SVT differentiation & P wave localisation – Not as hard as you think

Answer: Focal Atrial Tachycardia arising from the mid Crista Terminalis or Right Inferior Pulmonary Vein.

Firstly, we must differentiate what type of SVT this is. To do this, we must determine if it is a “short RP” tachycardia or a “Long RP tachycardia”

 

Definitions to know:

1. Short RP tachycardia = “RP interval is < PR interval” –> indicates AVJRT or AVRT
2. Long RP tachycardia = “RP interval is > PR interval” –> indicates Focal AT

 

This particular ECG demonstrates a “long RP tachycardia”. In most circumstances this indicates a Focal Atrial Tachycardia.

Additionally, the upright P wave in the inferior leads makes this EXCEPTIONALLY unlikely to be AVJRT or AVRT, since these tachycardias usually produce negative P waves in the inferior leads, even in the unusual circumstances whereby they generate a long RP tachycardia.

 

Hence we can be quite certain that this tachycardia is a Focal Atrial tachycardia.

 

To determine the origin of Focal Atrial Tachycardia we must look at the following Leads:

1. V1 (RA vs LA origin)

2. II/III/aVF (superior or inferior origin)

3. V2-V4 (anterior vs posterior origin)

4. Lead 1 (or II vs III) –> Rightward or leftward origin within the chamber determined by V1

So firstly, V1:

When V1 is upright, this indicates a Left Atrial or Crista Terminalis/SA Nodal origin.

 

• The LA is the most posterior structure in the chest. This means that Focal AT originating from the LA is always positive, as it depoalrises the atria towards V1.

 

• Positive P waves in V1 can also be found in the SA nodal or Crista Terminalis region of the RA. Hence a positive P wave in V1 should be indicate either a Crista Terminalis or LA origin.

 

Secondly, II/III/aVF

When the inferior leads are slightly positive, it indicates a slightly superior structure/origin, such as an inferior pulmonary vein in the LA, or Mid Crista Terminalis in the RA.

 

• Focal AT originating from superior structures generate a positive P wave in the inferior leads. Even the inferior pulmonary veins generally produce a slightly positive P wave in the inferior leads, as these structures tend to sit quite high in the atria.

 

Thirdly, V2-V4

Positive P waves in leads V2-V4 indicate a posterior origin, such as the pulmonary veins in the LA or Crista Terminalis in the RA, as these structures generate depolarization wavefronts moving towards the chest wall.

 

• Anterior structures such as the tricuspid or Mitral annuli often produce negative P waves in V2-V4, as these focii depolarise the atria AWAY from the chest wall.

 

Lastly, Lead 1 (&/or II vs III)

Lead II is more positive than lead III, when the origin of tachycardia is located in a more rightward position.

 

• This is seen in the example ECG we see above, indicating a rightward strcuture such as the Crista Terminalis or Right Pulmonary Vein.

 

• Lead I is positive when tachycardia originates from the rightward structures, as atrial depolarization occurs from right to left.

 

My two cents

This ECG demonstrates a clear long RP interval with upright P waves inferiorly. Both of these features almost always represents Focal AT.

The origin of tachcyardia can be broken down as follows:

• V1 – Positive –> Therefore LA or Crista Terminalis
• II – Slightly positive –> Therefore Mid Crista terminalis or Left/Right Inferior Pulmonary Vein or top of the Mitral Annulus or Left Atrial Appendage.
• V3 – Positive –> Therefore Posterior structure. Left/Right InferiorPulmonary Vein or Mid Crista Terminalis.
• Lead 1 – Positive –> Therefore Rightward structure. Right Inferior Pulmonary Vein or Mid Crista Terminalis
• Lead II > Lead III amplitude –> Therefore Rightward structure. Right inferior Pulmonary Vein or Mid Crista Terminalis.

 

I’ve included a free pdf from our ECG Course, summarising P wave localisation. Keep this handy as a future reference and have a practice localising the SA Node location during sinus rhythm!

P-Wave-Localisation summary pdf

Thanks for tuning in

Cheers
Mitch & CPP Team

 

P.s Thankyou to Emily for making the cheat sheet pdf :)

Must-read references to understand this subject :)

Kistler PM, Roberts-Thomson KC, Haqqani HM, Fynn SP, Singarayar S, Vohra JK, Morton JB, Sparks PB, Kalman JM. P-wave morphology in focal atrial tachycardia: development of an algorithm to predict the anatomic site of origin. J Am Coll Cardiol. 2006 Sep 5;48(5):1010-7.

Kistler PM, Chieng D, Tonchev IR, Sugumar H, Voskoboinik A, Schwartz LA, McLellan AJ, Prabhu S, Ling LH, Al-Kaisey A, Parameswaran R, Anderson RD, Lee G, Kalman JM. P-Wave Morphology in Focal Atrial Tachycardia: An Updated Algorithm to Predict Site of Origin. JACC Clin Electrophysiol. 2021 Dec;7(12):1547-1556.