This is an interesting set of tracings, as it shows the effect of adenosine on AV conduction, but in addition the effect on atrial refractoriness. The first tracing is clearly atrial flutter with an atrial rate of 270/min, and a ventricular rate of 135/min. The second recording shows much faster atrial activity (Note the distance between the turquoise arrows and the yellow arrows which mark out 3 atrial cycles). The second tracing is either atrial fibrillation (a know result of adenosine) or a faster atrial flutter (type II flutter) with variable block. Both of these effects could be seen as a result of the decrease in the atrial refractory period by the stimulation of the adenosine.