What is the defining characteristic of a second-degree heart block type I (Wenckebach)?

The defining characteristic of second-degree heart block type I, also known as Wenckebach, is the progressive lengthening of the PR interval until a QRS complex is dropped. This phenomenon occurs because the conduction through the AV node becomes progressively delayed with each successive heartbeat until the electrical impulse fails to reach the ventricles, resulting in a dropped beat.

The pattern of PR interval lengthening is crucial in identifying this type of block and helps differentiate it from other types of heart block. In practice, this manifests as a repetitive cycle where you can measure the increasing duration of the PR interval, making it distinctive.

The other options describe different conditions or heart block types. A constant PR interval with occasional dropped beats would pertain to a different type of block, while complete dissociation between atrial and ventricular activity describes third-degree heart block. Sudden loss of QRS complexes may suggest a more immediate conduction issue or a different arrhythmia, but it does not fit the progressive lengthening seen in Wenckebach.