Final answer:
The PR interval stays constant in a first-degree AV heart block, characterized by a delay in conduction but eventual passage of all impulses from the atria to the ventricles, as seen by an abnormally long but constant PR interval on the ECG.
Step-by-step explanation:
In which AV heart block does the PR interval stay constant? The PR interval stays constant in a first-degree AV heart block. In this type of block, there is a delay in the conduction between the SA and AV nodes, but all the electrical impulses eventually reach the ventricles. This delay is represented on the ECG by an abnormally long but constant PR interval.
In contrast, a second-degree AV heart block will have some P waves not followed by a QRS complex, while others appear normal, indicating that not all atrial contractions are followed by ventricular contractions. A third-degree AV heart block, also known as a complete heart block, is characterized by a complete dissociation of atrial activity from ventricular activity, showing no consistent PR interval as the P waves are not followed by the QRS complexes.