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At least 289 records · Page 16Linked to original sources
PubMed1991

An eikonal-curvature equation for action potential propagation in myocardium.

We derive an "eikonal-curvature" equation to describe the propagation of action potential wavefronts in myocardium. This equation is used to study the effects of fiber orientation on propagation in the myocardial wall. There are significant computational advantages to the use of an eikonal-curvature equation over a full ionic model of action potential spread. With this model, it is shown that the experimentally observed misalignment of spreading action potential "ellipses" from fiber orientation in level myocardial surfaces is adequately explained by the rotation of fiber orientation through the myocardial wall. Additionally, it is shown that apparently high propagation velocities on the epicardial and endocardial surfaces are the result of propagation into the midwall region and acceleration along midwall fibers before reemergence at an outer surface at a time preceding what could be accomplished with propagation along the surface alone.

Action Potentials↗
PubMed1988

The effects of slip velocity at a membrane surface on blood flow in the microcirculation.

Closed-form solutions are presented for blood flow in the microcirculation by taking into account the influence of slip velocity at the membrane surface. In this study, the convective inertia force is neglected in comparison with that of blood viscosity on the basis of the smallness of the Reynolds number of the flow in microcirculation. The permeability property of the blood vessel is based on the well known Starling's hypothesis. The effects of slip coefficient on the velocity and pressure fields are clearly depicted.

Blood Flow Velocity↗
PubMed1990

In vitro validation of a simple tomographic technique for estimation of percentage myocardium at risk using methoxyisobutyl isonitrile technetium 99m (sestamibi).

With the advent of technetium 99m-labeled myocardial blood flow agents, there is a need for a simple technique for quantitation of infarcted or jeopardized myocardium (IM). This study provides an in vitro validation of a simple technique based upon the analysis of three short-axis slices through the heart following emission computed tomography. All acquisitions were performed using a static cardiac phantom containing pertechnetate Tc 99m. Activity in the phantom was adjusted so that the count density and myocardial-to-background ratio were comparable to those observed in patients. Plastic insets (range of sizes = 4%-72% of myocardium) were used to simulate transmural infarctions. Eighteen studies were acquired, each over 180 degrees into a 64 x 64 matrix. Data were reconstructed using a Ramp Hanning filter with cut off at 0.7 times the Nyquist frequency. Short-axis slices of the myocardium were then generated, and representative apical (A), mid-ventricular (MV), and basal (B) slices were selected. For each slice, a circumferential profile was generated, and the average radius (R) was measured. The fraction (F) of the profile falling below a threshold value was considered to represent IM. Total IM was given by % IM = 100 x (RB FB + RMV FMV + 0.67 RA FA)/(RB + RMV + 0.67 RA), where the subscripts to R and F refer to the relevant short-axis slices. For a threshold set at 60% of peak, measured IM agreed closely with true IM (R2 = 0.98, measured IM = 1.01 x true IM - 1.35). Measurement of % IM was not distorted by variations in slice radius or in slice selection.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1995

Simultaneous dual-isotope technetium-99m/thallium-201 cardiac SPET imaging using a projection-dependent spilldown correction factor.

A spilldown correction method is proposed for the thallium-201 window image in simultaneous dual-isotope technetium-99m/thallium-201 single-photon emission tomographic (SPET) imaging based on a single acquisition into three energy windows. In this method, images are simultaneously acquired in two standard energy windows over the 99mTc and 201Tl photopeak regions and a third spilldown window adjacent to the 201Tl window. Using a Monte Carlo simulation of SPET, the fractional amount of 99mTc and 201Tl spilldown in the 201Tl window with respect to the total counts from the spilldown window, k12, was calculated for simulated images of point sources at varying depths within a water-filled elliptical tub phantom. When applied to experimental acquisitions, k12, multiplied by the total counts from the spilldown window, is then subtracted from the 201Tl window image to produce the corrected image. However, for successful applications in SPET, k12 must be determined on a projection-by-projection basis since k12 is depth dependent. Thus, a regression relation was obtained between k12 and the total count ratio of the spilldown to 99mTc windows, k23. The spilldown correction method was applied to 201Tl photopeak images of an extended source distribution in uniform and nonuniform attenuating media with dual-isotope 99mTc/201Tl and single-isotope 201Tl. A marked improvement in image contrast was observed between the corrected and uncorrected 201Tl window images. The average count ratio of uncorrected dual-isotope 201Tl/single-isotope 201Tl was 3.08 for uniform and 2.99 for non-uniform attenuating media.(ABSTRACT TRUNCATED AT 250 WORDS)

Heart↗
PubMed1993

Experimental validation of a new quantitative method for the analysis of infarct size by cardiac perfusion tomography (SPECT).

Using global constraints and dynamic programming, a new model-based segmentation algorithm was developed to determine myocardial borders and basal plane. The segmented image is transformed to a countrate polar map and the infarct size (I.S.) is determined by comparison with a reference polar map. In order to evaluate our method the algorithm was applied to heart phantoms, to software simulations and to animal studies. In the last experiments, Tc-99m Sestamibi was used as a perfusion agent. The total myocardial volume and infarct size of a Jasczack phantom were overestimated, especially when I.S. was expressed in absolute rather than relative values. It was proven by software simulations of cardiac Spect studies that those errors were mainly due to finite resolution effects causing a clear overestimation of myocardial thickness. Implementation of a constant thickness in the algorithm resulted in a much better correlation with actual values. In a dog experiment the size of total myocardial volume of the area at risk during occlusion and of the final infarct size after thrombolysis was correlated with the histologic values obtained by planimetry after TTC staining. In 8 studies, an excellent correlation between the histologic area at risk versus the estimated perfusion defects was obtained (r = 0.97). The automatic delineation of myocardial borders and valve plane was excellent even when perfusion defects were present. Manual intervention was only necessary in certain slices where a clear overlap between liver and myocardium was present in the dog studies. Segmental polar maps expressing count rate and volume information provided a visual and quantitative tool to evaluate the influence of thrombolysis in acute ligation experiments. It is concluded that the new algorithm is ready to be used in a clinical environment for the quantitative evaluation of perfusion defects after acute myocardial infarction and for the follow-up of the therapeutic strategy.

Animals↗
PubMed1987

Haemodynamic stress in lateral saccular aneurysms. An experimental study.

The flow velocities in lateral glass and silastic aneurysm models were quantitatively measured with the non-invasive laser Doppler method. The influences of the elasticity of the wall, the pulse wave and the properties of the perfusion medium on the intra-aneurysmal circulation were investigated. As shown previously, the inflow into the aneurysm arose from the downstream lip and was directed toward the centre of the fundus. Backflow to the parent vessel took place along the walls of the fundus. With non-pulsatile perfusion, flow velocities in the centre of the standardized aneurysms varied between 0.4 and 2% of the maximum velocity in the parent vessel. With pulsatile perfusion, flow velocities in the centre of the fundus ranged between 8 and 13% of the flow velocity in the axis of the parent vessel. Flow velocities in the aneurysms were slower with a macromolecular perfusion medium with blood like properties compared to a glycerol/water solution. Flow velocity measurements near the aneurysmal wall allowed the estimation of the shear stresses at critical locations. The maximum shear stresses at the downstream lip of the aneurysm were in the range of the stresses measured at the flow divider of an arterial bifurcation. The present results suggest that in human saccular aneurysms intra-aneurysmal flow and shear stress on the wall are directly related to the pulsatility of perfusion, i.e. the systolic/diastolic pressure difference and that the tendency to spontaneous thrombosis depends on the viscoelastic properties of the blood, namely the haematocrit.

Blood Flow Velocity↗
PubMed1992

The hyperaemic response to a transient reduction in cerebral perfusion pressure. A modelling study.

A mathematical model of cerebral blood flow and the cerebrospinal fluid circulation is described which permits the study of phenomena caused by dynamic changes in cerebrovascular autoregulatory or cerebrospinal fluid compensatory reserves. A transient decrease in cerebral perfusion pressure was produced by carotid artery compression. Comparison of the computer simulations with clinical and experimental data, reported elsewhere, suggests that the transient hyperaemic response (THR) is proportional to the strength of the autoregulatory response. The relationships between the magnitude and time course of the THR, and the period and level of reduction in CPP were studied. This model suggests that simple clinical tests based on the examination of THR using transcranial Doppler velocity measurements are of potential value for the non-invasive assessment of the autoregulatory reserve.

Blood Flow Velocity↗
PubMed1992

Cardiac muscle fiber force versus length determined by a cardiac muscle crossbridge model.

A mathematical model incorporating Huxley's sliding filament crossbridge muscle model coupled with parallel and series elastic components was simulated to examine force-length relations under different external calcium concentrations. Several researchers have determined experimentally in both papillary muscle preparations and in situ heart experiments that the calcium concentration (or effective concentration from inotropic agents) will affect the strength and convexity of the cardiac muscle fiber force-length relations. Simulations were performed over a several-order-of-magnitude range of calcium concentrations in isometric contractions and these showed that the force-length curve convexity was changed. Simulation results demonstrated that increasing the stiffness in the model contractile element or series elasticity element did not change the force-length convexity. Increasing the series elasticity element stiffness did slightly change the shape of the force-length curve. The model predicts that the curve convexity changes as a result of the calcium-troponin interactions.

Calcium↗
PubMed1996

Automated determination of left ventricular volume curves from bi-plane digital angiography without explicit use of edge detection algorithms.

Automated computation of left ventricular (LV) global and regional function using contrast angiography has not yet become a routine procedure with the advent of digital cardiac imaging systems. We describe a new technique to compute LV volume curves which does not require the use of manual or semi-automated detection of endocardial borders and provides on-line implementation of volumetric curves and computation of pressure volume loops during catheterization. The approach uses the concepts of variable entropy (or information) of left ventricular images throughout the cardiac cycle. LV volume curves are computed with an interpolation scheme using those LV volume curves of a patient data base which are associated with the closest variation in entropy in the RAO projection to the analyzed patient data according to a simple metric. Computed LV volume curves were correlated with those obtained with manual tracing. Left ventricular ejection fraction (LVEF), time to end systole (TES) and angiographic cardiac output (CO) were compared to those obtained with the manual method. Results using a data base of 365 patients revealed excellent correlation (r = 0.97) between manually derived volume curves and volume curves computed with the automated technique within a large range of LVEFs. In 87% of all cases the computed LVEF values were found within +/- 10% of the value obtained with the gold standard method. The systolic phase of the volume curves showed that 81% of all cases had the same accuracy. The TES showed much more variation due to undersampling of the cardiac cycle in time (r = 0.71).

Algorithms↗
PubMed1993

CO2 cerebrovascular reactivity as a function of perfusion pressure--a modelling study.

A mathematical model is described that demonstrated the properties of cerebral vascular resistance and compliance expressed as a function of cerebral perfusion pressure (CPP) and arterial CO2 partial pressure (PaCO2). The hypercapnic induced shift of the lower limit of autoregulation to a higher range of CPP, as shown by this model, is a useful characteristic that facilitates the differentiation between normal and impaired autoregulation described previously in experimental studies. Dynamic properties of cerebrovascular circulation derived from the relationship between pulse wave of CBF waveform and CPP have been analysed at different levels of PaCO2-phenomenon, being often described as dependence of blood flow velocity pulsatility index on the autoregulatory reserve. The model was also used to interpret interhemispheric asymmetry of CBF reactivity to changes in arterial concentration of CO2 in patients with carotid artery stenosis.

Blood Flow Velocity↗
PubMed1990

Major pitfalls in Doppler investigations. Part II. Low flow velocities and colour Doppler applications.

Many pitfalls result from the limited ability of Doppler instruments to record low flow velocities. These include a misleading resistance or pulsatility index due to diastolic cut-off and taking no signal to equal no flow assuming that no signal means no flow. Comparison of actual flow velocities as measured in an in-vitro system (range: 0.8 to 3.4 cm/s) with the lowest recordable spectral or colour signals in 3 Duplexscanners showed that reduced sensitivity to low flow velocities is not only dependent on the high pass ("wall") filter setting, Doppler frequency or angle of incidence, but also on factors such as vessel diameter, impairing the signal to noise ratio. Characteristic errors of colour flow mapping (misleading vascular anatomy, imitation of pathological findings, erroneous exclusion of flow) are due to partial volume effect, limited temporal and velocity resolution, changing angle of incidence, aliasing and failure to detect low flow velocities.

Adult↗
PubMed1993

Abnormal wall strain at distal end-to-side anastomoses.

Cyclic stretch has been demonstrated to induce proliferative and secretory activities by cultured arterial endothelial and smooth muscle cells, cellular processes that contribute to the development of intimal hyperplasia. A model of an end-to-side anastomosis was developed to examine the hypothesis that regions of the artery at such anastomoses are subjected to focally increased cyclic stretch, which may stimulate the development of intimal hyperplasia. Polytetrafluoroethylene grafts were anastomosed end to side to latex rubber tubes that have elastic properties similar to those of the human femoral artery. Pulse waves with physiologic pressure, rate, and contour were applied, and systolic and diastolic diameters were measured in two planes at longitudinal intervals. Circumferential strain imposed on the latex "artery" was calculated at each interval. Strain imposed perpendicular to the suture line was also measured. Circumferential strain was consistently maximal at a distinct region of the "artery" along the proximal third of the anastomosis (6.0 +/- 1.1% vs. 3.3 +/- 0.5% at other regions of the "artery"). The maximal strain across the suture line was found at precisely the same region (3.9 +/- 0.3% vs. 2.0 +/- 0.4%). The anastomotic region of the recipient artery in a distal end-to-side anastomosis is subjected to cyclic circumferential strains two times greater than those experienced by the remainder of the artery. This corresponds to a common location of intimal hyperplasia. Such strains may be a stimulus for intimal hyperplasia.

Anastomosis, Surgical↗
PubMed1993

Differences in carotid shunt flow rates and implications for cerebral blood flow.

A wide variety of carotid shunts are available for use in extracranial carotid surgery. Since it is commonly assumed that when properly positioned all shunts are equal in ability to protect the brain from cerebral ischemia, the choice of shunt is usually based on handling characteristics. However, after an intraoperative stroke occurred in a patient, we compared shunt flow rates using a simple and reproducible method of measurement. A mock circuit was created using a saline-filled fluid reservoir connected to the particular shunt being tested via 1/2-inch tubing. Hydrostatic pressure across the shunt was varied by changing the height of the reservoir, and the flow was collected over 30-second intervals. Multiple flow rate measurements were performed for each shunt with pressure gradients varying from 25 to 150 cm H2O. The data show significant hemodynamic differences among commercially available carotid shunts. A pressure gradient of 75 cm H2O produced a 2.8-fold variation in the amount of fluid delivered by various shunts. Minimal cerebral blood flow requirements and the possibility of underperfusion require that the surgeon consider such data in choosing an appropriate carotid shunt.

Blood Flow Velocity↗
PubMed1987

Relationship between chamber mechanical properties and mean pressure-mean flow diagram of the left ventricle.

We undertook a theoretical analysis of the source resistance of the left ventricle represented in a mean pressure-mean flow (P-Q) diagram, using the chamber properties established in terms of the pressure-volume relationship. This analysis showed that P-Q pairs of points should lie above the linear function proposed by Elzinga and Westerhof. A third-order polynomial function would theoretically explain better than a linear relation or a parabolic fit the curved shape of experimentally obtained P-Q relationships. The analysis resolves the discrepancy between Elzinga and Westerhof's theoretical concept of linear source resistance and the actual nonlinear P-Q relationship.

Animals↗
PubMed1989

Mechanics of a thin walled collapsible microtube.

The purpose of this study is to measure the transmural pressure-cross sectional area relation of micro tubes (240 microns diameter) and to compare the measured perfusion pressure-flow relation with the pressure-flow relation calculated from the experimental pressure-cross sectional area relation. The microtubes are made by dipping a glass mould in a latex solution and glueing their outside ends to the inside of glass pipettes. The pressure-cross sectional area relation is determined both with a microplethysmograph (pressure-volume relation) and the microscope (pressure-diameter relations). Heparinized blood is used to include the rheological properties of blood as a perfusion medium. Static pressure-flow relations are obtained with a constant velocity piston pump for two values of external pressure (0 and 10 kPa) and with two downstream resistor settings (0 and 380 kPa cm-3 sec). The calculated pressure-flow relations using length and the experimental pressure-cross sectional area relation, Poiseuille's law, and accounting for the diameter- and shear-dependent viscosity compared well with the relations obtained from the experiments. It is also found that the pressure-flow relation shows an apparent zero flow pressure axis intercept (the extrapolation of the pressure-flow relation to the pressure axis), which can therefore be explained on the basis of the shape of the pressure-area relations.

Hemodynamics↗
PubMed1989

The Korotkoff sound.

As the auscultatory method of blood pressure measurement relies fundamentally on the generation of the Korotkoff sound, identification of the responsible mechanisms has been of interest ever since the introduction of the method, around the turn of the century. In this article, a theory is proposed that identifies the cause of sound generation with the nonlinear properties of the pressure-flow relationship in, and of the volume compliance of the collapsible segment of brachial artery under the cuff. The rising portion of a normal incoming brachial pressure pulse is distorted due to these characteristics, and energy contained in the normal pulse is shifted to the audible range. The pressure transient produced is transmitted to the skin surface and stethoscope through deflection of the arterial wall. A mathematical model is formulated to represent the structures involved and to compute the Korotkoff sound. The model is able to predict quantitatively a range of features of the Korotkoff sound reported in the literature. Several earlier theories are summarized and evaluated.

Auscultation↗
PubMedDate not supplied

Parameter identification in coronary pressure flow models: a graphical approach.

The confident identification of parameters is important in the practical application of physiological models. However, the task of parameter identification is often complicated by interactions among parameters and by the fact that the sensitivity of the model to changes in a given parameter is generally a function of all the other parameters. Here we illustrate a graphical approach to parameter identification that allows the modeler to visualize the behavior of the model, the sensitivity functions, and certain functions characteristic of parameter interdependence. The visual display can be generated over any desired portion of parameter space. The technique is applied to a simple, four-parameter, myocardial pump model of the coronary circulation. The results indicate that over specified ranges of parameters, it is possible to distinguish among the four parameters of the model: the ratio of proximal-to-distal resistance, alpha; the overall resistance of the vascular bed, R; the compliance of the vascular bed, C; and a parameter, kappa, relating tissue pressure to left ventricular pressure. It was found that in order to identify all parameters uniquely, it was necessary to regress upon both coronary inflow and outflow.

Blood Pressure↗
PubMedDate not supplied

Effects of myocardial contraction on coronary blood flow: an integrated model.

The effects of myocardial contraction on the coronary flow are studied by means of an integrated structural model of left ventricular (LV) mechanics, coronary flow, and fluid and mass transport. This model relates global LV performance, and in particular coronary flow dynamics, to myocardial composition and structure and contractile sarcomere activity. Extravascular pressure is identified with hydrostatic tissue pressure, i.e., intramyocardial pressure (IMP), and is determined by the dynamics of myocardial contraction and fluid transport. Consistent with available experimental data, changes in myocardial function and contractile state are simulated by changing the sarcomere contractile properties or changing the LV loading conditions. The model's predictions are successfully compared with a wide range of experimental studies; all but one were performed at a constant coronary perfusion pressure and maximal vasodilation. The results indicate a dominant effect of the myocardial contractile state on coronary flow and a dissociation between coronary compression and LV cavity pressure (LVP) when the pressure is controlled by load changes. However, when active sarcomere contraction is regionally impaired by lidocaine, LVP plays an important role in the coronary flow characteristics. The model adequately predicts observations on the effect of cardiac contraction on systolic and diastolic coronary flows, as well as the role of LVP at different loading and contractile conditions. The analysis supports the hypothesis that coronary compression, as mediated through IMP, is independent of LV loading conditions and depends on myocardial contractility and coronary perfusion pressure.

Biomechanical Phenomena↗