alternating R-R intervals is admittedly not perfect (ie, The R-R interval between beats #5-6 appears to be slightly longer than the other BLUE line R-R intervals). But overall — the fact that so many of the alternating R-R intervals repeat — suggests that this is not due to chance (and that some form of conduction is present)!

Figure-2: Schematic representation in which the RED and BLUE lines that I have drawn are constant in length. Note alternating longer-then-shorter R-R intervals — in which the longer R-R intervals (BLUE lines) are of similar (albeit not exact) length — and shorter R-R intervals (RED lines) are equal in length. This is not due to chance!

Continuing with the Ps, Qs & 3Rs:

• All 8 beats in today's rhythm have a narrow QRS complex. That said — odd-numbered beats look slightly different than the even-numbered beats. Realizing that we are only looking at a single lead — the fact that the QRS complex looks so narrow in this lead II suggests that all beats in today's tracing are supraventricular!
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• PEARL #1: Although possible that the reason for the slightly different QRS appearance of odd- vs even-numbered beats is due to aberrant conduction — it is more likely that the reason for this slight difference in QRS morphology is that beats #1,3,5,7 arise from a different site than beats #2,4,6,8.

I next looked for P waves:

• This is most easily done by use of calipers. Set your calipers to a P-P interval suggested by the distance between any 2 P waves that you can clearly see. Doing so allows you to "walk out" a fairly regular atrial rhythm (with slight variation in the P-P interval, consistent with sinus arrhythmia — as suggested by the RED arrows in Figure-3).

Figure-3: I've added RED arrows to highlight sinus P waves that "walk out" throughout the entire rhythm strip. There is slight sinus arrhythmia.

Finally — the last of the 5 KEY parameters: 

• The last parameter to rhythm assessment is the 3rd R = Related. We are looking to see if P waves are at all related to neighboring QRS complexes.
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• Figure-4 — shows that there is a "relationship" of P waves to neighboring QRS complexes — in the form of a constant (and normal) PR interval preceding beats #3,5,7 (ie, short horizontal RED lines of equal duration in Figure-4). This confirms that these beats are sinus-conducted.
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• NOTE: I was uncertain in Figure-4 — about the relationship (if any) between P waves highlighted by the YELLOW arrows — and their neighboring QRS complexes ( = beats #2,4,6,8) — because there is variation in the distance from these P waves to the next QRS (as suggested by the BLUE lines in Figure-4). 

Figure-4: Beats #3,5,7 are sinus-conducted, because the PR interval preceding these beats is constant. In contrast — beats #2,4,6,8 are unlikely to be conducted, because the PR interval preceding these beats is not constant.

IMPRESSION: The "Short" Answer

Take another LOOK at today's tracing (Figure-5 — reproduced from Figure-1). Thus far — We have established the following:

• Today's rhythm is supraventricular — and there is "group beating" in a bigeminal pattern (ie, alternating longer-then-shorter R-R intervals). 
• An underlying sinus rhythm is present (As shown in Figure-3 — fairly regular P waves are seen throughout the lead II rhythm strip = sinus arrhythmia).
• Odd-numbered beats are sinus-conducted.
• Even-numbered beats appear to represent junctional escape beats — since the PR interval preceding these beats is very long and not constant. In addition — the QRS complex is slightly different (less negative) than the narrow QRS of sinus-conducted beats. This pattern is consistent with "Escape-Capture" Bigeminy — in which the initial beat in each pair (ie, beats #2,4,6,8) is a junctional "escape" beat — and the 2nd beat in each pair represents "capture" (ie, sinus conduction) of an on-time sinus P wave (as seen for beats #3,5,7).
• There is also some form of 2nd-Degree AV Block — because a number of on-time P waves do not conduct despite having adequate opportunity to do so. 

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Beyond-the-Core: Some of you may have noticed that the PR interval preceding beat #1 looks to be slightly longer than the PR interval preceding the other odd-numbered beats in today’s tracing. As noted in the legend to Figure-1 — I've digitized today's tracing to improve visualization using PMcardio. Unfortunately, the original tracing that I received was folded — therefore slightly distorted. Although I suspect that the PR interval before beat #1 is the same as that of the other odd-numbered beats — I admittedly cannot prove this. 

• This is the “reality” of our current world, in which transmission of smart phone photos via the internet offers the amazing advantage of near-instant consultation with experts all over the world — but which unfortunately may lead to suboptimal resolution due to the need for a “quick” smart phone photo (without the time for precise ECG scanning). 
• That said — Regardless of whether the PR interval preceding beat #1 is (or is not) the same duration of the PR interval preceding beats #3,5,7 — We know that the identical PR interval preceding beats #3,5,7 is not due to chance (and that at least these 3 beats are conducted!). Therefore — my “Bottom Line” interpretation of today's tracing is the same!

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BOTTOM LINE for the “Short” Answer: 

The “group beating” pattern in today’s rhythm is consistent with “Escape-Capture” Bigeminy — due to some form of 2nd-degree AV block. 

• We can not determine from this single tracing if this represents a “high-grade” form of AV block (because we never see 2 P waves in a row that fail to conduct despite being given an adequate chance to do so). 
• That said — the overall ventricular rate of today’s rhythm is slow (ie, between 40-50/minute) — in this 60-year old man who presented with an orthopedic injury and “1 or 2 episodes of syncope”. 
• Further evaluation of this patient is clearly indicated — but considering the presence of symptoms + the overall slow ventricular rate — unless a "fixable" cause is found (ie, drug-induced bradycardia, recent ischemia/infarction, sleep apnea, hypothyroidism, etc.) — a pacemaker may be needed.
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• To EMPHASIZE: Although the above written “breakdown” of today’s case is long — an experienced interpreter using calipers should be able to arrive at my “Bottom Line” conclusion in no more than 1-2 minutes!

Figure-5: Taking another LOOK at today's rhythm.

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The LADDERGRAM:

For those who want to know more regarding my thoughts on the likely mechanism of today’s complex rhythm — A picture is worth 1,000 words! It's easiest to illustrate the mechanism I favor with step-by-step construction of a laddergram — which I do in Figures-6 through -12.

• NOTE: The clinical reality — is that complex arrhythmias sometimes have more than a single possible explanation. We simply can not always be 100% certain of the mechanism of an arrhythmia from the surface ECG. That said — IF we are able to construct a laddergram that demonstrates a physiologicallly sound supporting explanation — then we have established at least one plausible solution.

Figure-6: It is usually easiest to begin a laddergram by marking out the path of sinus P waves (vertical RED lines that are parallel to the onset of each P wave — which I show in the Atrial Tier). I’ve drawn these lines vertical — because speed of conduction through the atria is rapid.

Figure-7: Since all QRS complexes in this tracing are narrow — all beats are supravenricular! The large BLUE arrows show my landmark for entering QRS complexes — which is to drop a vertical line from the onset of each QRS down to the Ventricular Tier. Note that the RED lines that I've drawn in the Ventricular Tier are nearly vertical — since conduction of these supraventricular impulses through the ventricles is rapid. Note the pattern (ie, “group” beating) of the near-vertical arrows in this Ventricular Tier (ie, alternating longer-then-shorter R-R intervals, consistent with today’s bigeminal rhythm).

Figure-8: Now that I’ve filled in the Atrial Tier (with each of the RED-arrow sinus P waves) — and the Ventricular Tier (with each of the narrow QRS complexes in today’s tracing) — it’s time to begin “solving” the laddergram, which is done by filling in the middle (AV Nodal) Tier. I begin with those beats that I am virtually certain are sinus-conducted (ie, beats #1,3,5,7).

Figure-9: As previously discussed — I believe beats #2,4,6 and 8 represent junctional escape beats — because: i) These beats occur at the end of longer R-R intervals which are of similar duration; ii) The PR intervals preceding these beats are extremely long; iii) As per Figure-4 — the PR intervals preceding these beats are not constant; and, iv) While all QRS complexes in today’s tracing are narrow — QRS morphology of these even-numbered beats is slightly different (ie, less negative) compared to the QRS morphology of odd-numbered beats. As noted above in PEARL #1 — this difference in QRS morphology is often an important clue suggesting a different site of origin. In the laddergram — I illustrate my theory that beats #2,4,6,8 are junctional escape beats with small BLUE circles that originate from within the AV Nodal Tier.

Figure-10: I’ve added BLUE butt-ends to show that those P waves that occur in the middle of the longer R-R intervals — are prevented from conducting to the ventricles by the junctional escape beats.

Figure-11: In addition to preventing conduction of those P waves that occur in the middle of the longer R-R intervals — retrograde conduction arising from these junctional escape beats (dotted BLUE butt-ends) — also prevents conduction of the next on-time P waves.

Figure-12: Final laddergram. The mechanism for the bigeminal group beating in today's case is the result of an "Escape-Capture" rhythm. The first beat in each pair represents a junctional "escape" beat. Retrograde conduction from junctional beats #2,4,6,8 prevents conduction of the next on-time P waves (explaining why P waves c,f,i, and l don't conduct). The on-time P waves that follow ( = d, g, j) are able to conduct normally (ie, P waves d,g,j “capture” the ventricles) to produce beats #3,5,7.

P.S.: There are a number of potential mechanisms that may produce an "escape-capture" rhythm. Today's rhythm just shows one of these. Links to related Blog Posts below show others.

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Acknowledgment: My appreciation to Praneet Manekar and Bharat Rewaria (from Amritsar, India) for the case and these tracings.

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Related 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 50 laddergram cases — many with step-by-step sequential illustration).
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• 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).

• 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).