Final answer:
The PR interval on an ECG measures the time from the beginning of the P wave to the start of the QRS complex, reflecting atrial to ventricular conduction. Normal PR interval ranges from 0.12 to 0.20 seconds and its prolongation may indicate a first-degree AV block.
Step-by-step explanation:
The PR interval on an electrocardiogram (ECG) represents the period of time from the onset of atrial depolarization (beginning of the P wave) to the start of ventricular depolarization (beginning of the QRS complex). It normally ranges from 0.12 to 0.20 seconds. The PR interval reflects the time it takes for the electrical impulse to travel from the sinus atrial (SA) node through the atria, the atrioventricular (AV) node, and into the ventricles. If there is a delay in this conduction, such as with a first-degree AV block, it would be indicated by a prolonged PR interval.
Abnormal PR intervals can indicate various cardiac conditions. For example, a PR interval that exceeds 0.20 seconds could be indicative of a first-degree heart block, where there's a delay in the conduction from the SA node to the AV node. A PR interval that is too short may suggest an accessory pathway between the atria and ventricles, such as in Wolff-Parkinson-White syndrome.