Temperature distribution in atherosclerotic coronary arteries: influence of plaque geometry and flow (a numerical study)

Detection of the vulnerable coronary atheromatous plaque. Where are we now?

Atherosclerosis is a progressive process with potentially devastating consequences and has been identified as the leading cause of morbidity and mortality, especially in the industrial countries. The underlying mechanisms include endothelial dysfunction, lipid accumulation and enhanced inflammatory involvement resulting in plaque disruption or plaque erosion and subsequent thrombosis. However, it has been made evident, that the majority of rupture prone plaques that produce acute coronary syndromes are not severely stenotic. Conversely, lipid-rich plaques with thin fibrous cap, heavily infiltrated by inflammatory cells have been shown to predispose to rupture and thrombosis, independently of the degree of stenosis. Therefore, given the importance of plaque composition, a continuously growing interest in the development and improvement of diagnostic modalities will promptly and most importantly, accurately detect and characterize the high-risk atheromatous plaque. Use of these techniques may help risk stratification and allow the selection of the most appropriate therapeutic approach.

Numerical analysis of the cooling effect of blood over inflamed atherosclerotic plaque

Atherosclerotic plaques with high likelihood of rupture often show local temperature increase with respect to the surrounding arterial wall temperature. In this work, atherosclerotic plaque temperature was numerically determined during the different levels of blood flow reduction produced by the introduction of catheters at the vessel lumen. The temperature was calculated by solving the energy equation and the Navier-Stokes equations in 2D idealized arterial models. Arterial wall temperature depends on three basic factors: metabolic activity of the inflammatory cells embedded in the plaque, heat convection due to luminal blood flow, and heat conduction through the arterial wall and plaque. The calculations performed serve to simulate transient blood flow reduction produced by the presence of thermography catheters used to measure arterial wall temperature. The calculations estimate the spatial and temporal alterations in the cooling effect of blood flow and plaque temperature during the measurement process. The mathematical model developed provides a tool for analyzing the contribution of factors known to affect heat transfer at the plaque surface. Blood flow reduction leads to a nonuniform temperature increase ranging from 0.1 to 0.25 degrees Celsius in the plaque/lumen interface of the arterial geometries considered in this study. The temperature variation as well as the Nusselt number calculated along the plaque surface strongly depended on the arterial geometry and distribution of inflammatory cells. The calculations indicate that the minimum required time to obtain a steady temperature profile after arterial occlusion is 6 s. It was seen that in arteries with geometries involving bends, the temperature profiles appear asymmetrical and lean toward the downstream edge of the plaque.

Diagnosis and treatment of coronary vulnerable plaques

Thin-capped fibroatheroma is the morphology that most resembles plaque rupture. Detection of these vulnerable plaques in vivo is essential to being able to study their natural history and evaluate potential treatment modalities and, therefore, may ultimately have an important impact on the prevention of acute myocardial infarction and death. Currently, conventional grayscale intravascular ultrasound, virtual histology and palpography data are being collected with the same catheter during the same pullback. A combination of this catheter with either thermography capability or additional imaging, such as optical coherence tomography or spectroscopy, would be an exciting development. Intravascular magnetic resonance imaging also holds much promise. To date, none of the techniques described above have been sufficiently validated and, most importantly, their predictive value for adverse cardiac events remains elusive. Very rigorous and well-designed studies are compelling for defining the role of each diagnostic modality. Until we are able to detect in vivo vulnerable plaques accurately, no specific treatment is warranted.

Calculation of arterial wall temperature in atherosclerotic arteries: effect of pulsatile flow, arterial geometry, and plaque structure

Background

This paper presents calculations of the temperature distribution in an atherosclerotic plaque experiencing an inflammatory process; it analyzes the presence of hot spots in the plaque region and their relationship to blood flow, arterial geometry, and inflammatory cell distribution. Determination of the plaque temperature has become an important topic because plaques showing a temperature inhomogeneity have a higher likelihood of rupture. As a result, monitoring plaque temperature and knowing the factors affecting it can help in the prevention of sudden rupture.

Methods

The transient temperature profile in inflamed atherosclerotic plaques is calculated by solving an energy equation and the Navier-Stokes equations in 2D idealized arterial models of a bending artery and an arterial bifurcation. For obtaining the numerical solution, the commercial package COMSOL 3.2 was used. The calculations correspond to a parametric study where arterial type and size, as well as plaque geometry and composition, are varied. These calculations are used to analyze the contribution of different factors affecting arterial wall temperature measurements. The main factors considered are the metabolic heat production of inflammatory cells, atherosclerotic plaque length l_p, inflammatory cell layer length l_mp, and inflammatory cell layer thickness d_mp.

Results

The calculations indicate that the best location to perform the temperature measurement is at the back region of the plaque (0.5 ≤ l/l_p ≤ 0.7). The location of the maximum temperature, or hot spot, at the plaque surface can move during the cardiac cycle depending on the arterial geometry and is a direct result of the blood flow pattern. For the bending artery, the hot spot moves 0.6 millimeters along the longitudinal direction; for the arterial bifurcation, the hot spot is concentrated at a single location due to the flow recirculation observed at both ends of the plaque. Focusing on the thermal history of different points selected at the plaque surface, it is seen that during the cardiac cycle the temperature at a point located at l/l_p = 0.7 can change between 0.5 and 0.1 degrees Celsius for the bending artery, while no significant variation is observed in the arterial bifurcation. Calculations performed for different values of inflammatory cell layer thickness d_mp indicate the same behavior reported experimentally; that corresponds to an increase in the maximum temperature observed, which for the bending artery ranges from 0.6 to 2.0 degrees Celsius, for d_mp = 25 and 100 micrometers, respectively.

Conclusion

The results indicate that direct temperature measurements should be taken (1) as close as possible to the plaque/lumen surface, as the calculations show a significant drop in temperature within 120 micrometers from the plaque surface; (2) in the presence of blood flow, temperature measurement should be performed in the downstream edge of the plaque, as it shows higher temperature independently of the arterial geometry; and (3) it is necessary to perform measurements at a sampling rate that is higher than the cardiac cycle; the measurement should be extended through several cardiac cycles, as variations of up to 0.7 degrees Celsius were observed at l/l_p = 0.7 for the bending artery.

An analytic and numerical study of intravascular thermography of vulnerable plaque

Intravascular thermography has been proposed as a method for detecting vulnerable plaque. A marker of vulnerability in a plaque is inflammation, which is believed to reduce its mechanical stability. It has been hypothesized that this inflammation leads to a higher metabolic rate and therefore higher heat generation, causing increased temperature in the vicinity of the plaque. This temperature increase could be measured intravascularly using a temperature sensor, e.g., a thermistor or a thermocouple. The aim of this study is to present a thorough mathematical and physical analysis of the thermal distribution that can be expected in the plaque under various physiological conditions. To get reasonable predictions on the expected temperature distributions, idealized models with simple geometries are solved analytically. More realistic models, with more complex geometries, are solved numerically using the finite element method (FEM). Based on this analysis, the maximum temperature increase that can be expected in a plaque due to increased metabolism is less than 0.1 K.

Influence of catheter design on lumen wall temperature distribution in intracoronary thermography

Intracoronary thermography is a currently used vulnerable plaque detection method. We studied how catheter design and catheter location influence the temperature readings, and thus its capacity to detect vulnerable plaques. Finite element calculations were performed on geometries representing the coronary artery, the vulnerable plaque and the catheter. Catheter material, diameter and location with respect to the plaque were varied. Both flow and no-flow situations were studied. Maximal lumen wall temperature difference without a catheter (DeltaT=0.12 degrees C, flow=75 cm(3) min(-1)) was considered the reference. Presence of a 1.0mm nitinol catheter right under the plaque increased DeltaT to 0.14 degrees C, whereas a 1.0 mm polyurethane catheter increased DeltaT to 0.51 degrees C. The location at which a thermosensitive element should be placed for most optimal temperature readings during a pullback was shown to lie at the catheter edge for the nitinol catheter and at 1.1mm from the catheter edge for the polyurethane catheter. Temperature readings decreased to background temperature when the catheter was in close proximity but not overlapping the plaque. DeltaT decreased approximately by 70% when a gap of 0.2 mm existed between the catheter and the lumen wall. Occlusion of blood flow increased DeltaT values in all cases, but most pronounced for nitinol catheters. A polyurethane catheter increased the temperature readings, since its heat conductivity is lower than that of blood, which makes it a very good choice for heat source detection. Catheter design can contribute to enhanced temperature readings and thus can enable more optimal vulnerable plaque detection.

A numerical study on the influence of vulnerable plaque composition on intravascular thermography measurements

Intracoronary thermography is a technique that measures lumen wall temperatures for vulnerable plaque detection. In this paper the influence of vulnerable plaque composition on lumen wall temperatures was studied numerically. Concerning the vulnerable plaque heat generation, the location of the heat source and its heat production were varied. Concerning the heat transfer, the thermal properties of the lipid core and the location of the vasa vasorum were studied. The heat source location was the main determinant of the lumen wall temperature distribution. The strongest effect was noted when the heat producing macrophages were located in the shoulder region leading to focal spots of higher temperature. The maximal lumen wall temperature was mainly determined by the heat production of the macrophages and the cooling effect of blood. The insulating properties of the lipid core increased lumen wall temperatures when the heat source was located in the cap and the presence of vasa vasorum lowered the temperatures. These results show that the lumen wall temperature distribution is influenced by vulnerable plaque composition and that intracoronary thermography techniques require a high spatial resolution. To be able to couple temperature measurements to plaque vulnerability, intracoronary thermography needs to be combined with an imaging modality.

Assessment of Culprit Plaque Temperature by Intracoronary Thermography Appears Inconclusive in Patients With Acute Coronary Syndromes

OBJECTIVE

Safety and feasibility evaluation of intracoronary temperature measurements in patients with acute coronary syndromes (ACS) using a catheter based thermography system.

METHODS AND RESULTS

Thermography was performed in 40 patients with ACS. A 3.5-F thermography catheter containing 5 thermocouples measuring vessel wall temperature, and 1 thermocouple measuring blood temperature (accuracy 0.05 degrees C) was used. Gradient (deltaTmax) between blood temperature (T(bl)) and the maximum wall temperature during pullback was measured. The device showed satisfactory safety in ACS. Only in 16 patients (40%) deltaTmax was > or = 0.1 degrees C. In 23 patients (57.5%) the highest deltaTmax was found in the culprit segment. DeltaTmax between culprit and adjacent non-culprit segments was observed in patients with transient blood flow interruption during thermography (0.11+/-0.03 versus 0.08+/-0.01; P=0.04), in contrast to patients with preserved flow (0.07+/-0.03 versus 0.06+/-0.02; P=0.058).

CONCLUSIONS

The novel, technically sophisticated intracoronary thermography proved its safety and feasibility. However, we were not able to convincingly and consistently differentiate between different lesions at risk, despite a selection of lesions that should appear most distinct to differentiate. A systematic interruption of flow may be necessary to achieve diagnostic results consistently, although such requirement may unfavorably change the risk-to-benefit ratio of this developing technology.

Intracoronary Thermography for Detection of High-Risk Vulnerable Plaques

Up to two-thirds of acute myocardial infarctions develop at sites of culprit lesions without a significant stenosis. New imaging techniques are needed to identify those lesions with an increased risk of developing an acute complication in the near future. Inflammation is a hallmark feature of these vulnerable/high-risk plaques. We have shown that inflamed atherosclerotic plaques are hot and their surface temperature correlates with an increased number of macrophages and decreased fibrous-cap thickness. Multiple animal and human experiments have shown that temperature heterogeneity correlates with arterial inflammation in vivo. Several coronary temperature mapping catheters are currently being developed and studied. These thermography methods can be used in the future to detect vulnerable plaques, potentially to determine patients' prognosis, and to study the plaque-stabilizing effects of different medications.

Intracoronary thermography: does it help us in clinical decision making?

The concept of the "vulnerable" plaque has recently emerged to explain how quiescent atherosclerotic lesions evolve to cause clinical events. The morphologic and immunologic determinants specific for the vulnerable plaque have been reported: a large lipid core (>or=40% plaque volume) composed of free cholesterol crystals, cholesterol esters, and oxidized lipids impregnated with tissue factor; a thin fibrous cap depleted of smooth muscle cells and collagen; an outward (positive) remodeling; inflammatory cell infiltration of fibrous cap and adventitia (mostly monocyte-macrophages, some activated T cells, and mast cells); and increased neovascularity. Despite the large amount of information regarding the morphological characteristics of remote lesions, we lack studies with functional assessment of non-culprit lesions. Coronary thermography is a technique for functional assessment of coronary atherosclerotic plaques. Several catheter designs have been proposed. There are catheters with thermistor(s) and wires with thermal sensors at the distal tip. All designs have several advantages and disadvantages. Despite the current limitations of coronary thermography, we gained important pathophysiological and clinical information regarding the vulnerability of atheromatic plaques. It has been documented both experimentally and clinically that increased heat generation is associated with increased macrophage concentration within the plaque. The correlation between local inflammatory involvement and local heat generation has also been observed with the peripheral inflammatory markers such as C-reactive protein. Whether systemic treatment, with agents such as statins or interventional techniques, such as drug-eluting stents, will have an impact on stabilizing vulnerable plaques need to be determined in future studies. In conclusion, although there are several techniques for evaluating morphologically atheromatic plaques, thermography is a promising method for the functional assessment of vulnerable plaque and has been introduced into clinical practice, with a good predictive value for clinical events in patients with increased temperature in the atherosclerotic plaque.

Clinical imaging of the vulnerable plaque in the coronary arteries: new intracoronary diagnostic methods

Rupture of a vulnerable plaque is the main cause of acute coronary syndromes and myocardial infarction. The features of rupture-prone atherosclerotic plaques have been previously described by pathologists. However, identification of vulnerable plaques in vivo is essential to study their natural history and to evaluate potential treatment modalities. Coronary angiography is the gold standard for the diagnosis of coronary artery disease, but it is unable to distinguish between stable and unstable plaques and to accurately predict future cardiac events. This current perspective describes the recently developed invasive imaging techniques to detect atherosclerotic vulnerable plaques in the coronary tree.