[Dynamics of coronary blood flow in the presence of aortic valve disease].
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Aortic Valve↗
Biomedical subjects
R Beyar
Publications and source records attributed to R Beyar.
The physiologic meaning of relative coronary flow reserve.
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Blood Flow Velocity↗
Crossbridge dynamics in muscle contraction.
The study deals with the description of muscle contraction based on biochemical studies and describes four major approaches for coupling calcium kinetics with crossbridge (Xb) cycling. The analysis illuminates two controversial points: 1) the relationship between Xb attachment/detachment and Xb cycling, i.e., the transition between weak to strong conformations, and 2) the effect of calcium on Xb function: does it regulate Xb kinetics or Xb recruitment.
Animals↗
Integration of structure, function and mass transport in the myocardium.
A left ventricular (LV) model that integrates muscle mechanics, coronary flow, and fluid transport, and accounts for the three-phase (fiber-blood-interstitium) myocardial structure and composition, is used to study the interactions between the mechanics, coronary flow and fluid and mass transport in the myocardium. Theoretical simulations elucidate the effects of ventricular load, coronary perfusion pressure, and fluid and mass transport on ventricular performance and coronary dynamics. The analysis yields a direct relation between cardiac function and structure to cardiac mechanics, coronary flow, and intramyocardial fluid (and mass) transport, and allows to study the interactions between coronary flow, ventricular and myocardial mechanics and intramyocardial fluid shifts.
Animals↗
Hypertrophic cardiomyopathy: functional aspects by tagged magnetic resonance imaging.
Studies addressing the issue of regional function in hypertrophic cardiomyopathy patients (HCM) are reviewed. The relationship between regional wall thickness and function in these patients was studied by three dimensional (3D) tagged magnetic resonance imaging (MRI) utilizing the volume-element approach. Regional function was indexed by myocardial thickening and circumferential shortening and related to the local thickness and wall stress index. An inverse relationship was found between wall thickening and thickness as well as between circumferential shortening and wall thickness. Lower stresses were obtained for thicker myocardial segments. Function of the normal-thickness regions was enhanced in the HCM patients relative to the normal subjects. Thicker segments in patients with HCM are thus characterized by reduced systolic function, which occurs at segments with relatively low stress levels. This pattern is consistent with the hypothesis that the thick myocardial segments have reduced contractile activity, probably due to recently identified mutations in the gene responsible for production of beta heavy chain myosin as well as other contractile proteins.
Cardiomyopathy, Hypertrophic↗
Modulation of the arterial coronary blood flow by asynchronous activation with ventricular pacing.
UNLABELLED: This study aims to test the assumptions that: (1) coronary arterial flow is attenuated in an early activated region by ventricular pacing; (2) asynchronous mechanical activation caused by ventricular pacing under controlled perfusion pressure and intact coronary tone is associated with reduced coronary flow compared to atrial pacing; and (3) abolishment of vascular tone under controlled perfusion pressure diminishes the expected difference in blood flow between atrial and ventricular pacing. Blood flow velocity (BFV) in the left anterior descending (LAD) and the left circumflex arteries (CFX) and a wall thickening index were measured in 14 open-chest dogs under normal conditions, and constant perfusion pressure. Four pacing sites were used: right atrium (RAp), mid-right ventricle (RVp), mid-left ventricle (LVp), and left ventricular apex (Apexp). Pacing modes were either sequential ventriculoatrial (VA) (protocol A, n = 7), or sequential atrioventricular (AV) (protocol B, n = 7), with a shorter AV difference (30 msec) than normal. RESULTS: BFV was decreased in the LAD during RVp and Apexp pacing by 9.7%-12.9% versus RAp and by 11.6%-14.6% versus LVp (P < 0.05). No BFV variations were observed in the CFX. Flow velocity conductance (FVC = mean blood flow velocity divided by the mean aortic pressure) was higher by 16%-28% in the CFX for the three ventricular pacing sites versus the atrial pacing, and higher by 14.1% +/- 6.1% only in LVp versus RAp pacing in the LAD (P < 0.05). Wall thickening index reduced during ventricular pacing in all three ventricular sites by 50%-64% (P < 0.05) compared to atrial pacing. Under constant perfusion pressure, LAD blood flow decreased with ventricular pacing as compared to right atrial pacing; this was particularly pronounced during the diastolic phase (16.6%-45.5%, P < 0.02). Normalized oscillatory flow amplitude (OFAn) was reduced in RVp pacing compared to RAp and LVp pacing (16.2 +/- 3.5 and 21.7% +/- 4.1%, respectively, P < 0.03). The variations in blood flow and OFAn disappeared with adenosine-mediated maximum vasodilatation. SUMMARY: (1) Mean and phasic flows are reduced in the early activated LAD region by ventricular pacing (RVp, Apexp). (2) Under controlled perfusion pressure and intact vascular tone, ventricular pacing compromises blood flow compared with atrial pacing. (3) This effect disappears when vascular tone is eliminated by intracoronary injection of adenosine, suggesting that the coronary autoregulation is responsible for some of the effects.
Animals↗
An integrated model of LV muscle mechanics, coronary flow, and fluid and mass transport.
An integrated left ventricular (LV) model that accounts for the three-phase (fiber-blood-interstitium) myocardial structure and composition is used to study the interactions among myocardial mechanics, coronary flow, and fluid and mass transport. Effects of ventricular load, coronary perfusion pressure, and fluid and mass transport on ventricular performance and coronary dynamics are studied here. In agreement with experimental observations, the analysis shows that 1) coronary flow impediment is not significantly affected by changes in the afterload and preload at constant coronary perfusion pressures, 2) an increase in coronary perfusion pressure increases the intramyocardial pressure (IMP) as well as the mean flow and oscillatory flow amplitude, 3) contractility has a direct effect on IMP and coronary flow impediment, and 4) changes in blood osmolarity and lymphatic outflow, which may cause myocardial edema, affect both ventricular mechanics and coronary flow. Clearly, accounting for fluid and mass transport allows to study the interactions among coronary flow, ventricular and myocardial mechanics, and intramyocardial fluid shifts.
Animals↗
[Treatment of arterial stenosis with stents].
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Angioplasty, Balloon, Coronary↗
Self-expandable Nitinol stent for cardiovascular applications: canine and human experience.
The initial experimental and clinical experience with the cardiovascular self-expandable Nitinol stent (vascular and coronary versions) is described. The stent is designed as a helical coil with two terminal balls that are used for restraining it on the delivery catheter. Upon release, the stent self-expands immediately. A temporary stent version continues with a long wire that can be removed by pulling it as a straight wire through a small profile catheter. The stent uncoils in its own groove upon removal, a relatively atraumatic procedure. The stents have been studied in dogs and in peripheral arteries in patients. The results show a transient nonocclusive proliferative response to the stent that is maximal at 3-6 mo. The removability of the permanent stent has been proven in dogs. The preliminary results in patients are encouraging and demonstrate its feasibility for permanent and potentially temporary arterial support.
Animals↗
Analysis of coronary circulation under ischaemic conditions.
Coronary flow patterns and pressure/flow relationships in coronary vessels with arterial stenoses are examined by using a model that combines the flow in the epicardial arterial tree with the intramyocardial perfusion. By using appropriate resistive elements, the model allows for the autoregulation of the vascular bed and for the development of coronary collaterals. Arterial flow predictions are compared to canine data. Coronary stenosis is simulated by a local pressure drop caused by a combination of viscous and inertial forces; stenosis with a constant cross-sectional area is compared to a dynamic stenosis in which the cross-sectional area is a function of the instantaneous transmural pressure. Simulation results predict that the normal phasic flow patterns in the epicardial arteries are unaffected up to 73% reduction in cross-sectional area, while the average flow remains unchanged up to 90% area reduction. At the critical level of 90% rigid stenosis, the autoregulation is saturated and the phasic nature of the arterial flow is severely damped. Dynamic stenoses demonstrate hysteresis loops of the instantaneous pressure/flow relationship. Theoretical predictions of local and global values are in excellent agreement with experimental measurements, indicating that the proposed approach can be used to realistically describe the coronary flow in the ischemic heart.
Animals↗
Analysis of flow in coronary epicardial arterial tree and intramyocardial circulation.
A mathematical model combining the coronary flow in the epicardial arterial tree and the intramyocardial circulation is presented. The epicardial arterial tree is represented by a resistive capacitive network based on its realistic anatomy. The intramyocardial flow is affected by the pump action of the contracting myocardium through the extravascular compressive pressure (ECP), which, in turn, affects the dynamic resistance and compliance changes based on the relationship between the transmural pressure and the cross-sectional area of a vessel. The model accounts for the autoregulatory mechanism of the intramyocardial compartments (arteriolar, microvascular and venular) and is structured according to the epicardial coronary anatomy. Realistic coronary epicardial arterial flow patterns are obtained, which compare well to experimentally measured data in six dogs under basal conditions and during reactive hyperemic response. Simulations of the average transmural flow in the three intramyocardial vascular compartments show that the flow in the arterial side is predominantly diastolic, with a systolic retrograde component, and is dominantly systolic antegrade flow in the venular side, consistent with experimental data. Interestingly, the transmurally average microcirculatory flow is continuous, with very small change throughout the cardiac cycle, and is practically insensitive to changes in the model parameters. The model presents a quantitative tool that describes the dynamic patterns of coronary flow in relationship to muscular and extravascular parameters.
Animals↗
Relating coronary perfusion to myocardial function using three-dimensional reconstruction of heart and coronary arteries.
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Arterial Occlusive Diseases↗
Self-expandable and highly flexible nitinol stent: immediate and long-term results in dogs.
We sought to investigate the acute and long-term patency rates and the histologic response of coronary arteries to a self-expandable nitinol coil stent. Twenty-two stents were implanted. Angiographic patency was demonstrated acutely in all but one dog, in which the stent was released in a small branch (1 mm); mismatch in stent-to-artery diameters resulted in vessel closure. Two dogs died from anesthesia overdose and two from bleeding within 24 hours. All dogs were treated with aspirin (80 mg/day) and warfarin (2.5 mg/day) for up to 1 month. Sixteen dogs were monitored for 1 to 2 weeks, 1 month, 3 months, 6 months, and 1 year and underwent subsequent angiography and histopathologic examination. Angiographic artery dimensions measured immediately after stent implantation (2.72 +/- 0.4 mm) did not differ from those noted at follow-up (2.68 +/- 0.44 mm, p not significant). Histologic examination showed outward stent pressure compressing the internal elastic membrane and media in most cases. Intimal hyperplasia started at 2 weeks and was most apparent at 3 and 6 months. Mean intimal thickness was 30.7 +/- 10.9 mu, 141.8 +/- 105.4 mu, 227.1 +/- 104.1 mu, 211.8 +/- 99.1 mu, and 170.1 +/- 42.7 mu at 1 to 2 weeks and 1, 3, 6 and 12 months, respectively. Therefore the nitinol self-expandable stent provokes a moderate cellular proliferative response that reaches its maximum in 3 to 6 months without further progression.
Alloys↗
Quantitative sorting of normal and abnormal coronary flow wave form shapes.
The normal phasic flow wave form in an epicardial coronary artery has a distinct characteristic shape, which reflects the interaction between the coronary tree, myocardial function and hemodynamic conditions. Since clinical measurements of phasic coronary wave forms are becoming available, determination of abnormal coronary flow wave forms is important. We suggest here an objective and automatic method to discriminate between normal and abnormal flow wave forms based on the Karhunen-Loève Transform (KLT), and experimentally tested it. The normal flow domain was represented by the resting flow waves measured in the left anterior descending arteries in 31 anesthetized dogs. The abnormal flow conditions, imposed and tested experimentally, were varying stenosis severity and severely reduced left ventricular pressure. In addition, the effects of reactive hyperemia on the shape of the flow were examined. The sorting index was based on the mean-square error (MSE) calculated for each flow signal based on a truncated KLT expansion. The results show excellent discrimination between the normal and the abnormal groups. During reactive hyperemia, however, MSE did not change significantly. These results indicate that the shape of abnormal coronary flow wave forms can be identified and discriminated from normal wave forms.
Animals↗
In-vivo study of the mechanical properties of epicardial coronary arteries.
This study proposes a method to examine the in-vivo pressure-volume (P-V) relationship of an epicardial coronary artery. With the proximal left anterior descending (LAD) artery in a dog occluded, the distal LAD flow oscillates around zero. Integration of the flow with respect to time yields the intraluminal arterial segment volume changes in the region between the site of occlusion and the flow probe. The distal LAD pressure is measured via a diagonal branch. The segmental P-V relationship exhibits a hysteresis loop. The area bound by the hysteresis loop corresponds to the energy loss on the arterial wall during one cycle (12.63 +/- 8.25 [erg.cm-1], n = 7), while the dynamic arterial compliance is calculated based on the ratio of the volume to pressure excursions (1.13 +/- 0.73 [ml.mmHg-1.cm-1.10(-4)]). These results represent first estimates of the in-vivo mechanical properties of the coronary arterial wall based on P-V measurements.
Animals↗
Pressure-flow characteristics of the coronary collaterals: a model study.
The pressure-flow relationship of the coronary collaterals is investigated by using an electrical analog model that combines the coronary epicardial arteries with the nonlinear characteristics of the intramyocardial circulation. The study aims to examine some controversial issues concerning the collateral circulation, including the transmural distribution of the collaterals, the distensibility of the collateral vessels (whether rigid or complaint), the effects of microcirculatory embolization, the collateral zero-flow pressure, and the nonlinearity of the collateral pressure-flow relationship. The study is carried out by simulating and comparing two basic experimental set-ups in which a coronary artery is ligated and the retrograde flow serves as an index of collateral flow. In the first "free-flow" setup, flow is allowed to bleed retrogradely against atmospheric pressure while perfusion pressure to the rest of the coronary arteries is varied over a wide range. In the second "back-pressure" setup, the coronary perfusion pressure is maintained at the control levels while the back pressure to the retrograde flow in the excised artery is varied. According to the analysis, the nonlinear pressure-flow relationships depend heavily on the experimental setup and are a function of the distensibility of the collaterals, which are distributed mainly on the epicardial surface, and the nonlinear contraction characteristics of the myocardium. The measured retrograde flow tends to underestimate the total collateral flow for the back-pressure setup because of antegrade flow escape.
Animals↗
Left ventricular wall thickness and regional systolic function in patients with hypertrophic cardiomyopathy. A three-dimensional tagged magnetic resonance imaging study.
BACKGROUND: Regional performance of the hypertrophied left ventricle (LV) in hypertrophic cardiomyopathy (HCM) is still incompletely characterized with studies variably reporting that the hypertrophied myocardium is hypokinetic, akinetic, or has normal function. Different imaging modalities (M-mode or two-dimensional echocardiography) and methods of analysis (fixed or floating frame of reference for wall motion analysis) yield different results. We assessed regional function in terms of systolic wall thickening and shortening and related these parameters to end-diastolic thickness using tagged magnetic resonance imaging and the three-dimensional volume-element approach. METHODS AND RESULTS: In 17 patients with HCM and 6 healthy volunteers, four parallel short-axis images with 12 radial tags and two mutually orthogonal long-axis images with four parallel tags were obtained at end diastole and end systole. After the LV endocardial and epicardial borders were traced, three-dimensional volume elements were constructed by connecting two matched planar segments in two adjacent short-axis image planes, accounting for translation, twist, and long-axis shortening. A total of 72 such volume elements encompassed the entire LV. From each of these elements, end-diastolic thickness and systolic function (fractional thickening and circumferential shortening) were calculated. The average end-diastolic thickness was 15.8 +/- 4.2 mm in patients with HCM, which was significantly greater than that in healthy subjects (8.6 +/- 2.1 mm, P < .001). Fractional thickening was significantly less in patients with HCM than in healthy subjects (0.31 +/- 0.22 versus 0.56 +/- 0.23, P < .001). There was a highly significant inverse correlation between fractional thickening and end-diastolic thickness that was independent of the type of hypertrophy or age group. Similar inverse relations were observed between circumferential shortening and end-diastolic wall thickness. CONCLUSIONS: The myocardium in patients with HCM is heterogeneously thickened and the fractional thickening and circumferential shortening of the abnormally thickened myocardium are reduced compared with healthy subjects. The decrease in fractional thickening and shortening is inversely related to the local thickness.
Adult↗
Myocardial mechanics and coronary flow dynamics.
The interaction between cardiac mechanics and coronary flow is highlighted here. Left ventricular (LV) structure and geometry are related to coronary flow dynamics and used in the analysis of experimental coronary flow data. The important role of the collagen mesh in the generation of the intramyocardial pressure (IMP), the pressure in the interstitial fluid, at a wide range of loading conditions is emphasized. The calculated IMP, based on a structural model of the LV myocardium, can explain most of the observed coronary compression characteristics under a variety of loading and contractility conditions. A more general compression function, the extravascular compressive pressure (ECP), is suggested to define coronary compression and is presented here based on the dynamics of the coronary inflow under constant perfusion conditions. Coronary compression is shown to be affected by fluid transport and the bi-directional coupling of coronary hemodynamics and IMP dynamics.
Animals↗