Final answer:
In a first-degree AV block, the PR interval is abnormally prolonged. Second-degree AV blocks show some P waves not followed by QRS complexes. In third-degree AV blocks, P waves and QRS complexes are completely disassociated, leading to variable or indiscernible PR intervals.
Step-by-step explanation:
The PR Intervals for each type of atrioventricular (AV) block are characteristic features that help in the diagnosis of these conditions. In a first-degree AV block, there is a delay in the transmission of electrical impulses from the sinoatrial (SA) node to the AV node, which is evidenced by an abnormally long PR interval on an electrocardiogram (ECG). A second-degree AV block, which can be further classified into types I and II, presents with dropped beats where some P waves are not followed by a QRS complex, indicating that some impulses do not propagate to the ventricles. Finally, a third-degree AV block, also known as a complete heart block, displays a complete disassociation between the P waves and QRS complexes, meaning that there is no relationship between atrial and ventricular activity, and the PR interval timing varies or is not discernible as the P waves and QRS complexes occur independently of each other.