Identifying the P wave is the key to decode any narrow QRS tachycardia . Though the the relationship to p and qrs is vita , many times it is not easy to relate them.More easily one may get a clue to the mechanism by analysing P wave timing .This is the basis of calling narrow qrs tachycardia as short RP and long RP.
Wonder . . . why the relation “P to R” became “R to P” here !
Since in the common narrow qrs tachycardias AVNRT/AVRT , atria activates the atria in a retrograde manner , we look for the relationship of qrs complex on subsequent P wave . Hence the interval between R to P become the focus.
In other words RP interval indicates retrograde conduction property of AV tissue .
If it is slow the P wave will be well separated from QRS .
If it is fast it will be close to QRS complex .
If it is ultra fast as in some AVNRT ,it can fall within the qrs complex and completely invisible .
(The so called r’ prime in classical AVNRT is nothing but a distorted p wave on the terminal qrs complex.)
Based on RP interval the following classification is used (List is incomplete)
Short RP Tachycardia
- AVNRT (Slow-Fast )
Long RP tachycardia
- Atypical AVNRT(Fast -slow)
- Atrial tachycardia*
- Sinus tachycardia*
- SA nodal re-entry*
- Some forms of AVRT
* Please note , here the P wave is not determined by the preceding qrs unlike other tachycardia in the list.
What is the cut off point to call it is Short RP /Long RP ?
It is arbitrary . Following may help
If RP interval > PR interval it is long RP.
If the absolute RP interval is > 100 ms with the heart rate of > 160 it would generally Indicate a long RP tachycardia .
The timing of retrograde P can be very complex than we believe as the following factors heavily influence it.
- The autonomic tone
- Site of retrograde atrial breakthrough point .
- Atrial size ,
- Atrial refractionaries
- Effect of drugs
- Intact-ness of inter atrial conduction
- Chances of the retrograde atrial activation capturing Internodal pathway
The P wave location in narrow qrs tachycardia is primarily determined by the retrograde VA conduction and less on the antegrade AV conduction . Looking at the interval between R and P is a quick way of getting the VA conduction in the bed side.
Once we get an idea how the VA circuit conducts , we can narrow down the possibilities in Narrow qrs tachycardias !
What determines the morphology of retrograde P waves in AVNRT/AVRT ?