Ventricular Tachycardia? Or SVT with Aberrancy?

This was texted to me, by a recent residency graduate, with the following information: 

“70-something w hx of TAVR (aortic valve replacement) and Stroke. SVT w/aberrancy or VT?”

What do you think?

Here is what I texted back: “SVT.  Give adenosine.”

Response: “How did you tell?”

My answer: “Very rapid initial part of the QRS.  Look at the right precordial leads: classic LBBB.  aVR is negative and the initial deflection of the q-wave in aVR is only 20 ms”

I did not add: “except that the QRS in lateral leads V5 and V6 is negative.  Although negative, there is a monophasic R-wave in the other lateral leads, I and aVL, typical of LBBB.  The onset of the QRS in right precordial leads to the nadir of the S-wave is only about 40-50 ms.  This is impossible in VT.”

One is tempted to immediately conclude that this is VT because: 

1.  The patient is older with cardiac disease, which generally favors VT
2.  One might be deceived into thinking there is concordance of the QRS in V1-V6 (all QRS in the same direction, which appears to be true).  This is NOT TRUE however: concordance is NOT present.  Concordance can only be called when the QRS is all one direction or all the other direction.  If there is an RS-wave, then there cannot be concordance. Here, there are RS-waves in every precordial lead.

Answer:  SVT with aberrancy.  This was proven when a previous ECG in sinus was found and had the exact same LBBB morphology.

Explanation:
Typical VT (in contrast to fascicular VT) starts in myocardium, not in Purkinje fibers (conducting fibers).  Therefore, the initial impulse takes some time to work its way through myocardium and therefore is traveling slowly (so it take time and therefore many milliseconds to traverse).  When it finally reaches conducting fibers, it then goes fast (narrow).

So in VT:
–The initial part of the QRS is slow (wide, with relatively small voltage change per change in time), and the latter part of the QRS is fast (narrow, with relatively large change in voltage per change in time).

In SVT with aberrancy, the opposite is true: 
The impulse is coming from above, from or through the AV node, and thus using conducting fibers.  In SVT with aberrancy, there is some obstruction to the conduction such as RBBB or LBBB, but it is still able to go down some rapid pathway (the left bundle in RBBB or right bundle in LBBB) and only LATER must go through myocardium to arrive at the remainder of the heart.
Thus:
–The initial part of the QRS is fast (narrow, with relatively large voltage change per change in time), and the latter part of the QRS is slow (wide, with relatively small change in voltage per change in time).

Another good rule:
If it looks exactly like LBBB or RBBB, then it is SVT with aberrancy.  But beware: if it looks like LBBB but has a wide r-wave in V1, then it is VT.  If it looks like RBBB, but has a wide r-wave in V1, then is is probably VT.

Finally, strictly speaking, this is not “aberrancy” because there was pre-existing BBB; aberrancy implies that the LBBB or RBBB is a result of the tachycardia, not a baseline finding.  But of course until you find that old ECG, you will not know.  So that is of academic significance.

Management:

Of course either VT or SVT with aberrancy can be treated with electrical cardioversion, but unless the patient is in severe shock or pulmonary edema, you have time to give adenosine.  If it is VT, adenosine is safe.  Adenosine is NOT safe if the rhythm is irregular and polymorphic, implying possible Atrial fibrillation with WPW.

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