What is the most likely diagnosis in a patient with gradually progressive PR interval prolongation followed by a dropped QRS complex?

When considering a patient with a clinical presentation of gradually progressive PR interval prolongation followed by a dropped QRS complex, the diagnosis that best fits this description is second-degree Mobitz Type I (Wenckebach) atrioventricular block.

In this type of block, the key characteristic is that the PR interval progressively lengthens with each successive beat until a QRS complex is dropped. This pattern happens because of a gradual increase in the interval of conduction through the atrioventricular (AV) node until the node fails to conduct the electrical impulse, leading to a missed beat—exemplified by the dropped QRS.

This behavior sets Mobitz Type I apart from other types of heart blocks. First-degree AV block features a consistent PR interval without any dropped beats, while Mobitz Type II typically presents with a consistent PR interval but drops QRS complexes without prior lengthening. Lastly, third-degree AV block shows complete dissociation between atrial and ventricular activity and does not involve a pattern of progressive PR interval changes.

Understanding these differences is crucial in accurately identifying the type of AV block based on the presented electrocardiogram (ECG) findings and clinical symptoms.