This is an interesting EKG as it shows paced atrial rhythm with a bipolar atrial pacemaker. In this case the lead configuration is different that that shown in the x-ray as a second atrial lead was added in the right atrial appendage and the atrium paced in a bipolar mode between the tips of the two leads. This combination seems to have decreased the episodes of tachycardia with block. The pacemaker is set at 80/min. to prevent the tachycardia and note the variation in the size and polarity of the atrial pacing spikes. This is an indication of the limitation of the EKG machine to record pacemaker pacing spikes which are brief. Note the markers on the top of the tracing which fail to detect the pacer for each beat. A pacemaker spike is usually less then 0.5 ms (compare with the QRS which is more than 80ms) and it is even more difficult to record when it is bipolar as is the case here.