ECG Blog #379 — Why Tachy on Telemetry?

I was sent the rhythm strip shown in Figure-1 — obtained from telemetry monitoring. I did not know the history.

• Can YOU explain what happens after beat #4?

Figure-1: Multi-lead rhythm strip obtained from telemetry monitoring.

MY Thoughts on the Rhythm Strip in Figure-1:

The first 4 beats in this 11-beat rhythm strip are sinus — as determined by the presence of regular upright P waves, with a constant PR interval in lead II (RED arrows in Figure-2).

• The rate of this underlying sinus rhythm is ~57/minute (ie, The R-R interval between these first 4 beats is slightly more than 5 large boxes — therefore a rate slightly less than 300/5 = about 57/minute).
• The last beat in Figure-2 is another sinus beat ( = RED arrow before this last beat #11).
• In between the first 4 sinus-conducted beats and the last sinus-conducted beat — is a wide beat ( = beat #5) — which is followed by a run of 5 narrow QRS complexes at a rapid rate (of ~120/minute). A short pause is seen after beat #10.
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• NOTE: Because it is possible for ectopic foci arising from different sites to manifest a similar QRS morphology in one or two leads — but to look very different in other leads — it is worthwhile taking another look at the original 5-lead tracing in Figure-1, to ensure that QRS morphology stays the same for simultaneously-recorded beats #6-thru-10 in each of the 5 leads shown in this multi-lead rhythm strip.
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• The wide beat in Figure-2 (ie, beat #5) — appears to be a PVC (Premature Ventricular Contraction) because in addition to being wide and very different in morphology compared to sinus-conducted beats — beat #5 is not preceded by a premature P wave.

QUESTION:

• Can YOU explain what happens for beats #6-thru-10?

Figure-2: Today's rhythm begins with 4 normal sinus beatas — and ends with a final sinus-conducted beat (RED arrows that precede beats #1-thru-4 — and the RED arrow before beat #11). Beat #5 is a PVC.

What Happens with Beats #6-thru-10?

As noted above — the QRS complex of beats #6-thru-10 is narrow, and similar in morphology to the QRS of sinus-conducted beats in this tracing. These 5 beats represent a short run of an SVT (SupraVentricular Tachycardia) rhythm at a rate of ~120/minute.

• The KEY to recognizing the mechanism of today’s rhythm — lies with identifying associated atrial activity. Unlike the sinus-conducted beats in this tracing — no upright P wave precedes the 5 beats in this short run of SVT. Instead — YELLOW arrows highlight negative P waves that clearly precede beats #7-thru-10 (ie, theYELLOW arrows in Figure-3).

Important Concepts:

• Terminology — The terminology used for defining supraventricular rhythms can be confusing. As discussed in detail in ECG Blog #240 — the term, “SVT” — refers to any supraventricular rhythm (ie, with origin at or above the AV node) that manifests a rate of ≥100/minute. By this definition, a variety of rhythms may qualify as “SVTs” — including sinus tachycardia, atrial flutter or fibrillation, MAT, AVRT/AVNRT, among others. Thus, the term “SVT” is a generic (general) one — which is optimally used when we do not yet know the mechanism of a fast, supraventricular rhythm (which is why at this point in the process for today's rhythm — I favored simply calling beats #6-thru-10 a regular SVT at ~120/minute).
• Why Isn’t this a Run of Atrial Tachycardia? — To Emphasize: From this single tracing, we can not rule out the possibility that the negative P waves preceding beats # 7-thru-10 might represent a run of ATach (Atrial Tachycardia). That said — ATach is a far less likely explanation than retrograde conduction because: i) The P-P interval between the YELLOW arrows in Figure-3 is constant (whereas ATach typically begins with a “warm-up” period, in which the the atrial rate gradually accelerates); and, ii) The distance from the preceding QRS to these YELLOW-arrow P waves ( = the RP’ interval) is constant — suggesting that these YELLOW-arrow P waves are related to the preceding QRS (instead of being independent of the previous QRS — as would be likely with ATach).

QUESTION: 

• Take another LOOK at Figure-3. Do YOU see any indication of atrial activity after the PVC? If so — Does this provide a clue to the mechanism of today's rhythm?

Figure-3: YELLOW arrows before beats #7-thru-10 highlight atrial activity in the form of negative P waves.

Is There Atrial Activity After the PVC?

• The WHITE arrow in Figure-4 strongly suggests that there is a P wave after the PVC in today's rhythm. We know this — because the negative deflection under the WHITE arrow appears to manifest a similar RP' interval with respect to the QRS complex that comes before it, as noted for the RP' interval seen between the YELLOW-arrow retrograde P waves.

Figure-4: The WHITE arrow highlights the location of the 1st retrograde P wave in today's rhythm.

LADDERGRAM Illustration:

As discussed in detail in ECG Blog #240 — Reentry SVT rhythms (both AVNRT and AVRT) are most commonly initiated by a PAC that occurs at "just the right moment" so as to enable retrograde conduction up an alternative AV nodal pathway that allows a reentry circuit to be set up.

• This is best explained by schematic illustration in the laddergram that I propose in Figure-5. The unusual feature about today's case, is that instead of a PAC initiating the 5-beat reentry SVT rhythm — it is a PVC that does so!
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• The rhythm in Figure-5 begins with 4 normal sinus-conducted beats (which are beats #1-thru-4 in the laddergram). This is followed by beat #5 — which is a PVC (as determined because beat #5 is wide, not preceded by any premature P wave — and very different in morphology from all sinus-conducted beats in this tracing).
• As suggested by orientation of the BLUE arrow that starts from the ventricles — this PVC conducts retrograde all the way back to the atria to produce the negative deflection under the WHITE arrow (first series of dotted RED lines that depict retrograde conduction back to the atria). Because this retrograde conduction presumably travels over the slower AV nodal pathway — the resulting RP' interval (that produces the retrograde P waves under the YELLOW arrows) is long!
• Presumably — the timing of retrograde conduction from this PVC was "just right" — such that it found forward conduction back to the ventricles was possible — thereby establishing the reentry circuit that produced the 5 beat run of SVT that extends from beat #6 until beat #10.
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• Note that no retrograde P wave is seen after beat #10! This stops conduction over the reentry circuit — which terminates this short run of reentry SVT. Then, after a suitable short pause — the SA Node recovers sufficiently to produce the next sinus impulse (that results in sinus-conducted beat #11).

Figure-5: Laddergram illlustration of the probable mechanism in today's rhythm.

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

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Related ECG Blog Posts to Today’s Case:

• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
• ECG Blog #185 — Reviews the Ps, Qs, 3R Approach to Arrhythmia Interpretation.
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• ECG Blog #188 — Reviews the essentials for reading (and/or drawing) Laddergrams, with LINKS to numerous Laddergrams I’ve drawn and discussed in detail in other blog posts. 
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• ECG Blog #240 — Reviews the definition of what an "SVT" is — as well as illustrating the different types of reentry SVT rhythms (with assessment of the RP' interval).
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• ECG Blog #229 — Why AFlutter is commonly overlooked? 
• ECG Blog #137 — AFlutter with an unusual conduction ratio.  
• ECG Blog #138 — AFlutter vs Atrial Tachycardia. 
• ECG Blog #40 — Another regular SVT that turned out to be AFlutter.
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• González-Torrecilla et al: Ann Noninvasive Electrocardiol 16(1):85-95, 2011 — Reviews distinction between AVNRT vs AVRT and other regular SVT rhythms in patients without WPW.
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• Please check out the November 12, 2019 post in Dr. Smith's ECG Blog — in which I reviewed the case of a different kind of regular SVT Rhythm (AFlutter).