This is a two lead rhythm strip from a patient in the emergency room. The rhythm is sinus with second degree A/V block. Note the progressive lengthening of the pr interval and that the interval that brackets the blocked p wave is less than twice that of the RR interval. This recording suggests a Mobitz I A/V block, but some care has to be taken as the QRS that ends the pause in at least one case looks like a nodal escape beat.