ECG Blog #256 — Special Kind of Bigeminy

The ECG shown in Figure-1 was obtained from an older woman found to have a slow pulse. 

• Can you explain the rhythm?

Figure-1: ECG obtained from an older woman with a slow pulse.

 

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NOTE: Some readers may prefer at this point to listen to the 6:15-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-68).

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

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Today’s 2nd Audio Pearl = 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).

 

 

 

My Sequential Thoughts for Interpreting this Tracing:

As always — I began my systematic approach to the rhythm with assessment of the Ps, Qs and 3Rs (as discussed in detail in ECG Blog #185).

• P waves — Normal sinus P waves are not seen on this tracing (ie, No upright P wave in the long lead II rhythm strip is seen).
• The QRS complex is narrow for all beats in this tracing. This confirms that the rhythm is supraventricular.
• The rhythm is obviously not completely Regular — so the Rate varies. Stepping back a little bit from this tracing — it should be apparent that there is group beating from a bigeminal rhythm (ie, repetitive groups of 2 beats that are each separated by a pause of ~1.5 seconds). Given the length of these pauses — the overall heart rate is slow (ie, There is bradycardia).

 

NOTE: Although no sinus P waves are seen in Figure-1 — there is evidence of some other kind of atrial activity in the form of a negative P wave in the long lead II that occurs after the 1st beat in each group (See YELLOW arrows in Figure-2).

• Regarding the 3rd R in our Ps, Qs, 3R Approach (ie, the "Relation" between P waves and neighboring QRS complexes) — Doesn't it look as if the distance from the preceding QRS until each of these negative P waves is constant? These negative P waves in Figure-2 therefore are related to the QRS complex that preceeds them.
• These negative P waves are also related to the QRS complex that follows them — because PR interval preceding beats #2, 4, 6 and 8 in Figure-2 is constant. Therefore, these narrow beats (ie, beats #2, 4, 6 and 8) are being conducted!

Figure-2: I've added YELLOW arrows to highlight negative P waves that are seen after the 1st QRS complex in each group (See text).

 

PEARL #1: The 1st beat in each of the pairs of beats seen in Figure-2 must be a junctional escape beat because the rate is slow, the QRS complex of beats #1, 3, 5, 7 and 9 is narrow (therefore supraventricular) — and these beats are not preceded by a sinus P wave.

• In support that beats #1,3,5,7 and 9 are junctional (or His) escape beats — is the finding that the preceding R-R interval before each of these beats is the same (ie, just under 8 large boxes in duration — which corresponds to a junctional escape rate just under 40/minute).

 

PEARL #2: There are 2 possible explanations for the negative P waves (YELLOW arrows) that are seen in Figure-2. These possible reasons include:

• The negative P waves could be PACs that occur with a fixed coupling interval after each of the junctional escape beats.
• OR — The reason the P waves highlighted by YELLOW arrows are negative in the long lead II rhythm strip could be that these P waves are conducted retrograde (backward) from each of the junctional escape beats. Although impossible to prove from this single ECG — it would seem that this 2nd possibility is far more likely. As explained in ECG Blog #239 — Echo Beats are most likely to occur following a period of delayed conduction, and the RP' interval seen in Figure-2 (ie, distance from each of the junctional escape beats until the negative P wave that follows it) is clearly prolonged. This delayed retrograde conduction provides more opportunity for conditions to be "just right" to allow the retrograde impulse to "turn around" and conduct forward (ie, to produce a reciprocal or "echo" beat).

 

 

Deriving the LADDERGRAM:

A picture tells 1,000 words. The complex mechanism of today's case is best explained by step-by-step derivation of a Laddergram (See ECG Blog #188 for review on how to read and/or draw Laddergrams).

• Sequential legends over the next 7 Figures illustrate my thought process as I derived the final laddergram shown below in Figure-9.

 

Figure-3: It is usually easiest to begin a laddergram by marking the path of sinus P waves through the AtrialTier. However, in today's case — there are no sinus P waves! Instead — the only atrial activity is in the form of negative P waves (YELLOW arrows) that I suspect most likely represent retrograde atrial activity for the reasons stated above in Pearl #2. Since the most challenging part for constructing a laddergram is determining events within the AV Nodal Tier — I thought it best to save the AV Nodal Tier for last, and to begin by drawing in ventricular complexes — which I do in Figure-4.

 

Figure-4: Since all QRS complexes in this tracing are narrow — all beats are supravenricular! The large GREEN 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.

 

 

 

Figure-5: I've completed the Ventricular Tier with near-vertical RED lines corresponding to each of the 9 QRS complexes in the rhythm. Note the group beating for this bigeminal rhythm!

 

 

 

Figure-6: We've established that since the 1st beat in each pair is narrow and not preceded by a sinus P wave — that beats #1, 3, 5, 7 and 9 are junctional escape beats. I illustrate this with small BLUE circles that originate within the AV Nodal Tier.

 

 

 

Figure-7: Working on the assumption that each of the negative P waves (YELLOW arrows) reflect retrograde conduction arising from the junctional escape beats — I've drawn in dotted BLUE lines with appropriate timing to arrive back to the atria at the moment corresponding to occurrence of the negative P waves.

 

 

 

Figure-8: This leaves us having only to decide about the mechanism for supraventricular beats #2, 4, 6 and 8. As discussed and illustrated in ECG Blog #239 (Be sure to also check out the Audio Pearl in Blog 239)— the relative delay in retrograde conduction from each junctional escape beat provides ample opportunity for each retrograde impulse to "turn around" and conduct forward again to produce an Echo Beat (ie, slanted BLUE lines in the AV Nodal Tier that conduct forward to produce reciprocal beats #2, 4, 6 and 8).

 

 

 

Figure-9: Final laddergram. The mechanism for the bigeminal group beating in today's case is the result of an "Escape-Capture" rhythm, in which the first beat in each pair represents a junctional "escape" beat — in which retrograde conduction of the impulse on its way back to the atria is able to "turn around" and alsoconduct downward, thereby producing an Echo beat that "captures" the ventricles (to produce beats #2,4,6 and 8).

 

 

FOLLOW-UP to Today's Case:

A 24-hour Holter monitor was obtained on the patient in today's case. Results of this Holter recording confirmed that this older woman had Sick Sinus Syndrome — and a permanent pacemaker was implanted.

• As reviewed in Audio Pearl #69 (above) — establishment of the diagnosis of SSS requires ruling out other potential causes bradycardia. Other than a somewhat voluminous T wave in lead V4 — the 12-lead ECG in today's case (as shown in Figure-1) did not suggest acute ST-T wave changes. Work-up of this patient ruled out potentially "fixable" causes of the rhythm disorder (ie, no rate-slowing drugs — no recent ischemia-infarction — no hypothyroidism — no sleep apnea).
• Several ECG features consistent with the diagnosis of SSS are implied by the laddergram in Figure-9. These include: i) Presumption of marked sinus bradycardia (and/or prolonged sinus pauses) as the reason a slow junctional escape rhythm was able to take over; ii) In addition to a "sick" sinus node — the laddergram in Figure-9 suggests there is also a "sick AV node", as the rate of the junctional escape rhythm is slower than expected (ie, slighty less than 40/minute); and, iii) The long RP' interval following each of the junctional beats suggests that there may be a component of AV block.
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• P.S. (Beyond-the-Core): A final point to note for academic interest — is that QRS morphology for the junctional escape beats (ie, beats #1,3,5,7 and 9) — is slightly different than QRS morphology for each of the capture beats (ie, beats #2,4,6 and 8 — which clearly show slight differences compared to junctional beats in each of the 6 limb leads). Reasons for this slight difference in QRS morphology may be atributed to: i) aberrant conduction; or, ii) The fact that depending from which part of the AV Node the escape focus is arising from — the path (and therefore QRS morphology) of junctional escape beats may differ slightly.

 

 

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Acknowledgment: My appreciation to Feroz Haroon (from Kashmir, India) for the case and this tracing.

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Relevant ECG Blogs to Today's Audio Pearl:

• ECG Blog #185 — Reviews the Ps, Qs & 3R Approach to Systematic Rhythm Interpretation. 
• ECG Blog #188 — Reviews how to understand (and how to draw) Laddergrams! 
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• ECG Blog #239 — Reviews the concept of Echo Beats, and its clinical applications (showing an unusual bigeminal rhythm case of AV Wenckebach over dual AV nodal pathways, terminated by Echo beats).
• ECG Blog #232 — For review of a bigeminal rhythm due to subtle 3:2 AV Wenckebach. (NOTE: The Audio Pearl in this post is devoted to the concept of Bigeminal Rhythms). 
• ECG Blog #243 — For review of a bigeminal rhythm due to AFlutter with dual-level Wenckebach conduction out of the AV node.
• ECG Blog #252 — For review of a bigeminal rhythm due to atrial trigeminy with blocked PACs. 
• ECG Blog #206 — For review of a fascinating case of a bigeminal rhythm due to 3:2 AV Wenckebach with alternating Hemiblock.
• ECG Blog #163 — Escape-Capture Bigeminy (with sinus bradycardia and resultant junctional escape — and possibly also with SA block).