A 70 yo c/o of palpitations.
Image 1- ECG Case
Image 2- ECG Case with P waves marked and ladder diagram
It initially starts as sinus rhythm with a PAC. Later, an atrial arrhythmia (likely atrial tachycardia) in noted (black arrows). PRI interval initially is slightly longer compared during sinus rhythm but on the 3rd beat, there is marked prolongation of the PRI. After that, inverted P waves in the precordial leads can be seen right after the QRS (red arrows). Thus, we see a supraventricular tachycardia that is called AV nodal reentry tachycardia (AVNRT).
In some individuals, there exist 2 AV nodal pathways. One conducts the impulse fast but recovers slow or has a long refractory period (fast pathway or FP) and the other conducts slow but recovers fast or has short refractory period (slow pathway or SP). During sinus rhythm, the impulse is conducted via the fast pathway (red line).
Image 3 - Dual AV Node Physiology
ANVRT is almost always initiated by a premature atrial beat like a PAC. A PAC is blocked in the fast pathway (because the FP has not recovered from the previous sinus impulse) and it is conducted anterograde via the slow pathway (creating a long PRI in the surface ECG) and it is retrogradely conducted via the slow pathway (because the SP has already recovered). This completes the circuit and the AVNRT is initiated.
Image 4 - Initiation of AVNRT by a PAC
In our case, the first 2 AT beats are conducted with slightly longer PRI than during sinus rhythm. It traveled the SP but not only after the 3rd beat (markedly long PRI) that it was slow enough to allow reentry and initiated the AVNRT. This can probably be like initiation in the laboratory of an AVNRT by rapid atrial pacing.
Image 5 - 12L of the case during the tachyardia (AVNRT)
Josephson, Mark E. 2008. Clinical Cardiac Electrophysiology: Techniques and Interpretations, 4th Edition. Lippincott Williams & Wilkins