The Significance of a Prolonged PR Interval

The PR interval represents the time from the beginning of the P wave (atrial excitation) to the beginning of the QRS complex (ventricular excitation). Taking an average PR interval of 0.16 sec., let us dissect it into its component parts. The impulse from the SA node spreads down the three atrial specialized tracts and through the atrial myocardium to reach the AV node. The atrial myocardial excitation generates the P wave and this lasts 0.08 sec. There is a delay in the AV node itself, taking up another 0.04 sec., and then the excitation enters the common bundle of His. Thus the P-H interval (P to His bundle) takes up 0.12 sec. The procession down the intraventricular pathways is divided further; from His bundle to the right or left bundle branch takes 0.02 sec., and from each of these bundles to the right and left ventricles takes another 0.02 sec. before the QRS complex begins to form. Thus the H-QRS, or second part of the PR interval, comprises 0.04 sec. The P-H plus H-QRS embrace the full PR interval of 0.16 sec.