Electrocardiogram (ECG) showing atypical Mobitz type I (Wenckebach) atrioventricular (AV) block

Single lead electrocardiogram (ECG) strip showing an atypical Mobitz type I (Wenckebach) AV block with 18:17 ratio. The last three cycles of the group, the skipped beat (with the P wave lost in the T wave; arrow), and the first three cycles of the next group are shown. The last three cycles had a PR interval of 0.36 sec while the first three cycles showed PR intervals of 0.23, 0.32 and 0.34 sec with a decreasing R-R interval. This demonstrates the importance of comparing the PR interval of the last beat before the dropped QRS to the PR interval of the first and second beats of the new cycle. The PR interval is the longest in the beat before the dropped beat, shortest in the first beat of the cycle, and increases in the second beat.

Normal rhythm strip in lead II. The PR interval is 0.15 sec and the QRS duration is 0.08 sec. Both the P and T waves are upright.