What does sinus rhythm - borderline short pr interval mean on an ecg?

Usually nothing.: The pr interval, the time between the P wave (atria contract) and the qrs (ventricles contract) is typically between 0.12 and 0.20 seconds. If it is a bit shorter than 0.12 seconds but the ECG is otherwise normal, there is generally nothing to be concerned about. If you have episodes of heart racing, you should tell your doctor, as there may be a condition in which you are prone to fast rhythms.