PR interval is measured from the onset of P wave to the onset of QRS complex. It includes the P wave and the PR segment. Normal PR interval is from 120 to 200 ms. If it is prolonged, it is called as first degree atrioventricular block.
PR interval is the time taken for activation of the atria and conduction of the impulses down to the ventricles and initiate activation of the ventricles. This includes the physiological delay in conduction at the AV node. The delay allows complete emptying of the atria before the ventricles start contracting. The atrial help to ventricular filling acts like an atrial booster at the end of passive diastolic ventricular filling.
