Abstract Study Objectives The use of adenosine on failure of vagal maneuvers in patients with paroxysmal supraventricular tachycardia (PSVT) is recommended. The aim of the present study was to identify a possible prognostic index for the efficacy of adenosine in PSVT. Methods This retrospective study included 321 patients with PSVT, in whom vagal maneuvers failed to restore normal sinus rhythm and who received 6 mg adenosine, followed by 12 mg adenosine (repeated if necessary). A 2-step clustering algorithm was used to reveal nonapparent groupings. Various patients' characteristics were inserted into the model. Results A straightforward index has been chosen that would aid the distinction of patients failing to respond to adenosine. The formula, (age / heart rate at admission) + number of past PSVT episodes, was chosen as the one that combined the highest sensitivity (96.2%) and specificity (71.2%). Conclusions This prognostic index constitutes a useful and reliable bedside diagnostic tool to identify patients with PSVT who were less likely to respond to adenosine administration in the emergency setting.