What is the characteristic of a 1st degree heart block?

A first-degree heart block is characterized by a prolonged PR interval that measures greater than 0.20 seconds on an electrocardiogram (ECG). This condition indicates a delay in the electrical conduction through the atrioventricular (AV) node, but it does not result in any dropped beats. The key feature here is the consistent, prolonged PR interval, which remains the same throughout the ECG tracing.

While the other choices describe different types of conduction issues or heart blocks, they do not apply to a first-degree block. A fixed PR interval with a dropped beat would suggest a second-degree block, particularly a type known as Mobitz type II, where certain beats are missed. A progressively lengthened PR interval refers to a type of second-degree block known as Mobitz type I, or Wenckebach, where each successive beat has a longer PR interval until one beat is dropped. Lastly, a complete block between the SA and AV node describes a third-degree heart block, where there is no conduction from the atria to the ventricles, resulting in a complete dissociation between the two. Thus, the defining characteristic of a first-degree heart block is indeed the prolonged PR interval exceeding 0.20 seconds.