The operating room monitors that are currently available use digital signal processing techniques to acquire and display electrocardiograms (ECGs). As a result, pacemaker impulses are interpreted as noise and thus filtered from the final display. For these signals to be displayed, the monitor must be manually reconfigured away from the default state. However, the displaying of the pacemaker impulse remains a digital process, and the monitor "paints" this signal (and other noise) back into the displayed ECG as a pacemaker impulse. Two surgical cases are reported that demonstrate monitor malfunction in systems with digitally processed ECGs during application of nerve stimulator signals to the patient. In one case, application of a 100-Hz tetany signal completely abolished the displayed pacemaker pulses. In the second case, application of a 50-Hz tetany signal abolished appropriate display but caused the monitor to display artifactual pacemaker pulses. Knowledge of undesirable monitor behavior can reduce inappropriate therapy in the operating room.