INTegrated Assessment of intERmediate Coronary Stenoses by Fractional Flow rEserve (FFR) and Near-infraREd Spectroscopy (NIRS).

Background Objective of percutaneous coronary intervention (PCI) is the treatment of angiographically significant coronary stenoses in patients affected by stable or unstable coronary syndromes 1. When we face an intermediate stenosis (percentage diameter stenosis between 40% and 70%), revascularization is guided by the presence of inducible myocardial ischemia, detected by non - invasive stress tests or by assessment of fractional flow reserve (FFR). FFR is an index of the physiological significance of a coronary stenosis and is defined as the ratio of maximal blood flow in a stenotic artery to normal maximal flow. An FFR value of 0.80 or less identifies ischemia-causing coronary stenoses with an accuracy of more than 90%. FFR is a very accurate tool to identify functionally significant stenoses and to predict cardiovascular events in patients with stable coronary artery disease (CAD) and solid evidences demonstrate that PCI of lesions with FFR > 0.80 can be safely deferred (incidence of MACE < 1% with medical treatment only). Conversely, stenoses with an FFR <0.80 have a poor prognosis and require treatment by PCI 2-4. The afore-mentioned indications come from studies that mainly involved patients affected by stable CAD, but the role of FFR in the setting of acute coronary sindrome (ACS) is less clear. Hakeem et al. have recently shown that deferring percutaneous coronary intervention on the basis of non-ischemic FFR in patients with an initial presentation of ACS is associated with significantly worse outcomes than stable patients5.

FFR indeed has the ability to identify vessels with reduced coronary flow, but cannot detect atherosclerotic plaques with unstable features, which may present without flow limitation (FFR > 0.80) but can cause acute coronary events 6, 7. Since the prevalence of features of plaque instability (plaque volume > 70%, minimum luminal area <4 mm2, presence of a "thin cap fibroatheroma") increase with the increase of the severity of the stenosis, the decision of performing PCI for stenoses of clear angiographic severity seems rational and supported by solid evidence 8. Conversely, delaying PCI of intermediate stenoses on the basis of a negative FFR can be problematic, particularly in patients with ACS, where the only functional evaluation with FFR may not be sufficient in the presence of unstable plaques.

Differences in plaque composition between physiologically significant vs non-significant lesions have never been assessed in either stable and ACS patients.

Intracoronary Near-InfraRed Spectroscopy (NIRS) identifies lipid-rich plaques (LRP) with high sensibility and specificity. The technique, validated on autopsy specimens, is an effective tool to detect LRP in vivo, identifying those coronary atheromas that can potentially cause acute events 9. The NIRS system consists of a 3.2-F rapid exchange catheter (InfraReDx, Burlington,Massachusetts), a pullback and rotation device, and a console. The measurement of the probability of LRP for each scanned arterial segment is displayed as a map, with the x-axis indicating the pullback position in millimeters and the y-axis the circumferential position of the measurement in degrees. The algorithm displays the probability of lipid content at the interrogation site by using a false color scale from red (low probability) to yellow (high probability). The entire display is termed a "chemogram". Pixels containing insufficient informations are displayed as black. The ratio between the number of yellow pixels to the whole number of pixels except the black ones, multiplied by one - thousand, is the "Lipid Core Burden Index (LCBI)" of the analyzed artery segment. A value of LCBImax > 400 identifies a high lipid content in a given segment. In an autopsy study conducted on human aortic specimens, the technique reached 90% sensibility and 93 % specificity in the detection of lipid rich plaques9, opening new horizons in terms of risk stratification and therapy10-15. To provide a quantitative target suitable for algorithm construction and validation, a lipid core plaque of interest was defined as a fibroatheroma with a lipid core > 60° in circumferential extent, >200 µm thick, with a fibrous cap having a mean thickness < 450 µm16.

Aim of the study