This a recording rhythm strip from the same patient is after giving intravenous Adenosine. The tachycardia slows and becomes irregular and the QRS narrows. Then there is a pause and flutter waves are seen at a rate of 300/min. Note the PVCs that appear after the pause. The rhythm at the end of the recording is probably flutter with 2:1 block. It is entirely possible that the Adenosine increased the rate of this patient's flutter and hence the AV node was able to create a 2:1 block (see EKG below, note SVT in lead V1) and slow down the ventricular rate. The patient, after this recording reverted to sinus rhythm. It is not unusual that acceleration of atrial flutter with cardiac pacing produces a flutter that is less stable and which then converts spontaneously to sinus rhythm.