This is an electrocardiogram from a middle-aged woman treated for atrial fibrillation. The patient was doing well when this electrocardiogram was taken, and was taking digoxin and flecainide . The electrocardiogram shows an irregular rhythm which appears to be sinus. The grouping of the QRS complexes suggests a Mobitz type I AV block. In this case though, no A V block is seen and this may represent a sinus node exit block.