The tracing on the right was recorded immediately after the intravenous administration of adenosine. The rhythm is now sinus tachycardia with a rate of 120 and a PR interval of 0.14 seconds. This rapid response to the administration of adenosine indicates that the arrhythmia was due to a re-entry circuit involving the AV junction. Note that the QRS complex is normal and identical to that recorded during the tachycardia (tracing on the left). Also note that the ST segments are now normal.