This ECG is from a 16 year old female. She had no evidence of cardiac disease and was receiving no drugs. There is sinus arrhythmia with PP intervals that range from 0.76 to 1.0 seconds. The PR interval is prolonged, measuring 0.26 seconds. This most likely represents a physiologic increase in vagal tone. However, acute inflammation in the AV node by diseases such as acute rheumatic fever and Lyme disease may also cause prolongation of the PR interval.