Here is a good example of 60-cycle interference artifact on a sinus rhythm strip.  The artifact is caused by electrical interference from a nearby electrical appliance. Modern monitors are able to filter interference out, but it is still occasionally seen.  Even though we can still discern P waves in this strip, and we can see that the rhythm is irregular, possibly sinus arrhythmia, the artifact prevents us from accurately evaluating the strip.  Every effort should be made to identify the offending device.  In "ECG Basics", we attempt to stay "basic", but if any of our Gurus would like to comment on this in a more technical fashion, it is welcome.