ABSTRACT
Once the sinus node has generated an electrical stimulus, this must be transmitted through the atria, atrioventricular (AV) node and bundle of His to reach the ventricles and bring about cardiac contraction. Atrial ectopics arising near to the AV node will also have a shorter PR interval than normal sinus beats, but it will rarely be less than 0.12 s long. The presence of a short PR interval in isolation, with no history of re-entry tachycardia, is regarded as a normal variant of AV node conduction. Prolongation of the PR interval is a common finding and indicates that conduction through the AV node has been delayed. Normally, the PR interval is constant. In some conditions, however, the interval between P waves and QRS complexes changes, giving rise to a variable PR interval. The PR segment, between the end of the P wave and the start of the QRS complex, is usually flat and isoelectric.
