Final answer:
A 2nd degree Mobitz I AV block on an ECG is indicated by a cyclic pattern of progressively lengthening PR intervals until a P wave occurs without a subsequent QRS complex, after which the cycle repeats.
Step-by-step explanation:
On an ECG, a 2nd degree Mobitz I AV block, also known as Wenckebach phenomenon, is characterized by a progressive lengthening of the PR interval until a P wave is not followed by a QRS complex. This pattern then repeats, with the PR interval returning to its shortest duration and again progressively lengthening. Each cycle of increasing PR interval followed by a dropped QRS complex indicates the intermittent and progressive failure of the AV node to conduct atrial impulses to the ventricles.