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Biomedical subjects

K B Chandran

Publications and source records attributed to K B Chandran.

At least 19 recordsLinked to original sources
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↗
PubMed1989

Effect of systolic flow rate on the prediction of effective prosthetic valve orifice area.

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 the Aaslid and Gabbay formula for the effective orifice area of prosthetic heart valves. Pressure gradient, flow, and valve opening area measurements were performed on four 27 mm valve prostheses (two mechanical bileaflet designs, St. Jude and Edwards-Duromedics, an Edwards pericardial tissue valve, and a trileaflet polyurethane valve) each mounted in the aortic position of an in vitro pulse duplicator. With the known valve orifice area, a different discharge coefficient was computed for each of the four valves and 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. All discharge coefficients were observed to increase with increasing systolic flow rate. An empirical relationship of discharge coefficient as a linear function of systolic flow rate was determined through a regression analysis, with a different relationship for each valve and 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.

Blood Flow Velocity↗
PubMed1989

Effect of prosthetic mitral valve geometry and orientation on flow dynamics in a model human left ventricle.

Pulsatile flow dynamics through bileaflet (St Jude and Duromedics), tilting disc (Bjork-Shiley and Omniscience), caged ball (Starr-Edwards), pericardial (Edwards) and porcine (Carpentier-Edwards) mitral valves in a model human left ventricle (LV) were studied. The model human ventricle, obtained from an in situ diastolic casting, was incorporated into a mock circulatory system. Measurements were made at various heart rates and flow rates. These included the transvalvular pressure drop and regurgitation in percent and cm3 beat-1. The effect of valve geometry and the orientation of the valve with respect to the valve annulus was analyzed using a flow visualization technique. Qualitative flow visualization study indicates certain preferred orientations for the tilting disc and bileaflet valve prostheses in order to obtain a smooth washout of flow in the LV chamber.

Heart↗
PubMed1989

Experimental fluid dynamics of aortic stenosis in a model of the human aorta.

Aortic stenosis has been modelled in an in vitro, pulsatile mock circulatory system (MCS) using a porcine valvular prosthesis, and studied with a laser Doppler anemometer (LDA). The MCS incorporated an acrylic model of the human aorta made from a cadaveric casting in situ. A Carpentier-Edwards aortic valve prosthesis was placed in the MCS after being rendered stenotic by suturing of the valve cusps. Flow velocity profiles across the lumen of the aorta in the presence of aortic stenosis were determined using LDA at two preselected sites in the ascending aorta, and at one preselected site in the brachiocephalic artery. Results indicate that a strong systolic jet bordered by transient vortices with intensely reversed flows is produced distal to severely stenotic aortic valves, becoming less intense with a lesser degree of stenosis. Peak fluid velocities in the systolic jet were determined by LDA at distances of 2.6 and 5.6 cm from the valve inlet for a mean flow rate of 5.2 l min-1. Peak systolic pressure gradients and peak turbulent axial stresses were also determined and found to increase dramatically with stenosis. Furthermore, increasing degrees of stenosis also resulted in more severely disturbed flows in the brachiocephalic artery. Peak fluid velocities and their associated turbulent axial stresses in the systolic jets produced by aortic valvular stenosis are remarkably sensitive to even small changes in the calculated valve orifice areas, and can therefore be very useful in assessing the severity and progression of valvular disease. In addition, increasing degrees of aortic stenosis cause more turbulence to be transported into the brachiocephalic artery.

Aorta↗
PubMed1989

In vitro comparison of velocity profiles and turbulent shear distal to polyurethane trileaflet and pericardial prosthetic valves.

A comparative study of flow dynamics past biomer trileaflet valves and a pericardial bioprosthetic valve under steady and physiological pulsatile flow conditions in vitro is reported in this paper. The velocity profiles and the turbulent shear stresses distal to the valves were measured using laser Doppler anemometry. The authors' results showed that the velocity profiles distal to the trileaflet valves were similar to that measured distal to the pericardial valve. Higher magnitudes of absolute turbulent shear stresses were measured distal to the synthetic valves in comparison to the pericardial valves. However, when the stresses were nondimensionalized with respect to the orifice diameter at the inlet aspect, the stresses were comparable for all of the three valves. With design modifications to increase the orifice diameter at the inlet aspect of the polyurethane valves, the turbulent stresses distal to the valves can be minimized. Such in vitro studies on the flow dynamics past the polyurethane valves can provide information towards design changes to improve the performance characteristics of these valves. Polyurethane valves with flow characteristics comparable to the pericardial valves can be manufactured relatively inexpensively compared to mechanical or tissue valve prosthesis. Hence, the synthetic valves may be a viable alternative for short-term use in total artificial heart devices as a bridge to transplant.

Bioprosthesis↗
PubMed1989

An in vitro comparative study of St. Jude Medical and Edwards-Duromedics bileaflet valves using laser anemometry.

An in vitro comparative study of St. Jude (SJ) and Edwards-Duromedics (DM) Bileaflet valves was performed under steady and physiological pulsatile flow conditions in an axisymmetric chamber using Laser Doppler Anemometry (LDA). LDA measurements were conducted in two different orientations; in the first orientation, the LDA traverse was perpendicular and, in the second orientation, parallel to the tilt axis of the leaflets. The axial velocities were measured in both orientations at two different locations distal to the valves. The velocity profiles at peak systole show the presence of stronger vortex in the sinus region for flow past SJ valve in the first orientation compared to the DM valve. Velocity profile distal to the SJ valve in second orientation was relatively flat where as for the DM valve, a jet-like flow was present. The differences found in the velocity profiles between the two valves can be attributed to the differences in geometry with thicker leaflets, smaller angle of leaflets opening and the presence of the leaflet curvature for the DM valve. The results obtained in this study do not show any fluid dynamic advantages due to the curved leaflet geometry of the DM valve.

Biomechanical Phenomena↗
PubMed1988

An in vitro experimental comparison of Edwards-Duromedics and St Jude bileaflet heart valve prostheses.

An in vitro comparison of the hydrodynamic characteristics of the Edwards-Duromedics (DM) and St Jude (SJ) bi-leaflet aortic valve prostheses is presented. Aortic valves 27 mm in diameter were mounted in a pulse duplicator simulating physiological pulsatile flow using a glycerol solution as the blood analogue fluid. Mean trans-valvular pressure difference (TPD) in systole, the per cent regurgitation (PCR) and the projected valve orifice area (VOA) were compared at a range of flow rates and heart rates to reflect the functioning of the valve under resting as well as exercise conditions. Our results showed that the TDP for DM (0.99 +/- 0.28 kPa) valve was slightly larger than the SJ (0.81 +/- 0.19 kPa) valve (mean difference of 0.18 +/- 0.26 kPa, P less than 0.05). However, the PCR for the DM (13.6 +/- 7.8) valve was smaller than the SJ (17.9 +/- 9.0) valve (mean difference of 4.3 +/- 4.2%, P less than 0.01). Moreover, VOA for the DM (2.51 +/- 0.10 cm2) valve was smaller than the SJ (3.13 +/- 0.04 cm2) valve (mean difference of 0.63 +/- 0.10 cm3, P less than 0.001).

Aortic Valve↗
PubMed1988

Detection, localization, and quantitation of bioprosthetic mitral valve regurgitation. An in vitro two-dimensional color-Doppler flow-mapping study.

The usefulness of two-dimensional color-Doppler flow-imaging (2D Doppler) in the detection, localization, and quantitation of bioprosthetic mitral valve regurgitation is uncertain. Mitral bioprostheses, before and after the creation of transvalvular (n = 33), paravalvular (n = 17), or combined (n = 23) defects, were mounted in a pulsed duplication system (flow rates, 2.5-6.5 l/min; pulse rate, 70 beats/min). An Aloka 880 2D Doppler system (Japan) was used to image the regurgitant jets in the simulated left atrial chamber, analogous to images obtained with transesophageal echocardiography. Jet area was corrected to an estimate of stroke volume: 2D Doppler measurements were divided by [(valve effective orifice area) X (continuous-wave Doppler-determined mean diastolic filling velocity)]/pulse rate. Regurgitant fraction and regurgitant volume were measured by an electromagnetic flow probe. 2D Doppler correctly identified the presence and location of paravalvular regurgitation. In transvalvular and combined transvalvular-paravalvular defects, there were six incorrect interpretations, all having transvalvular regurgitant volumes less than 4 ml/beat. In the presence of transvalvular regurgitation, jet area, length, and width correlated linearly with regurgitant volume (r = 0.82, 0.80, and 0.68, respectively; p less than 0.0001) and regurgitant fraction (r = 0.62, 0.61, and 0.45, respectively; p less than 0.001). Correlations with regurgitant fraction were improved when 2D Doppler measurements were corrected for stroke volume (r = 0.78, 0.79, and 0.67, respectively; p less than 0.0001). Mitral bioprostheses with transvalvular defects were also studied at varying flow rates (3.2-7.5 l/min) and pulse rates (70, 90, and 110 beats/min). The correlation between jet area and regurgitant volume was improved with a second-order polynomial regression (r = 0.93, p less than 0.0001). Our conclusions are that 1) in this in vitro model analogous to transesophageal imaging, 2D Doppler accurately detects and localizes bioprosthetic mitral valve regurgitation; 2) in transvalvular bioprosthetic mitral valve regurgitation, 2D Doppler measurement of jet area has a curvilinear relation with regurgitant volume, and correlation with regurgitant fraction is improved with correction for stroke volume; and 3) in paravalvular bioprosthetic mitral valve regurgitation, correlations between 2D Doppler measurements and regurgitant volumes are weaker, possibly because of jet eccentricity.

Animals↗
PubMed1987

Finite element analysis of myocardial diastolic function using three-dimensional echocardiographic reconstructions: application of a new method for study of acute ischemia in dogs.

The effect of acute myocardial ischemia on the myocardial elastic modulus has been a matter of controversy. To evaluate this question, diastolic elastic modulus was assessed by finite element analysis of left ventricular geometry using three-dimensional echocardiographic reconstructions and right and left ventricular pressure recordings. Elastic properties were estimated before and after coronary occlusion in 6 open-chest dogs. Elastic modulus values were derived by means of a computer program that determined the global elastic modulus that best predicted the diastolic changes in left ventricular geometry. In the finite element analysis after coronary occlusion, two analyses were performed: one utilizing the control elastic modulus for all segments of the left ventricle and one in which ischemic (dyskinetic) segments were assigned a higher elastic modulus. Results showed that the control elastic modulus was a poor predictor of diastolic left ventricular expansion after coronary occlusion. The finite element analysis in which the ischemic segments were assigned a higher elastic modulus better predicted ischemic diastolic wall motion patterns. Error values (difference between predicted and actual left ventricular segmental diastolic motion) were: control, 1.9 +/- 0.3 mm (mean +/- SD), ischemia, 2.9 +/- 0.5 mm, and 2.2 +/- 0.4 mm using the stiffer elastic modulus for ischemic segments. Error values were significantly higher (p less than 0.05) under ischemic conditions when the control elastic modulus was uniformly applied compared with control and ischemia with dyskinetic segments assigned a higher elastic modulus. From these data, it is concluded that the myocardial diastolic elastic modulus is increased by ischemia and that this approach may allow clinical assessment of intrinsic muscle stiffness.

Animals↗