Supraventricular tachycardia. It's all in the p wave.
Electrophysiology Fellow
Cardiac electrophysiology fellow with an interest in the familial, words, and the history of electrophysiology.
Searching accessory pathways
Localising a (concealed) accessory pathway
We've talked before about localising accessory pathways by the characteristics of the delta wave. The question for today: can a concealed accessory pathway be localized using the surface ECG?
Here are two ECGs, one during sinus rhythm, the other during tachycardia. The differential diagnosis for the SVT is not broad. I can think of two possibilities. However, I know from an EPS study that this is a SVT using a concealed bypass tract.
What is the other possibility?
If this is an SVT using a concealed bypass tract, where is the bypass tract located?
Update: So, as mcg points out (comments) this tracing is suggestive of a left lateral accessory pathway. The differential diagnosis is atrial tachycardia, and the two cannot be differentiated on the surface ECG unless there is wobble in the tachycardia cycle length, which there is not in this case.
It is presumably concealed, but left lateral pathways with some antegrade conduction can have minimal preexcitation on the surface ECG due to delayed conduction to the atrial insertion. In this case we paced the CS catheter close to the atrial insertion without demonstrating preexcitation.
Something to memorize...
"Nodoventricular (NV) bypass tracts were described pathologically in 1937 by Mahaim and Benatt. Later, fibres connecting the AV node and the right bundle branch - nodo-fascicular (NF) - were recognised. Now it is known that the majority of clinically apparent bypass tracts thought to be NV or NF, are actually slowly conducting atrio-ventricular or atrio-fascicular pathways. True NV or NF tracts are extremely rare."
On the importance of the comma
Josephson. Another example - my last -
In regard to conditions which indicate the presence of a bypass tract:
"The ability to preexcite the atrium during SVT when the His bundle has been depolarized and is therefore refractory to the retrograde impulse, or the depolarization of the atria by a VPD delivered during ventricular pacing before retrograde activation of the His bundle…"
It reads like the VPD is delivered before retrograde activation of the His. It should be thus:
"The ability to preexcite the atrium during SVT when the His bundle has been depolarized and is therefore refractory to the retrograde impulse, or the depolarization of the atria, by a VPD delivered during ventricular pacing, before retrograde activation of the His bundle…"
Two important commas that change the meaning of the sentence. A non-electrophysiology trained editor is unlikely to pick up such nuances; but the careful editing of a friend?
May as well make this the learning point of the day: during ventricular pacing, a VPD that activates the atria before retrograde activation of the His bundle indicates the presence of an accessory pathway.
Practical application of accessory pathway localization
As an application of my former post:
Consider the Z plane first…
The ∆ wave in V1 is clearly negative, and clearly positive in V4, therefore transition occurs between these points. Thinking on Figure 1, negative ∆ waves in the praecordial leads occur in right sided and septal pathways.
nb. the terminal appearance of the p wave, and the ∆ in V2 and V3 is unusual, so I have avoided them - always a wise policy; best to state what you know first!
Now consider the frontal plane…
Try to imagine the ∆ vector, it is positive in II, negative in III, and probably biphasic (though predominantly positive) in aVF. It is also very positive in I and aVL. I'm picturing a vector that points leftward, and inferiorly. (Look at Figure 2 and think about the pathway that would fit such a vector best).
The corresponding pathway would be right anteroseptal, with this caveat: there is always overlap in the electrophysiological appearance between anatomically adjacent pathways. This could be a right free wall pathway that is more superiorly located.
The important point is this: this pathway can almost certainly be approached from the right side avoiding the need for an arterial puncture.
Final point, to accurately localize a pathway, the ∆ must be 'pure', i.e. not fused with native conduction. When the PR interval is ≤ 0.12 ms it is likely that the ∆ is not fused.
Thanks to ecglibrary.com for this example of Wolff Parkinson White.
Easy EP #2: Localisation of accessory pathways (bypass tracts) without algorithms
There are two main alternatives to the use of localisation algorithms - visualisation, and pattern-recognition. We all hope to use the latter eventually but while that ability develops, and partly to aid its development, visualisation is helpful. It also has the advantage of providing a link to the laboratory: the process is similar to transforming electrical signals from multiple catheters into the spatial dimension. Josephson splits the pathways into five main groups, based on distinct ECG patterns: left lateral (LL), left posterior (LP), posteroseptal (PS), anteroseptal (AS), and right free wall (RF).
First, consider the 5 pathways in the "Z" plane, which is probed by the praecordial leads. As the pathways move from left to right there is a general movement of the delta from being biphasic/negative in V1/V2 (right sided, septaL) to being positive in V1/V2 (left sided). As Josephson states: "variable lead placement, variations of body shape and/or size, and variations in heart size, location in the chest…" make the praecordial leads less reliable than the limb leads - therefore, better to use them as a general locator, rather than specific. Try to visualise the pathway as arising on the right/middle or the left.
Second, consider the pathways in the frontal "XY" plane that is examined by the limb leads. The initial direction of depolarisation is represented by a vector originating at the ventricular insertion of the tract and pointing toward the LV (arrows in Fig 2). Thus, left posterior (LP) pathways have negative delta waves in the inferior leads, and positive aVL ± lead I. Posteroseptal pathways generally have a more negative delta in lead III than II, while left posterior are more negative in II compared III.
The red arrow in Figure 1 represents the fact that PS pathways may be more right than left and vice versa. This is important for catheter procedures, and may influence the choice of approach. As the pathway moves to the right, lead III will be more negative than II; as it moves to the left, an opposite pattern will result.
[ Edit: One trick, that you just have to remember, is the difference between right free wall, and posteroseptal pathways, their axes in the frontal plane may be very similar (i.e. 0 → −60˚ or so). Usually, posteroseptal pathways will have an abrupt change in the R/S ratio from V1 (<1) to V2 (>1). Right free wall - usually - will not.]
The method is simple. Memorise the pictures, and the relative position of the ECG leads. Practise.
