How does a second-degree Mobitz Type II block appear on an ECG?

In a second-degree Mobitz Type II block, the essential feature observed on an ECG is the presence of intermittent non-conducted P-waves while maintaining a stable PR interval. This means that some P-waves fail to trigger a QRS complex, but the intervals between P-waves that are conducted remain consistent.

This distinction is critical because it helps differentiate Mobitz Type II from other arrhythmias. In Mobitz Type II, the PR interval remains unchanged, which is a significant hallmark. The regularity of the PR intervals indicates that the conduction issue occurs at the level of the AV node but does not exhibit the variable behavior seen in other types of heart blocks.

For example, a Mobitz Type I (Wenckebach) block would show varying PR intervals prior to a dropped QRS complex, while complete AV block would lead to dissociation between P-waves and QRS complexes without any reliable PR interval.

Understanding these characteristics aids in diagnosing the specific type of heart block and determining the appropriate management approach.