Coronary Plaque Rupture in Stable Coronary Artery Disease and Non-ST Segment Elevation Myocardial Infarction: An Optical Coherence Tomography Study

Background. Plaque rupture (PR) is the main cause of coronary thrombosis in non-ST segment elevation myocardial infarction (NSTEMI), but can be found in stable coronary artery disease (CAD). Our study compared the morphology and local inflammatory activity of ruptured plaques between stable CAD and NSTEMI patients using frequency-domain optical coherence tomography (FD-OCT). Methods. We retrospectively evaluated 70 plaques with PR at the FD-OCT (25 in stable CAD patients and 45 in NSTEMI patients). Main clinical, angiographic, and morphological features were compared. Results. Besides an overall equivalence in clinical and angiographic features (except for more smokers among NSTEMI patients), some important FD-OCT differences in plaque morphology emerged: PR in NSTEMI was characterized by more macrophage infiltrates (78% in NSTEMI patients vs 20% in stable CAD patients; P<.001) and intraluminal thrombosis (84% in NSTEMI patients vs 48% in stable CAD patients; P<.01). Quantitative analysis showed a higher density of macrophages in NSTEMI than in stable CAD patients: median max normalized standard deviation (NSD) was 0.0934 (IQR, 0.0796-0.1022) vs 0.0689 (IQR, 0.0598-0.0787); P<.01 and mean NSD was 0.062 (IQR, 0.060-0.065) vs 0.053 (IQR, 0.051-0.060); P<.001. Other morphological features did not differ between stable CAD and NSTEMI patients. Main FD-OCT quantitative parameters like minimal lumen area and plaque length were also equivalent between the 2 groups. Conclusions. Differences in morphological features of PR between stable CAD and NSTEMI patients suggest that local inflammation contributes to the unstable fate of the atherosclerotic plaque.

J INVASIVE CARDIOL 2021;33(11):E843-E850. Epub 2021 October 7.

Key words: coronary plaque rupture, FD-OCT, local inflammation

Coronary plaque rupture (PR), with exposure of prothrombogenic factors to the blood and consequent intraluminal thrombosis, represents the most frequent pathologic substrate of acute coronary syndrome (ACS), including non-ST segment elevation myocardial infarction (NSTEMI).¹ Yet, PR can also be found in patients with stable coronary artery disease (CAD) and in autopsy findings of subjects who died of non-cardiac causes. In fact, previous studies demonstrated that PR in many cases contributes to plaque growth rather than triggering a thrombotic vessel occlusion.^2,3

Thus, the reasons why some PRs, but not others, cause an ACS remain largely unknown. In previous studies comparing PR between ACS and stable CAD patients, plaque morphology was assessed by intravascular ultrasound (IVUS). These studies suggest that specific IVUS plaque features, such as a greater plaque burden, may influence the propensity of a PR to determinate an ACS.⁴ Other studies suggested that the coexistence of specific plaque morphology features⁵ and the over-expression of prothrombotic factors in blood can affect the probability that a PR causes an ACS rather than remaining clinically silent.⁶ Another important player is plaque inflammation, known to promote fibrous cap fragility.⁷

Frequency domain optical coherence tomography (FD-OCT) is the imaging technique that provides the highest resolution in the intravascular assessment of coronary circulation in humans.⁸ In fact, FD-OCT is able to reveal many cases of PR undetectable by IVUS⁹ and is more accurate than IVUS in evaluating plaque components. FD-OCT is also able to give an estimate of the presence and severity of local inflammation by assessing macrophage density in the plaque.¹⁰

The purpose of this study is to compare the morphology and the presence and intensity of local inflammation in coronary lesions exhibiting PR assessed by FD-OCT between NSTEMI and stable CAD patients in order to better understand the mechanisms responsible for the different clinical consequences of PR.

Study population. We retrospectively assessed morphological features of coronary plaques in consecutive patients enrolled at the Catholic University of the Sacred Heart of Rome FD-OCT registry. The FD-OCT registry includes patients with CAD who have undergone coronary FD-OCT imaging of de novo lesions in a native coronary artery. In the FD-OCT registry, the decision to perform FD-OCT was at the discretion of the operator. In particular, the main reasons for using FD-OCT were characterization of coronary plaques (eg, evaluation of the amount of calcium or lipids, presence of thin-cap fibroatheroma, plaque rupture, etc)  and optimization of PCI procedures (eg, evaluation of lumen diameters and stent apposition).

Exclusion criteria were presentation with ST-elevation myocardial infarction (the high amount of thrombus in many ST-elevation related plaques may interfere with the FD-OCT’s ability to analyze plaque morphology); significant hepatic or renal dysfunction; evidence of malignant disease; acute or chronic inflammatory disease; and cardiogenic shock.

NSTEMI patients had at least 2 episodes of angina at rest or 1 episode lasting >20 minutes during the preceding 48 hours as well as rise and fall of high-sensitivity troponin T levels. Stable CAD was defined as evidence of myocardial ischemia at stress tests with or without angina symptoms or angina equivalents and stable clinical status over the last 3 months.

In accordance with the guidelines, all NSTEMI patients were treated with aspirin (250 mg intravenous loading dose, followed by aspirin 75 to 100 mg daily) and a loading dose of a thienopyridine (clopidogrel 600 mg, ticagrelor 180 mg) and fondaparinux or enoxaparin. All stable CAD patients were on medication with oral aspirin 75-100 mg daily and clopidogrel 300 mg loading dose before the FD-OCT scan procedure.

In all patients, cardiovascular risk factors were carefully examined, including family history of CAD (first-degree relative with a coronary event), diabetes mellitus (patients who were receiving oral hypoglycemic agents and/or insulin or having a known fasting glucose value ≥126 mg/dL or postprandial 2-hour blood glucose value of ≥200 mg/dL), hypercholesterolemia (low-density lipoprotein cholesterol level ≥130 mg/dL or treated hypercholesterolemia), smoking (current), and hypertension (systolic blood pressure >140 mm Hg and/or diastolic blood pressure >90 mm Hg or treated hypertension). Previous myocardial infarction was defined as any kind of myocardial infarction that occurred prior to the last 3 months.

The following main angiographic features were collected: evidence of multivessel coronary disease (≥70% stenosis in at least 1 major epicardial vessel and ≥50% stenosis in at least 1 other major vessel); and proximal coronary segment involvement (proximal right coronary artery: from ostium to one-half the distance to the acute margin of the heart; proximal left anterior descending artery: proximal to and including first major septal branch; proximal left circumflex artery: proximal to and including origin of first obtuse marginal branch).

Procedural data. Coronary angiography was performed via the transradial or transfemoral approach with the use of a 6 Fr or 7 Fr sheath. Unfractionated heparin (initial weight-adjusted intravenous bolus of 60 IU/kg, with repeat boluses to achieve an activated clotting time of 250-300 seconds) was administered in all patients. In NSTEMI patients, the use of intracoronary or intravenous platelet IIb/IIIa inhibitors was left to the operator’s discretion. FD-OCT scan was to be performed prior to IIb/IIIa inhibitor administration to be enrolled in the study.

A 0.014˝ guidewire was placed distally in the target vessel and an intracoronary injection of 200 µg of nitroglycerin was performed before FD-OCT acquisition.

FD-OCT images were acquired before any balloon predilation by a commercially available system (C7 System; LightLab Imaging/St. Jude Medical) connected to an FD-OCT catheter (C7 Dragonfly; LightLab Imaging/St. Jude Medical), which was advanced to the culprit lesion. The FD-OCT run was performed in the same modality by all operators, using the integrated automated pullback device at 20 mm/s. During image acquisition, coronary blood flow was replaced by continuous flushing of contrast media directly from the guiding catheter at a rate of 4 mL/s for the left coronary artery and 3 mL/s for the right coronary artery with a power injector, in order to create a virtually blood-free environment.^11,12

FD-OCT image analysis. FD-OCT image analysis was performed offline by 2 expert investigators who were blinded to clinical data. All of the PRs identified at the FD-OCT involved “culprit” lesions in NSTEMI patients and angiographically significant lesions in stable CAD patients. In particular, among NSTEMI patients, 1 “culprit” lesion was identified on the basis of angiographic features, electrocardiographic ST-segment alterations, and/or regional wall-motion abnormalities on echocardiographic assessment. In stable CAD patients, angiographically significant lesions were identified as stenosis >50%. A systematic triple-vessel FD-OCT assessment was not performed in all patients. The atherosclerosis components were defined according to the expert review documents on methodology, terminology, and clinical applications of FD-OCT.^11,12

Quantitative analysis. The following parameters were measured for each lesion: lesion length; and minimal lumen area (MLA), defined as cross-sectional area at the smallest lumen area level.

Morphological analysis. Morphological analysis was performed every 1 mm along the entire plaque length. PR was identified as the presence of a clear fibrous cap discontinuity leading to a communication between the inner (necrotic) core of the plaque and the lumen.

The presence of calcified (signal-poor region with defined borders), fibrous (homogeneous, signal-rich region), and lipid (signal-poor region with diffuse borders) tissues within the plaques were recorded. Thrombus was defined as an irregular mass protruding into the lumen or adjacent to the luminal surface. The thinnest part of the fibrous cap was measured 3 times, and the average value was calculated. In PR, residual fibrous cap was identified as a flap between the lumen of the coronary artery and the cavity of plaque, and its thickness was measured at the thinnest part.

Thin-cap fibroatheroma was defined as a plaque with lipid tissue present ≥90° in any of the cross-sectional images within the plaque and the thinnest part of a fibrous cap measuring ≤65 µm.

Macrophage analysis. In order to detect the presence of local inflammation at the level of coronary plaques, we assessed the presence of macrophages in the lesion by FD-OCT. Macrophage density measurement was obtained by using a 300 x 125 µm² (lateral x axial) region of interest (ROI), located in the FD-OCT frames with evidence of macrophages.¹²

In particular, macrophages were visualized by FD-OCT imaging as signal-rich, distinct, or confluent punctate regions that exceed the intensity of background speckle noise and generate a backward shadowing. For fibrous caps of PRs with a thickness <125 µm²,¹³ the depth of the ROI was matched to the cap thickness. Median filtering was performed with a 3 x 3 square kernel to remove speckle noise. Kappa measurement of agreement for intraobserver and interobserver variability was 0.82 (P<.001) and 0.91 (P<.001), respectively.

In plaques with macrophages, quantitative evaluation of macrophage content was obtained by measuring the normalized standard deviation (NSD) known to have a high degree of positive correlation with histological measurements of macrophage content,^14,15 by using a dedicated software provided by St. Jude Medical. In particular, NSD was measured for each pixel within each cap using a 125 µm² window centered at the pixel location:

NSD_(x,y)=[Ơ_(x,y)125 µm² /(S_max–S_min)] x 100

Where NSD_(x,y) is the normalized standard deviation of the FD-OCT signal at pixel location (x,y), S_max is the maximum FD-OCT image value, and S_min is the minimum FD-OCT image value. Pixels within the 125 x 125 µm² window that did not overlap with the segmented cap were excluded.

Max NSD was defined as the value of NSD measured in the single frame with the highest macrophage concentration within a plaque. Mean NSD was obtained by calculating the mean value of NSD between all FD-OCT frames (acquired every 1 mm length) with evidence of macrophages inside the same plaque.

Statistical analysis. Continuous variables were expressed as mean ± standard deviation or median with interquartile range (IQR) if they followed a normal or non-normal distribution, respectively. Continuous variables were compared with unpaired t-test, whereas categorical variables were compared using the Chi-square test or Fisher’s exact test, as appropriate. SPSS software, version 16.0 (SPSS) was used for statistical analyses. A P-value <.05 was required for statistical significance.

The study flow chart is shown in Figure 1. During the reference period, 209 lesions in 172 stable CAD patients and 175 ACS lesions in 175 patients underwent FD-OCT coronary assessment of a de novo lesion. In the ACS group, 73 lesions were ruled out (25 due to the impossibility to obtain complete clinical information, 18 for poor imaging quality [eg, presence of artifacts at the FD-OCT images] and 30 for clinical presentation with STEMI). In the stable CAD group, 28 lesions were excluded (18 for incomplete data and 10 for poor FD-OCT imaging quality). Eventually, 45 lesions in the NSTEMI group and 25 lesions in stable CAD group showed PR at the FD-OCT and were enrolled in the study.

Clinical and angiographic data are reported in Table 1. Twenty-five patients with stable CAD and 45 patients with NSTEMI showed PR at the FD-OCT and were enrolled. The mean age and gender distribution were equivalent between the 2 groups. The prevalence of main cardiovascular risk factors — possible due to the small study population — was not statistically different between stable CAD and NSTEMI patients, except for a higher percent of active smokers in NSTEMI patients (49% in the STEMI group vs 24% in the stable CAD group; P=.047). History of previous myocardial infarction was similar between the 2 groups.  The main angiographic features, like multivessel disease and involvement of proximal coronary segments, were also not statistically different.

The FD-OCT characteristics of the plaques with evidence of PR are shown in Table 2. Parameters potentially able to influence hemodynamics and shear stress, such as MLA and plaque length, as well as the main plaque components and prevalence of thin-cap fibroatheroma, were similar in the 2 groups.

In sharp contrast, the prevalence of macrophage infiltration was higher in PR of NSTEMI patients as compared with stable CAD patients (78% in the NSTEMI group vs 20% in the stable CAD group; P<.001). Furthermore, quantitative analysis showed a significantly higher density of macrophages in the NSTEMI group. In fact, max NSD was 0.0934 (IQR, 0.0796-0.1022) in NSTEMI vs 0.0689 (IQR, 0.0598-0.0787) in stable CAD (P<.01) and mean NSD was 0.062 (IQR, 0.060-0.065) in NSTEMI vs 0.053 (IQR, 0.051-0.060) in stable CAD (P<.001).

PRs of NSTEMI patients were also characterized by a higher evidence of intraluminal thrombosis compared with stable CAD patients (84% in the NSTEMI group vs 48% in the stable CAD group; P<.01).

As shown in Figure 2 and Table 3, we divided the studied population into 4 subgroups based on the simultaneous presence of macrophages and thrombus in the PR, the presence of only one of them, or the absence of both. The subgroup analysis showed a significantly greater number of NSTEMI patients with co-presence of macrophages and thrombus (71% in the NSTEMI group vs 12% in the stable CAD group; P<.001), whereas the stable CAD patients were more represented in the subgroups with thrombus without macrophages (13% in the NSTEMI group vs 36% in the stable CAD group; P=.04) and in the subgroup without either (7% in the NSTEMI group vs 44% in the stable CAD group; P<.001).

The main finding of this study is that coronary plaques with evidence of PR assessed by FD-OCT exhibit some important differences according to clinical presentation of stable CAD vs NSTEMI. Indeed, PR in NSTEMI patients is characterized by a higher intraplaque macrophage infiltration suggestive of intense local inflammatory activity and greater prevalence of thrombosis as compared with stable CAD patients.

PR is the prevalent substrate of coronary thrombosis in nearly 50% of patients with ACS.¹ Autopsy and imaging studies demonstrate that many PRs remain clinically silent, which can also be found in patients with stable CAD.^16,17 To date, the exact mechanisms responsible for the discrepancy between apparent plaque instability and clinical stability remain largely unknown.

The consequence of PR is related to local thrombogenicity, which in turn is influenced by local inflammation.^18,19 Accordingly, previous studies showed that tissue factor expression, which is a major activator of the coagulation cascade, is greater in coronary plaques of patients with unstable presentation compared with those with stable clinical presentation.^20,21 Moreover, Lee et al⁶ showed that both stronger tissue factor expression and specific plaque morphologies at IVUS (greater plaque burden and higher remodeling index) were predominant in ruptured plaques of patients with ACS as compared with stable CAD patients.

Previous studies showed that local inflammation with macrophage infiltration plays a key role in plaque instability.^22,23 In fact, macrophages secrete metalloproteinases that degrade the fibrous cap and promote thrombus formation, releasing tissue factor.^24,25

This study is the first comparison of PR morphology between NSTEMI and stable CAD patients using FD-OCT, which, compared with IVUS, provides images with greater resolution and allows the detection of intraplaque macrophage infiltrates. In the current study, FD-OCT analysis showed that PRs in NSTEMI patients have a significantly greater prevalence of both macrophage infiltration and luminal thrombosis as compared with stable CAD patients. Moreover, in plaques with evidence of macrophages, a quantitative assessment of macrophage infiltrates by measuring the normalized standard deviation (NDS) showed a higher density of macrophages in PRs of NSTEMI patients than in stable CAD patients.

Furthermore, in the subgroup analyses according to the simultaneous presence of macrophages/thrombus, only one of them, or the absence of both, it emerged that a significantly higher number of PRs in stable CAD patients showed thrombus without macrophages or the absence of both compared with NSTEMI patients. This suggests that the absence of local inflammation, even in case of atherothrombosis, might determine the clinical stability of an unstable/ruptured plaque.

It is worth noting that in our study, 20% of patients with NSTEMI did not have evidence of plaque inflammation, suggesting that other non-inflammatory mechanisms can be responsible for plaque fissure, as recently proposed by Crea and Libby.⁷ Furthermore, in 16% of patients with NSTEMI, we failed to demonstrate coronary thrombus ,although this might simply be a consequence of the antithrombotic treatment given to the patients. Conversely, intraplaque inflammation and atherothrombosis are not exclusive characteristics of NSTEMI patients. In fact, even in stable CAD patients, FD-OCT analysis was able to detect macrophage infiltration and plaque-thrombosis, in 20% and 48% of cases, respectively. It remains rather unclear why these plaque features did not cause clinical instability in these patients.

With the exception of differences in the prevalence in coronary thrombosis and plaque inflammation, the current study failed to find other morphological differences at the site of PR, including minimal luminal area, plaque composition, and prevalence of thin-cap fibroatheroma, between NSTEMI and stable CAD patients. These findings confirm that functional alterations rather than anatomic factors determine the transition from clinical stability to clinical instability.¹

In contrast with previous studies,²⁶ we found a substantial equivalence in the prevalence of clinical risk factors and previous history of myocardial infarction between the 2 groups, except for a higher percentage of active smokers in the NSTEMI group. The small number of patients enrolled in the study may explain the lack of an impact of clinical risk factors on vulnerable plaques.

Study limitations. Several limitations can be found in our study. First, the small number of plaques investigated and the retrospective nature of the study may introduce some important biases and suggest that our results are only hypothesis generating. Second, we did not analyze the presence of plaque erosion (or calcified nodule erosion), besides plaque rupture, as a substrate for clinical destabilization in the NSTEMI group. Third, we did not perform a systematic triple-vessel FD-OCT analysis, so we do not have data about non-culprit lesions in the NSTEMI group. Fourth, our analysis of macrophages, based on ROI, can potentially generate significant bias due to the presence of back scatter. Indeed, a recent article by Phipps et al²⁷ suggested that the diagnostic accuracy of OCT for such bright spots with sharp attenuation is not specific for macrophages. However, in this article, we demonstrated a good agreement for intraobserver and interobserver variability in the macrophage evaluation. Fifth, in the unstable group, we have only FD-OCT data about patients with NSTEMI. Future large studies that analyze plaque morphology differences between unstable angina and NSTEMI are needed. Sixth, NSTEMI patients received a stronger antithrombotic therapy than stable CAD patients before undergoing FD-OCT analysis, which may have influenced intraluminal thrombosis. However, the prevalence of thrombosis was higher in NSTEMI patients despite the stronger antithrombotic therapy. Seventh, inflammatory biomarkers (such as C-reactive protein) are not available in our study, but may be useful in the future to investigate the relationship between systemic inflammation, local inflammation, and ACS risk.

This study investigated the FD-OCT morphological features of ruptured plaques between stable CAD and NSTEMI patients. Data have shown that PR in NSTEMI patients, compared with stable CAD patients, is characterized by a higher prevalence of macrophage infiltration and thrombus. These findings have demonstrated that coronary clinical instability could be more associated with plaque inflammation, rather than plaque vulnerability, and mainly related to other morphological and functional factors.