ECG Blog #312 — Bigeminy but Not Mobitz I

How would YOU interpret the lead II rhythm strip shown in Figure-1?

Figure-1: How would you interpret this rhythm strip?

MY Approach to the Rhythm in Figure-1:

As always — I favor a systematic approach to every arrhythmia that I encounter, encompassed by the memory aid, "Watch Your Ps, Qs and the 3Rs" (See ECG Blog #185). That said, looking at this rhythm — I could not fail to notice that there is "group beating" in a supraventricular (ie, narrow QRS) bigeminal pattern.

PEARL #1: As discussed in ECG Blog #232 — recognition of a bigeminal supraventricular pattern when the 1st beat in each pair is conducted, should suggest the following differential diagnosis:

• Sinus rhythm with atrial or junctional bigeminy (ie, every-other-beat is a PAC or a PJC).
• Sinus rhythm with atrial trigeminy — in which every-third P wave is a PAC that is "blocked" (non-conducted).
• Some form of SA ( = Sino-Atrial) Block.
• Mobitz I, 2nd-Degree AV Block ( = AV Wenckebach) with 3:2 AV conduction.
• Mobitz II, 2nd-Degree AV Block (with non-conduction of every 3rd P wave).

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Beyond-the-Core: Technically, there are a few additional causes of a bigeminal rhythm that need not be considered in today's case, because the rhythm is either not strictly supraventricular — or — because the 1st beat in each group is not sinus-conducted. These additional causes include:

• Ventricular bigeminy (ie, every-other-beat is a PVC).
• Atrial fibrillation, atrial tachycardia or atrial flutter with Wenckebach conduction.
• "Escape-Capture" (the 1st beat in each group is a junctional or ventricular escape beat — followed by a conducted beat).

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PEARL #2: Another way to look at the bigeminal pattern of group beating that we see in Figure-1 — is that each group of 2 beats is separated by a short pause.

• The commonest cause of a pause is a blocked PAC. In clinical practice — the finding of blocked PACs is far more common than any form of AV block.
• As a result, within seconds of seeing today's tracing — my thoughts were that statistically, the most likely cause of the bigeminal rhythm with short pauses that is seen in Figure-1 — would be atrial trigeminy, in which every-third P wave is a blocked PAC. I'd therefore want to address this possibility early in my assessment. 

 

At this point — I applied the Ps, Qs, 3R Approach:

• P waves — are present (RED arrows in Figure-2 highlight upright sinus P waves in lead II).
• The QRS is narrow — which confirms that the rhythm is supraventricular.
• The rhythm in Figure-2 is not "Regular" — so the Rate varies. That said — there is group beating, with a repetitive pattern of alternating short-long intervals. P waves are Related to neighboring QRS complexes — in that each QRS complex in this tracing is preceded by a sinus P wave with a fixed and normal PR interval. However, the P-P interval is not regular.

Figure-2: I've added RED arrows to Figure-1 to highlight sinus P waves.

Continuing with My Systematic Approach to the Rhythm:

Now that we have systematically assessed the 5 parameters in the Ps, Qs, 3R Approach — it's time to consider the entities included in the differential diagnosis for a bigeminal rhythm that was presented above in PEARL #1:

• Despite the clinical reality expressed in PEARL #2 above — there is no evidence that the rhythm in Figure-2 represents atrial trigeminy, with every third beat being a PAC that is non-conducted. That is — there is no deflection suggestive of a non-conducted PAC in the T wave of beats #1, 3, 5, 7 and 9.
• The rhythm in Figure-2 is neither Mobitz I nor Mobitz II 2nd-degree AV block. The reason we can easily exclude 2nd-degree AV block — is that the atrial rhythm is not regular! (RED arrows in Figure-2). With rare exceptions, for there to be AV block — the P-P interval should at least be fairly regular, and this is definitely not the case in this tracing That is — no P wave is seen near the mid-point of each pause, as would be expected if some form of 2nd-degree AV block was present.
• Additional reasons why the rhythm in Figure-2 is not a form of AV block are: i) The PR interval is not increasing (as it should be if this was Mobitz I); and, ii) The QRS is not wide (as it should be if this was Mobitz II).
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• The rhythm in Figure-2 is not junctional bigeminy — because the P wave in lead II is positive everywhere (The P wave in lead II would be negative if there were junctional beats). This also rules out "escape-capture" — because there are no junctional escape beats.
• The rhythm is not atrial bigeminy — because P wave morphology of every P wave is the same (ie, with a distinct notch at its midpoint). In addition — the PR interval in front of every beat remains the same.
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• By the process of elimination — this leaves us to consider some form of SA block as a likely etiology for the rhythm in Figure-2.

CAVEATS (Beyond-the-Core): 

Unfortunately — We only have access to the 7 second rhythm strip that is shown in Figure-2. We do not have access to a simultaneously-recorded 12 lead ECG.

• It is true that PACs which originate near the SA node may closely resemble sinus P waves in one or more leads. Technically speaking, without the benefit of a 12-lead ECG — I can not completely rule out the possibility of atrial bigeminy, in which the reason the P waves before beats #1,3,5,7 and 9 so closely resemble the P wave of sinus beats — is that the PACs originate so near to the SA node.
• For the same reason — I can not completely exclude the possibility that the reason for the short pauses in Figure-2 might be blocked PACs that are not evident in the single lead being monitored. And although no notched deflection suggestive of a blocked PAC is seen in Figure-2 — it is true that the ST-T wave of beats #1,3,5,7 and 9 appears to be flatter than the ST-T wave of beats #2,4,6,8.
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• That said — I think the rhythm in Figure-2 is most consistent with SA Block.

The Concept of SA Block:

Wenckebach phenomena, in which there is progressive delay in conduction until conduction fails — is not limited to the Mobitz I type of 2nd-degree AV block that occurs out of the AV node.

• Among the other types of Wenckebach conduction — is SA Block of the Wenckebach type. Instead of progressive delay in the PR interval until eventually a P wave fails to conduct to the ventricles (as occurs with the Mobitz I form of 2nd-degree AV block) — with SA Wenckebach, there is progressive delay in the time it takes sinus node impulses to get out of the SA node — until eventually, exit of an impulse out of the SA Node is completely blocked.
• Because this progressive delay in sinus node impulses is completely contained within the SA node — SA block is not seen on the surface ECG. This makes it much more challenging to diagnose SA block.

The mechanism of SA Wenckebach is best explained by use of a Laddergram. I illustrate this concept in Figure-3 — in which I propose a laddergram to explain the ECG findings for today's rhythm:

• Note the addition in Figure-3 of an "extra" Tier to the laddergram — to illustrate progressive delay of sinus node impulses trying to get out of the SA Nodal Tier. Note that every-third SA nodal impulse is blocked.
• Of the 2 sinus node impulses in each group that are able to make it out of the SA Node — conduction of these impulses on their way to the ventricles now proceeds normally, as these impulses pass through the Atria — the AV Node — and the Ventricles. Thus, my proposed laddergram suggests that the rhythm in today's case represents SA Wenckebach with 3:2 SA conduction (ie, 2 out of every 3 SA nodal impulses is able to make it out of the SA Node).

Figure-3: My proposed laddergram for the rhythm in today's case.

NOTE: It may be easiest to think of SA block as an "exit" block that occurs at the level of the SA Node. In the same way that 2nd-degree AV block may be of the Mobitz I and Mobitz II types — so it is with SA block:

• With the Mobitz II form of AV block — instead of progressive increase in the PR interval until a beat is dropped, the PR interval remains constant until one or more on-time P waves is non-conducted.
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• With SA block of the Mobitz II type — sinus node impulses are conducted out of the SA Node without progressive delay — until suddenly, one or more on-time sinus node impulses is not able to make it out of the SA Node. As a result, with SA block of the Mobitz II type — the pause due to non-conduction of one or more P waves out of the SA Node will be some "fixed ratio" of the shortest P-P interval (ie, the result of 2:1, 3:1 or other ratio "exit" block). This is in contrast to SA Wenckebach — in which the pause is less than twice the shortest P-P interval

The "essentials" of SA Block are summarized in Figure-4. I emphasize the following:

• SA block is not common! I count on my fingers and toes the number of times I've seen true SA block over my decades of looking for this rhythm disorder. But as per my proposed laddergram in Figure-3 — I believe this is the most logical explanation for the mechanism of today's rhythm.
• The significance of SA block depends on the clinical setting in which it occurs. This may be a benign conduction disturbance when the pause containing non-conducted sinus impulses is short and occurs in an otherwise healthy and asymptomatic individual. In contrast — when associated with other conduction system disorders in a symptomatic patient with underlying heart disease — clinical outcome may be significantly influenced by the presence of SA block. In older individuals with syncope — SA block may be a component of Sick Sinus Syndrome. That said, given the absence of information in today's case — clinical significance (if any) of the rhythm in Figure-3 is unknown.

Figure-4: Essentials of SA Block (Modified from Grauer: ACLS-2013-ePub).

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Related ECG Blog Posts to Today’s Case: 

• ECG Blog #185 — Reviews the Ps, Qs and 3R Approach to Systematic Rhythm Interpretation.
• ECG Blog #188 — for Review on how to Read (and/or Draw) Laddergrams (with links to more than 50 examples of explained laddergrams).
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• ECG Blog #232 — Reviews the concept of a Bigeminal Rhythm (which may be due to Atrial or Ventricular Bigeminy, Wenckebach conduction — or other causes — Listen to Audio Pearl #47).
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• ECG Blog #163 — Reviews a case of "escape-capture" bigeminy, in which SA block might be operative.
• ECG Blog #256 — Reviews another case of "escape-capture" bigeminy (in which retrograde conduction from junctional escape results in "capture").
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• ECG Blog #164 — Reviews a case Mobitz I 2nd-Degree AV Block, with detailed discussion of the "Footprints" of Wenckebach.
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• ECG Blog #33 — Reviews a case showing blocked and aberrantly-conducted PACs. 
• ECG Blog #66 — Reviews a case showing blocked and aberrantly-conducted PACs. 
• ECG Blog #147 — Reviews a case showing blocked PACs. 
• ECG Blog #57 — Reviews a case showing atrial bigeminy with blocked PACs.