ECG Blog #140 – (PVC – Atrial Bigeminy – PAC – LAHB – RBBB – LPHB – Aberration).

The ECG in the Figure-1 was obtained from an otherwise healthy 20-year old man. The patient was asymptomatic. This ECG was obtained when an irregular heartbeat was noted on routine exam.

• How would you interpret this­­ tracing?
• Are these multifocal (multiform) PVCs?
• How certain are you of your answer?

Figure-1: 12-lead ECG obtained from a previously healthy 20-year old man with an irregular heartbeat. Are these multiform PVCs?

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Interpretation: The underlying rhythm is sinus. Virtually every-other-beat occurs early and looks different, with at least some degree of QRS widening. That said, these are not PVCs (Premature Ventricular Contractions). Instead, the rhythm is atrial bigeminy, in that every-other-beat is a PAC (Premature Atrial Contraction).

• We know that the different-looking beats here are not PVCs — because: i) these early beats are preceded by premature P waves (RED arrows in lead II of Figure-2); and, ii) QRS morphology of the early beats is highly characteristic for aberrant conduction (See below). 

Distinguishing between early-occurring wide beats that are ventricular in etiology (ie, PVCs) vs PACs that are wide because of aberrant conduction is at times a challenging task! 

• We have previously reviewed this topic in detail in earlier ECG Blog posts (See ECG Blog #14 and Blog #15). This particular case applies principles brought forth from those previous ECG Blog posts that convincingly demonstrates the widened beats in Figure-1 are PACs with aberration.

Figure-2: The ECG from Figure-1 has been labeled (See text for full explanation).

 

Why We KNOW that Aberration is Present in Figure-2:
The most convincing evidence for establishing that aberrant conduction is the etiology of early-occurring widened beats in this tracing lies with recognition of premature P waves. In contrast, PVCs are not preceded by premature P waves. RED arrows in lead II of Figure-2 highlight the presence of a premature P wave with constant PR interval before each early beat (ie, beats #1,3,5,7).

• The fact that deflections highlighted by red arrows in lead II occur at this exact point in virtually all other simultaneously-recorded limb leads establishes that these deflections are real and not artifact.
• Note: It is common to see PR interval prolongation for PACs (as is the case here) — since early-occurring atrial activity is clearly more likely to encounter some delay as it traverses an AV node that may not yet have recovered full conduction properties.

Aberrant Conduction is most likely to take the form of some type of bundle branch block and/or hemiblock pattern. As a result — attention to QRS morphology may sometimes help to distinguish between aberrancy vs ventricular beats.

• By far — the most common form of aberrant conduction manifests a RBBB (Right Bundle Branch Block) pattern. This is because under normal circumstances, the right bundle branch tends to have the longest RP (Refractory Period) — which means, that an early-occurring impulse (ie, a PAC) has the greatest chance to arrive at the AV node at a time when the right bundle branch is still refractory. That said, any form of conduction defect may be seen with aberrant conduction — depending primarily on the relative length of the RP for the various conduction fascicles in a given patient. As a result — there may be LBBB (Left Bundle Branch Block) aberration; aberration of either hemifascicle ( = LAHB or LPHB = Left Anterior HemiBlock or Left Posterior HemiBlock aberration) — or, any combination thereof (ie, RBBB/LAHB or RBBB/LPHB aberration).
• KEY POINT: If early beats manifest highly typical QRS morphology for any one or any combination of conduction defect — this is highly suggestive that the widened beat (or run of beats) is supraventricular with aberrant conduction. The converse, however — is not true! That is, aberrant conduction is still possible even if QRS morphology of early beats does not resemble any form of conduction defect(s). In such cases — other ECG criteria will have to be used for distinguishing between ventricular vs supraventricular etiology. 

Unfortunately, Figure-2 does not provide us with an uninterrupted long-lead rhythm strip. That said, it should be obvious that every-other-beat (ie, each odd-numbered beat) manifests a different and widened QRS morphology.

• Attention to premature beats #9 and 11 in lead V1 reveals a highly characteristic RBBB morphology, especially for beat #9 — which shows initial small, narrow r wave; followed by S wave that descends below the baseline in V1; followed by a terminal tall R’ (ie, taller “right rabbit ear”).
• Note the presence of a terminal wide S wave in simultaneously-occurring beats #9 and #11 in lead V6. In contrast, beat #13 in lead V1 is only minimally different in morphology from normally conducted beats #8,10 and 12. As might be expected, there is no terminal wide S wave for beat #13 in simultaneously-occurring lead V6. ECG recognition of this very highly characteristic RBBB morphology in leads V1 and V6 for early-occurring beats #9 and 11 is virtually diagnostic of RBBB aberration.
• Turning our attention to the limb leads — we know (even without seeing a simultaneously-recorded lead V1) that beats #1, 3, 5 and 7 are also all conducted with a typical RBBB pattern, because each of these beats manifests a terminal wide S wave similar to that we have already intermittently seen in lead V6. What is especially fascinating about this tracing — is that while early beats #1 and 3 only manifest RBBB aberration — beat # 5 manifests RBBB/LPHB aberration — and, beat #7 manifests RBBB/LAHB aberration. We say this, because of the typical very deep straight S wave in lead I of beat #5 that occurs in association with a qR pattern in leads II,III (highly typical of LPHB!) — whereas we see an rS complex with predominant negativity in leads II and III for beat #7 (highly typical of LAHB!).
• And then there is beat #13 — which despite being early, occurs without a RBBB pattern, and with no more than minimal aberration.

BOTTOM LINE: PVCs do not do what we see here. More than the already diagnostic presence of premature P waves preceding each early beat (best seen in the limb leads) — changing QRS morphology of every-other-beat manifesting multiple variations of highly typical conduction defect morphology establishes with 100% certainty that the rhythm in Figures-1,2 is Atrial Bigeminy with varying forms of Aberrant Conduction.

• Otherwise, judging from QRST morphology in the normally conducted beats — there do not appear to be any acute changes on this tracing.

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Additional Reading: 

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• See also ECG Blog #14 and Blog #15).