Early PAC – JET or AVJRT?

Answer: Early PAC terminated tachycardia without advancing the immediate His signal. Highly indicative of AVJRT (Typical AVNRT) rather than Junctional Ectopic Tachycardia (JET).

Timing of the PAC:

This manoeuvre typically involves either

1. Early PAC  (>55ms pre expected His signal).
2. His-Synchronous PAC

• The above example is an “Early PAC” occurring 120ms pre expected His signal.

An early PAC which terminates tachycardia strongly suggests AVJRT (Typical AVNRT).

• This is especially true when it occurs without immediate His advancement

• It is still true in the setting of immediate His advancement followed by termination such as in the example below.

(NOTE: Examples of “Immediate” & “Subsequent” His signal nomenclature can be found at the bottom of this post.)

A single PAC can terminate a re-entrant arrhythmia by interrupting it at the critical moment. In this case, it interrupts AVJRT by forcing the retrograde limb of tachycardia (the Fast pathway) to depolarise antegradely.

AVJRT Terminations with Early PAC

Early PAC response in JET

• Since JET is driven by an automatic ectopic focus rather than a reentrant circuit, a PAC is unlikely to terminate the tachycardia.

• How often do you terminate an automatic Focal AT or VT with a single paced beat? Rarely, right? It is very unlikely that you will successfully overdrive suppress an automatic arrhythmia such as JET with a single PAC.

• Furthermore, in the example above, the PAC never reached the His bundle (we know this since it did not advance the immediate His), so how could it terminate a focal arrhythmia arising from the His region? This makes it even less likely to be JET.

• An early PAC which advances the immediate His without terminating tachycardia is suggestive of JET as a diagnosis.

• Advancement of immediate His must occur via antegrade conduction over the fast pathway (which is the retrograde limb of AVJRT & would therefore terminate tachycardia)

JET Response to EARLY PAC

My Two Cents

• An early PAC which terminates tachycardia suggests AVJRT.
• This is especially true if it terminates tachy without advancing the immediate His signal.
• An early PAC which does not terminate tachycardia, DESPITE advancing the immediate His Signal CANNOT be AVJRT. JET is far more likely.

Thanks for tuning in :)
Cheers
Mitch & CPP Team

References:

Much of these concepts, including the first EGM were not originally my own, but gathered from work published by others. Here are some recommended references.

Padanilam BJ, Manfredi JA, Steinberg LA, Olson JA, Fogel RI, Prystowsky EN. Differentiating junctional tachycardia and atrioventricular node re-entry tachycardia based on response to atrial extrastimulus pacing. J Am Coll Cardiol. 2008 Nov 18;52(21):1711-7. 

Alasti M, Mirzaee S, Machado C, Healy S, Bittinger L, Adam D, Kotschet E, Krafchek J, Alison J. Junctional ectopic tachycardia (JET). J Arrhythm. 2020 Jul 27;36(5):837-844. 

Defining “Immediate” & “Subsequent” His.

• Immediate His signal is the His signal which occurs immediately after the PAC stimulus.
• Subsequent His signal is the His signal which occurs subsequent to the immediate His signal.