11 yr old boy. s/p Repair of Sinus venosus ASD. Started with AAI pacing at 80 bpm in the operating room. You set out to determine the underlying rhythm. The following strips are availabe for analysis. Second-from-the-last panel has atrial lead electrogram in V1. (Click on the images to enlarge).
Last panel image is from Principles of Clinical Electrocardiography by Mervin J. Goldman. Lange Medical Publications. 11th ed. 1982.