Dizzying Differentials – SVT diagnosis

Answer: Orthodromic AVRT.

Differentiating the mechanism of SVT has implications for medical therapy, & ablation strategy.

A regular narrow complex tachycardia in a young patient is most likely AVJRT, AVRT, or Focal AT, listed from most to least likely. Both AVRT and AVJRT are common across genders and age groups, but AVRT is more prevalent in young males in their teenage years, while AVJRT is more frequent in females aged 20-40 years.

• Although rarer tachycardias like Junctional Ectopic Tachycardia (JET) or Dual AV Nodal Non-Reentrant Tachycardia (DAVNNRT) are possible, they are less likely unless specific indicators suggest them. As a physician once noted, “Common things are more common.”

To diagnose the specific SVT, we need to identify P waves and determine if it is a short or long RP tachycardia.

•  In lead II, deeply inverted ST segments likely represent an inverted P wave. This can be confirmed in V2 and V5, where notching within the ST segment is highly suspicious of a hidden P wave. This indicates a short RP SVT, making AVJRT and AVRT more likely and Focal AT less likely.

Now that we have established it is a short RP SVT, we need to differentiate between AVJRT and AVRT.

• In typical AVJRT, the P wave is mostly buried within the QRS complex because AVJRT involves near simultaneous atrial and ventricular depolarisation (with less than 70ms difference in timing). Thus, only a small portion of the P wave may be visible at the end of the QRS complex.

• In AVRT, the atrium usually depolarises a little after the ventricle depolarises. This means the P wave is often buried in the early or mid ST segment as you see here. Therefore this ECG is likely Orthodromic AVRT.

• Orthodromic refers to ventricular depolarisation occurring via the AV node (resulting in a narrow QRS complex) and retrograde atrial activation occurring via the Accessory Pathway.

My two cents:
In combination with knowledge on demographic & symptom profile, establishing the RP interval can help predict the mechanism of SVT & arrive at a specualtive diagnosis.

In this case, a young male presenting with a short RP interval, where the RP interval is within the ST segment rather than the QRS complex. The demographic & location of the P wave is most indicative of orthodromic AVRT as the diagnosis.

An even shorter RP would suggests AVJRT, whereas a long RP would suggest Focal AT unless there were other factors which suggested re-entry as the mechansim such as the mode of SVT initiation (a subject for a later Beat Box tune!).

Thanks for tuning in :)
Cheers
Mitch & CPP Team

For more information, check out either our ECG course here, or our intro to EP here,or SVT in EP here.