Final answer:
A prolonged PR interval on an ECG indicates a delay in electrical conduction between the atria and ventricles, typically a first-degree AV block. The normal PR interval should be between 120 to 200 milliseconds; anything longer suggests an abnormality in cardiac conduction that may need further investigation.
Step-by-step explanation:
When observing an electrocardiogram (ECG), a prolonged PR interval can be indicative of an atrioventricular (AV) block, specifically a first-degree AV block. The PR interval represents the time taken for an electrical impulse to travel from the atria to the ventricles, starting with the P wave (indicating atrial depolarization) and ending just before the QRS complex (signaling ventricular depolarization). A normal PR interval duration is between 120 to 200 milliseconds. If prolonged, it suggests a delay in the AV node's conduction of the electrical impulse from the atria to the ventricles, which may require further clinical evaluation.
Type and severity of AV blocks are often determined by analyzing the patterns on the ECG. In a first-degree AV block, you would observe a consistent but prolonged PR interval. This is different from other blocks, such as a second-degree AV block where some P waves are not followed by a QRS complex, or a third-degree block where there is a complete disassociation between atrial and ventricular activities, shown by no correlation between P waves and QRS complexes.