Cepstrum analysis of reflected pressure waves in stenosed arteries.
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Animals↗
Biomedical subjects
S Einav
Publications and source records attributed to S Einav.
The effect of varying degrees of stenosis on the characteristics of turbulent pulsatile flow through heart valves.
Many problems and complications associated with heart valves are related to the dynamic behavior of the valve and the resultant unsteady flow patterns. An accurate depiction of the spatial and temporal velocity and rms distributions imparts better understanding of flow related valve complications, and may be used as a guideline in valve design. While the generalized correlation between increased turbulence level and the severity of the stenosis is well established, few studies addressed the issue of the intermittent nature of turbulence and its timing in the cardiac cycle, and almost none assessed the effect of a progressive stenosis on the flow characteristics through heart valves. In this experimental work we simulated the type of flow which is present in normal and stenosed valves and conducted a comprehensive investigation of valve hemodynamics, valvular turbulence and morphology under varying degrees of stenosis. The characteristics of valves and stenoses were simulated closely, to achieve the flow conditions that initiate turbulent flow conditions. Laser Doppler anemometry (LDA) measurements were carried out in a pulse duplicator system distal to trileaflet polyurethane prosthetic heart valves, installed at mitral and aortic positions. The effect of the degree of the stenosis was comparatively studied through the structure of the turbulent jets emerging from normal and stenotic heart valves. Maximum turbulence level was achieved during the decelerating phase and correlated to the severity of the stenosis, followed by relaminarization of the flow during the acceleration phase. The intermittent nature of the turbulence emphasized the importance of realizing the timing of the turbulence production and its spatial location for optimizing current valve designs. The plug flow through the normal aortic valve prosthesis was replaced by jet like behavior for a 65% stenosis, with the jet becoming narrower and stronger for a 90% stenosis. The morphology of the velocity and turbulence waveforms was found to be governed by the stenosis geometry and the valve position (aortic, mitral).
Aortic Valve↗
Preliminary experimental evaluation of an inverse source imaging procedure using a decoupled coil detector array in magnetic resonance imaging.
Recently, we have discussed several of the aspects involved in the detector array concept in magnetic resonance imaging where an image is obtained by applying inverse source procedures to the data assembled by an array of coil detectors surrounding the object. In this work we describe an experimental setup, where a detector array was constructed of 9 coils, that gives a coarse resolution of 3 x 3 pixels. By measuring the induced current signals over this array of coils, a relationship is established between the set of signals and the structure of the body under investigation. Through matrix inversion, reconstruction of the original source from the detected signals is possible. In this preliminary experimental setup, we did not use an actual nuclear magnetic resonance signal. Instead, a miniature solenoid was used as a source, to simulate a precessing magnetic moment.
Biomedical Engineering↗
A squeeze flow phenomenon at the closing of a bileaflet mechanical heart valve prosthesis.
In vivo cavitation in cardiovascular flow fields may occur under very unusual circumstances as a localized transient phenomenon which are confined to very small regions in the vicinity of the valve body or leaflet surface. The violent collapse of cavitation bubbles induces local erosion that may lead to structural damage. The fluid mechanical factors that may cause in vivo cavitation inception in mechanical heart valve (MHV) prostheses are investigated. It is established that the closing velocity of the leaflet holds the key to MHV cavitation. During the final phase of valve closing, the fluid mass in the gap space between the closing occluder and the valve's body is squeezed into motion by the rapidly approaching boundaries. The flow pattern created by this motion (termed 'squeeze flow'), is found to be related to the valve geometry, and the impact velocity of the closing leaflet. Given the closing velocity of the leaflet and the geometry of the MHV, computational flow dynamics (CFD) are made to determine the velocity distributions in the gap flow field of a bileaflet MHV in the mitral position. A two dimensional, time dependent model of the gap space show that flow velocity in the gap space can reach values as high as 30 ms-1 in regions near the edge of the inflow surface of the Edwards Duromedics (ED) MHV leaflet. This high speed stream ejected from the gap channel can create the conditions that characterize cavitation. The location of the isolated high speed region corresponds to the surface erosion that was observed in a number of damaged ED-MHV explants.
Acceleration↗
Transition to turbulence in pulsatile flow through heart valves--a modified stability approach.
The presence of turbulence in the cardiovascular system is generally an indication of some type of abnormality. Most cardiologists agree that turbulence near a valve indicates either valvular stenosis or regurgitation, depending on the phase of its occurrence during the cardiac cycle. As no satisfying analytical solutions of the stability of turbulent pulsatile flow exist, accurate, unbiased flow stability criteria are needed for the identification of turbulence initiation. The traditional approach uses a stability diagram based upon the stability of a plane Stokes layer where alpha (the Womersley parameter) is defined by the fundamental heart rate. We suggest a modified approach that involves the decomposition of alpha into its frequency components, where alpha is derived from the preferred modes induced on the flow by interaction between flow pulsation and the valve. Transition to turbulence in pulsatile flow through heart values was investigated in a pulse duplicator system using three polymer aortic valve models representing a normal aortic valve, a 65 percent stenosed valve and a 90 percent severely stenosed valve, and two mitral valve models representing a normal mitral valve and a 65 percent stenosed valve. Valve characteristics were closely simulated as to mimic the conditions that alter flow stability and initiate turbulent flow conditions. Valvular velocity waveforms were measured by laser Doppler anemometry (LDA). Spectral analysis was performed on velocity signals at selected spatial and temporal points to produce the power density spectra, in which the preferred frequency modes were identified. The spectra obtained during the rapid closure stage of the valves were found to be governed by the stenosis geometry. A shift toward higher dominant frequencies was correlated with the severity of the stenosis. According to the modified approach, stability of the flow is represented by a cluster of points, each corresponding to a specific dominant mode apparent in the flow. In order to compare our results with those obtained by the traditional approach, the cluster of points was averaged to collapse into a single point that represents the flow stability. The comparison demonstrates the bias of the traditional stability diagram that leads to unreliable stability criteria. Our approach derives the stability information from measured flow phenomena known to initiate flow instabilities. It differentiates between stabilizing and destabilizing modes and depicts an unbiased and explicit stability diagram of the flow, thus offering a more reliable stability criteria.
Aortic Valve↗
Analysis of air flow patterns in the human nose.
The nasal cavity is the main passage for air flow between the ambient atmosphere and the lungs. A preliminary requisite for any investigation of the mechanisms of each of its main physiological functions, such as filtration, air-conditioning and olfaction, is a basic knowledge of the air-flow pattern in this cavity. However, its complex three-dimensional structure and inaccessibility has traditionally prevented a detailed examination of internal in vivo or in vitro airflow patterns. To gain more insight into the flow pattern in inaccessible regions of the nasal cavity we have conducted a mathematical simulation of asymmetric airflow patterns through the nose. Development of a nose-like model, which resembles the complex structure of the nasal cavity, has allowed for a detailed analysis of various boundary conditions and structural parameters. The coronal and sagittal cross-sections of the cavity were modeled as trapezoids. The inferior and middle turbinates were represented by curved plates that emerge from the lateral walls. The airflow was considered to be incompressible, steady and laminar. Numerical computations show that the main air flux is along the cavity floor, while the turbinate structures direct the flow in an anterior-posterior direction. The presence of the turbinates and the trapezoidal shape of the cavity force more air flux towards the olfactory organs at the top of the cavity.
Humans↗
An experimental study of pulsatile pipe flow in the transition range.
The study of pulsatile flows is relevant to many areas of applications. Typical applications include aerodynamics, biofluid mechanics, wind flows, and gas transport. Transition to turbulence during pulsatile flow is physiologically and clinically important. It has been suggested as a possible mechanism to enhance the transport of gases during high-frequency ventilation, may be related to valvular regurgitation and heart murmurs and to post stenotic dilatation and aneurysms. Measurements in a pulsatile pipe flow with a superimposed mean flow are reported. Data were taken in a water flow with mean Reynolds numbers in the range of 0 < Re(m) < 3000, oscillating Reynolds numbers of 0 < Re omega < 4000, and Stokes parameter 7 < lambda < 15. Velocity profiles of various phases of the flow, condition for flow reversal, and pressure losses were measured. The adequacy of a quasi-steady-state model is discussed. Condition for transition is determined by visually inspecting velocity signals at the centerline.
Biomechanical Phenomena↗
Numerical model of blood flow through a bifurcation: interaction between an artery and a small prosthesis.
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Arteries↗
Real-time abdominal fetal ECG recording using a hardware correlator.
A real-time fetal ECG monitoring system using abdominal recording is presented. The system is based on an IBM AT compatible personal computer. The computer lacks the performance required for real-time analysis. Therefore, a new design of a fast hardware correlator board was developed to enhance the computer throughput. The technique is based on a cross-correlation procedure. An averaged maternal ECG waveform is derived using the cross-correlation function for the waveform's alignment. With this procedure a template signal corresponding to one complete maternal ECG is obtained. The averaged maternal ECG is then subtracted from the abdominal signals. Thus, it is possible to detect all the fetal ORS complexes in spite of their coincidence with the maternal ECG. An average fetal ECG is then extracted to improve the signal-to-noise ratio, making it possible to recognize fetal P and T waves.
Algorithms↗
Pulse transmission and impedance characteristics of a non-uniform circulatory model.
In recent years, the technique of non-uniform transmission lines has been utilized in the synthesis of lines feeding antennae from transmitters, thereby maintaining matching over a wide band of frequencies and with varying load. For this reason, an electrical model based on that technique stems from the resemblance between the two systems: in both the feeding medium is non-uniform, and it is necessary to maintain a good response over a wide range of frequencies. Our non-uniform transmission line model of the arterial system, introduced in an earlier publication, supplies comprehensive answers to many questions dealing with the research into this system. Whereas in our earlier model the ohmic resistance R was considered small and not considered in the calculations, it is included in the present study. We have calculated the variation of the input impedance with the frequency and distance from the source, the effect of occluded main branches, augmentation of the pressure wave, the relationship between body size and heart rate and the matching of impedances at large bifurcations. We found that our calculated results agree very well with the quantitative results measured by other investigators in the field.
Blood Circulation↗
Calculation of the mutual induction between coplanar circular surface coils in magnetic resonance imaging.
The induced current in a coil due to the presence of alternating currents in an adjacent coil is calculated here for a case of two similar and coplanar, circular coils. By using an overlapping configuration, the mutual inductance is nullified as is predicted theoretically and confirmed experimentally.
Image Enhancement↗
Hemodynamic aspects of obliterative processes in peripheral blood vessel--rigid and soft narrowing.
Hemodynamic aspects of obliterative processes in peripheral blood vessels were studied on a mechanical model built of distensible tubing, with a fixed peripheral resistance, through which citrated blood was circulated by pulsatile flow. Hemodynamics of progressive focal stenosis, elongated soft stenosis, and elongated rigid stenosis were assessed. By the use of a hydrodynamic model and a series of in vitro experiments, we have measured the pressure and flow characteristics, and calculated the pressure and energy losses for the various stenotic sites. The critical stricture was found to be larger for a rigid stenosis than a soft stenosis. The length of the stenosis was also an important factor. Increasing the length of a rigid stenosis, for example, by 50 percent resulted in an increase of 25 percent in the flow through the stenosis. The energy dissipation was determined as a preferred indication for several parameters such as: pressure drop, pulsed flow, pulse rate, and the geometry and mechanical properties of the stenosis.
Arterial Occlusive Diseases↗
Steady flow through collapsible tubes: measurements of flow and geometry.
Compliant tubes attain a complex three-dimensional geometry when the external pressure exceeds the internal pressure and the tube is partially collapsed. A new technique for remote measurement of dynamic surfaces was applied to classical experiments with collapsible tubes. This work presents measurements of the three-dimensional structure of the tube as well as pressure and flow measurements during static loading and during steady-state fluid flow. Results are shown for two tubes of the same material and internal diameter but with different wall thicknesses. The measured tube laws compare well with previously published data and suggest the possible existence of a similarity tube law. The steady flow measurements did not compare well with the one-dimensional theoretical predictions.
Compliance↗
Numerical schemes for unsteady fluid flow through collapsible tubes.
The study of fluid flow through compliant tubes is a fluid-structure type problem, in which a dynamic equilibrium is maintained between the fluid and the tube wall. The analogy between this flow and gas dynamics initiated the use of a number of numerical methods which were originally developed to solve compressible flow in rigid ducts. In this study we investigate the solutions obtained by applying the Lax-Wendroff and MacCormack schemes to one-dimensional incompressible flow through a straight collapsible tube. The time-evolving numerical results were compared with exact steady-state solutions. For boundary conditions which were held fixed after a prescribed rise time, the unsteady numerical solution converges to the exact steady-state solution with very good accuracy. The stability and accuracy of all the methods depend on the amount of viscous pressure loss dictated by wall friction. Flows with undamped oscillations cannot, however, be solved with these techniques.
Elasticity↗
Shear stress related blood damage along the cusp of a tri-leaflet prosthetic valve.
Blood flowing through a prosthetic heart valve can be damaged by flow-induced shear forces. Fluid dynamics variables and geometric factors play an important role in the evaluation of shear-stress-related blood damage. Central-flow prosthetic valves have been considered as an optimal replacement for mechanical and biological valves. Recently it was shown that shear stress distribution along the surface of a polyurethane cusp reaches values that can damage the blood elements. A mathematical model correlating the effects of shear stresses on blood corpuscles with clinical findings was employed in vitro. The model can be applied to the effects of blood-surface interaction and is of clinical relevance.
Blood Cells↗
Wall shear stress distribution along the cusp of a tri-leaflet prosthetic valve.
High levels of wall shear stress on the surface of valvular cusps can cause mechanical damage to the blood cells and the cusp surfaces. The shear stresses are also responsible for mechanical failure of prosthetic heart valves. Qualitative measurements of wall shear stress in the vicinity of the leaflets are thus essential for diagnosis of suspected complications and provide important information for the design and fabrication of bioprosthetic heart valves. For this purpose we measured the velocity distribution along the inside wall of the cusps of a tri-leaflet heart valve with a two colour laser Doppler anemometer system. The wall shear stresses on the cusp surface were computed and found to range from 80 to 120 N/m2 during the ejection phase. Wall shear stresses of up to 180 N/m2 were measured in loci of cusp flexure and the accelerated boundary layer. The results of this study show a correlation between the high shear stress loci and the clinically (animal) observed regions of cusp calcification.
Blood Flow Velocity↗
Simulation of airway closure during forced vital capacity.
Airway closure, which disconnects peripheral respiratory units from the trachea, has been observed during expiration to residual volume. It is attributed to dynamic compression that may cause unstable collapse and closure of small airways. During forced vital maneuvers, airway closure is expected to be more significant owing to the maximum expiratory effort. In the present study we have added a simulation of airway closure to the model developed by Elad and associates which simulated flow limitation during forced expiration. Progressive closure is simulated by variation in the number of branches and their cross-sectional areas rather than by change in tube law. The results demonstrate that peripheral airway closure may explain the reduction in maximal flow rate at small lung volumes. It can reproduce either the abrupt fall in maximal expiratory flow-volume curves as observed in dog lungs or the gradual decrease that has been observed in humans.
Airway Resistance↗
Fringe mode reflectance laser Doppler microscope system.
A reflectance fringe mode Laser Doppler microscope anemometer (LDMA) has been designed and tested. A novel beam splitter permits adjustment of the direction and separation of the two laser beams which form the fringe sensing volume, and furnishes the control needed to compensate for chromatic aberration and variation in optical quality. Tests conducted in a narrow 28:1 aspect rectangular flow channel filled with water showed close agreement between the predicted and measured velocity profiles. Velocity measurements, made alternately in the reflectance and transmittance modes using the same sensing volume in the channel, revealed deviations between the modes of less than 1.1% for water and 2.3% for whole blood. The deviation for the in vivo experiment was better than 2.8%. Velocity profiles were measured alternately in the same sensing volume, (dimensions were 10 x 12 microns) by reflectance and transmittance LDMA in 40 hamster cheek-pouch arterioles, ranging from 65 to 140 microns in diameter. These showed close agreement throughout the range of measured vessels. Reflectance LDMA makes non-invasive, high resolution velocity measurements in the range of 0.03 to 40 cm/s possible in small blood vessels near the surface of opaque organs and tissues. The accuracy and reproducibility of the readings were calculated to be better than 1.1%. Reflectance fringe mode LDMA greatly extends the applicability of LDMA because sites where transmittance LDMA can be applied are limited.
Animals↗