Final answer:
A second-degree Type I AV block, or Wenckebach, is characterized by a delay in electrical impulses between the atria and ventricles, resulting in a progressive lengthening of PR intervals on the ECG. The dropped beats create a pattern of slowing down and restarting ventricular contractions.
Step-by-step explanation:
A second-degree Type I AV block, also known as Wenckebach, is a type of AV block that occurs when the electrical impulses between the atria and ventricles are delayed and some of the atrial beats are blocked from reaching the ventricles.
This can be seen on an ECG as progressively lengthening PR intervals until a P wave is eventually blocked and a QRS complex is dropped.
The dropped beats create a pattern in which the rate of ventricular contractions slows down before restarting again.