What defines a Mobitz type II heart block on an ECG?

Mobitz type II heart block is characterized by a consistent and unchanging PR interval with intermittent dropped beats, which is indicative of the condition's hallmark feature. In this scenario, the PR interval remains constant prior to a dropped QRS complex, meaning that there is no progressive lengthening or alteration of the PR intervals leading up to the dropped beat. This pattern indicates that although the conduction through the AV node is impaired, it remains stable until a block occurs, resulting in a QRS that fails to follow a P wave.

Understanding this characteristic is crucial in differentiating Mobitz type II from other types of heart blocks, particularly Mobitz type I (Wenckebach), which features progressively lengthening PR intervals before a drop occurs, and complete heart block (third-degree block), where there is no relationship between P waves and QRS complexes. Thus, recognizing the stable nature of the PR interval in Mobitz type II heart block is essential for accurate diagnosis and management of patients presenting with this condition.