Generation of ECG in common form of atrioventricular nodal reentrant tachycardia (AVNRT)

Ladder diagram of the common (slow-fast) form of AVNRT showing transmission of the impulse in atrial, atrioventricular (AV) nodal, and ventricular tissue and the resultant ECG. The first two cycles show the normal sinus beat (A₁) conducting through the fast pathway (F) to the ventricle (V₁) with the impulse blocked in the slow pathway. The premature atrial beat (A₂ on ladder diagram, P on ECG) is timed critically; it finds the fast pathway refractory but is able to conduct through slow pathway, resulting in a long PR interval, and a ventricular depolarization (V₂). It is also able to reenter and conduct retrogradely up the now recovered fast pathway, resulting in an inverted P wave (A*) that is buried in the QRS complex. This atrial echo beat (A*) then reenters the now recovered slow pathway, which conducts anterogradely and results in ventricular depolarization (V*). The cycle repeats, and AV nodal reentry is established.