In the beginning, there are sinus beats that are conducted with 1st degree AV block. These beats are followed by ectopic atrial contractions which are not conducted through AV node (blocked PACs). However, an ectopic atrial contraction eventually triggers an AVNRT, a regular narrow complex tachycardia with relatively long RP intervals due to slow conduction through retrograde pathway in the AV node.

Special Thanks to my dear friend salvatore salvatore for providing the diagram.