Epicardial adipose tissue thickness is an indicator for coronary artery stenosis in asymptomatic type 2 diabetic patients: its assessment by cardiac magnetic resonance

The study included 100 Korean asymptomatic diabetic patients who were enrolled in a prospective trial that evaluated the role of endothelial progenitor cell count in CAD [11]. Study methods were described in detail in previous reports. None of the participants had a history of cardiovascular disease (angina, myocardial infarction (MI), cerebrovascular diseases (CVD), or peripheral artery disease (PAD)), malignancy, or severe renal or hepatic disease. They answered the Rose questionnaire [12] and reported no chest pain or equivalent symptoms. The Institutional Review Board of Yonsei University College of Medicine approved this study, and all subjects provided informed consent in accordance with the ethical committee and the Korean Good Clinical Practice guidelines.

The patients’ records were reviewed to verify the duration of diabetes. The body mass index (BMI) was calculated as weight divided the square of height (kg/m²), and waist circumference was measured at the midpoint between the lateral iliac crest and the lowest rib. Hip circumference was measured at the maximal protrusion of the greater trochanter, and waist-to-hip ratio (WHR) was calculated as the ratio of waist-to-hip circumference. Blood pressure was measured twice with a mercury sphygmomanometer on the right upper arm after resting for at least 10 minutes in sitting position, taking the average values of systolic and diastolic blood pressures. Metabolic syndrome was defined according to criteria of the Adult Treatment Panel III (ATP III) of the National Cholesterol Education Program (NCEP’s). The modified ATP III definition of metabolic syndrome requires the presence of at least three or more of the following five components: 1) elevated waist circumference ≥ 90 cm (male), ≥ 80 cm (female); 2) elevated triglyceride (TG) > 150 mg/dL (1.695 mmol/L); 3) reduced high-density lipoprotein (HDL) cholesterol < 40 mg/dL (male), < 50 mg/dL (female); 4) hypertension or elevated blood pressure ≥ 130/85 mm Hg; or 5) diabetes or elevated fasting plasma glucose ≥ 5.6 mmol/L [13].

Laboratory methods were described in detail in previous reports [11]. Briefly, all blood samples were obtained in the morning after a 12-hour overnight fast, and we performed a standardized mixed meal stimulation test in all subjects using commercial liquid nutritional supplements (Ensure, Meiji Dairies Corporation, Tokyo, Japan; total 500 kcal, 17.5 g fat, 68.5 g carbohydrate, and 17.5 g protein) [14]. Blood samples were collected at 0 and 90 min after taking the liquid supplement (basal and stimulated levels, respectively) for glucose, TG, insulin, and C-peptide analyses. Plasma glucose was measured using the glucose oxidase method. Plasma TG, total cholesterol, HDL- cholesterol, blood urea nitrogen, creatinine, AST, and ALT levels were assayed using a routine Hitachi 7600 autoanalyzer (Hitachi Instruments Service, Tokyo, Japan). Low-density lipoprotein cholesterol (LDL- cholesterol) was calculated using the Friedewald equation. Serum glycated albumin (GA) was determined by an enzymatic method using an albumin-specific proteinase, ketoamine oxidase, albumin assay reagents (LUCICA GA-L, Asahi Kasei Pharma Co., Tokyo, Japan), and a Hitachi 7699 Pmodule autoanalyzer. The coefficient of variation (CV) was 1.43%. Serum glycated hemoglobin (HbA1c) was measured by high-performance liquid chromatography (HPLC) using a Variant II Turbo system (Bio-Rad Laboratories, Hercules, CA). The reference interval for HbA1c was between 4.0 and 6.0%, while that for GA was between 11.0 and 16.0%. Serum insulin and C-peptide levels were measured in duplicate using an immunoradiometric assay (IRMA) method (Beckman Coulter, Fullerton, CA). The homeostasis model assessment of insulin resistance (HOMA-IR) was computed as follows: fasting insulin (μIU/ml) × fasting glucose (mmol/ml)/22.5.

CMR was performed using a 3.0 Tesla magnetic resonance imaging (MRI) unit (Achieva 3.0 T TX, Philips Medical Systems, The Netherlands) with a 32-channel receiver coil. Myocardial ischemia was evaluated based on first-pass myocardial perfusion MR images acquired during adenosine stress (140 μg/kg/min for 3 minutes) and at rest with bolus injection of 0.01 mmol/kg of gadopentetate dimeglumine (Magnevist®, Bayer Schering Pharma, Berlin, Germany) for each session. After the perfusion study, whole heart coronary MR angiography (3-dimensional turbo field echo sequence with navigator gating) was performed without additional injection of the contrast medium. Cine MRI was performed with a balanced steady-state free-precession sequence along the cardiac short axis and horizontal long axis. Delayed enhancement MRI was obtained to assess myocardial viability using a 3-dimensional phase-sensitive inversion recovery sequence 10 minutes after the rest perfusion study.

CMR images were independently evaluated by two experienced cardiac radiologists, who were not informed of the identity or any clinical information about the patients. After independent evaluation of the images, final conclusions were made by consensus interpretation. Abnormal findings on CMR included inducible ischemia, myocardial infarction, global left ventricular (LV) systolic dysfunction, and coronary artery stenosis. Inducible ischemia was defined as a perfusion deficit induced by adenosine stress on first-pass perfusion images that was reversed on resting perfusion images with no evidence of delayed myocardial hyper-enhancement on delayed enhancement images. Myocardial infarction was defined as an area with hyper-enhancement consistent with coronary distribution on delayed enhancement MRI. Silent myocardial ischemia by CMR was defined as an evidence of inducible ischemia or myocardial infarction. Global LV systolic dysfunction was defined as LV ejection fraction < 45% measured from cine MRI. Based on coronary MR angiography, the degree of stenosis was qualitatively defined as significant, suspected, minimal, or normal. Focal areas of marked signal loss, signal reduction, or stenosis of ≥ 50% in the vessel diameter were used as the criteria for significant stenosis [13, 14].

Fat thickness in the left atrioventricular groove was measured as previously mentioned in studies by other investigators [15, 16]. The measurements were performed at the end-diastolic phase on the horizontal long-axis plane in cine MRI. Maximal EAT thickness was determined as measured from the myocardial surface to the pericardium (perpendicular to the pericardium) (Figure 1).

All statistical analyses were performed with PASW statistics software (version 18.0; SPSS Inc., Chicago, IL). Continuous variables with a normal distribution were expressed as mean ± SD, and discrete variables were expressed as percentages. The relationship between EAT thickness and other metabolic parameters was examined using Pearson’s correlation coefficient (R). Statistical comparisons between groups according to significant coronary artery stenosis or silent myocardial ischemia were performed using the Student’s t test or Mann–Whitney U test for continuous variables and a χ2 test for discrete variables. We then used multivariate logistic regression analysis to assess whether the associations between EAT thickness and the presence of significant coronary artery stenosis or silent myocardial ischemia were independent of age, gender, and traditional coronary risk factors. A P- value < 0.05 was considered significant.

Table 1 shows the baseline characteristics of the study participants. The final population consisted of 100 subjects (51 males and 49 females; mean age 56 ± 7 years). Their mean duration of diabetes was 8.4 ± 6.7 years. The levels of serum HbA1c and GA were 7.0 ± 0.9% and 16.7 ± 3.9%, respectively. The mean body mass index was 25.3 ± 3.2 kg/m² (range: 18.7 – 35.6 kg/m²), and the mean WHR was 0.91 ± 0.06 (range: 0.78 – 1.10). Of these 100 patients, 78 had metabolic syndrome. A total of 24 patients had significant coronary artery stenosis defined as stenosis ≥ 50% on coronary MR angiography, and 14 patients had silent myocardial ischemia on CMR images (1 with silent myocardial infarction, 11 with inducible ischemia, and 2 with both). Ten patients had both silent myocardial ischemia and significant coronary artery stenosis. The average thickness of the layer of EAT at the left atrioventricular groove was 11.9 ± 2.3 mm (range: 6.8 – 17.9 mm, 12.0 ± 2.1 mm in males and 11.8 ± 2.5 mm in females). The mean LV ejection fraction was 68 ± 3%, and all patients had preserved LV systolic function (range: 56 – 77%). The mean LV mass was 83 ± 21 g (range: 43 – 165), which was significantly greater in males than in females (94 ± 19 g vs. 71 ± 16 g, p<0.001). None of the women was a smoker in this study.

Subjects with any anatomical (stenosis ≥ 50%) or functional abnormality had a consultation with a specialized cardiologist. Among 28 patients recommended to undergo invasive coronary angiography, 20 patients were submitted for invasive coronary angiography, and 8 were submitted for revascularization (7 percutaneous coronary interventions and 1 coronary artery bypass graft).

EAT thickness was positively and independently correlated with metabolic parameters including BMI, waist circumference, waist-to-hip ratio, systolic blood pressure, postprandial glucose, fasting/postprandial triglyceride, HbA1c level, and HOMA-IR. However, EAT thickness was not correlated with either LDL-cholesterol or HDL-cholesterol levels (Table 2).

We divided the subjects into two different classes: 1) subjects with or without significant coronary artery stenosis, and 2) subjects with or without silent myocardial ischemia. As shown in Table 3, patients with significant coronary artery stenosis had significantly higher age, WHR, systolic blood pressure, total cholesterol, and GA than those without significant coronary artery stenosis. EAT thickness was significantly higher in the patients with stenosis than without stenosis (13.0 ± 2.6 mm vs. 11.5 ± 2.1 mm, p = 0.01). When subjects were divided based on presence of myocardial ischemia, only BMI was statistically different between the patients with and without silent myocardial ischemia (27.3 ± 3.8 kg/m²vs. 25.0 ± 3.0 kg/m², p = 0.033) (Table 4A). In a subgroup analysis based on gender, female patients with myocardial ischemia had a higher EAT thickness than those without ischemia (14.1 mm (IR 2.5) vs. 11.5 mm (IR 3.4), p = 0.031). Similar EAT thickness was found between males with ischemia and without ischemia (11.9 mm (IR 4.2) vs. 12.5 mm (IR 2.9), p = 0.947) (Table 5). Multivariate logistic regression analysis revealed that EAT thickness was an independent indicator of significant coronary artery stenosis as assessed by coronary MR angiography, after adjusting for traditional risk factors (OR 1.403, p = 0.026) (Table 6).