ECG Blog #501 (15) — Is this a Run of VT?

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NOTE: I’ve decided to update and republish several of my favorite cases from years past. (Today's post is an improved version of ECG Blog #15 — first published in 2011).

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The rhythm in Figure-1 — is from a right-sided MCL-1 monitoring lead.

• Does a run of VT (Ventricular Tachycardia) begin with beat #6?
• How certain are you of your diagnosis?

Figure-1: Does a run of VT begin with beat #6?

MY Thoughts on the Rhythm in Figure-1:

The underlying rhythm (as suggested by the first 5 beats) — is sinus tachycardia at ~105/minute (ie, The QRS is narrow and regular, with an R-R interval for the first 5 beats of just under 3 large boxes — and with P waves showing a fixed PR interval before each QRS for beats #1-5). 

• After beat #5 — Sinus rhythm is interrupted by a run of a regular WCT rhythm (Wide-ComplexTachycardia).  
• The rate of this regular WCT rhythm is ~200/minute (the R-R interval of every-other-beat for the WCT run is ~3 large boxes — so half the rate = 300÷3 =100 X 2 ~200/minute).
• We don't know for how long this run lasts — since it is still ongoing as the rhythm strip ends after beat #14.
• Sinus P waves are absent during the WCT, but — Doesn't it look like some form of atrial activity is present?

PEARL #1: The finding of atrial activity during a run of a regular WCT rhythm does not necessarily mean that the rhythm is supraventricular. This is because both reentry SVT rhythms, as well as VT may conduct retrograde with 1:1 VA conduction.

• Whereas retrograde P waves are generally negative in the inferior leads — they are usually positive in a right-sided lead such as aVR, V1 or MCL-1. Therefore — the small, upright pointed deflections that we seem to see occurring in the middle of the R-R interval during the WCT (ie, starting after beat #6) — do not tell us the answer.

PEARL #2: The width of QRS complexes during the tachycardia that begins with beat #6 clearly looks wider than it is for the first 5 sinus-conducted beats. That said — it does not look "very" wide.

• That said — We only see 1/12 of the ECG in Figure-1. Part of the QRS may lie on the baseline. When it does — then the QRS may "look" narrow in some leads, whereas in reality — the QRS may be very wide in other leads. 
• "12 leads are better than one!" (ie, When possible, if your patient is hemodynamically stable — Try to get a 12-lead ECG during the tachycardia! ).
• The above said, we might suspect that the rhythm in Figure-1 is supraventricular — because the initial deflection of QRS complexes during the WCT run is upright and narrow (very much like it is for the first 5 sinus-conducted beats). 
• In addition — S waves of the QRS during the run are steeply (rapidly) descending. This suggests supraventricular conduction — whereas with VT, the initial QRS deflection tends to be wider (because the impulse is not arising from within the ventricular conduction system — and is therefore is slower to depolarize the ventricles). 
• The above said — exceptions exist — and "12 leads are better than one" for determining IF the QRS is (or is not) truly wide during a tachycardia.

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KEY Point: How does the WCT run begin in Figure-1? 

(ie, What do we see just before beat #6?).

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ANSWER:

Note the RED arrow that I've added in Figure-2!

• This red arrow notches the T wave that precedes the run of widened beats ( = the T wave of beat #5). This is the "tell-tale" notching of a PAC (Premature Atrial Contraction) — that in Figure-2, precipitates a reentry SVT ( = AVNRT or AVRT).
• In contrast — the onset of VT is not preceded by a PAC.

Figure-2: Answer to Figure-1.

PEARL #3: To Emphasize: The most common cause of a regular WCT rhythm is ventricular tachycardia! That said — there are times when we can definitely exclude VT from consideration.  The rhythm in Figure-2 is one of those times. 

• QRS widening during a tachycardia may occur because of rate-related aberrant conduction (See below). The best way to diagnose aberrant conduction — is by identifying a premature P wave at the onset of the tachycardia. The RED arrow in Figure-2 does just that.
• PACs are sometimes difficult to identify — because they may be partially (or totally) hidden within the preceding T wave. But not in Figure-2. The way that we can be sure that the notching highlighted by the RED arrow is "real" (and not the result of artifact) — is that none of the 4 preceding T waves of the sinus-conducted beats show any hint of notching.
• And, as previously noted — 2 additional findings consistent (albeit not diagnostic) of a supraventricular etiology for beats #6-thru-#14 are: i) that the amount of QRS widening during the run appears to be minimal — and, ii) that the initial deflection (both the small, slender r wave and the steep downslope of the S wave) is similar in morphology to the initial part of the QRS of sinus beats.

Laddergram Illustration:

Appreciation of the mechanism of a reentry SVT is best conveyed by laddergram (as shown in Figure-3):

• Beats #1-thru-5 are sinus conducted.
• Beat #6 is a PAC. This starts the sequence — with retrograde conduction back to the atria (dotted lines that arise from within the AV nodal tier). 
• When "the timing is just right" — the mechanism by which a PAC may initiate a run of a reentry SVT — is if this early beat arrives at the AV node and finds the fast pathway still to be refractory, such that conduction to the ventricles occurs over the slow pathway — whereby a self-sustaining reentry loop is potentially established.

Figure-3: Laddergram illustration of a reentry SVT — in which a PAC precipitates establishment of a self-sustaining reentry loop.

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So Why is the QRS Wide in Figure-2?

The reason QRS complexes are wide in the run of SVT that begins with beat #6 — is that the fast heart rate precipitates a run of rate-related aberrant conduction. As shown in Figure-4 — Depending on when during the period of repolarization a PAC occurs — there are 3 possibilities for conduction:

• Possibility #1: Premature Impulse A — occurs so early as to fall within the ARP (Absolute Refractory Period). Because the entire conduction system is still in an abolute refractory state — premature impulse A is "blocked" (ie, non-conducted to the ventricles).
• Possibility #2: Premature Impulse C — occurs after the refractory period is over.  As a result — a PAC occurring at Point C will conduct normally (with a narrow QRS that looks identical to other sinus beats on the tracing).
• Possibility #3: Premature Impulse B occurs at an intermediate point during the RRP (Relative Refractory Period). A PAC occurring at Point B will therefore conduct aberrantly (ie, with QRS widening) — because only part (but not all) of the ventricular conduction system has recovered.
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• PEARL #4: Most often PACs that occur during the RRP will conduct with some form of bundle branch block and/or hemiblock pattern (reflecting that part of the cnduction system which has not yet recovered).

Figure-4: Absolute and Relative Refractory Periods (ARP & RRP) — explaining why beat A is blocked — and beat B is conducted with aberration.

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Putting It All Together:

The MCL-1 rhythm in today's case begins with 5 sinus-conducted beats. This is followed by a regular WCT run of at least 9 beats, until the rhythm strip ends.

• The WCT manifests a rate of ~200/minute. This rate is faster than usual for sinus tachycardia in an adult — and "off" for untreated AFlutter with 2:1 AV conduction (untreated AFlutter most often manifesting a ventricular rate close to 150/minute). This strongly suggests that the differential diagnosis is between a reentry SVT rhythm (ie, AVNRT or orthodromic AVRT) with QRS widening as a result of rate-related aberrant conudction — vs — VT.
• As shown by the RED arrow in Figure-2 — a "tell-tale" PAC initiates the run of wide beats. This virtually confirms that the WCT is supraventricular due to a reentry SVT (as per the laddergram in Figure-3).

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ADDENDUM:

• Included below is more on aberrant conduction.

ECG Media PEARL #28 (4:45 minutes Video) — Reviews WHY some early beats and some SVT rhythms are conducted with Aberration (and why the most common form of aberrant conduction manifests RBBB morphology).

• NOTE: I have excerpted a 6-page written summary regarding Aberrant Conduction from my ACLS-2013-ePub. This appears below in Figures-5, -6, and -7).
• CLICK HERE — to download a PDF of this 6-page file on Aberrant Conduction. 

Figure-5: Aberrant Conduction — Refractory periods/Coupling intervals (from my ACLS-2013-ePub).

 

Figure-6: Aberrant Conduction (Continued) — QRS morphology/Rabbit Ears.

 

Figure-7: Aberrant Conduction (Continued) — Example/Summary.

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