When P waves are visible must specify their number and position.
There are 3 possibilities:
Look for electrical QRS complexes alternance (variation of the amplitude of the QRS complex between beats). Electrical QRS complexes alternance is in > 90 % AVRT.
If no electrical alternans is present look for P' waves and measures RP' et P'R intervals.
If there is an alternance of QRS complexes (alternation of QRS complex amplitude or axis between beats) it is in more than 90% of the cases a tachycardia using an accessory pathway AVRT.