What is the key distinction between complete heart block and Mobitz II on ECG?

• A. Mobitz II is defined by complete AV block with no conduction
• B. In Mobitz II, PR intervals are constant in conducted beats
• C. Complete heart block shows intermittent dropped QRS with varying PR
• D. Complete heart block has no AV conduction; Mobitz II has intermittent dropped QRS with a constant PR in conducted beats

Answer: (D)

The key distinction is whether AV conduction is completely lost or only intermittently impaired. Mobitz II preserves some conduction: some P waves are conducted to the ventricles, producing a QRS with a constant PR interval, but there are occasional non-conducted P waves that result in dropped QRS complexes. Complete heart block, on the other hand, has no AV conduction at all—the atria and ventricles beat independently (AV dissociation), so there is no fixed relationship between P waves and QRS and no predictable PR interval for conducted beats. This difference matters because Mobitz II can still conduct sometimes and has a risk of progressing to complete block, often requiring pacing, whereas complete heart block typically requires pacing immediately due to the complete loss of AV conduction.