Experimental evaluation of mechanical haemolysis with Starr-Edwards, Kay-Shiley and Björk-Shiley valves

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 × 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

Analysis of 3D transient blood flow passing through an artificial aortic valve by Lattice-Boltzmann methods

The development of flow instabilities due to high Reynolds number flow in artificial heart valve geometries inducing high strain rates and stresses often leads to hemolysis and related highly undesired effects. Geometric and functional optimization of artificial heart valves is therefore mandatory. In addition to experimental work in this field it is meanwhile possible to obtain increasing insight into flow dynamics by computer simulation of refined model problems. After giving an introductory overview we report the results of the simulation of three-dimensional transient physiological flows in fixed geometries similar to a CarboMedics bileaflet heart valve at different opening angles. The visualization of emerging complicated flow patterns gives detailed information about the transient history of the systems dynamical stability. Stress analysis indicates temporal shear stress peaks even far away from walls. The mathematical approach used is the Lattice Boltzmann method. We obtained reasonable results for velocity and shear stress fields. The code is implemented on parallel hardware in order to decrease computation time. Finally, we discuss problems, shortcomings and possible extensions of our approach.

A comparison of the hydrodynamical behaviour of three heart aortic prostheses by numerical methods

The behaviour of the blood flow passing through artificial heart values in aortic position is numerically simulated by discretizing the Navier-Stokes equations for viscous incompressible flow through the finite element method. Three different artificial valves, Starr-Edwards, St Jude and disc valves, are modelled by using the finite element method as well as the Navier-Stokes equations for viscous incompressible flow. The blood flow behaviour is characterized by discretizing both planar and axisymmetric domains of the value devices. The streamlines as well as the velocity distributions are studied and compared. Likewise, the pressure patterns are analysed and compared. Some numerical examples of the three valves are presented and discussed herein.

Hemolytic differences among artificial cardiac valves used in a ventricular assist pump

Ventricular assist devices (VADs) required for cardiac support may produce clinically significant hemolysis. VAD valves differ in both mechanics and hemodynamics. Therefore, we examined a ball valve, a modified tilting disc (MTD) valve, a polyurethane trileaflet valve, and a Björk-Shiley monostrut valve to determine their degrees of hemolysis. The valves were tested in a Pierce-Donachy VAD which pumped fresh bovine blood through a mock loop. Blood samples were analyzed for hematocrit and plasma hemoglobin, from which the indices of hemolysis were calculated. A one-way analysis of variance indicated significant differences between certain valves. The MTD was the most hemolytic. No significant hemolytic difference was found between the trileaflet and monostrut valves despite their different designs. The monostrut valve and the MTD valve were hemolytically very different despite their similar design. This study suggests that the valve type significantly affects the hemolysis produced by the VAD.

The St. Jude experience

This paper covers our experience with the use of the St. Jude prosthetic heart valve from November 1979 through August 1983 in 91 patients operated on for aortic and mitral valve replacement. Nonfatal complications included hemorrhagic sequela due to anticoagulation, with an annual rate of 1 percent (1.4 percent per 100 patient years), thromboembolism with an annual rate of 0.8 percent (0.87 percent per 100 patient years), sternal infection 1 percent, operative cardiovascular accident 1 percent, and pericardial tamponade 1 percent. Operative mortality was 1 percent, early mortality (within 30 days) was 3 percent, and late mortality was 3 percent, with a total overall mortality of 7 percent. Excluding two patients who died from noncardiac causes, the overall mortality was 5 percent. The mortality rate per year was 2 percent. The survival rate 3.8 years postoperatively was 89 percent for mitral valve replacement patients and 93 percent for aortic valve replacement patients, for an overall 38 year survival rate of 92 percent. All patients were anticoagulated with warfarin. There were no instances of valve failure, replacement, or serious hemolysis. Eighty-three percent were active or working with a New York heart functional class I. In our experience, the complication rate with the St. Jude valve is as low or lower than that for any other mechanical prosthetic cardiac valve available in the world today.

In-vitro wall shear measurements at aortic valve prostheses

Wall shear distributions during the cardiac cycle at the valve rings of Starr-Edwards, Björk-Shiley and Lillehei-Kaster aortic valves are measured and compared with thresholds reported for shear-induced trauma of blood components. Further, for the disk valves, the influence of pulse rate on wall shear stresses is evaluated. Hot film anemometry with flush-mounted wall shear probes is used as measurement technique in a pulsatile flow mock circuit. The experimental systolic data support the better hemodynamic characteristics of the disk valves over the ball valve also with respect to the threshold shear stresses of flow induced blood trauma. These results are confirmed by postoperative clinical studies, where lower LDH-values are found with the disk than with the ball valves. During diastole, however, high shear stresses are measured and calculated at the valve ring of the Björk-Shiley prosthesis, which can be referred to the non-overlapping closing mechanism. This result is discussed with respect to the often observed thrombus formation at the disk downstream of the smaller orifice of the Björk-Shiley valve.

The Björk-Shiley heart valve prosthesis. Flow characteristics of the new 70 degrees model

Thrombus formation on the outflow face and tissue overgrowth along the sewing ring adjacent to the minor outflow region are major clinical pathologic problems observed with the Björk-Shiley tilting disc valve. In the hope of reducing these pathologic conditions the convexo-concave Björk-Shiley valve was designed. A new modification to the convexo-concave model has further improved most of the fluid dynamic characteristics of the valve. In vitro flow studies indicate an average improvement in pressure drop characteristics of about 20 to 30%. Velocity measurements made with a laser-Doppler anemometer in the immediate downstream vicinity of the modified convexo-concave valve indicate that the design changes have further reduced the size of the stagnation zone near the outflow face of disc. It was observed, however, that the regurgitation across the valve increased by about one to two percentage points.

Chronic haemolysis following combined mitral and aortic valve replacement. A randomized study between the Björk-Shiley and Lillehei-Kaster disc valve prostheses

Intravascular haemolysis was studied in a randomized series of 35 patients examined 12--24 months after combined mitral and aortic valve replacements with either the Björk-Shiley (pyrolytic carbon) tilting disc valve (19 patients) or the Lillehei-Kaster pivoting disc valve (16 patients). Red blood cell count, haemoglobin concentrations, and serum bilirubin varied within normal ranges with a few exceptions. Haptoglobin was absent or reduced in 86%, while elevated values for serum lactate dehydrogenase were found in 82%. The increase in serum lactate dehydrogenase activity was moderate and indicated a normal or only slightly reduced red cell lifespan in most patients. Although the intravascular haemolysis was of little clinical significance, the increase in lactate dehydrogenase was significantly higher in patients with the Lillehei-Kaster prosthesis than in patients with the Björk-Shiley prosthesis, indicating a slightly shorter lifespan in the former group. For both models, however, intravascular haemolysis was less severe than previously reported after combined aortic and mitral valve replacement with ball valve prosthesis.

The Björk-Shiley aortic prosthesis: flow characteristics, thrombus formation and tissue overgrowth

Thrombus formation and tissue overgrowth were observed in nine Björk-Shiley aortic prostheses recovered six months or longer after implantation. These pathologic findings may be attributed to the flow characteristics of the prosthesis. The open disc of the valve separates the flow into two unequal regions. Varying degrees of thrombus formation were observed in the minor outflow region, including the depression in the aortic face of the disc and the metal strut bridging this area. Tissue overgrowth was noted along the perimeter of the prosthesis adjacent to the minor outflow region. That overgrowth further reduced the available cross section for flow in this already constrained area. In vitro velocity measurements with a laser-Doppler anemometer identified a zone of stagnation about 20 mm wide near the aortic face of the disc. The average velocities in the major and minor outflow regions were around 100 and 25 cm/sec, respectively, and the corresponding peak-shear stresses were approximately 700 and 150 dynes/cm2. There is reason, then, to attribute the thrombus formation and tissue overgrowth to the stagnation zone and the low shear in the minor outflow region.