Thomas M. Farmakis

Haemodynamic factors and the important role of local low static pressure in coronary wall thickening

UNLABELLED BACKGROUND/STUDY OBJECTIVES: The purpose of our study was to investigate the possible correlation between blood flow physical parameters and the wall thickening in typical human coronary arteries. METHODS Digitized images of seven transparent arterial segments prepared post-mortem were adopted from a previous study in order to extract the geometry for numerical analysis. Using the exterior outline, reconstructed forms of the vessel geometries were used for subsequent computational fluid dynamic analysis. Data was input to a pre-processing code for unstructured mesh generation. The flow was assumed to be two-dimensional, steady, laminar with parabolic inlet velocity profile. The vessel walls were assumed to be smooth, inelastic and impermeable. Non-Newtonian power law was applied to model blood rheology. The arterial wall thickening was measured and correlated to the wall shear stress, static pressure, molecular viscosity, and near wall blood flow velocity. RESULTS Wall shear stress, static pressure and near wall velocity magnitude exhibit negative correlation to wall thickening, while molecular viscosity exhibits positive correlation to wall thickening. CONCLUSION There is a strong correlation between the development of vessel wall thickening and the blood flow physical parameters. Amongst these parameters the role of local low wall static pressure is predominant.

Spatial and phasic oscillation of non-newtonian wall shear stress in human left coronary artery bifurcation : an insight to atherogenesis

ObjectiveTo investigate the wall shear stress oscillation in a normal human left coronary artery bifurcation computational model by applying non-Newtonian blood properties and phasic flow. MethodsThe three-dimensional geometry of the investigated model included the left main coronary artery along with its two main branches, namely the left anterior descending and the left circumflex artery. For the computational analyses a pulsatile non-Newtonian flow was applied. To evaluate the cyclic variations in wall shear stress, six characteristic time-points of the cardiac cycle were selected. The non-Newtonian wall shear stress variation was compared with the Newtonian one. ResultsThe wall shear stress varied remarkably in time and space. The flow divider region encountered higher wall shear stress values than the lateral walls throughout the entire cardiac cycle. The wall shear stress exhibited remarkably lower and oscillatory values in systole as compared with that in diastole in the entire bifurcation region, especially in the lateral walls. Although the Newtonian wall shear stress experienced consistently lower values throughout the entire cardiac cycle than the non-Newtonian wall shear stress, the general pattern of lower wall shear stress values at the lateral walls, particularly during systole, was evident regardless of the blood properties. ConclusionsThe lateral walls of the bifurcation, where low and oscillating wall shear stress is observed, are more susceptible to atherosclerosis. The systolic period, rather than the diastolic one, favors the development and progression of atherosclerosis. The blood viscosity properties do not seem to qualitatively affect the spatial and temporal distribution of the wall shear stress.

Wall shear stress gradient topography in the normal left coronary arterial tree: possible implications for atherogenesis

SUMMARY Objective: Wall shear stress gradient (WSSG) in vitro has shown its importance in atherogenesis, probably as a local modulator of endothelial gene expression.The purpose of this study is to numerically analyse the WSSG distribution over the normal human left coronary artery (LCA) tree. Research design and methods: A three-dimensional computer generated model of the LCA tree, based on an averaged human data set extracted from angiographies, was adopted for finite-element analysis. The LCA tree includes the left main coronary artery (LMCA), the left anterior descending (LAD), the left circumflex artery (LCxA) and their major branches. Results: In proximal LCA tree regions where atherosclerosis frequently occurs, low WSSG appears. At distal segments, the WSSG increases substantially due to increased velocity resulting from increased vessel tapering. Low WSSG occurs at bifurcations in regions opposite the flow dividers, which are anatomic sites predisposed for atherosclerotic development. Conclusions: This computational work determines, probably for the first time, the topography of the WSSG in the normal human LCA tree. Spatial WSSG differentiation indicates that low values of this parameter probably correlate to atherosclerosis localization. However, further studies are needed to clarify the role of WSSG in atherogenesis.

Molecular Viscosity in the Normal Left Coronary Arterial Tree. Is It Related to Atherosclerosis

The purpose of this study is to elucidate, probably for the first time, the distribution of molecular viscosity in the entire left coronary artery (LCA) tree. The governing mass, momentum, and energy flow equations were solved by using a previously validated 3-dimensional numerical (finite-element analysis) code. High-molecular-viscosity regions occur at bifurcations in regions opposite the flow dividers, which are anatomic sites predisposed for atherosclerotic development. Furthermore, high-molecular-viscosity values appear in the proximal regions of the LCA tree, where atherosclerosis frequently occurs. The effect of blood flow resistance, due to increased blood viscosity, gives rise to increased contact time between the atherogenic particles of the blood and the endothelium, probably promoting atherosclerosis. Observations suggest that, whole viscosity distribution within the coronary artery tree may represent a risk factor for the resulting atherosclerosis. This distribution can become a possible tool for the location of atherosclerotic lesions.

Evaluation of a Doppler-Derived Index Combining Systolic and Diastolic Left Ventricular Function in Acute Myocardial Infarction

Assessment of left ventricular (LV) function is crucial in the immediate postinfarction period. The authors evaluated the clinical applicability of the Doppler-derived myocardial performance index (MPI, defined as the sum of isovolumic contraction and relaxation times divided by LV ejection time) in patients with acute myocardial infarction (AMI) as to whether this index reflects the severity of LV dysfunction in this subgroup of patients. Post-AMI patients (n = 33) were compared with age- and sex-matched healthy subjects (n = 35). Within 24 hours of the AMI and 1 month thereafter, patients underwent 2D and Doppler echocardiography. Patients were divided into group A (Killip Class I, n = 22) and group B (Killip Class II-III, n = 11). The authors measured the LV ejection fraction (EF), diastolic indices (transmitral E and A waves, E/A ratio, deceleration time [DT], isovolumic contraction time [IVCT], isovolumic relaxation time [IVRT], MPI, LV end-systolic and end-diastolic volume indices [ESVi and EDVi] and wall motion score index [WMSi]). One-year mortality was also assessed. There was no significant difference concerning E and A waves, E/A ratio, and IVRT between the 2 groups. There were highly statistical differences at day 1 for EF (59.3 ± 6.7% vs 36.8 ± 4.5%, p<0.0001), DT (0.160 ± 0.030 sec vs 0.127 ± 0.022, p < 0.005), MPI (0.344 ± 0.084 vs 0.686 ± 0.120, p < 0.0001), ESVi (28.4 ± 3.9 mL/m2 vs 46.2 ± 8.4, p < 0.001), and WMSi (1.58 ± 0.06 vs 1.88 ± 0.35, p = 0.05), which persisted after 1 month. One-year mortality was significantly (0 vs 27.3%, p<0.01) lower in group A patients. This study shows that the MPI, reliably indicated LV dysfunction post-AMI, significantly correlated with clinically determined functional class, and possibly has some prog nostic implication.

Molecular viscosity distribution in the left coronary artery tree

The purpose of this study is to elucidate, probably for the first time, the distribution of the molecular viscosity in the entire left coronary artery (LCA) tree. A three-dimensional computer model of the normal LCA tree, reported previously, was adopted for subsequent numerical analysis. The governing mass, momentum and energy flow equations were solved using a previously validated numerical (finite-element analysis) code. The calculated results show high molecular viscosity regions appearing in zones opposite to all left coronary artery flow dividers on either of the two sides. These are regions where atherosclerotic plaques usually develop. Viscosity values change throughout the flow field. The distribution of high molecular viscosity along the walls is in agreement with the prone to atherosclerosis regions.

Diastole and its beneficial role in coronary atherogenesis

The purpose of this study is to elucidate the role, which wall shear stress (WSS) differentiation between diastole and systole plays in coronary atherosclerosis. A finite-element analysis of the 3D, pulsatile, non-Newtonian, haemodynamics of the normal human left main coronary artery bifurcation, based on published data, is performed. The time averaged mean WSS in the entire left coronary artery bifurcation region ranged from 0.086 to 5.97 N/m2. Arterial WSS was significantly lower on the lateral walls of the bifurcation, for all tested time steps of the pulse cycle. The distribution of low WSS along the walls is in agreement with the common locations of atheroma. It is the systolic period, rather than the diastolic one, which is probably associated with the development of atherosclerotic plaques, due to significantly lower WSS values during systole.

Computational Haemodynamics Of Left Coronary Artery

The purpose of our study was to quantitatively analyze the three-dimensional haemodynamics of the normal human Left Coronary Artery (LCA) tree. Physical parameters analyzed, amongst others, include: wall shear stress, wall stress and blood viscosity. The LCA tree includes: Left Anterior Descending, Left Circumflex Artery and their major branches. A 3D computer model of the normal LCA tree was adopted for subsequent numerical analysis. Discharges from distal branches were set proportional to the third power of the corresponding branch diameter. The blood was treated as a non-Newtonian fluid obeying to the power law. The governing mass, momentum and energy flow equations were solved using a previously validated numerical code. Low Wall Stress Gradients (WSG) and low Wall Shear Stress Gradients (WSSG) occur nearby to and at bifurcations, particularly at regions opposite the flow dividers, which are anatomic sites predisposed for atherosclerotic development. The Wall Stress (WS) decreases at distal LCA parts while the Wall Shear Stress (WSS) distribution reveals low values opposite to the flow dividers while increased values appear at distal tree parts. The viscosity distribution is highly affected from the blood flow conditions. Transactions on Biomedicine and Health vol 6,