Active vs passive rhythms as an explanation of bigeminal rhythm with similar P waves. We describe the criteria for differential diagnosis between 3:2 sinoatrial block from atrial bigeminy due to an ectopic focus in the sinus or parasinus zone. In the 3:2 sinoatrial block the RR interval of the basic rhythm is similar to the short R-R interval of the paired rhythm. In atrial bigeminy, the R-R interval of the basic rhythm is similar to the long R-R interval of the paired rhythm. 