Study design Investigators aim to perform a prospective, single-center, investigator-initiated, randomized study to compare the Adenosine-induced coronary vasodilatation after the loading dose of Ticagrelor either Prasugrel during the Percutaneous Coronary Intervention. Patients with acute coronary syndrome undergoing Percutaneous Coronary Intervention will be enrolled in the study and will be randomized, in a 1:1 ratio, to receive a loading dose of Ticagrelor (180 mg) or Prasugrel (60 mg). In patients with non-ST elevation myocardial infarction these drugs will be administered only when the coronary anatomy will be known, to avoid bleeding due to prasugrel, in patients suitable for coronary artery bypass grafting as recommended by European Society of Cardiology guidelines (Class IB) (10). In patients with ST elevation myocardial infarction, instead, prasugrel and ticagrelor will be administrated before the procedure, according to the European Society of Cardiology guidelines (Class IB) (11). Coronary Flow Reserve will be recorded by intracoronary Doppler Flow Wire before the stent implantation and after the procedure at baseline and 2-minute later adenosine intravenous administration at incremental doses of 50, 80, 110 and 140 ug/Kg/min with 2 minutes interval between infusions.