A total of 566 consecutive patients who had undergone gray-scale and VH-IVUS in NCL in ACS patients and in TL in SA patients between January, 2008 and September, 2011 were identified from the Chonnam National University Hospital VH-IVUS registry database. We compared VH-IVUS findings between 290 ACS-NCL and 276 SA-TL. The presence of SA was determined according to the Canadian Cardiovascular Society classification. Unstable angina and non-ST segment elevation myocardial infarction are considered to be closely related conditions whose pathogenesis and clinical presentations are similar, but of differing severity (i.e., they differ primarily in whether the ischemia is severe enough to cause sufficient myocardial damage to release detectable quantities of a marker of myocardial injury, most commonly troponin I, troponin T, or the MB isoenzyme of creatine phosphokinase). Once it has been established that no biochemical marker of myocardial necrosis has been released, the patient with an ACS may be considered to have experienced unstable angina, whereas the diagnosis of non-ST segment elevation myocardial infarction is established if a marker of myocardial injury has been released.5) ST-segment elevation myocardial infarction is a clinical syndrome defined by characteristic symptoms of myocardial ischemia, in association with persistent electrocardiographic ST-elevation and the subsequent release of biomarkers of myocardial necrosis. Diagnostic ST-elevation in the absence of left ventricular hypertrophy or left bundle-branch block is defined by the European Society of Cardiology/American College of Cardiology Foundation/American Heart Association/World Heart Federation Task Force for the Universal Definition of Myocardial Infarction as a new ST-elevation at the J point in at least 2 contiguous leads of ≥2 mm (0.2 mV) in men or ≥1.5 mm (0.15 mV) in women in leads V 2-3, and/or of ≥1 mm (0.1 mV) in other contiguous chest leads or the limb leads.6) We excluded patients with Thrombolysis in Myocardial Infarction (TIMI) 0-1 flow, coronary artery bypass graft lesion, chronic total occlusion, restenosis after stenting, important systemic disease such as systemic lupus erythematosus, amyloidosis, sarcoidosis, human immunodeficiency virus infection, and malignancies, and so on, or serum creatinine >2.5 mg/dL. NCL in ACS patients was defined as plaques viewed on an angiogram that had not been treated. TL in SA patients was defined as a coronary lesion whose diameter stenosis by quantitative coronary angiography (QCA) was greatest if the patient had multivessel disease. In the present study, we included all TLs, including both treated lesions and non-treated lesions, in SA patients. Plaques with more than a 30% diameter stenosis as compared with the reference diameter by IVUS were included in our study. Each plaque was separated by at least 5 mm from the edge of any other plaque or implanted stent edge. The protocol was approved by the institutional review board. Hospital records of all patients were reviewed to obtain clinical demographics and medical history.

Peripheral blood samples were obtained before coronary angiography using direct venipuncture. The blood samples were centrifuged, and serum was removed and stored at -70℃ until the assay could be performed. The serum levels of total cholesterol, triglyceride, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol were measured using standard enzymatic methods. High-sensitivity C-reactive protein was analyzed turbidimetrically with sheep antibodies against the human C-reactive protein; this has been validated against the Dade-Behring method.7) Serum N-terminal pro-B-type natriuretic peptide was measured using an electrochemiluminescence sandwich immunoassay method with an Elecsys 2010 analyzer (Roche Diagnostics, Mannheim, Germany).

Coronary angiogram was analyzed with validated a QCA system (Phillips H5000 or Allura DCI program, Philips Medical Systems, Eindhoven, the Netherlands). With the outer diameter of the contrast-filled catheter as the calibration standard, the reference diameter and minimal lumen diameter were measured in diastolic frames from orthogonal projections. Perfusion was evaluated according to TIMI criteria.8)

Gray-scale and VH-IVUS data were analyzed by 2 independent observers. The levels of reproducibility for external elastic membrane (EEM), lumen, and plaque plus media (P&M) cross-sectional areas (CSAs) using the Spearman rank-order correlation coefficients were 0.96, 0.97, and 0.97, respectively. Similarly, for plaque components by VH-IVUS, reproducibility for the fibrous (FT), fibro-fatty (FF), dense calcium (DC), and NC volume measurements using the Spearman rank-order correlation coefficients were 0.93, 0.94, 0.93, and 0.93, respectively.

Quantitative volumetric gray-scale and VH-IVUS analyses were performed across the entire lesion segment, and cross-sectional analyses were performed at the minimum lumen sites and at the largest NC sites. We analyzed the IVUS images and measured the diseased segment (normal to normal). Conventional quantitative volumetric gray-scale IVUS analysis was performed according to the American College of Cardiology Clinical Expert Consensus Document on Standards for the Acquisition, Measurement and Reporting of Intravascular Ultrasound Studies.9) Measurements were made at every 1-mm interval in the region of interest, which was defined as the segment between the most normal looking distal to proximal reference sites within 5 mm, proximally and distally, to the lesion. References were single slices with the largest lumen and smallest plaque CSAs within 10 mm proximally and distally. Hypoechoic plaque was less bright than the adventitia. Hyperechoic noncalcified plaque was as bright as or brighter than the adventitia without acoustic shadowing, and hyperechoic calcified plaque was brighter than the adventitia with acoustic shadowing. When there was no dominant plaque composition, the plaque was classified as mixed. Volumetric data were generated by the software using Simpson's method. EEM and lumen CSAs were measured. P&M CSA was calculated as EEM minus lumen CSA. Plaque burden was calculated as P&M divided by EEM CSA. VH-IVUS analysis classified the color-coded tissue into four major components: green (FT); yellow-green (FF); white (DC); and red (NC).1)10-12) VH-IVUS analysis was reported as a percentage of plaque area or volume. We defined TCFA as a NC ≥10% of the plaque area in at least 3 consecutive frames without overlying fibrous tissue in the presence of ≥40% plaque burden.1)

The Statistical Package for the Social Sciences (SPSS) for Windows, version 15.0 (SPSS Inc., Chicago, IL, USA) was used for all analyses. Continuous variables were presented as the mean value±1SD; comparisons were conducted by a Student's t-test or nonparametric Wilcoxon test if the normality assumption was violated. Discrete variables were presented as percentages and relative frequencies. Comparisons were conducted by chi-square statistics or Fisher's exact test as appropriate. Multivariate analysis was performed to determine the independent predictors of TCFA. All variables with p<0.1 in the univariate analysis were entered into the multivariate analysis. A p of <0.05 was considered statistically significant.

The baseline characteristics are summarized in Table 1. Patients with ACS had lower left ventricular ejection fraction, higher white blood cell count, higher fibrinogen, higher high-sensitivity C-reactive protein, higher low-density lipoprotein cholesterol, and higher apolipoprotein B levels compared with those with SA. There were no significant differences in previous medications between both groups.

Coronary angiographic findings are as summarized in Table 2. There were no significant differences in target vessel, TIMI flow grade, reference vessel diameter, or minimal lumen diameter between both groups. ACS-NCL had lower diameter stenosis compared with SA-TL.

Gray-scale IVUS results are summarized in Table 3. Plaque burden was significantly smaller at the proximal reference and at the minimum lumen site and at the largest NC site in ACS-NCL compared with SA-TL. Plaque morphology was significantly different between both groups. No significant differences were observed in volumetric parameters between both groups.

At the minimum lumen site, %NC and %DC areas were significantly greater. In contrast, absolute FF, and the %FT and %FF areas were significantly smaller in ACS-NCL compared with SA-TL (Fig. 1). At the largest NC site, although there were no significant differences in the absolute plaque areas, the %NC area was significantly greater in ACS-NCL compared with SA-TL (Fig. 2). Although there were no significant differences in the absolute plaque volumes, %NC volume was significantly greater in ACS-NCL compared with SA-TL (Fig. 3). TCFA was observed more frequently in ACS-NCL compared with SA-TL (27.6% vs. 18.1%, p=0.032).

Multivariate analysis was performed to identify the independent predictors of TCFA and the results are summarized in Table 4. The following variables were tested (all with p<0.10 in univariate analysis), including clinical presentation (ACS vs. SA), diabetes mellitus, white blood cell count, fibrinogen, high-sensitivity C-reactive protein, low-density lipoprotein cholesterol, and minimum lumen site plaque burden. Independent predictors of TCFA by multivariate analysis were ACS {odds ratio (OR): 2.204, 95% CI: 1.321-3.434, p=0.021} and high-sensitivity C-reactive protein (OR: 1.101; 95% CI 1.058-1.204, p=0.035).