This tracing was recorded immediately following the intravenous administration of adenosine. It now shows sinus rhythm with a normal P wave and a PR interval of about 0.16 sec. The QRS complex and T wave are unchanged from those shown on then previous page. The prompt response to adenosine suggests that the arrhythmia was due to AV nodal re-entry and that the retrograde P waves were not obvious because they were hidden within the QRS complex.