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At least 1,153 records · Page 64Linked to original sources
PubMed1991

In vitro validation of Doppler indices using blood and water.

Using an in vitro flow model, volume flow and pressure were measured to compare calculated resistance with simultaneously measured continuous-wave Doppler indices during imposed changes in downstream resistance to pulsatile flow. With stepwise reductions in flow, the peak (S) and trough (D) points of the maximum shifted-frequency envelope fell in parallel in a linear fashion until D reached zero. Pressure remained constant, and the pressure waveform remained unchanged. As calculated resistance increased, resistance index (RI) and pulsatility index (PI) both increased in a linear fashion until the point at which D became zero. However, S/D ratio was not linear, and the slope increased as resistance increased. The results obtained from microspheres suspended in water and with heparinized sheep's blood were similar. The results indicate that, with reductions in flow at constant pressure in vitro, S/D ratio does not relate to calculated resistance in the same linear manner as PI and RI.

Animals↗
PubMed1994

Improved cardiac anthropomorphic phantom.

An improved cardiac sonographic phantom is described that simulates the anatomy of the human heart for applications as a low cost quality assurance and training tool in echocardiography. The major new feature in the phantom is the development of a polyurethane sponge left ventricle, which mimics the myocardium during sonographic imaging while undergoing hydraulic pumping. The left ventricle, including prosthetic mechanical mitral and aortic valves, is housed in an optically transparent Lexan chamber or in a tissue mimicking polyurethane rubber torso. Additional new features include a port for transesophageal imaging, and an adjustable calibrated stroke volume on the piston pump, which can serve as a standard for echocardiographic stroke volume measurement. B-scans of the phantom show excellent image quality in all three standard echocardiographic views as well as in transesophageal scanning.

Echocardiography↗
PubMed2003

Modifying the cosine model for nonstationary blood flow signal simulation.

OBJECTIVE: The most important features of the Doppler signal are its nonstationary nature and randomness, which make simulating these kinds of signals difficult. Our intent was to create realistic simulations of these signals. METHODS: We generated the Doppler signal for the carotid artery by using the cosine model. We also modified the cosine model to simulate the femoral Doppler signal for the whole cardiac cycle. RESULTS: In auditory comparisons, the simulation results compared favorably with the original data. No appreciable differences were found between the real clinical signals and the simulations for the normal carotid and femoral arteries. However, the calculated normalized root mean square errors were high. CONCLUSIONS: The cosine model is simple to implement and requires input of only the spectrum shape variation during a cardiac cycle. In addition, it is more informative than other modeling methods and therefore is preferred over other methods.

Blood Flow Velocity↗
PubMed1999

Static state hemodynamic variables estimation model for the moving-actuator type total artificial heart. Part. I--cardiac output estimation.

Cardiac output estimation is a very important study for the artificial heart. In this paper, we developed a cardiac output estimation model for the moving-actuator type total artificial heart (MA-TAH) that was developed at Seoul National University Hospital. The proposed model is simple and provides beat-by-beat mean cardiac output estimation. Moreover, it uses non-invasively acquired signals. Model parameters were adjusted with in vitro data by least mean square (LMS) algorithm. Results showed that the proposed scheme gives a mean estimation error of about 0.1 (l/min) for the given data. This ensures the suitability of the proposed model.

Cardiac Output↗
PubMed1999

Cerebral blood flow regulation in REM sleep: a model for flow-metabolism coupling.

The pattern of metabolic and circulatory changes occurring during REM sleep in the whole brain is also observed at a regional level in different instances of functional activation. This pattern is characterized by an increase in metabolic rate, blood flow, glucose and oxygen uptake, the increase in glucose uptake generally exceeding oxygen uptake. A model of interpretation is presented, based on the assumption that substantial limitation to oxygen diffusion exists in the brain. According to the model, microregions lying at mid-distance between capillaries may become hypoxic, depending on metabolic rate and blood-cell PO2 difference. At increasing metabolic rates, O2 consumption in pericapillary microregions increases and the PO2 drop becomes steeper. As a consequence, in microregions far from capillaries a decrease in O2 availability occurs, in concomitance with the increase in metabolic rate, so that non-oxidative glucose metabolism develops locally. A similar spatial PO2 pattern forms in the case of arterial hypoxia, when capillary PO2, and then blood-cell PO2 difference, is reduced. The hypoxic microregions are the source of vasodilatatory messages, the consequent vasodilatation increasing average capillary PO2 and then favoring O2 diffusion to the tissue. Oxygen thus appears to be a better candidate than glucose as a mediator of blood flow-metabolism coupling. This is supported by its higher extraction fraction and by the fact that, in physiologic conditions, arterial hypoxia (and not hypoglycemia) acts on cerebral blood flow. Moreover, the diffusion capacity of glucose in the brain is higher than that of oxygen, so that diffusion limitation is more likely to occur for oxygen. The present model allows consistent organization of the stereotyped changes in cerebral blood flow and glucose and oxygen uptake occurring both in REM sleep and in other instances of brain activation.

Animals↗
PubMed1999

The porous, guidewire-directed, detachable aneurysm liner: a new concept in the endovascular treatment of intracranial aneurysms.

A new device for the endovascular treatment of aneurysms is described. It consists of a guidewire-directed porous liner or bag, detachably mounted on a microcatheter and designed to be inserted into an aneurysm and to be filled with detachable coils or other embolic agents. Several prototypes have been made. Preliminary in vitro and in vivo experiments have demonstrated its behavior in relatively wide-necked aneurysms.

Animals↗
PubMed1999

Statistical method for the estimation of cerebral blood flow using the Kety-Schmidt technique.

The Kety-Schmidt technique for the measurement of cerebral blood flow (CBF) has been in use for many years, but efficient statistical methods for producing estimates of CBF have received little attention. This paper proposes simple statistical models for this problem and explores their properties using data from a recent study of severe head injury in children. The method, which can readily be implemented on a personal computer, allows the uncertainty in the estimate of CBF to be quantified.

Cerebrovascular Circulation↗
PubMed1999

Valve orifice area alone is an insufficient index of aortic stenosis severity: effects of the proximal and distal geometry on transaortic energy loss.

BACKGROUND AND AIMS OF THE STUDY: Standard measures of hemodynamic severity of aortic valve stenosis vary widely among patients with and without clinical symptoms. Our hypothesis is that valve orifice area alone is not the sole determinant of adverse clinical outcome. Stenotic orifice area ratio is ratio of the cross-sectional stenotic orifice area to the down-stream, ascending aorta cross-sectional area. Determination of workload together with aortic valve orifice area ratio might improve risk stratification among asymptomatic patients with critical aortic stenosis. Accordingly, application of both parameters together might be useful in guiding management decisions in this condition. METHODS: In this study the dependency of transaortic fluid mechanical energy transfer (one component of left ventricular workload) on aortic valve orifice area is shown using modeling and experimental techniques. RESULTS: For a stroke volume of 62 ml at a heart rate of 60 beats/min, the piston work (analogous to left ventricular work) increased by 17% as the stenotic orifice area ratio decreased from 0.60 to 0.25, by 35% as the ratio fell from 0.25 to 0.20, and by 73% as the ratio fell from 0.20 to 0.10. CONCLUSIONS: As predicted by the fundamental fluid mechanical theory, simulated left ventricular work and energy loss in aortic stenosis are influenced not only by the effective stenotic valve orifice area, but also by the geometry of the inflow and outflow conduits, proximal and distal to the valve. These findings might explain clinically observed discrepancies between valve orifice area and the onset of the classical symptoms of severe aortic stenosis that reflect the left ventricular workload. Consideration of the left ventricular work in addition to the effective valve orifice area should enhance clinical evaluation, prognostication and risk stratification among patients with severe aortic stenosis.

Aorta↗
PubMed1999

Papillary muscle misalignment causes multiple mitral regurgitant jets: an ambiguous mechanism for functional mitral regurgitation.

BACKGROUND AND AIMS OF THE STUDY: The study aim was to test the hypothesis that asymmetric alignment (misalignment) of the papillary muscles is sufficient to cause incomplete mitral leaflet coaptation and functional mitral regurgitation (MR). METHODS: Different spatial relationships between the papillary muscles and the mitral annulus were investigated in isolated porcine mitral valves in vitro to assess the impact on mitral valve competence. The systolic occlusional leaflet area (OLA) needed to cover the mitral orifice and the anterolateral (ACOM) and posteromedial (PCOM) commissural portion (OLA(ACOM), OLA(PCOM)) were assessed by 2D echocardiography to quantitate incomplete mitral leaflet coaptation. The regurgitant fraction (RF) and MR jet location were assessed by a flow meter and color Doppler ultrasound. RESULTS: Posterolateral dislocation of the posteromedial papillary muscle impaired mitral leaflet coaptation at the corresponding half-portion of the mitral orifice (OLA(PCOM): 351-397 mm2 versus 296 mm2 (normal); p < 0.001) and modified the contralateral part (OLA(ACOM): 354-387 mm2 versus 304 mm2 (normal); p <0.001). The mitral leaflet coaptation line moved in apical and posterior directions, creating a commissural MR orifice at the PCOM side. At the ACOM side, anterior leaflet prolapse and restricted posterior leaflet mobility created an additional commissural regurgitant jet (RF = 0.11-0.13). Symmetrical papillary muscle misalignment restricted mitral leaflet mobility on both sides of the orifice in a synergistic manner (OLA(PCOM): 416-459 mm2 and OLA(ACOM): 427-489 mm2; both p <0.001 versus normal). The central MR jet orifice, which extended towards both commissures, caused more significant MR (RF = 0.15-0.26). CONCLUSIONS: Papillary muscle misalignment caused mitral regurgitant jet ambiguity with an anterior MR jet location following posteromedial papillary muscle displacement. These findings may improve understanding of the relation between myocardial lesion and mitral regurgitant jet location and thereby facilitate rational strategies for valvular interventions.

Animals↗
PubMed1999

Temporal properties of the hemodynamic response in functional MRI.

Today, most studies of cognitive processes using functional magnetic resonance imaging (fMRI) adopt an event-related experimental design. Highly flexible stimulation settings require new statistical models where not only the activation amount, but also the time course of the measured hemodynamic response is analyzed. It is possible to obtain statistically valid descriptions of single hemodynamic responses from a robust nonlinear estimation procedure. Focus is placed on the temporal behaviour of the hemodynamic response: relative temporal order, changes induced by modification of the experimental context, and interindividual differences. Example analyses from recent fMRI studies underline the usefulness of this approach.

Adult↗
PubMed2000

[Computer simulation programs as an alternative for classical nerve, muscle and heart experiments using frog tissues].

Courses in Physiology include different methodical approaches such as exercises with living animals, experiments using organs or tissues from killed or slaughtered animals, application of diagnostic techniques in humans and theoretical seminars. In addition to these classical approaches computer programs for multimedia simulation of nerve, muscle and heart physiology are now a regular component of courses in Physiology at the School of Veterinary Medicine in Hannover. It is the aim of the present paper to give the first experiences about these new components.

Animal Testing Alternatives↗
PubMed2000

Can induction of systemic hypotension help prevent nidus rupture complicating arteriovenous malformation embolization?: analysis of underlying mechanism achieved using a theoretical model.

BACKGROUND AND PURPOSE: Nidus rupture is a serious complication of intracranial arteriovenous malformation (AVM) embolotherapy, but its pathogenetic mechanisms are not well described. An AVM model based on electrical network analysis was used to investigate theoretically the potential role of hemodynamic perturbations for elevating the risk of nidus vessel rupture (Rrupt) after simulated AVM embolotherapy, and to assess the potential benefit of systemic hypotension for preventing rupture. METHODS: Five separate hypothetical mechanisms for nidus hemorrhage were studied: 1) intranidal rerouting of blood pressure; 2) extranidal rerouting of blood pressure; 3) occlusion of draining veins with glue; 4) delayed thrombosis of draining veins; and 5) excessively high injection pressures proximal to the nidus. Simulated occlusion of vessels or elevated injection pressures were implemented into the AVM model, and electrical circuit analysis revealed the consequent changes in intranidal flow, pressure, and Rrupt for the nidus vessels. An expression for Rrupt was derived based on the functional distribution of the critical radii of component vessels. If AVM rupture was observed (Rrupt > or = 100%) at systemic normotension (mean pressure [P] = 74 mm Hg), the theoretical embolization was repeated under systemic hypotension (minor P = 70 mm Hg, moderate P = 50 mm Hg, or profound P = 25 mm Hg) to assess the potential benefit of this maneuver in reducing hemorrhage rates. RESULTS: All five pathogenetic mechanisms under investigation were able to produce rupture of AVMs during or after embolotherapy. These different mechanisms had in common the capability of generating surges in intranidal hemodynamic parameters resulting in nidus vessel rupture. The theoretical induction of systemic hypotension during and after treatment was shown to be of significant benefit in attenuating these surges and reducing Rrupt to safer levels below 100%. CONCLUSION: The induction of systemic hypotension during and after AVM embolization would appear theoretically to be of potential use in preventing iatrogenic nidus hemorrhage. The described AVM model should serve as a useful research tool for further theoretical investigations of AVM embolotherapy and its hemodynamic sequelae.

Blood Pressure↗
PubMed2000

Physiological basis of flow dependence of Gorlin formula valve area in aortic stenosis: analysis using an hydraulic model of pulsatile flow.

BACKGROUND AND AIM OF THE STUDY: The study aim was to clarify the basis of the cardiac output dependence of aortic valve area calculated with the Gorlin formula which has been reported in patients with aortic stenosis. Clinical and experimental studies which have attempted to differentiate between a change in physical orifice area, versus a defect in the Gorlin formula as the cause of cardiac output related variations in Gorlin valve area in aortic stenosis have yielded conflicting results. METHODS: We employed a numerical model of pulsatile flow in which the total instantaneous transvalvular gradient was the sum of the convective and viscous pressure losses and pressure recovery beyond the stenosis. By analogy with other hydraulic devices, viscous losses due to stenosis were modeled by the term KfV(EXP), where V is flow velocity. Kf and EXP were determined for various orifices by adjusting these two parameters to obtain excellent fit between curves of the orifice discharge coefficient based upon the expression KfV(EXP), and empirically measured orifice discharge coefficient curves which have been published in the engineering literature. Mean systolic transvalvular gradient was calculated from the total instantaneous transvalvular gradient values for an assumed jet area, and an assumed systolic time-velocity flow profile. This mean gradient was substituted into the Gorlin equation to find the apparent Gorlin valve area at cardiac outputs varying from 0 to 10 l/min for a range of where V is assumed true areas between 0.5 and 2.0 cm2. RESULTS: For functional valve areas <1.5 cm2, viscous losses resulted in at most a 10-12% fall in apparent Gorlin valve area when cardiac output was decreased from 5 to 2.5 l/min. In addition, maximum viscous losses did not result in a pressure-flow relationship which was closer to linear than to quadratic. which the CONCLUSION: Clinically significant changes in valve area with flow are due to orifice area changes rather than Gorlin formula flow variability. Moreover beyond the Gorlin valve area is preferred over valve 'resistance' for assessing stenosis severity. In low cardiac output states, output should be increased to the normal range before Gorlin valve area is measured.

Aortic Valve↗
PubMedDate not supplied

[Modelling cerebrovascular circulation from the viewpoint of autoregulation mechanisms].

The rationale for cerebral blood flow modelling is to extrapolate the possible effects of the pharmacological agents and significance of different pathologic states. Most authors present linear, statistical models. Such modelling does not take into consideration many important factors. Autoregulation of cerebral circulation is the most important one. It maintains cerebral blood flow on a constant level despite significant changes of systemic pressure. Influence of autoregulation is necessary for evaluation of the different anatomical configurations of the main supplying arteries and the cerebral arterial circle of Willis. This paper presents original, non-linear and dynamic model of the cerebral circulation with factor of autoregulation. Comparison of the data from the modelling and from the clinical observations shows high correlation.

Blood Pressure↗
PubMed2001

The Batista procedure. Theoretical analysis and clinical implications.

BACKGROUND: The Batista procedure leads to dramatic early improvement in left ventricular function in some patients and a worsening in function in others. The theoretical and actual clinical effects of the procedure on early postoperative left ventricular function remain controversial. The purpose of this study is to utilize an appropriate mathematical model to determine the effects of the Batista procedure on stroke volume and myocardial wall stress. Our hypothesis is that the preoperative end-systolic stress (ses) is an important predictor of early postoperative myocardial function after this procedure. A corollary is that an index related to ses may be useful in selecting patients for this procedure. METHODS: An analysis of the Batista procedure is developed, based upon a spherical membrane model of the ventricle. This model shows how ventricular dilatation distorts the systolic and diastolic pressure-volume relations. RESULTS: Dilatation initially improves ventricular performance; but further dilatation, beyond a critical value, produces an unstable state with sharply falling performance. For a ventricle operating significantly beyond the point of critical dilatation, our theoretical results suggest that the Batista procedure not only reduces myocardial stress but may improve stroke volume. The end-systolic stress, ses is an indicator of how close a ventricle is to the critical dilatation point. CONCLUSIONS: There is a theoretical basis for the Batista procedure. Resection of myocardium not only decreases wall stress but may improve stroke volume for sufficiently dilated and depressed ventricles. Patients with markedly elevated end-systolic stress may benefit most from the Batista procedure.

Cardiac Surgical Procedures↗
PubMed2001

Laser Doppler velocimetry and flow visualization studies in the regurgitant leakage flow region of three mechanical mitral valves.

Streak line flow visualization and laser Doppler velocimetry (LDV) were conducted in the regurgitant leakage flow region of 3 mechanical heart valve types: CarboMedics, Medtronic Hall, and St. Jude Medical. Streak line flow visualization identified regions of high regurgitant flow, and LDV measurements were focused on those locations. Maximum regurgitant flow velocities after valve closure ranged from 0.7 to 2.6 m/s, and maximum Reynolds shear stress after valve closure ranged from 450 to 3,600 dyne/cm2. These data indicate that leakage flows can generate turbulent jets with elevated Reynolds stresses even in bileaflet valves.

Blood Flow Velocity↗