William H. Corcoran

Pressure drops across prosthetic aortic heart valves under steady and pulsatile flow—In vitro measurements

Abstract Pressure drops across ten prosthetic aortic heart valves have been measured under both steady and pulsatile flow using two different Newtonian liquids having viscosities of 0.01 and 0.035 dyn sec/cm 2 , respectively. The difference in viscosities of the test fluids seemed to have negligible effect within experimental error on the pressure drops over a range of flow rates from 83 to 500 cm 3 /sec. The correlation coefficients between the pressure drop experiments conducted with the two liquids were at least 0.99. The experimental results and the theory developed (equations 12 and 14) indicate that for a given aortic valve it is possible to predict peak systolic and mean systolic pressure drops from experimental pressure drop data for steady flow in the same channel. Correlation coefficients of at least 0.98 were obtained between the experimental and predicted values.

In vitro velocity measurements in the vicinity of aortic prostheses

Abstract A laser-Doppler anemometer was used for in vitro measurements of velocity profiles in the vicinity of aortic prostheses at steady flow rates of 167 cm 3 /sec to 417 cm 3 /sec. Results showed that at these high flow rates the wall shear stresses were about 1 × 10 3 dyn/cm 2 in the near vicinity of the aortic prostheses studied. Also maximum turbulence-intensity levels of 25–50% were measured, and they decayed to about 15–20% at 120 mm downstream from the valves. The experimentally observed wall-shear stresses were of a magnitude that could damage the endothelial lining of the ascending aorta and the formed elements of blood, at least when they adhere to the wall of the aorta. In the near vicinity of the aortic prostheses, maximum values of bulk-turbulent-shear stresses estimated from turbulence-intensity measurements and simple flow models were in the range of 10 2 –10 3 dyn/cm 2 . Turbulent shears in that range could damage red-blood cells and platelets and lead to thrombus formation. With pulsatile flow, where there is an acceleration phase, the shear stresses would even be larger.

Experimental measurements of turbulence spectra distal to stenoses

Abstract Turbulence spectra were measured downstream from stenoses simulated by placing plastic cylinders inside a rigid tube. The inside diameter of the tube was 3 8 in. and the plastic cylinders had various orifice sizes. Measurements were made for water. and the flow rates giving Reynolds numbers in the range of 800–2000 were investigated. A hot-film probe was used to monitor the turbulence. Turbulence spectra were obtained by processing the signal with a digital computer which utilized a fast Fourier processor. Comparisons of turbulence spectra were made for various severities of simulated stenoses. flow rates and various locations of the probe. Contributions of the turbulence to the generation of sound are briefly discussed.

In vitro flow dynamics of four prosthetic aortic valves: a comparative analysis.

The velocity fields downstream of four prosthetic heart valves were mapped in vitro over the entire cross-section of a model aortic root using laser Doppler anemometry. THe Björk-Shiley 60 degrees convexo-concave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis were examined under both steady and pulsatile flows. Velocity profiles under steady flow conditions were a good approximation for pulsatile profiles only during midsystole. The pulsatile flow characteristics of the four valves showed variation in large scale flow structures. Comparison of the valves according to pressure drop, shear stress and maximum velocities are also provided.

Sulfur compounds in coal

The literature on the chemical structure of the organic sulfur compounds (or functional groups) in coal is reviewed. Four methods were applied in the literature to study the sulfur compounds in coal: direct spectrometric and chemical analysis, depolymerization in drastic conditions, depolymerization in mild conditions, and studies on simulated coal. The data suggest that most of the organic sulfur in coal is in the form of thiophenic structures and aromatic and aliphatic sulfides. The relative abundance of the sulfur groups in bituminous coal is estimated as 50:30:20%, respectively. The ratio changes during processing and during the chemical analysis. The main effects are the transformation during processing of sulfides to the more stable thiophenic compounds and the elimination of hydrogen sulfide.

Laser-doppler anemometer to study velocity fields in the vicinity of prosthetic heart valves

Laser-Doppler anemometry is relatively new technique which is used for measuring velocity fields. It has major applications in the field ofin vitro biofluid mechanics. The laser-Doppler anemometers have many advantages compared with the traditional hot-wire or hot-film anemometers which are still mainly used in studies of biofluid mechanics. A laser-Doppler anemometer (I.d.a) system which can be used to measurein vitro velocity and shear-stress profiles in the vicinity of prosthetic heart valves is described. Accurate velocity measurements in the vicinity of prosthetic heart valves are very scarce, and the use of I.d.a systems will facilitate acquisition of these data.

Turbulence downstream from the Ionescu-Shiley bioprosthesis in steady and pulsatile flow.

The turbulence generated downstream from an aortic Ionescu-Shiley bioprosthesis has been investigated in vitro with both steady and pulsatile flow; Instantaneous point velocities were measured using laser-Doppler anemometry (LDA) at numerous preselected locations in the flow. The mean and RMS velocities from these data at each location were then used to estimate the laminar and turbulent shear in the bulk flow as a function of radial position on a cross-section of the flow system downstream from the mounted prosthesis. Estimated total shear stresses were found in the bulk flow that were of sufficient magnitude to possibly cause haemolysis and initiate platelet chemical-release reactions. For steady flow and at peak pulsatile flow, maximum total shear stresses were estimated to be 120 N m−2 and 100 N m−2, respectively, over more than 5 per cent of the flow cross-section. The spatial distribution of the elevated shear stresses correlates well with the valve superstructure. It is concluded that these elevated stresses are a direct consequence of the notable flow constriction generated by the valve’s fully opened leaflets.

In vitro velocity measurements down strem from the lonescu-Shiley aortic bioprosthesis in steady and pulsatile flow

Velocity measurements were made in vitro using laser Doppler anemometry (LDA) downstream from an lonescu-Shiley (IS) bioprosthetic aortic heart valve. Velocity measurements were made in both steady and pulsatile flow. A systematic, flow mapping approach to the measurement methodology showed that the IS valve generated a large jetlike flow constriction. The acceleration ratio, defined as the maximum mean velocity for the IS valve divided by that for no valve obstructing the flow, was as high as 2·4 for steady flow and 2·6 for pulsatile flow. It was concluded that the IS valve generated a flow quite unlike that observed by other in vestigators for the natural human aortic valve, after which the leaflet design of the IS valve was modelled. In addition, a comparative analysis of steady and pulsatile results was undertaken. It was found that the pulsatile flow results for the systolic ejection interval could be divided into three phases, denoted early, mid, and late systole, as defined by the flow structure at the data plane location. Only during midsystole were the pulsatile flow results approximated by the steady flow results. Also, it was found that the magnitude of the flow disturbance measured in steady flow tended to be an upper bound on that measured for pulsatile flow.

A quantitative method for thein vitro study of sounds produced by prosthetic aortic heart valves Part I: analytical considerations

A method is presented for analysing sounds producedin vitro by prosthetic aortic heart valves, and a one-dimensional harmonic model is described. Procedures for estimating physical parameters of the model are outlined for the case of transient and nontransient sounds, and a computational method is described for making comparisons between two general sounds. The fast Fourier transform provides a satisfactory means for the basic transformation to the frequency domain. Useful representations of the acoustical information that are considered are the original time and amplitude plots. power-density spectra, power-distribution functions, a three-dimensional surface of power-frequency-time, sections of these three-dimensional surfaces, and a three-dimensional power-distribution surfaces. Note is made that each representation is useful for indicating specific acoustical characteristics which may be important when either comparing or describing sounds. The spectra provide an accurate means for estimating the parameters of the model and provide clearer comparisons when compared to the time-amplitude plots. This fact is most clearly shown by the three-dimensional powerdifference surface. This surface provides a very convenient means for the overall comparison of two sounds.

In vitro velocity measurements in the near vicinity of the Bjork-Shiley aortic prosthesis using a laser-Doppler anemometer

In this study,in vitro velocity measurements in the near vicinity of a Björk-Shiley aortic valve, one of the more commonly used aortic valve prostheses, were made using a laser-Doppler anemometer. The velocity measurements identified a zone of stagnation, about 20 mm wide, immediately downstream from the fully open disc. The measurements also showed that the flow through the valve was divided into two unequal regions, namely, the major and minor outflow regions. Because of the low flow in the minor outflow region, the shear stresses along the perimeter of the valve in that region were considerably lower than the shear stresses along the sewing ring of the major outflow region. Pathologic studies of nine recovered Björk-Shiley aortic valves indicated varying amounts of thrombus formation on the outflow face of the disc and excess growth of endothelial tissue along the perimeter of the minor outflow region. If the large stagnation zone and the relatively low shear in the minor outflow region which were observed in thein vitro measurements also existin vivo, they could lead to the clinically observed thrombus formation and tissue overgrowth, respectively.