Here is a sample of an EKG taken over the telephone (some resolution is lost on a Web page display). This is a continuous 50-second sample which can be read like text, left-to-right and top-to-bottom. Note the large "cross-hairs", which was used to mark points for measurement and for cut-and-paste. The already-measured interval near the screen center shows 860 milliseconds between two firings of the pacemaker.  A trained technician would use several such measurements to determine whether the pacemaker was functioning correctly.