The 2-lead rhythm strip shown in Figure-1 was obtained from the Holter monitor of a middle-aged adult.
- Does this tracing show a 4-beat run of VT or aberrant conduction?
- How certain are YOU of your diagnosis?
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| Figure-1: A simultaneously-recorded 2-lead rhythm strip, obtained during Holter monitoring. Does this tracing show a 4-beat run of VT or aberrant conduction? |
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NOTE #1: Some readers may prefer at this point to listen to the 6:00 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-34).
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Today's ECG Media PEARL #34 (6:00 minutes Audio) — Are YOU really seeing AV Dissociation in that WCT Rhythm? (ie, Is the rhythm VT?).
MY Thoughts regarding the Rhythm in Figure-1:
The underlying rhythm in this 2-lead Holter tracing is sinus — as indicated by normal-appearing upright P waves with a fixed PR interval preceding each of the narrow beats (RED arrows in Figure-2).
- After the first 2 sinus beats — the rhythm is interrupted by a 4-beat run showing a very different QRS morphology (ie, beats #3-thru-6).
- Sinus rhythm resumes with beat #7 for the last 3 beats in the tracing.
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| Figure-2: I’ve labeled atrial activity from Figure-1 (See text). |
What is the Cause of the 4-Beat Run?
I am 100% certain that the 4 different-looking beats that are seen in Figure-2 constitute a short run of NSVT ( = Non-Sustained Ventricular Tachycardia). My reasons for saying this are the following:
- Statistically ( = according to the literature) — When the underlying rhythm is sinus, sudden occurrence of a wide run of different-looking beats (as we see in Figure-2) will turn out to be VT at least 80% (if not 90%) of the time. Therefore — Always assume VT until proven otherwise (ie, Our “mindset” should be that we need to prove aberrant conduction — rather than the other way around).
- There is no reason for aberrant conduction to occur in Figure-2. Aberrant conduction of supraventricular beats is most often seen when the run of wide beats occurs early in the cycle (ie, when the coupling interval is short) — because this increases the chance that the run will begin during the RRP (Relative Refractory Period). But in Figure-2 — the first wide beat ( = beat #3) occurs late in the cycle, at a time when it is very unlikely that the conduction system will still be refractory. Thus, there is no “reason” for aberrant conduction. (NOTE: For more on WHY certain supraventricular beats conduct with aberration — Please see ECG Blog #211).
- QRS morphology for the different-looking beats shows marked QRS widening and a totally different appearance. Supraventricular beats that conduct with aberration often manifest a similar initial direction and slope of the QRS complex — because supraventricular beats often initially conduct down the normal AV nodal pathway until they encounter that part of the conduction system that is refractory. But, as opposed to the rapid R wave upslope that we see for sinus beats in both lead A and lead B of Figure-2 — beats #3-thru-6 show a marked difference in the very initial part of the QRS complex.
- I am uncertain which specific leads rhythms A and B in Figure-2 correspond to. The most common leads monitored are a standard lead II — and either a right- or left-sided lead (corresponding to lead V1 or MCL-1 — or — lead V5 or V6 or MCL-6). Knowing the specific lead(s) used in monitoring helps — because aberrant conduction is much more likely when QRS morphology is consistent with some type of conduction defect (ie, RBBB, LBBB, LAHB, LPHB, or some combination thereof).
- I suspect rhythm A corresponds to a lead II recording — in which case the excessively wide initial R wave and terminal S wave fragmentation are not characteristic of LAHB conduction. And, regardless of the lead that rhythm B corresponds to — QRS morphology would not be typical for any known form of conduction defect. Thus, QRS morphology in Figure-2 is atypical for any of the common forms of conduction defects — and, this is another factor against aberrant conduction. (NOTE: This atypical QRS morphology by itself does not rule out the possibility of aberrant conduction — but it does make aberrant conduction less likely).
- Most Convincingly: There is AV Dissociation in Figure-2! This is subtle — but it is present (PINK arrows). Calipers are needed to find this! — but the notching that is highlighted by PINK arrows in rhythm A — corresponds perfectly to subtle notching that is also seen in rhythm B (as shown by the vertical BLUE timelines) — and — these PINK arrows occur with a P-P interval that is perfectly consistent with the slight underlying sinus arrhythmia. Demonstration that sinus P waves continue on-time throughout the rhythm strip in both leads A and B confirms that there are independent atrial and ventricular rhythms. This constitutes AV dissociation (ie, P waves that transiently are not related to neighboring QRS complexes) — and identification of AV dissociation proves that beats #3-thru-6 constitute a 4-beat run of NSVT.
P.S. — Some Definitions:
- There are many different names for early-occurring ventricular beats. I favor the term PVC (Premature Ventricular Contraction).
- 2 PVCs in a row are called a ventricular Couplet.
- 3 PVCs in a row are called a ventricular Salvo or a “Triplet”.
- The definition of “VT” (Ventricular Tachycardia) — is 3 or more PVCs in a row.
KEY Point: Rather than simply noting the presence of “VT” — it is important to describe the rhythm by a number of KEY parameters. These include:
- Duration of the VT run (ie, How many beats or for what period of time does the VT last?);
- Rate of the VT?
- Regularity of the wide rhythm? (ie, Not all VT is regular — as seen for the 4-beat run in today’s case).
- QRS Morphology? (ie, Do all VT beats look the same — or is there changing morphology?).
- What happened just before? — and immediately after the VT episode? (ie, Was the patient in sinus rhythm or AFib before VT began? Was there a “post-ectopic” pause after the run terminated?)
- Are there recurrent episodes of the VT rhythm?
- Has there been antiarrhythmic treatment?
- Importantly: What is the patient's hemodynamic status during the VT episode? (ie, Is the patient symptomatic? Hemodynamically stable or unstable?)
Some Final Terms:
- NSVT (Non-Sustained VT) — The occurrence of 3 or more PVCs in a row up until the point when VT becomes “sustained”.
- Sustained VT — Definitions for what constitutes “sustained” VT vary, depending on duration of the VT episode and/or the number of beats in the run. Most authorities reserve designation as “sustained” VT for runs lasting at least 30 seconds and/or VT that causes hemodynamic instability.
- Editorial Comment: My Simplified Definition — IF the run of VT is long enough to give YOU palpitations, then it is probably “sustained” VT.
- “Slow” vs “Fast” VT — This gets into the definition of whether the VT rhythm is more likely to be associated with hemodynamic consequences or not. In general — AIVR ( = Accelerated IdioVentricular Rhythm or “slow” VT) will often be surprisingly well tolerated — whereas “fast” VT (usually associated with a ventricular rate of at least 130/minute) is much more likely to be associated with hemodynamic consequences.
- NOTE: For more on AIVR and “slow” vs “fast” VT — Please check out ECG Blog #108.
In SUMMARY regarding Today’s Case:
- The rhythm in Figure-1 shows an underlying sinus arrhythmia that is interrupted by a 4-beat run of an irregular NSVT at ~110-130/minute.
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Acknowledgment: My appreciation to Zineddine Chleghoum (from Alger, Algeria) for the case and this tracing.
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Related ECG Blog Posts to Today’s Case:
- ECG Blog #211 — WHY does Aberrant Conduction occur?
- ECG Blog #133 — Illustrates use of Fusion beats and AV Dissociation in the diagnosis of VT.
- ECG Blog #128 — Reviews the concept of Fusion Beats.
- ECG Blog #129 — Late-cycle (End-diastolic) AIVR and Fusion beats in diagnosis of ventricular beats.
- ECG Blog #134 — A very subtle example of AV Dissociation to prove VT.
- ECG Blog #108 — About AIVR — and review of why retrograde P waves with 1:1 VA conduction do not constitute AV “dissociation”.
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