How is a first-degree AV block defined?

A first-degree AV block is characterized by a prolonged PR interval on the electrocardiogram (ECG) that remains consistent, meaning there are no dropped beats. The PR interval is the time it takes for electrical impulses to travel from the atria through the AV node before reaching the ventricles. In a first-degree AV block, this interval exceeds 200 milliseconds, indicating a delay in conduction through the AV node but still allowing all impulses to be conducted successfully to the ventricles.

The absence of dropped beats differentiates first-degree AV block from other types of heart block. In more advanced blocks, such as second-degree AV block (types 1 and 2) or third-degree AV block, drops in ventricular beats can occur. Therefore, the key features to remember about first-degree AV block are the prolonged PR interval and the consistent presence of QRS complexes corresponding to every P wave, reinforcing that there are no missed beats. This makes option C the correct definition of a first-degree AV block.