As just mentioned, the presence of retrograde P waves beyond the end of the QRS complex is one clue to the existence of an AV nodal bypass tract serving as the retrograde limb of the re-entry circuit. Another and more definitive clue is the presence of ventricular pre-excitation during sinus rhythm.
This ECG is from a 21 year old female with a history of palpitations and paroxysmal tachycardia but no other indication of heart disease. The rate of this narrow complex tachycardia is 220 beats per minute and There are no obvious P waves. Thus, it is impossible to determine from this ECG alone the correct diagnosis of this narrow complex tachycardia or whether it is due to enhanced automaticity or reentry.