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Plaque characterisation by Virtual Histology intravascular ultrasound analysis in patients with type 2 diabetes
  1. K Nasu,
  2. E Tsuchikane,
  3. O Katoh,
  4. H Fujita,
  5. J-F Surmely,
  6. M Ehara,
  7. Y Kinoshita,
  8. N Tanaka,
  9. T Matsubara,
  10. Y Asakura,
  11. K Asakura,
  12. M Terashima,
  13. T Suzuki
  1. Department of Cardiology, Toyohashi Heart Center, Toyohashi, Japan
  1. Dr K Nasu, Department of Cardiology, Toyohashi Heart Center, 21-1, Gobudori, Oyama, Toyohashi, Aichi, 441-8530 Japan; nasu{at}heart-center.or.jp

Abstract

Objectives: To evaluate the in-vivo plaque composition and characteristics in patients with type 2 diabetes mellitus (DM) using Virtual Histology intravascular ultrasound (VH IVUS).

Methods: In 90 patients with stable angina pectoris, de novo target vessels were studied and plaque components were analysed. Patients were divided into two groups: a diabetic group (36 vessels) and a non-diabetic group (54 vessels).

Results: The percentage area of necrotic core and dense calcium were significantly larger in the DM group than the non-DM group (necrotic core: 11.0% (interquartile range (IQR): 7.2–15.2%) vs 7.6% (IQR 5.6–13.2%), p = 0.03; dense calcium: 5.6% (IQR: 2.3–7.3%) vs 2.9% (IQR: 1.7–4.9%), p = 0.01). The DM group presented with a significantly higher presence of at least one VH IVUS-derived thin-cap fibroatheroma (VHD-TCFA) (75% vs 41%, p = 0.001) and VH IVUS-derived fibrocalcific atheroma (VHD-FCA) (75% vs 40%, p = 0.001). In the DM group, 53% of the vessels had both VHD-TCFA and VHD-FCA, which was significantly higher than non-DM group (17%, p = 0.0004).

Conclusions: Coronary plaque characteristics in DM patients showed an increased amount of dense calcium and necrotic core, as well as a higher frequency of VHD-TCFA and VHD-FCA. Atherosclerosis of the target vessel was more advanced in diabetic patients.

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Type 2 diabetes mellitus (DM) is a well established risk factor for development of coronary heart disease.1 Patients with DM also have a higher mortality from cardiovascular complication such as acute coronary syndrome than non-DM patients, and two-thirds of people with DM die of vascular disease.2 The main cause of acute coronary syndrome is the disruption of a thin-cap fibroatheroma with a large necrotic core and a fibrous cap <65 μm.3 Furthermore, the presence of radiologically detectable arterial calcification is a strong marker of future cardiovascular events in diabetic patients.4 Greyscale intravascular ultrasound (IVUS) is a useful modality to evaluate the morphology of atherosclerotic plaques and the vessel wall.5 However, the evaluation of low echogenic regions, which is thought to represent the composition of lipid-containing and mixed plaque, is significantly limited by greyscale intravascular ultrasound.5 Virtual Histology (VH, Volcano Therapeutics, Rancho Cordova, CA, USA) IVUS uses spectral analysis of the radiofrequency ultrasound backscatter signals which allows for the identification of four different types of atherosclerotic plaque components: fibrous, fibro-fatty, dense calcium and necrotic core.6 7 A direct comparison between in vivo VH IVUS and in vitro histopathology tissue sample obtained by directional coronary atherectomy was shown to have a 87–97% accuracy for each plaque component.8 The primary objective of this study is to evaluate coronary plaque characterisation in the target vessels of DM patients using VH IVUS.

METHODS

Study population

Between October 2004 and July 2005, pre-interventional VH IVUS images were obtained for 440 consecutive patients in our institution. To standardise lesion characteristics between DM and non-DM groups and avoid the effect of thrombus for VH IVUS analysis, inclusion and exclusion criteria were defined as follows:

Inclusion criteria: (1) patients with stable angina pectoris (CCS I or II); (2) patients with a native de novo single atherosclerotic lesion that was considered to be the culprit lesion (>75% angiographic stenosis by visual estimation).

Exclusion criteria: (1) chronic total occluded lesions; (2) target lesions with angiographically determined thrombus.

A total of 118 patients were eligible using these criteria; however, 28 patients were excluded from analysis for technical reasons; in 18 patients, the border detection could not be described because of poor image quality, and a motorised pull-back system was not used in 10 patients. Therefore, the study population contained 90 patients. Type 2 DM was defined as follows: (1) adult-onset hyperglycaemia; (2) a fasting blood glucose of patients with diet therapy alone of more than 126 mg/dl; (3) patients receiving treatment with hypoglycaemic medications (insulin or oral hypoglycaemic agents). The institutional ethics committee approved the study and all patients gave written, informed consent.

Data acquisition and medication

All patients were given aspirin (100 mg/day) and ticlopidine (200 mg/day) for at least a week before the procedure. During the procedure, heparin was given as a bolus of 150 U/kg with additional boluses to 2000 U/hour. All intravascular ultrasound imaging were performed after administration of 200 µg of nitroglycerin. For the IVUS procedure, a 20-MHz, 3.2F, phased-array IVUS catheter (Eagle Eye, Volcano Therapeutics, Rancho Cordova, CA, USA) was used. After placing the IVUS catheter at a point which was at least 30 mm from the coronary ostium as well as distal to the target lesion, the catheter was pulled back to the coronary ostium using a motorised pull-back system at 0.5 cm/s. During pullback, greyscale IVUS was recorded and raw radiofrequency data were captured at the top of the R wave for reconstruction of the colour-coded map by a VH IVUS data recorder (Volcano Therapeutics, Rancho Cordova, CA, USA). The greyscale IVUS movie and captured radiofrequency data were written on a CD-R and DVD-R, respectively.

Greyscale and VH IVUS analyses

Measurements were made for the target vessel, which was defined as the segment from distal to the target lesion to the coronary ostium, evaluating a minimal length of 30 mm. Manual contour detection of both the lumen and the media-adventitia interface was performed by an experienced analyst who was blinded to baseline clinical and angiographic lesion characteristics. Intraobserver analysis in random samples of 45 patients was performed by the experienced analyst at least 2 weeks apart. Intraobserver differences in measurements of vessel cross-sectional area (CSA) and lumen CSA were 3.6% (SD 2.2%) and 3.9% (2.5%), respectively. Vessel CSA and lumen CSA were calculated, and the difference between the two values was defined as plaque plus media CSA. Percentage plaque plus media CSA was defined as plaque plus media CSA divided by vessel CSA. Atherosclerotic coronary plaques between the lumen and media-adventitia contours were characterised automatically with the use of custom-built software (IVUSLab software, Volcano Therapeutics, Rancho Cordova, CA, USA) which uses classification trees based on mathematical autoregressive spectral analysis of IVUS backscattered data, as described previously.7 Fibrous areas were marked in green, fibro-fatty in yellow, dense calcium in white, and necrotic core in red on the VH IVUS image. The absolute value of each plaque component was also calculated automatically by the software. The relative value was defined as the area of each plaque component divided by the total amount of plaque area. Geometrical and compositional data were obtained for each CSA and an average was calculated for each target vessel.

Definition of plaque types

VH IVUS-derived thin-cap fibroatheroma (VHD-TCFA) and VH IVUS-derived fibrocalcific atheroma (VHD-FCA) were defined as lesions fulfilling the following criteria in at least three consecutive cross-sectional images by two experienced, independent observers.

VHD-TCFA: (1) percentage necrotic core area >20% without evidence of fibrous cap; (2) percentage plaque plus media CSA ⩾40% (fig 1A). We selected a cut-off value based on previous pathological and in vivo VH IVUS findings.911

Figure 1 Intravascular ultrasound and typical Virtual Histology (VH) intravascular ultrasound (IVUS) images of VH IVUS-derived thin-cap fibroatheroma (VHD-TCFA) and VH IVUS-derived fibrocalcific atheroma (VHD-FCA). VHD-TCFA: (A-1) greyscale IVUS image of target lesion. Note the heterogeneity of the plaque. (A-2) Colour-coded map of A-1 reconstructed by VH IVUS. Note a necrotic core rich plaque without evidence of fibrous cap. VHD-FCA: (B-1) greyscale intravascular ultrasound image at target lesion. Note the significant deep calcium plate with acoustic shadow. (B-2) Colour-coded map of B-1. Note the confluent dense calcium at bottom of the plaque.

VHD-FCA: (1) percentage dense calcium area >10% with percentage necrotic core area <5% in VH IVUS images; (2) percentage plaque plus media CSA⩾40%; (3) confluent calcific plate with an arc of >90° in corresponding greyscale IVUS images because a previous study using IVUS showed that intermediate and severe calcific lesions had a calcium plate with an arc of >90°12 (fig 1B).

For the random samples of 45 patients, intraobserver analysis was performed at least 2 weeks apart and interobserver analysis was performed by two experienced, independent observers. There were no intraobserver and interobserver differences in evaluating the presence of VHD-TCFA and VHD-FCA.

Statistical analysis

Distribution normality of continuous data was assessed with the Shapiro-Wilk W-test. Continuous data are reported throughout this body of the text and in the tables as mean (SD) or medians with 25 and 75 interquartile range. Categorical data were expressed as a number or frequencies of occurrence. Comparison of continuous variables were performed by two-tailed unpaired Student t-test for normally distributed variables and by Mann-Whitney test for variables with skewed distribution. The χ2 test or Fisher’s exact test for sparse data was used for comparing frequency of occurrence. SPSS 11.0 (SPSS, Inc, Chicago, IL, USA) was used for data analysis. A probability value of less than 0.05 was considered to indicate statistical significance.

RESULTS

Patient and lesion characteristics

The DM group included 36 vessels (36 patients) and the non-DM group included 54 vessels (54 patients). The baseline patient and lesion characteristics are compared between the two groups in table 1. The value of fasting blood glucose and glycosylated haemoglobin in the DM group was higher than those in the non-DM group. Of the diabetic group, 22% (n = 8), 64% (n = 23), and 14% (n = 5) of patients were treated with insulin, oral hypoglycaemic agents and diet, respectively. The other patient and lesion characteristics were well matched.

Table 1 Baseline patient and lesion characteristics

Greyscale and VH IVUS data

Quantitative data

Table 2 shows mean quantitative greyscale and VH IVUS measurements. There were no differences in length of the imaged segment. Of the results of greyscale IVUS measurements, percentage plaque plus media CSA in the DM group was significantly larger than that in the non-DM group (DM: 58.8% (5.4%) vs non-DM: 55.4% (7.7%), p = 0.01). Of the results of VH IVUS measurements, both absolute and relative values of necrotic core and dense calcium were significantly larger in the DM group.

Table 2 Mean greyscale IVUS findings and plaque compositions in DM and non-DM groups

Qualitative data

In the DM group, 75% of vessels had at least one VHD-TCFA, which was significantly greater than those in the non-DM group (40%, p = 0.001). Similarly, VHD-FCA was observed more frequently in the DM group than in the non-DM group (75% vs 40%, p = 0.001). In addition, both VHD-TCFA and VHD-FCA in the target vessel were observed more frequently in the DM group (53% vs 17%, p = 0.0004).

DISCUSSION

The main findings of this study are, firstly, that diabetic patients have larger necrotic core and dense calcium in the target vessels than non-diabetic patients. Secondly, DM is associated with a higher frequency of VHD-TCFA and VHD-FCA in the target vessels.

Incidence of vulnerable plaque in target vessel of diabetic patients

Diabetes is associated with a twofold to fourfold increase in the risk of developing coronary artery disease and cardiovascular mortality compared to the general population.13 In general, acute coronary events are thought to be induced either by erosion or rupture of the lesion with exposure of the necrotic core. A postmortem study showed that necrotic core burden in diabetic patients was larger than that of non-diabetics.14 In this study, the incidence of necrotic core was significantly higher in VH IVUS images of the target vessel for diabetic patients. This finding is in agreement with these previous ex vivo studies and a recent clinical study showing the significant increase in lipid-rich content in coronary atherectomy specimens from patients with diabetes mellitus.15

It is well established that coronary occlusion and myocardial infarction most frequently evolve from mild to moderate plaque,16 17 which are more lipid-rich and vulnerable to rupture than larger plaques.17 18 A previous study analysing a non-culprit vessel by VH IVUS showed VHD-TCFA was common in non-culprit vessels of patients undergoing percutaneous intervention in other vessels.19 Indeed, intermediate lesions could be most dangerous; however, they by far outnumber the significantly stenotic plaque which became occluded more frequently than did the intermediate lesions.20 On the other hand, a previous study using IVUS demonstrated that diabetes was an independent predictor of plaque rupture in patients with stable angina pectoris.21 In this study, the prevalence of VHD-TCFA in target vessels of DM patients was significantly higher than in non-DM patients. Thus, DM patients may have a high risk of acute coronary syndrome because of the disruption of thin-cap fibroatheroma. Therefore, the evaluation of the target vessel and the stabilisation of the target lesion by systemic medication or interventional treatment may be important to improve the prognosis of DM patients.

Calcification in plaque of diabetic patients

The use of electron-beam computed tomography for the detection of coronary artery calcium has been studied previously and the extent of coronary artery calcium strongly correlates with the overall magnitude of atherosclerotic plaque burden22 and the development of subsequent coronary events.23 Diabetic individuals, especially, have higher coronary artery calcium than non-diabetics24 and the presence of any degree of coronary artery calcium in DM patients portended a higher risk for all-cause mortality than in non-DM patients.25 From pathological findings in postmortem studies, the fibrocalcific atheroma is defined as the end stage of process of atheromatous plaque rupture and/or erosion with healing and calcification.26 In addition, another postmortem study also showed that the average area of calcium in the coronary artery lesion site, which had more than 50% cross-sectional luminal narrowing with a diabetes individual, was larger than that with non-diabetes.14 The present study showed the larger burden of dense calcium and the higher incidence of VHD-FCA in the target vessel of the diabetic group and supported the previous findings by electron-beam computed tomography and pathology. The stage of atherosclerosis in the target vessel of diabetic patients may have progressed more than that of non-diabetic patients.

However, the relevance of plaque calcification to plaque rupture is controversial. In previous studies using IVUS, lesser degrees of calcium were observed in the culprit lesion of patients with acute coronary syndrome compared with lesions of patients with stable coronary artery disease.26 To the best of our knowledge, the effect of calcification for plaque rupture has not been established; however, there is one hypothesis in a previous report.27 As the degree of calcification increases, the interfaces between calcified (rigid) and non-calcified (soft) regions within the plaque and failure stress, which is predicted as a possible cause of plaque rupture, tends to occur at these interfaces. However, calcification beyond the point at which the rigid regions coalesce may be associated with decreasing risk of plaque rupture. Thus, coronary calcification may be one of the responses against acute coronary events.

Study limitations

This study has several limitations. Firstly, this study is a single centre, retrospective study. A study of larger patient populations from various centres is warranted to confirm these data. Secondly, although averages of analysed length in both groups were over 50 mm, those were only small segments of the entire coronary artery. Thirdly, the present VH IVUS technology is unable to differentiate thrombus from other plaque components. The subjects of this study were patients with stable angina pectoris and lesions with angiographically detected thrombus was excluded; however, this algorithm relies on the placement of two borders—namely, the luminal border and the media-adventitia border, so that small thrombus within these two borders might lead to incorrect tissue characterisation. Fourthly, the axial resolution of VH IVUS (>150 μm) is not sufficient to detect a thin cap (<65 μm). The hypothesis that contact of necrotic core with the lumen represents a thin cap needs to be confirmed with a comparative histopathological study. Finally, differences in vasomotor tone could have contributed to measurements of vessel and lumen dimensions. However, nitroglycerin was administered before angiography and intravascular ultrasound analysis, and differences in vasomotor tone should not have significantly affected measurements of quantitative analysis.

CONCLUSIONS

In this study, VH IVUS identified the increased necrotic core and dense calcium that were associated with diabetes mellitus. In addition, VHD-TCFA and VHD-FCA were more frequent in diabetic than in non-diabetic patients. Overall, advanced atherosclerosis was observed more frequently in the target vessels of diabetic patients.

REFERENCES

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Footnotes

  • Funding: None.

  • Competing interests: None.

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