Image 1 : The ECG of the middle-age man who was diagnosed as myocarditis.
Image 2 - ladder diagram
As we can see, beats # 1, 2 are normally conducted complexes. Beat #3 is an interpolated PVC. Beat # 4 is sinus rhythm but with the prolonged PRI and beat 5 is also a PVC with retrograde atrial capture and then the cycle repeats. Notice that beats # 3,8,13 are interpolated PVCs and beats # 4,9,14 have the prolonged PRI. The possible interpretation for the prolonged PRI of beat 4,9,14 is the concealed retrograde conduction of the premature ventricular complex. The sinus P waves after the interpolated PVCs reach the AV node during its relative refractory period ( caused by the concealed retrograde conduction of the interpolated PVCs), so they take much more time to penetrate the AV node. That phenomenon created the long PRI (beats #4, 9 and 14).