Opening and closure characteristics of different types of stented biological valves.

BACKGROUND: In an increasingly senescent population stented biological valves have regained renewed popularity because of the absence of anticoagulation, while the stented design allows for safe and easier implantation. Constructed bovine pericardial valves as well as valves with porcine cusps are used, both of which exhibit good clinical results although degeneration still appears. While clinical hemodynamic studies did not show particular differences between both valves types, the opening and closure behavior of native cusps and artificially constructed pericardial leaflets is different. It is unclear whether these phenomena account for differences in load and stress which may influence onset and course of degeneration. MATERIAL AND METHODS: Edwards Perimount (EP) and Medtronic Mosaic (MM) heart valves with diameters of 21 mm, 23 mm, and 25 mm were investigated in a pulse duplicator. Movements of the valves were visualized with a high-speed camera (1000 frames/sec). Mean transvalvular gradient (mm Hg), dissipated power (mW), and power transfer by stretching (mW), mean orifice area (mm2), opening time (ms), and closure time (ms) were analyzed in a range of cardiac outputs from 1.4 l/min to 6.3 l/min and 70 beats per minute. RESULTS: Closure times were generally longer than opening times for both valve types. Opening time of EP valves was longer than opening time of the MM valves of the same size (EP23: 31.2 +/- 2.5 ms; MM23: 12.7 +/- 0.1 ms). With respect to closure times, however, there were no marked differences between all valves (EP23: 69.3 +/- 2.0 ms; MM23: 63.2 +/- 6.3 ms). Smaller sized Perimount valves exhibited lower mean transvalvular gradients than Mosaic valves of the same size (EP23: 7.21 +/- 0.07 mm Hg; MM23: 10.5 +/- 0.15 mm Hg). In larger sizes these differences diminished. Power transfer to the valve's structures was significantly enhanced in EP valves (EP23: 134 +/- 1.3 mW; MM23: 64 +/- 0.9 mW). CONCLUSIONS: While valves with constructed pericardium showed lower mean transvalvular gradients, particularly in the smaller sizes, this valve type exhibited alterations of movement performance in contrast to porcine valves. It can be speculated that constant power transfer to the valve's structures may result in an earlier degeneration because of the impact of the increased load and stress on the suspension apparatus of the constructed pericardial leaflets.

Different tilting disc valves show similar rotation-dependent impairment in hemodynamic performance under a tilted implantation position.

BACKGROUND: Aortic annulus calcification can promote tilted implantation of mechanical valves. This study evaluates the hemodynamics of tilting disc valves under this condition. METHODS: 23 mm and 25 mm Ultracor (UC) and Medtronic-Hall-Easy-Fit (MH) valves were investigated in a pulse-duplicator under physiological conditions. Mean pressure gradient (dP(mean)), systolic energy loss (dW(sys)), effective orifice area (EOA), closure (V(Cl)), leakage (V(L)), and total regurgitation volume (V(R)) were assessed. Valves were independently positioned at five axial rotations (0 - 180 degrees , zero defined as major orifice facing the top of the "tilt-ramp") and three tilt angles (0 degrees, 10 degrees, 20 degrees) by lifting the prosthesis in the noncoronary sinus. RESULTS: Diameter-enhanced MH valves exhibited a better systolic performance but a higher regurgitation than corresponding UC valves. Moderate tilting showed a rotation-independent increase in dP(mean) and dW(sys) and a decrease in V (R) and EOA with no fundamental differences between valve types. Further tilting caused small additional changes at 90 - 180 degrees rotation. At 0 degrees rotation, however, dramatic regurgitation occurred throughout. CONCLUSION: Tilting worsened systolic performance regardless of valve type. It should therefore be avoided. Due to extensive regurgitation at 0 degrees rotation, this position should be corrected whenever tilting is inevitable.

[Model fluids of blood for in vitro testing of artificial heart valves]. / Modellfluide für Blut bei der In-vitro-Testung künstlicher Herzklappen.

The increasing development and implantation of artificial organs subject to perfusion with human blood once implanted (grafts, heart-valve prostheses and assist systems) require extensive testing of hydrodynamic performance in mock circulation models. As human blood ist not always available in the necessary quantities, different fluids (water, saline or glycerine solutions) are employed for measurements of flow characteristics. However, these model fluids do not possess the non-Newtonian rheological properties of blood. In addition, they do not allow estimation of possible blood damage. Aqueous solutions of high molecular weight polyacrylamides (PAA) have rheological properties similar to blood, displaying also molecular degradation due to shear stress in the flow. Therefore, they were used as model fluid for blood. Different model solutions were compared to blood with regard to their influence on characteristic flow parameters of mechanical heart valves. Likewise, the shear damage of erythrocytes could be compared to flow-induced polymer degradation. It was shown that PAA solutions in definite concentrations are suitable models for blood, not only in terms of non-Newtonian rheology, but also in terms of estimation of hemolytic potential of artificial heart valves.

Different types of aortic stenosis and simulation of their morphological-hydrodynamic interdependence--in vitro study with allografts and stenotic valve models.

Biological valves display a dependence of valve resistance and valve area on flow and a phase shift between systolic flow through the valves and pressure difference across the valves. The pressure-flow relations of stenosed valves raise questions about the "best measure of stenosis". There is a need for quantitative evaluation of the hydrodynamic performance of homografts and allografts. In the present paper, we report on in vitro studies of the hydrodynamic behavior of homografts from human donors, allografts from different animal species as well as three valve models. Valve model I was designed to simulate flow-dependence of valve area, valve model II was designed to simulate restricted valve opening independent of flow, and valve model III was designed to simulate a flow-dependent movement of valve root in flow direction. Among other aspects, the effect of increased viscosity of the test fluid on the pressure difference and the effects of water absorption by valve tissue on valve characteristics were investigated. The results of the present studies clearly indicate that any biological valve may be modelled as a serial connection of a model I type valve and a model II type valve. From the results, the dependence of the characteristic pressure-flow relationship of a valve on valve size and valve distensibility can be clearly seen and the clinical significance of the characteristic coefficients of the pressure-flow relationship of a valve can be elucidated. Further, it was shown that the characteristic phase shift between flow and pressure difference displayed by biological valves is due to their movable valve plane similar to that of valve model III.

[Characterization of mechanical in vitro hemolysis and sub-hemolysis. 2: Variables of state and dimensionless characteristic values of hemolysis]. / Charakterisierung der mechanischen In-vitro-Hämolyse und -Subhämolyse. Teil 1: Zustandsgrössen und dimensionslose Kennzahlen der Hämolyse.

Blood damaging effects of artificial perfusion devices such as assist devices, heart valve prostheses, for example, must be evaluated in vitro before being used in the clinical setting. For this purpose, mainly animal blood has been used, and a number of associated problems are currently being discussed. Differences in the use of the term hemolysis--meaning breakdown of erythrocytes or increased plasma hemoglobin, result in incompatibility among different authors. In addition, subhemolytic damage and its quantification has not been investigated to any extent. Another problem are the differences in the mechanical fragility of erythrocytes from different animal species, and the question of transferability to the in vivo situation. Furthermore, the variability of mechanical stability within a given species is often greater than the differences between one species and another. International efforts are now being made to standardize haemolytic test conditions and the present study is meant as a contribution to this. In the first part we describe an extension of our LYSE number model. Characteristically, the model uses dimensionless similarity numbers, LY and MY, thus making the results obtained under different test conditions comparable with one another. The LY number reflects the breakdown of cells (decreasing hematocrit), the MY number an increase in plasma hemoglobin. Differences between LY and MY are an indication of subhemolytic events.

Mechanical degradation of polyacrylamide solutions as a model for flow induced blood damage in artificial organs.

Human or animal blood is normally used as a test fluid for the in vitro evaluation of hemolysis by artificial organs. However, blood has some disadvantages (large biological variability and problems with cleaning the devices). For that reason, we searched for a reproducible technical fluid with blood-like flow characteristics that exhibits similar shear depending destruction. In this study, a direct comparison between erythrocyte damage of bovine blood and shear-induced degradation of polyacrylamide solution is given. A uniform shear field was applied to the fluids using a shear device with a plate-plate geometry. It was shown that similarities exist between erythrocytes disaggregation and breakdown of super molecular structures in polymer solutions, caused by mechanical stress. In both cases steady low shear viscositity was diminished and the elastic component of complex viscosity of blood and polymer solutions has been reduced. There is a correlation between shear-induced hemolysis of bovine blood and mechanical polymer-degradation, which depends on the applied shear stresses.

Shear stress related hemolysis and its modelling by mechanical degradation of polymer solutions.

Human or animal blood is normally used as a test fluid for in vitro evaluation of hemolysis by artificial organs. However, blood has some disadvantages (no transparence for visualization of the flow field, large biological variability and problems with cleaning the devices). For that reason, it would be of advantage to have a reproducibile transparent technical fluid with blood like flow characteristics and that exhibits similar shear depending destruction. We have shown that solutions of long-chaining polymer molecules are destroyed by heart valve prostheses in a similar manner as blood corpuscules. In the presented study, a direct comparison between erythrocyte destruction of bovine blood and degradation of Polyacrylamid molecules in a 300 ppm solution is given. A uniform shear field was applied to the fluid using a plate-plate geometry shear device. This device allows a variation of shear stress and shear time in the range of 15-500 N/m2 and 64-516 ms, respectively A correlation between the index of hemolysis (ratio of free plasma hemoglobin to total hemoglobin) and the percentage viscosity decrease (derived from low shear viscosity at a shear rate of 0.01/s) was found. Thus, PAA solutions are suitable model fluids for in vitro estimation of the damaging effects of artificial devices.

In vitro testing of artificial heart valves: comparison between Newtonian and non-Newtonian fluids.

The in vitro testing of artificial heart valves is often performed with simple fluids like glycerol solutions. Blood, however, is a non-Newtonian fluid with a complex viscoelastic behavior, and different flow fields in comparable geometries may result. Therefore, we used different polymer solutions (Polyacrylamid, Xanthan gum) with blood-like rheological properties as well as various Newtonian fluids (water, glycerol solutions) in our heart valve test device. Hydrodynamic parameters of Björk-Shiley heart valves with a tissue annulus diameter (TAD) of 21-29 mm were investigated under aortic flow conditions. Major results can be summarized as follows. The mean systolic pressure differences depend on the model fluids tested. Closing time and closing volume are not influenced by the rheological behavior of fluids. These parameters depend on TAD and the pressure differences across the valve. In contrast, rheological behavior has a pronounced influence upon leakage flow and leakage volume, respectively. Results show furthermore that the apparent viscosity data as a function of shear rate are not sufficient to characterize the rheological fluid behavior relevant to hydrodynamic parameters of the heart valves investigated. Therefore, similarity in the yield curves of non-Newtonian test fluids mimicing blood is only a pre-requisite for a suitable test fluid. More information about the viscous and elastic component of the fluid viscosity is required, especially in geometries where a complex flow field exists as in the case of leakage flow.

[Differentiated evaluation of heart valve stenosis by expanded Bernoulli equation--in vitro studies of model stenoses]. / Differenzierte Bewertung von Herzklappenstenosen durch die erweiterte Bernoulli-Gleichung--in-vitro-Untersuchungen an Modellstenosen.

A suitable measure for the hydrodynamic assessment of heart valve stenoses must be independent of flow and should correspond to the morphology of the stenoses. The "effective orifice area" according to Gorlin does not fulfil this requirement, generally, because it is constant only under special conditions. This suggests the development of a multidimensional stenosis model. The idea for doing so is based on hydrodynamic evaluation of different elementary stenosis types in comparison with a valve that behaves like Gorlin's theory. The Bernoulli equation can than be expanded definitely and one gets a set of unknown stenosis parameters corresponding to the elementary stenoses. The clinical relevance of these must be evaluated by morphological evidence and by similarity of the flow-pressure drop characteristics as compared to real heart valve stenoses. A suitable reference valve is the Björk-Shiley valve. This valve was combined with evident elastic and stiff obstacles to opening with the result of flow-pressure drop characteristics similar to biological valves written in terms of flow q: [formula: see text] where q and q2 are mean flow and mean square flow through the valve, respectively. Empirical results reported in the literature can be explained as special cases of the stenosis model as demonstrated by examples. The proposed equation can be interpreted in physically founded terms in contrast with an empirical one. It gives rise to a differentiated evaluation of heart valve stenosis by orifice area (c2), elastic properties of shape and material (c1) and pre-stress (c0) independent on flow. The model can be extended step by step as required.

[Mechanical hemolysis caused by artificial organs--comparison of in vitro hemolysis studies and their application to in vivo conditions]. / Mechanische Hämolyse durch künstliche Organe--Vergleichbarkeit von In-vitro-Hämolyseuntersuchungen und ihre Ubertragbarkeit auf die Verhältnisse in vivo.

Changes of plasma concentration are often used for in vitro characterisation of the hemolytic potency of artificial organs and apparatus. Different indices of hemolysis are derived from Hb concentration, which, in general, depend on experimental conditions and cannot be compared quantitatively or used to describe the in vivo damage. In this paper we propose a similarity number called "lysis number" that is independent of experimental conditions. It describes the probability for a single blood cell to be completely destroyed in a single pass through the corresponding artificial assist system. The concept is based on the steps: 1. Definition of "lysis number" as an index of hemolytic performance of artificial organs or implants. 2. Description of more complex hemolytic damaging processes (different hemolytic steps) that may be in series or parallel and definition of an effective lysis number. 3. Experimental in vitro estimation of each of the processes in consecutive steps. 4. Calculation of total hemolysis of the complex system using the linkage rules. 5. Application to in vivo by an appropriate differential equation in RBC mas taking into account mechanically-induced hemolysis rate, survival time of normal RBC and erythropoetic generation rate.

[Testing the hydrodynamic properties of heart valve prostheses with a new test apparatus]. / Testung hydrodynamischer Eigenschaften von Herzklappenprothesen mit einem neuen Prüfstand.

A new test apparatus for the testing of artificial heart valves makes it possible to realize both physiological flow through the valve during the flow period, and physiological pressure differences across the artificial valve during the closing phase, both for aortic and mitral valve positions, with freely selectable stroke volume and heart frequency. The major hydraulic parameters of the valve are measured directly (pressure drop, closing volume and closing time, leakage volume) or are calculated (e. g. energy loss). In association with the demonstration of the suitability of the HKP+ test apparatus for the measurement of hydraulic parameters in accordance with ISO 5840, the hydraulic properties of various types and sizes of valve with respect to pressure drop (resistance), together with their closing behaviour and leakage flow, are described for different fluids, and the correlations shown.

In vitro hemolysis by mechanical heart valve prostheses with tilting disc.

Hemolytic and subhemolytic blood damage by mechanical heart valve prostheses have been observed in both clinical and in vitro investigations. A direct comparison between these studies is not possible. Nevertheless the transfer of some in vitro results to the behaviour of the valve in situ may be performed considering the similarity principle. This requires the use of dimensionless similarity numbers such as the plasma's hemoglobin concentration (PHb) or others, instead of dimensioned parameters. To evaluate the in vitro hemolysis of valve prosthesis a test chamber filled with human banked blood was used. An artificial ventricle ensuring an oscillatory flow through the valve was also used. The rise of PHb was evaluated in terms of a similarity number, called the lysis number. This number describes the probability of destroying a single red blood cell participating once in the hemolytic process under consideration. The lysis number, a Björk-Shiley valve (TAD 29), was found to be in the order of 2 x 10(-4). From this, the survival time of erythrocytes in patients with an artificial heart valve was estimated. It was found to be in the order of 20 d of T50 Cr in agreement with clinical results.

[Three-dimensional visualization of stationary flow behind artificial heart valves]. / Dreidimensionale Sichtbarmachung stationärer Strömungen hinter künstlichen Herzklappen.

The visualization and quantitative analysis of flow offers a possibility for the hydrodynamic characterization of artificial heart valves. Different types of valves can be compared if velocity profile and the turbulent shear stress caused by the prosthesis are known. The tracer technique was selected, since it permits visualization also of turbulent flow through the valve. With the aid of a simple optical device the three-dimensional flow pattern behind the valve is determinable. The main features of the method are: The regions of interest can easily be identified. Velocity profiles can be determined and shear stress and turbulence intensities estimated. The experimental setup is simple, calibration is not necessary, and it can be used for turbulent flows. The method can be used only with transparent fluids and vessels; measurements in blood are not possible. Because of the large number of measuring points required the method is very time-consuming. The use of an automatic picture analyzing system would make it possible to increase the number of pictures processed, and thus increase resolution. The velocity profile of a three-finger-valve, the TAD 29, was established at a distance of 20 mm from the ring, and compared with known profiles from the literature. The valve has an opening angle of 70 degrees. All typical regions for the flow of an artificial heart valve, such as jet, stagnation gone, backflow and turbulence were demonstrated.

[Hydrodynamic in vitro comparison of a new artificial heart valve with the Björk-Shiley valve (standard)]. / Hydrodynamischer in-vitro-Vergleich einer neuen künstlichen Herzklappe mit der Björk-Shiley-Klappe (Standard).

Measurements performed to compare a newly developed tilting disc valve with the Björk-Shiley valve included velocity profiles downstream of the heart valves, valve-induced flow turbulence and pressure drop across the opened valves. The velocity profiles measured with pulsed Doppler ultrasound are similar, although they do not permit a quantitative comparison of the valves. The interpretation of the 90 degrees-component of Doppler signals as a measure of the turbulence permits a quantitative comparison without the need for extensive measurements. However, only large vortices are recorded, so that our turbulent shear stresses are lower than these reported in the literature. The pressure drop across the opened valve is a measure of the energy loss, and important parameters for the valve can be derived from it. The pressure drop is dependent on the test conditions, and is therefore not a characteristic constant of the valve. The transformation of the power law Q = C delta P beta into a relation between Re- and Eu-number gives a nondimensional similarity number that is characteristic for tilting disc valves. Its verification requires more investigations, involving variation of valve size and the viscosity of the test fluid.

Leakage of mechanical heart valve prostheses of Björk-Shiley type: in vitro investigations using Newtonian fluids.

The leakage of mechanical heart valve prostheses with tilting disc occluders was investigated. A U tube apparatus and a quasi-stationary method were used for measuring the backflow pressure characteristics. The method has several advantages over pulsatile or oscillatory techniques described in the literature. We have attempted to interpret the results of our measurements in terms of laminar and turbulent losses of energy. This leads to dimensionless loss numbers for valve leakage which are independent of arbitrary experimental conditions.

Fission and refusion of red blood cells using a micropipette technique.

In micropipette experiments with small capillaries and moderate high pressure difference (approximately 1000 Pa) cell fragmentation (fission) of human red blood cells without hemolysis was observed by TV-system for a large number of fresh red blood cells of different donors. After separation, the fragment moves away from the residual cell. In seven cases this process was evaluated quantitatively and was shown that the rate of the fragment was constant in time. Two mechanisms for this phenomenon are discussed. In particular cases a spontaneous re-fusion with the residual cell body in the capillary can be observed. In our opinion probably protein-depleted membrane surfaces arise and membrane fusion is possible simply by mechanical contact without additional electric fields and/or fusion agents.