During the course of a physiology laboratory, a student finds that her PR interval is 0.24 second. Concerned, she takes her own ECG again an hour later and sees an area of the ECG strip where the PR interval becomes longer and longer. Performing an ECG measurement on herself for a third time, she sees an area of the strip where a P wave is not followed by a QRS or T; farther along in the strip, however, a normal pattern reappears. What do you think these recordings indicate? i understand the first 3 reading that they indicate an AV block but why would it suddenly go back to normal?