Refer images above

There are two pathways,Fast pathway is blue is efficient and rapid but has Refractory period of 330 milliseconds. So once a train goes through the rails it takes 330 milliseconds for it to be ready. And slower pathway does work slowly but gets ready fast so it’s Refractory period is 270 milliseconds, this is indicated in Red.

This patient has sinus rate of 600 ms so the fast pathway being ready will conduct and PR will be shorter is 130 milliseconds as fast pathways conducts faster. This is an APC which happens 280 Msec after previous sinus discharge not it can’t use the blue line so what can it do , will Red slow pathway is ready for it and it goes slowly over the red pathway. Remember it’s so slow that it take 60 millisecond to travel in the pathways and by the time it comes to point of convergent at the AV grove blue line is ready to be used in retrograde direction. So Retrograde P wave conducts rapidly after the QRS indicted by negative P wave just after QRS.

Adenosine slows AV nodal conduction. And atrial tachycardia has nothing to do with AV node . So abrupt termination more or less rules out Atrial tachycardia.

Our problem is to explain a mechanism that permits such a short PR.Antidromic AVRT permits such a short PR but QRS would be widened as there is direct ventricular stimulation and that is not the case.And orthodromic AVRT has to use normal Fast pathway and we know that that entails the delay of 130 msec in the antegrade limb so that’s not possible

If at the site where slow pathways depolarises the retrograde fast pathway and the HIs pukinje system , there was a delay in the his His purkinje system the ventricular depolarisation would occur slightly later that retrograde atrial depolarisation so it is common AVNRT with slight delay in His purkinje system

Complex Case