Biomedical subjects

A M Brendzel

Publications and source records attributed to A M Brendzel.

10 recordsLinked to original sources

PubMed2000

Particle image velocimetry investigation of intravalvular flow fields of a bileaflet mechanical heart valve in a pulsatile flow.

BACKGROUND AND AIM OF THE STUDY: Our previous studies of bileaflet mechanical heart valves (MHV) explanted from sheep revealed patterns of localized platelet aggregation on valve surfaces, which may have clinical relevance. Since flow phenomena may promote localized platelet aggregation, an evaluation of flow within a valve lumen was conducted. METHODS: Phase-locked particle image velocimetry (PIV) measurements were obtained within the lumen of a 'mitral' model bileaflet MHV with transparent acrylic leaflets and housing, in a pulsatile flow loop. Instantaneous, two-dimensional flow maps of a central plane, perpendicular to the flow and leaflet pivot axes, were obtained at discrete times during the simulated cardiac cycle. Flow conditions were cardiac output, 3.5 l/min; rate, 72 beats/min; and systolic duration, 300 ms, using blood analog fluid refractive index-matched to acrylic. Leaflet closing velocities and angles were found using double-exposure imagery, and maximum leaflet closing velocity was extrapolated from regression analysis. RESULTS: During full opening, flow within the three lumenal orifices formed a three-peak axial velocity profile. Vorticity was concentrated in shear layers adjacent to downstream leaflet surfaces and in downstream wakes. Forward flow peak velocity was 90 cm/s, with a steep velocity gradient in the central orifice. During closing, the central-gap regurgitant flow formed a jet (peak velocity, 144 cm/s). High vorticity occurred near leaflet leading and trailing edges. During full closure, first a transient (<3 ms) 'stopping vortex' developed near the leaflet trailing edge, followed by a wall jet which formed at the leaflet-housing junction. Maximum leaflet closing velocity was 1.4 m/s. CONCLUSION: Localized jets, steep velocity gradients, high vorticity and vortex recirculation have been observed in vitro near model MHV surfaces. In vivo, each of these flow phenomena, when occurring near valve surfaces, may promote localized platelet aggregation. For the acrylic leaflets, maximum velocity was comparable with results reported for pyrolytic carbon leaflets. PIV of fully transparent models is a promising method for evaluating lumenal flows.

Cardiac Output↗

PubMed1999

The new St. Jude Medical regent mechanical heart valve: laboratory measurements of hydrodynamic performance.

BACKGROUND AND AIM OF THE STUDY: The St. Jude Medical (SJM) mechanical heart valve was the first bileaflet valve with leaflets and orifice all fabricated from pyrolytic carbon. In 1992, a supra-annular cuff configuration, the SJM mechanical heart valve Hemodynamic Plus Series (HP), was introduced, which gave a one-size gain in orifice area over the standard valve. In a continuing effort to improve hydrodynamic performance, a new enhanced configuration of the mechanical valve - the SJM Regent valve - has been developed. The blood-contacting components of the SJM Regent valve have the same design features as those of the standard or HP valve, but the orifice lumen area has been increased, without compromising structural integrity, by a shift in the cuff-retaining orifice rims. METHODS: Valve performance has been measured using a laboratory simulation of the left heart system. Three samples of both valve types (SJM Regent, HP) for sizes 17 mm to 27 mm, were tested at a cardiac output of 5 l/min. Supplementary steady-flow measurements were made at flow rates between 5 and 30 l/min. Pressure and flow signals were recorded and several performance parameters, including pressure gradient, effective orifice area (EOA), insufficiency (regurgitant volume as a percent of forward flow volume) and energy loss, were computed. RESULTS: All sizes of the SJM Regent valve showed improved hydrodynamic performance compared with the HP valve. For the small sizes (17, 19, 21 mm), the pressure gradient of the SJM Regent decreased by 37%, 36% and 39%, respectively; the EOA increased by 22%, 22% and 21%; and the total transaortic energy loss decreased by 33%, 31% and 31% compared with the HP valve. The regurgitation for both valves ranged between 4% and 8% of forward flow volume over the 17-27 mm size range. CONCLUSIONS: Hydrodynamic measurements confirm the gain of the SJM Regent valve over the HP valve that is expected based on the increased geometric orifice area. Given its basic similarity of design, but with an increased EOA as demonstrated in vitro, the SJM Regent valve is anticipated to provide an increased level of clinical benefit.

Biocompatible Materials↗

PubMed1994

Cyclic fatigue and fracture in pyrolytic carbon-coated graphite mechanical heart-valve prostheses: role of small cracks in life prediction.

A fracture-mechanics based study has performed to characterize the fracture toughness and rates of cyclic fatigue-crack growth of incipient flaws in prosthetic heart-valve components made of pyrolytic carbon-coated graphite. Such data are required to predict the safe structural lifetime of mechanical heart-valve prostheses using damage-tolerant analysis. Unlike previous studies where fatigue-crack propagation data were obtained using through-thickness, long cracks (approximately 2-20 mm long), growing in conventional (e.g., compact-tension) samples, experiments were performed on physically small cracks (approximately 100-600 microns long), initiated on the surface of the pyrolytic-carbon coating to simulate reality. Small-crack toughness results were found to agree closely with those measured conventionally with long cracks. However, similar to well-known observations in metal fatigue, it was found that based on the usual computations of the applied (far-field) driving force in terms of the maximum stress intensity, Kmax, small fatigue cracks grew at rates that exceeded those of long cracks at the same applied stress intensity, and displayed a negative dependency on Kmax; moreover, they grew at applied stress intensities less than the fatigue threshold value, below which long cracks are presumed dormant. To resolve this apparent discrepancy, it is shown that long and small crack results can be normalized, provided growth rates are characterized in terms of the total (near-tip) stress intensity (incorporating, for example, the effect of residual stress); with this achieved, in principle, either form of data can be used for life prediction of implant devices. Inspection of the long and small crack results reveals extensive scatter inherent in both forms of growth-rate data for the pyrolytic-carbon material.

Graphite↗

PubMed1990

Cyclic fatigue-crack propagation, stress-corrosion, and fracture-toughness behavior in pyrolytic carbon-coated graphite for prosthetic heart valve applications.

Fracture-mechanics tests were performed to characterize the cyclic fatigue, stress-corrosion cracking, and fracture-toughness behavior of a pyrolytic carbon-coated graphite composite material used in the manufacture of cardiac valve prostheses. Testing was carried out using compact tension C(T) samples containing "atomically" sharp precracks, both in room-temperature air and principally in a simulated physiological environment of 37 degrees C Ringer's lactate solution. Under sustained (monotonic) loads, the composite exhibited resistance-curve behavior, with a fracture toughness (KIc) between 1.1 and 1.9 MPa square root of m, and subcritical stress-corrosion crack velocities (da/dt) which were a function of the stress intensity K raised to the 74th power (over the range approximately 10(-9) to over 10(-5) m/s). More importantly, contrary to common perception, under cyclic loading conditions the composite was found to display true (cyclic) fatigue failure in both environments; fatigue-crack growth rates (da/dN) were seen to be a function of the 19th power of the stress-intensity range delta K (over the range approximately 10(-11) to over 10(-8) m/cycle). As subcritical crack velocities under cyclic loading were found to be many orders of magnitude faster than those measured under equivalent monotonic loads and to occur at typically 45% lower stress-intensity levels, cyclic fatigue in pyrolytic carbon-coated graphite is reasoned to be a vital consideration in the design and life-prediction procedures of prosthetic devices manufactured from this material.

Biocompatible Materials↗

PubMed1982

Electrical determination of viability in saline-treated mouse myeloma cells.

Suspension of mouse myeloma cells in phosphate buffered saline (PBS) induced a significant amount of cell death. The lethal effects of PBS include an increase in cell lysis, a decreased ability of cells to exclude trypan blue, and a decrease in the colony-forming ability of these cells. Dead cells were also detected on a Coulter counter by the increase in the fraction of cells with a smaller electrical size distribution (ESD). Comparing mixtures of live and dead cells by ESD and trypan-blue exclusion showed a high correlation of electrical size with viability (correlation coefficient = 0.98). Sizing of PBS-treated cells by light microscopy suggested that the altered ESD of the PBS-treated cells was due to a downward shift in the volume distribution. Light-scattering experiments also suggested a decrease in the size of cells after PBS treatment. An increase in permeability of the cell membrane may also contribute to these results. We conclude that electrical sizing is an excellent indicator of physical changes that result from PBS-induced cell death, and is an effective method for distinguishing live and dead mouse myeloma cells after PBS treatment.

Animals↗

PubMed1980

Effects of lipid-soluble substances on the thermotropic properties of liposome filtration.

Filtration of dimyristoyl phosphatidylcholine or dipalmitoyl phosphatidylcholine liposomes at various temperatures from 3 to 60 degrees C revealed a discontinuous change in filtration behavior centered about the gel-to-liquid crystal transition temperature. This change was continuous at temperatures immediately above or below the transition temperature. Although pure dipalmitoyl phosphatidylcholine liposomes are in the gel state at 22 degrees C, passage of liposomes composed of dipalmitoyl phosphatidylcholine and cholesterol through the filters at 22 degrees C gave results similar to those obtained with liquid-crystal liposomes. Low cholesterol concentrations were nearly as effective as high concentrations in producing this behavior; this observation is consistent with a shear mechanism for reduction of liposome size, since the stress induced by passage of the otherwise rigid liposome through a small pore would be relieved by fracture at a lattice imperfection. Liposomes composed of egg phosphatidylcholine and cholesterol were retained by the filters to a slightly greater extent than pure egg phosphatidylcholine liposomes; these results are consistent with the known condensing effect of cholesterol on liquid-crystal lipsomes and a shear mechanism occurring with filtration. Liposomes composed of dipalmitoyl phosphatidylcholine and either dipalmitoyl phosphatidic acid or dicetyl phosphate were filtered at 22 degrees C; they showed a filtration characteristic similar to liquid-crystal liposomes. Inclusion of the water-soluble dyes eosin Y or Evans blue in dipalmitoyl phosphatidylcholine liposomes resulted in filtration at 22 degrees C which was similar to that observed for liquid-crystal liposomes. The dyes, sodium fluorescein, 6-carboxyfluorescein and fluoresceinisothiocyanate dextran, did not alter 22 degrees C liposome filtration.

Cholesterol↗

PubMed1980

Liposome filtration. Dependence on transition temperature.

Liposomes formed by vortexing and passed through polycarbonate surface retention membranes showed appreciable differences in filtration behavior depending on the temperature of filtration relation relative to the liposome gel-liquid crystal transition temperature. Below transition, liposomes were filterable and size distributions could be determined; the cumulative volume distributions were log-normal. Above transition, liposomes were not filterable: smaller liposomes were formed until a limiting size was reached. These results suggest that liquid crystal liposome size distributions cannot be determined by filtration. This filtration behavior is a physical property of liposomes, related to the gel-liquid crystal transition, not previously reported. This property could be exploited as a new method for controlling liposome size distributions, but the implications for lipid membranes, including biological membranes, are general.

Filtration↗

PubMed1978

Chemically shifted singlet oxygen spectrum.

The estimated light emission spectrum was determined for a singlet oxygen (1O2)-producing system, NaOCl + H2O2, alone and in the presence of tryptophan and bovine serum albumin. Tryptophan and bovine serum albumin caused a decrease in the red emission of 1O2 and an increase in the amount of shorter wavelength light. This effect was due to chemiluminescence rather than fluorescence. Arachidonic acid caused a similar spectral shift, while guanosine demonstrated a late chemiluminescent reaction of predominantly short wavelength light in the presence of 1O2.

Hydrogen Peroxide↗

PubMed1978

Use of a unique chemiluminescence spectrometer in a study of factors influencing granulocyte light emission.

Factors contributing to variability in chemiluminescence (CL) measurements of phagocytizing granulocytes were identified and controlled. Observed CL was decreased by light quenching caused by red blood cells, hemoglobin, phagocytizable particles, and granulocytes. Increases in CL were achieved by opsonization with heat labile and heat stable (antibody) serum factors, reaction temperatures in the range of 37-40 degrees C, and mixing. The effects of controlled temperature variation and mixing were studied with a chemiluminescence spectrometer uniquely designed for such studies. Features of this spectrometer which make it more suitable than the previously employed scintillation spectrometers for the observation of granulocyte and other chemiluminescent systems include; (1) the ability to measure CL immediately upon reaction initiation; (2) simplicity of photomultiplier tube exchange; and (3) built-in optical filter holders for spectral analysis.

Animals↗

PubMed1977

Chemiluminescence spectra of human myeloperoxidase and polymorphonuclear leukocytes.

The light emission spectra of myeloperoxidase-H2O2-Cl- and phagocytizing polymorphonuclear leukocytes were estimated by a computer simulation technique by using light transmittance data from nine band-pass filters. No shift in the chemiluminescence spectrum of either system was observed during the course of the reactions, suggesting that the light-generating mechanisms remain constant after their initiation. Transmittances were virtually identical for both myeloperoxidase-H2-O2-Cl- and polymorphonuclear leukocyte reactions, suggesting that the light-generating mechanisms are identical and leading to the estimation of nearly identical spectra. Both spectra were broad, with maximum light emission near 570 nm.

Computers↗