Thursday, December 8, 2022

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".

  • 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).

MApproach 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!

  • 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.

  • 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.

  • 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-5reproduced 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-3fairly 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. 

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!

BOTTOM LINE for theShort” 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.

  • 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.


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.


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



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).

  • 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 #315Escape-Capture Bigeminy (from marked sinus bradycardia).
  • ECG Blog #144Escape-Capture Bigeminy (from 2nd-degree AV block of uncertain severity).


  1. Amazing post. Thank you for directing my attention to this. Coincidentally, I have just received the ECG of a young boy with Rhabdomyolysis with escape-capture and this post helped me to interpret the arrhythmia. I am getting the whole series of ECG and will share with you,,,,wow, you open my eyes to another "wonder" in ECG. No wonder you have so many views and you have students from every corner of the globe, Thank you for your labor of love,

    1. THANKS so much for the kind words! We ALL are learning every day! (I learn lots from the tracings that other people send me — :)