A 12-lead ECG with a simultaneously recorded lead II rhythm strip. The downward arrows show the P waves of a regular sinus rhythm with a rate of 58 beats per minute. The QRS complexes show group beating with three QRS complexes for every 4 P waves. The numbers below Lead II are the corresponding RR intervals. The numbers above Lead II are the corresponding PR intervals. The duration of the RR interval spanning the QRS complex pause (1920 msec) is less than twice the duration of any other RR interval. The durations of the PR intervals in each group gradually increase (300 msec then 380 msec then 420 msec). The durations of the RR intervals after the pause get progressively shorter (1080 msec then 1040 msec) because the increments by which the PR intervals get longer are progressively less (80 msec then 40 msec).
Image courtesy of L. Brent Mitchell, MD.