Even though artificial heart valve implants have a history of some 30 years, there is to this day no ideal valve substitute. Each of the categories of substitutes used has its own advantages as well as problems. Since my last review on the subject, that appeared in this journal, was some 13 years ago (Lim, 1977), it is perhaps appropriate to provide an update on the status of cardiac valve replacement for the general local readership.
In the fabrication of a bioprosthetic heart valve from bovine pericardial tissues, the tissues are subjected to suturing. The stress-strain response of sutured as well as unsutured strips of this tissue were examined. The stress-strain response was determined using a tensile-testing machine. It was found that suturing weakens the tissue in that sutured strips are more extensible, exhibit a lower stress at rupture and a lower final elastic modulus. In addition, it was also found that the bigger the suture/needle size used the greater the decrease in tissue strength. In all, tissue strength was observed to decrease by 22 to 59% in this study. The weakening of the tissue is attributed to the puncture holes created by the surgeon's needle which create regions of weakness. This response of bovine pericardial tissue to suturing should be given due consideration in the fabrication of a bioprosthetic heart valve using this tissue.
Hemodynamics have long been implicated in atherogenesis. The studiesreported here seek to explain the mechanisms for the formation ofatherosclerotic plaque in an aortic bifurcation. Flow studies were made ina model constructed from plexiglass to represent an aortic bifurcation. Under steady flow conditions at inflow Reynolds numbers of 80-1250,the streamline flow patterns and the boundary layer separation zones wereinvestigated in relation to the location of atherosclerotic plaques clinicallyfound at regions in the human aortic bifurcation. The streamline flowswere visualized by a slow injection of dye over the cross section of the tubeentrance and along the tube walls. The studies revealed a complex flowfield where secondary flows, induced by the centrifugal and viscous forces,cause the fluid to move towards the inner walls of the aortic bifurcation. The effect was more clearly seen with increasing Reynolds number. Boundary layer separation zones were observed to occur at the outercorners of the branching. The nature of the separation zone formed wasfound to be dependent on Reynolds number. The residence time of fluidparticles within such a separation zone was estimated by measuring thewashout time of a bolus of dye injected at strategic locations along the tubewalls. The residence time was found to decrease exponentially withincreasing Reynolds number. These observations provide strong support forthe role of flow separation in the accumulation of LDL and plateletaggregation within the aortic bifurcation.
As the packing structure of lipid molecules in the liposomes will vary in the presence of ions, it is expected that the density of lipid and the effective volume of lipid molecules in the dispersions will also vary, albeit minutely. Density measurements of lipid-water dispersions with the addition of Ca(2+) ions were determined accurately. The effect of Ca(2+) ions on the molecular packing structure of the liposomes was elucidated from the results obtained. The results for the density of the lecithin in the dispersions with and without the addition of Ca(2+) ions are, respectively, 1.0782 and 1.0579 g cm(-3) at 25 degrees C; and 1.0048 and 0.9961 g cm(-3) at 50 degrees C. The average values of the effective molecular volume of lecithin in the dispersions with and without the addition of Ca(2+) ions are, respectively, 1.131E-21 and 1.152E-21 cm(3) at 25 degrees C; and 1.213E-21 and 1.224E-21 cm(3) at 50 degrees C.