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

OBJECTIVE: To 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. METHODS: The 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. RESULTS: The 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. CONCLUSIONS: The 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 in normal left coronary artery tree.

Despite the fact that the role of wall shear stress (WSS) as a local mechanical factor in atherogenesis is well established, its distribution over the entire normal human left coronary artery (LCA) tree has not yet been studied. A three-dimensional computer generated model of the epicardial LCA tree, based on averaged human data set extracted from angiographies, was adopted for finite-element analysis of the Navier-Stokes flow equations treating blood as non-Newtonian fluid. The LCA tree includes the left main coronary artery (LMCA), the left anterior descending (LAD), the left circumflex artery (LCxA) and their major branches. In proximal LCA tree regions where atherosclerosis frequently occurs, low WSS appears. Low WSS regions occur at bifurcations in regions opposite the flow dividers, which are anatomic sites predisposed for atherosclerotic development. On the LMCA bifurcation, at regions opposite to the flow divider, dominant low WSS values occur ranging from 0.75 to 2.25 N/m2. High WSS values are encountered at all flow dividers. This work determines, probably for the first time, the topography of the WSS in the entire normal human LCA epicardial tree. It is also the first work determining the spatial WSS differentiation between proximal and distal normal human LCA parts. The haemodynamic analysis of the entire epicardial LCA tree further verifies the implications of the WSS in atherosclerosis mechanisms.

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.

Wall pressure gradient in normal left coronary artery tree.

The three-dimensional wall pressure gradient (WPG) of the normal human left coronary artery (LCA) tree is quantitatively analysed. A model LCA tree, based on 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. The WPG is calculated using 44,452 nodes throughout the tree extension. The governing flow equations were solved using a validated numerical code. WPG as well as wall shear stress gradient (WSSG) were calculated at all available bifurcation regions. In proximal LCA tree regions where atherosclerosis frequently occurs, low WPG appears. At distal segments, WPG increases substantially due to increased velocity resulted from increased vessel tapering. Low WPG occurs at bifurcations in regions opposite the apexes, which are anatomic sites predisposed for atherosclerotic development. Endothelial cells respond to the combined effects of locally low WPG and low WSSG and provide a mechanism promoting atherosclerosis. This computational work determines probably for the first time the topography of the WPG in the normal human LCA tree. Spatial WPG differentiation indicates that locally low values of this physical parameter probably correlate to atherosclerosis localization.

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

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 at bifurcations in regions opposite the flow 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 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.

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 prognostic implication.