This is a phenomenon you need to recognize.
When a patient has an implanted pacemaker and experiences the equivalent of a brady-asystolic cardiac arrest there will still be apparent complexes on the monitor, like the ones displayed here. Be cognizant of how these complexes look compared to a typical paced rhythm because they are easy to miss, resulting in delayed recognition of cardiac arrest.
Luckily, in my experience, the monitors in my department are actually surprisingly good at detecting these changes and alarming "ASYSTOLE" in spite of the presence of electrical activity.
I didn't know that early in my career, however, the first time I saw this. Thinking it was a false alarm I sauntered over to the patient's room and was shocked when I opened the door and saw the patient exhibiting agonal respirations.
Most of the time when the monitors alarm at my department our first step is to try and troubleshoot at the monitor bank and see if it's artifact or legit (>99% of the time I'd say it's artifact). If there's a paced rhythm and the monitor alarms, however, my first step is to instead go immediately to the bedside to check the patient.
It's too easy to overlook badness otherwise.http://ecgrhythms.wordpress.com/2014/11/03/ppm_no_pulse/