Unpacking SA-VA for SVT: How it Works, When it Doesn’t & some Controversy in-between

VAVA response excludes Focal AT, indicating likely AVJRT(AVNRT) or AVRT. In the absence/addition to a measured Post Pacing Interval (PPI –TCL), the SA – VA measurement can be used where the Stim-HRA time during RV apex pacing is compared to the VA time during tachycardia to differentiate between AVJRT & AVRT.

How do we use SA – VA measurements?

• SA – VA <85ms indicates AVRT.
• SA – VA(tach) >85ms indicates AVJRT.

In this example, the SA – VA = 150ms, lending evidence to AVJRT rather than AVRT.

• Importantly, the measurement in the above example has been made using the earliest A, rather than the HRA EGM (which was the validated method). This illustrates that whilst technically incorrect, you can appreciate that the relative difference between the earliest A and the HRA is the same for both the SA & the VA measurement. This means that the SA – VA calculation would have roughly the same result, so long as a consistent atrial EGM is used to measure SA & VA time. 

How does the SA – VA manoeuvre work?

• SA time is a measurement of how long a pacing stimulus takes to travel from the RV apex to the atrium.

• VA time during RV apex pacing (SA) and VA during AVJRT should be dissimilar.

• This is because RV pacing VA conduction involves “sequential” conduction through the distal His-Purkinje system, up the RV septum & through the AV node. On the other hand, during AVJRT, VA conduction is simply via the AV node and does not involve the RV septum & the distal His Purkinjie system (The VA time is a pseudo-interval in AVJRT). Hence, during AVJRT, VA conduction is significantly shorter when compared to pacing from the RV apex.

• During orthodromic AVRT, VA conduction occurs via the interventricular septum, & then through the accessory pathway. This closely approximates the route of RV apex pacing stimulus in the presence of a retrograde accessory pathway. Hence there should be <85ms of difference between orthodromic AVRT & RV apex pacing VA time.

When does SA-VA let you down?

• During AVJRT with a bystander accessory pathway. If a retrogradely conducting septal accessory pathway is present, AVJRT & RV pacing may result in a relatively similar VA time, resulting in SA-VA <85ms. This can misrepresent the tachycardia as AVRT, when in fact, it was AVJRT with a bystander accessory pathway.

My Two Cents:

SA – VA is a staple manourve in the EP lab. It is also low hanging fruit, because it can be performed without sustained tachycardia & in the absence of successful entrainment. Perhaps a controversial opinion, but unless patients are exhibiting marked variability in intra-atrial conduction velocity, I honestly think using a consistent Atrial EGM is a reasonable substitute for using a HRA EGM if needed. For these reasons, it is an incredibly versatile measurement and there are few reasons not to routinely consider it when differentiating SVT.

Thanks for tuning in :)
Cheers
Mitch & CPP Team

Further explanation, details & examples can be found in:

EP in Practice – Program 2

Michaud G, Tada H, Chough S, et al. Differentiation of atypical atrioventricular node re-entrant tachycardia from orthodromic reciprocating tachycardia using a septal accessory pathway by the response to ventricular pacing. J Am Coll Cardiol. 2001 Oct, 38 (4) 1163–1167.