ANSWER
The ECG reveals an atrial-sensed and ventricular-paced rhythm of 83 beats/min. In this case, the pacemaker is functioning appropriately as programmed.
Pacemaker code consists of three letters: The first refers to the chamber(s) paced, the second to the chamber(s) sensed, and the third to the pacemaker’s response to a sensed beat. This patient has a pacing lead in the right atrium and one in the right ventricle and is programmed DDD. Each D in this case stands for dual: The first to indicate that both leads are programmed to pace, the second to indicate that both chambers may be sensed, and the third to indicate that the response to sensing can be either to inhibit or trigger a ventricular-paced beat in response to what happens in the atrium. Hence, there are four possible scenarios with a DDD pacemaker: AS-VS (atrial sensed-ventricle sensed; eg, intrinsic AV conduction requiring no pacing), AS-VP (atrial sensed-ventricle paced), AP-VS (atrial paced-ventricle sensed), and AP-VP (atrial paced-ventricle paced).
In this case, the atrial rate (83 beats/min) is faster than the pacemaker’s lower programmed rate. In order to see atrial pacing on the ECG, the intrinsic atrial rate would have to be less than the programmed rate of 60 beats/min. As soon as the pacemaker senses atrial conduction (either spontaneous or paced), it starts a timer (programmed at 130 ms in this case). If there is no spontaneous ventricular depolarization by the end of the timer, the pacemaker delivers an impulse to the ventricle, resulting in a paced ventricular beat. An often-made mistake (as this case illustrates) is the assumption that if one does not see pacing spikes, the pacemaker is not functioning properly.