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Biomedical subjects

K B Chandran

Publications and source records attributed to K B Chandran.

At least 37 records · Page 2Linked to original sources
PubMed1995

In vivo assessment of nonlinear myocardial deformation using finite element analysis and three-dimensional echocardiographic reconstruction.

In vitro data have shown that the myocardium exhibits nonlinear passive stress-strain relationship and a non-linear pressure-volume relationship. A finite element (FE) analysis and optimization algorithm was used on three-dimensional reconstructed left ventricular (LV) geometry using echocardiographic images, along with hemodynamic measurements, in seven closed-chest dogs to show a nonlinear stress-strain relationship in vivo. Our analysis included the computation of Poisson's ratio from the measured volumetric strain of the myocardium and a simulated pericardial pressure load ("equivalent pericardial pressure") applied to the epicardial surface of the reconstructed LV. LV geometry was reconstructed in three or four incremental time steps in diastasis and the myocardium was assumed to be homogeneous, isotropic, and linearly elastic during these short intervals in this initial study. Simultaneous LV chamber pressure and equivalent pericardial pressure were incorporated into the algorithm to predict actual LV expansion. Computations were performed iteratively at each interval to compute the optimized elastic modulus. By performing the FE analysis and optimization at each interval (a step-wise linear analysis approach), a linear relationship between the myocardial elastic modulus and LV chamber pressure was derived (r = .87 to .98). Such a linear relationship is equivalent to an exponential myocardial stress-strain relationship in vivo. Detailed measurement of nonhomogeneous regional deformation are becoming possible with the advent of sophisticated imaging techniques. The methodology described in this study, with appropriate modifications in the FE analysis and optimization algorithms, can be applied to assess the complex three-dimensional pressure-deformation characteristics in vivo.

Algorithms↗
PubMed1994

A method for automatic edge detection and volume computation of the left ventricle from ultrafast computed tomographic images.

RATIONALE AND OBJECTIVES: Detection of endocardial and epicardial borders of the left ventricle (LV) using various imaging modalities is time-consuming and prone to interpretive error. An automatic border detection algorithm is presented that is used with ultrafast computed tomographic images of the heart to compute cavity volumes. METHODS: The basal-level slice is identified, and the algorithm automatically detects the endocardial and epicardial borders of images from the basal to the apical levels. From these, the ventricular areas and chamber volumes are computed. The algorithm uses the Fuzzy Hough Transform, region-growing schemes, and optimal border-detection techniques. The cross-sectional areas and the chamber volumes computed with this technique were compared with those from manually traced images using canine hearts in vitro (n = 8) and studies in clinical patients (n = 27). RESULTS: Though the correlation was good (r = .88), the algorithm overestimated the LV epicardial area by 4.8 +/- 6.4 cm2, though this error was not statistically different from zero (P > .05). There was no difference in endocardial areas (r = .95, P > .05). The algorithm tended to underestimate the end-diastolic volume (r = .94) and the end-systolic volume (r = .94), although these errors were not statistically different from zero (P > .05). The algorithm tended to underestimate the ejection fraction (r = .80), although this error was not statistically different from zero (P > .05). CONCLUSIONS: Automatic detection of myocardial borders provides the clinician with a useful tool for calculating chamber volumes and ejection fractions. The algorithm, with the corrections suggested, provides an accurate estimation of areas and volumes. This algorithm may be useful for contour border identification with ultrasound, positron-emission tomography, magnetic resonance imaging, and other imaging modalities in the heart, as well as other structures.

Algorithms↗
PubMed1994

Cavitation dynamics of mechanical heart valve prostheses.

Nine different mechanical mitral heart valves were chosen in order to study cavitation dynamics in detail in an in vitro flow system simulating a single event of mitral valve closure. The transvalvular pressure (ventricular minus atrial pressure) rise rate averaged during the valve closing period was used as an index of the loading rate. A series of photographs in the vicinity of the inflow surface of the valve were attempted during the bubble appearance period using a stroboscopic visualization technique. The in vitro study revealed three sources of cavitation initiation in the design of the mechanical heart valves tested: stop (seat stop or seating lip), inflow strut, and clearance (gap formed between the occluder and the housing or between the two occluders in the closed position). Among these, the occluder stop design was the most critical to cavitation since all valves having the stop at the edge of the major orifice area showed a higher intensity of cavitation and threshold loading rates below the estimated normal physiological value. The analysis of bubble locations and dynamics led us to propose that the fluid squeezing effect between the occluder and the stop in the housing and the streamline contraction effect along the clearance are factors responsible for cavitation incipience.

Heart Valve Prosthesis↗
PubMed1994

Transient behavior analysis of a mechanical monoleaflet heart valve prosthesis in the closing phase.

In this paper, an analysis of the dynamics in the closing phase of the occluder of a mechanical monoleaflet heart valve prosthesis is presented. The dynamic analysis of the fluid in the vicinity of the occluder was based on the control volume approach. The backflow velocity of the fluid was computed by applying the continuity and momentum equations in the unsteady state. By considering the fluid pressure and gravity as external forces acting on the occluder, the moment equilibrium on the occluder was employed to analyze the motion of the occluder during closing and the force of impact between the occluder and the guiding struts. The computed magnitudes of the occluder tip velocities, as well as the backflow of the fluid during the closing phase using this model, were in agreement with previously reported experimental measurements. The maximum impact force between the occluder and guiding struts of 140-280 N was determined to occur during the initial impact for a duration of 35-45 microseconds. The results of such model studies may be extended for the analysis of the endurance limit of the valve prostheses as well as to determine the mechanical stresses on the formed elements and the incipience of cavitation bubbles during the closing phase of the valve function.

Biomechanical Phenomena↗
PubMed1994

Pressure field in the vicinity of mechanical valve occluders at the instant of valve closure: correlation with cavitation initiation.

In order to analyse cavitation dynamics during the closing phase of mechanical heart valve prostheses, an experimental study was performed to simulate a single closing event of the occluder with pressure measurements close to the occluder surface on the inflow side at the instant of valve closure. Cavitation bubble visualization was also performed using a stroboscopic lighting technique and photographic recording with tilting disc and bileaflet valve geometries. Large negative pressure transients were measured in the major orifice region of tilting disc valves and in the peripheral edges of bileaflet valves. The intensity of cavitation increased with increasing loading rate (transvalvular pressure rise rate) during the closing phase and with increasing valve size. The squeezing of fluid between the occluder and the seat stop or seating lip in the region where the occluder velocity at closure is at its maximum, and the resulting local flow dynamics are suggested as mechanisms for initiation of cavitation. The fluid forced through the clearance region due to the instantaneous large pressure difference across the occluder may be another source of large negative pressures and cavitation. The present in vitro study also suggests that cavitation may be the cause of platelet and red blood cell destruction, and ensuing thrombus formation associated with mechanical heart valves.

Blood Pressure↗
PubMed1993

Application of finite-element analysis with optimisation to assess the in vivo non-linear myocardial material properties using echocardiographic imaging.

An application of finite-element analysis with an optimisation technique to assess the myocardial material properties in diastasis in vivo is described. Using the data collected from an animal model, the three-dimensional geometry of the left ventricular chamber, at several times in diastole, was reconstructed. From the measurement of the ventricular chamber pressure during image acquisition, finite-element analysis was performed to predict the expansion during diastasis. Initially, by restricting the motion of the epicardial nodes and computing the reaction forces, an 'equivalent pericardial pressure' was determined and applied in subsequent analysis. The duration of diastasis was divided into three or four intervals and the analysis was performed at each interval to assess the material properties of the myocardium. Using such a step-wise linear approach, the non-linear material properties of the myocardium during passive expansion was determined. Our results demonstrated that the computed 'equivalent pericardial pressure' increased with and was smaller than the corresponding left ventricular chamber pressure. The passive myocardium exhibited a linear tangent modulus against chamber pressure relationship which is equivalent to an exponential stress/strain relationship, similar to those suggested by in vitro studies.

Animals↗
PubMed1993

Dynamic behavior analysis of mechanical monoleaflet heart valve prostheses in the opening phase.

In this paper, fluttering behavior of a mechanical monoleaflet tilting disk heart valve prostheses during the opening phase was analyzed. The impact between the occluder and the guiding strut at the fully open position was included in the analysis with a Bjork-Shiley monoleaflet aortic valve. The motion of the valve occluder was modeled as a rotating system, and equations were derived by employing the moment equilibrium principle. Forces due to lift, drag, gravity, and buoyancy were considered as external forces acting on the occluder. The 4th-order Runge-Kutta method was used to solve the governing equations. The results demonstrated that the occluder reaches the steady equilibrium position only after damped vibration. Fluttering frequency varies as a function of time after opening and is in the range of 8-84 Hz. Valve opening appears to be affected by the orientation of the valve relative to gravitational force. The opening velocities are in the range of 0.56-1.37 m/sec and the dynamic loads by impact of the occluder and the strut are in the range of 60-190 N.

Aortic Valve↗
PubMed1993

Flow dynamics in the human aorta.

The aorta is the major blood vessel transporting blood pumped by the left ventricle to the systemic circulation. The tricuspid aortic value at the root of the aorta provides a centralized flow with nearly uniform velocity profile into the ascending aorta. The aorta consisting of the ascending limb, the aortic arch, and the descending segment is a vessel of complex geometry including curvature in multiple planes, branches and bifurcation as well as taper. The understanding of the development of blood flow in this distensible vessel has been the subject of several theoretical as well as experimental investigations. Flow development in the aorta and in the branch vessels has been of interest in delineating the role of wall shear stresses on the etiology of atherosclerosis. In this paper, a review of the current status on our understanding of the complex flow dynamics in the aorta is presented. With the advent of transesophageal echocardiography and magnetic resonance velocity mapping, further evidence of the presence of secondary flows even in the descending aorta has been reported. The importance of the effect of secondary flow in the descending aorta on the perfusion of distal blood vessels (such as superior mesenteric and renal arterial branches) as well as in the iliac bifurcation is also included in the discussion.

Aorta↗
PubMed1992

A Doppler guided retrograde catheterization system.

A Doppler guided retrograde catheterization system was developed to accurately catheterize the aortic root and left ventricular chamber without X-ray. This system consists of a 20 MHz, 0.076 mm thick x 1.016 mm diameter pulsed Doppler crystal integrated into the tip of a 100 cm multipurpose triple lumen catheter. Two lumens (0.61 mm) are used for electrodes; a third lumen (1.245 mm) may be used for guidewire and pressure determination; and the system is attached to a flow velocimeter. In an aortic arch flow model, the principles of Doppler signal guidance were confirmed with flow toward the catheter tip demonstrating positive signals and flow away from the catheter tip demonstrating negative signals. The magnitude and polarity (direction) of the detected phasic and mean velocities were utilized to guide catheterization in six dogs. Using the reversal of Doppler signal polarity to indicate branch entry and manipulating the catheter so as to maintain maximal positive axial velocity, the Doppler catheter was successfully advanced from the femoral artery to the aortic valve. Branches detected by the Doppler system were confirmed by fluoroscopy. The aortic valve was audible when approached and the left ventricular chamber was recognized by its characteristic pressure waveform. The Doppler guided retrograde catheterization system offers new technology to perform left heart catheterization without X-ray and may prove useful in a variety of settings including the development of invasive ultrasonic diagnostic and therapeutic technology.

Animals↗
PubMed1992

Finite-element analysis of arterial anastomoses with vein, Dacron and PTFE grafts.

A finite-element simulation of an end-to-end artery/graft anastomosis has been presented in this study to evaluate the distribution of compliance and stresses in the vicinity of the anastomosis due to any mismatch in compliance characteristics. The arterial wall was assumed to be made of linear isotropic elastic material in this simplified model and a static analysis was performed with a mean arterial pressure loading of the artery-graft model. Anastomoses to vein grafts and both Dacron and polytetrafluoroethylene (PTFE) grafts were studied. The results suggested the presence of a hypercompliant zone on the arterial side and a region of high tensile stresses in the wall on the graft side of the anastomosis. The presence of a hypercompliant zone has been reported from previous in vivo studies. The hypercompliance was larger with Dacron and PTFE grafts compared with that with the vein graft. However, larger tensile stresses were present in the wall of the vein graft compared with the synthetic grafts. The analysis further showed that increasing the diameter of the graft compared with the host artery to increase flow through the implant will result in a significant increase in the hypercompliance on the arterial side. Such simulation studies may prove valuable in studying the effects of compliance mismatch and suggest ways to improve the design of small diameter vascular grafts.

Anastomosis, Surgical↗
PubMed1992

Numerical simulation of steady flow in a two-dimensional total artificial heart model.

In this paper, a numerical simulation of steady laminar and turbulent flow in a two-dimensional model for the total artificial heart is presented. A trileaflet polyurethane valve was simulated at the outflow orifice while the inflow orifice had a trileaflet or a flap valve. The finite analytic numerical method was employed to obtain solutions to the governing equations in the Cartesian coordinates. The closure for turbulence model was achieved by employing the k-epsilon-E model. The SIMPLER algorithm was used to solve the problem in primitive variables. The numerical solutions of the simulated model show that regions of relative stasis and trapped vortices were smaller within the ventricular chamber with the flap valve at the inflow orifice than that with the trileaflet valve. The predicted Reynolds stresses distal to the inflow valve within the ventricular chamber were also found to be smaller with the flap valve than with the trileaflet valve. These results also suggest a correlation between high turbulent stresses and the presence of thrombus in the vicinity of the valves in the total artificial hearts. The computed velocity vectors and turbulent stresses were comparable with previously reported in vitro measurements in artificial heart chambers. Analysis of the numerical solutions suggests that geometries similar to the flap valve (or a tilting disk valve) results in a better flow dynamics within the total artificial heart chamber compared to a trileaflet valve.

Biomechanical Phenomena↗
PubMed1991

Stress distribution on the cusps of a polyurethane trileaflet heart valve prosthesis in the closed position.

In this paper, a finite element analysis of the stress distribution on the cusps of a polyurethane trileaflet heart valve prosthesis in the closed position is presented. The geometry of the valve was modified from a relationship proposed by Ghista and Reul (J. Biomechanics 10, 313-324, 1977). The effects of variations in stent height, leaflet thickness and coaptation area on the stress distribution were also analyzed. Analyses were performed with both rigid and flexible stents for the trileaflet valve in order to delineate the effect of stent flexibility on the leaflet stress distribution. The results showed that regions of stress concentration were present near the commissural attachment similar to those predicted with the bioprostheses. The stresses on the leaflets were reduced by increasing the stent height with both rigid and flexible stents. Selectively increasing the leaflet thickness near the commissures and also increasing the coaptation area did not prove to reduce the leaflet stresses when the stent flexibility was taken into account. The possible effect of high stresses on the structural integrity of polyurethane leaflets and its relationship with calcification is yet to be investigated.

Heart Valve Prosthesis↗
PubMed1991

Velocity and turbulence measurements past mitral valve prostheses in a model left ventricle.

Thrombogenesis and hemolysis have both been linked to the flow dynamics past heart valve prostheses. To learn more about the particular flow dynamics past mitral valve prostheses in the left ventricle under controlled experimental conditions, an in vitro study was performed. The experimental methods included velocity and turbulent shear stress measurements past caged-ball, tilting disc, bileaflet, and polyurethane trileaflet mitral valves in an acrylic rigid model of the left ventricle using laser Doppler anemometry. The results indicate that all four prosthetic heart valves studied create at least mildly disturbed flow fields. The effect of the left ventricular geometry on the flow development is to produce a stabilizing vortex which engulfs the entire left ventricular cavity, depending on the orientation of the valve. The measured turbulent shear stress magnitudes for all four valves did not exceed the reported value for hemolytic damage. However, the measured turbulent shear stresses were near or exceeded the critical shear stress reported in the literature for platelet lysis, a known precursor to thrombus formation.

Blood Flow Velocity↗
PubMed1991

Quantitative shape descriptors of left-ventricular cine-CT images.

In order to assess regional diastolic function of the left ventricle (LV), we use LV cine-CT images to build finite element models. To quantitatively evaluate the accuracy of our geometric reconstruction technique used in building these models, we introduce a new measure of shape-similarity, and compare and contrast the results obtained from the new measure to the results obtained from traditional shape-similarity measures. All of these measures are used to compare the endocardial and epicardial LV contours obtained from cine-CT images of canine hearts with those obtained from video images of the same hearts. Our results show that our imaging procedure accurately reproduces shape, and further suggest that the new descriptor has the sensitivity and resolution required to distinguish between images separated by as little as 3 mm.

Animals↗
PubMed1991

In vitro hemodynamic analysis of flexible artificial ventricles.

An in vitro fluid dynamic study was performed to compare the hemodynamic characteristics of a rigid and a flexible total artificial heart. The artificial ventricles were incorporated into a mock circulatory system, and pressure signals within the ventricular chamber, proximal to the inflow valve and distal to the outflow valve, were obtained. The instantaneous flow rate through the inflow and outflow valves was measured with electromagnetic flow probes. Flow visualization studies performed on the flexible ventricle suggested a vortical motion within the chamber with a smooth washout of fluid in the next pumping phase, but flow disturbances were observed near the wall of the ventricle as well as near the outflow valve. The rate of pressure increase (dP/dt) was smaller in the flexible ventricle as compared with the rigid ventricle for comparable flows and heart rates. The results of the present study indicated that the flexible ventricle with polyurethane valves, having the advantage of ease of implantation and cost savings, can be a viable alternative as a bridge to transplant.

Heart, Artificial↗
PubMed1990

Functional chiral asymmetry in descending thoracic aorta.

To determine whether rotational blood flow or chiral asymmetry exists in the human descending thoracic aorta, we established the ability of color Doppler ultrasound to detect rotational flow in a tornado tube model of a vortex descending fluid column. In a model of the human aortic arch with a pulse duplicator, color Doppler was then used to demonstrate that rotational flow occurs first in the transverse arch and then in the proximal descending thoracic aorta. With the use of color Doppler esophageal echocardiography, 53 patients (age range, 25-78 years; mean age, 56.4 years) were prospectively examined for rotational flow in the descending thoracic aorta. At 10 cm superior to retro-left ventricular position, 22 of 38 patients (58%) revealed rotational flow with obvious diastolic counterclockwise rotation but less obvious systolic clockwise rotation. At 5 cm superior to retro-left ventricular position, 29 of 46 patients (63%) revealed rotational flow with a tendency toward systolic clockwise and diastolic counterclockwise rotation. At the retro-left ventricular position, 47 of 53 patients (89%) revealed rotational flow, usually of a clockwise direction, occurring in systole. Our data suggest that aortic flow is not purely pulsatile and axial but has a rotational component. Rotational flow begins in the aortic arch and is carried through to the descending thoracic aorta, where flow is chirally asymmetric with systolic clockwise and diastolic counterclockwise components. These data demonstrate an aortic rotational flow component that may have physiological implications for organ perfusion.

Adult↗
PubMed1989

Prediction of stenotic valve orifice area: an in vitro study on a bioprosthesis.

The Gorlin equation for the hemodynamic assessment of valve area is commonly used in cardiac catheterization laboratories. A study was performed to test the prediction capabilities of the Gorlin formula, as well as those of the Aaslid and Gabbay formulas for the effective orifice area of a porcine valve of varying degrees of stenosis. Pressure gradient, flow, and valve opening area measurements were performed on Carpentier-Edwards porcine valve prostheses (made stenotic by suturing at the commissures) mounted in the aortic position of an in vitro pulse duplicator. With the known valve orifice area, a discharge coefficient was computed for each of the three orifice area formulas. After some theoretical considerations, it was proposed that the discharge coefficient would be a function of the flow rate through the valve. The discharge coefficient was observed to increase with increasing systolic flow rate. An empirical relationship of the discharge coefficient as a linear function of the systolic flow rate was determined through a regression analysis, with a different relationship for each orifice area formula. Using this relationship in the orifice area formulas improved the accuracy of the prediction of the effective orifice area with all three formulas performing equally well.

Aortic Valve Stenosis↗