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Biomedical subjects

E C Eckstein

Publications and source records attributed to E C Eckstein.

At least 19 recordsLinked to original sources
PubMed1996

A fluid dynamics study of the Trac catheter.

A fluid mechanics study of the Trac catheter was done using a 10 x scaled model. Wall pressure distributions and particle and dye flow visualization studies show that a primary vortex develops due to the spinning tip, and a secondary flow recirculates material along the chamber wall to the base of the chamber and back to the tip. Strong, nearly instantaneous mixing occurs throughout the vortex for speeds above 50,000 rpm scaled for the full-scale catheter. Close-contact pressure studies also show that wall pressures of the order of 0.4 MPa may be obtained for a full-scale catheter.

Angioplasty↗
PubMed1996

Ceftazidime-resistant Klebsiella pneumoniae isolates recovered at the Cleveland Department of Veterans Affairs Medical Center.

The rate of ceftazidime resistance among Klebsiella pneumoniae isolates recovered from patients at the Cleveland Department of Veterans Affairs Medical Center increased from 6% in the first quarter of 1993 to 28% in the first quarter of 1994. The outbreak was hospitalwide, with the highest rates of resistance occurring on wards where ceftazidime was administered most frequently. Although many plasmid patterns were observed in the clinical isolates, molecular epidemiological analysis with use of pulsed field gel electrophoresis revealed substantial similarities between the strains; this finding suggested that most of the strains-if not all of them-were derived from the original clone. The addition of piperacillin/tazobactam to the hospital formulary and educational efforts focused on minimizing the administration of ceftazidime were associated with a marked decrease in the drug's use and a concomitant decrease in the percentage of ceftazidime-resistant isolates. We have not yet observed a significant rise in the rate of resistance to piperacillin/tazobactam among clinical isolates of K. pneumoniae.

Ceftazidime↗
PubMed1994

An estimated shape function for drift in a platelet-transport model.

Prior work has shown that concentration profiles of platelets in flowing whole blood and of platelet-sized beads in flowing blood suspensions can include near-wall excesses. A model to describe this phenomenon was built about a single-component convective diffusion equation. To incorporate redistribution to preferred sites by shear flows of red cell suspensions, the model used a drift shape function (in addition to the commonly used augmented diffusion coefficient). This paper reports experiments that provide an average concentration profile from which the shape function for that model is calculated; the experiments and shape function are for the particular conditions of 40% hematocrit, platelet-sized latex beads (2.5 microns diameter), tube ID of 217 microns, and a wall shear rate of 555 s-1. Less precise estimates of the shape function obtained from data of previous studies indicate that the shape function is similar for the hematocrit of 15%.

Biophysical Phenomena↗
PubMed1994

Transient lateral transport of platelet-sized particles in flowing blood suspensions.

Concentration profiles of 2.5 microns latex beads were measured to demonstrate lateral transport of platelet-sized objects in flows of blood suspensions; the flows had equivalent Poiseuille wall shear rates (WSRs) from 250 to 1220 s-1. Each experimental trial began with a steady flow of suspension without beads in a thin-walled capillary tube (219 microns ID; 10.2 microns SD). The tube entrance was then switched to a reservoir containing suspension of equal hematocrit, but with beads, for a short interval of flow at the same WSR. This process established a paraboloidal tongue of labeled suspension with a transient concentration gradient at its surface. The tube and contents were rapidly frozen to fix the suspended particles in flow-determined locations. Segments of frozen tube were collected at distances from the entrance corresponding to 13%, 39%, and 65% of the axial extent of the ideal paraboloidal tongue. Concentration profiles were estimated from distances measured on fluorescence microscope images of cross-cut tube segments. Experiments used tubes either 40 or 50 cm long, suspension hematocrits of 0, 15, or 40%, and bead concentrations in the range of 1.5-2.2 x 10(5)/mm3. Profiles for 0% hematocrit suspension, a dilute, single-component suspension, had features expected in normal diffusive mixing in a flow. Distinctly different profiles and more lateral transport occurred when the suspensions contained red cells; then, all profiles for 13% extent had regions of excess bead concentration near the wall. Suspension flows with 40% hematocrit exhibited the largest amount of lateral transport. A case is made that, to a first approximation, the rate of lateral transport grew linearly with WSR; however, statistical analysis showed that for 40% hematocrit, less lateral transport occurred when the WSR was 250 s-1 or 1220 s-1 than 560 s-1, thus indicating that the rate behavior is more complex.

Biological Transport, Active↗
PubMed1993

Anatomic and biomechanical properties of human lumbar dura mater.

Determinants of dural defects subsequent to deliberate or accidental dural puncture include the equipment, techniques, and the inherent anatomic and biomechanical properties of dura mater. These properties were studied in specimens of human and canine lumbar dura mater in an attempt to delineate the structure of the tissue and to characterize its behavior in biomechanical terms. Human dura had a longitudinal orientation on gross appearance, and was confirmed microscopically to be composed of longitudinal lamella of collagen and elastin fibers. Longitudinal tensile strength and stiffness were greater than transverse tensile strength and stiffness, which is consistent with the dura's apparent anatomic structure and functional requirements. Additional biomechanical testing of the dura demonstrated the property of relaxation which is a characteristic of a viscoelastic material. Significant differences were observed between human and canine dural properties, suggesting limited value of this animal model. Integration of these observed anatomic and biomechanical properties of the lumbar dura provides a greater understanding of dural puncture and may explain previous and often confusing clinical and experimental findings.

Adolescent↗
PubMed1991

Model of platelet transport in flowing blood with drift and diffusion terms.

A drift term is added to the convective diffusion equation for platelet transport so that situations with near-wall excesses of platelets can be described. The mathematical relationship between the drift and the fully developed, steady-state platelet concentration profile is shown and a functional form of the drift that leads to concentration profiles similar to experimentally determined profiles is provided. The transport equation is numerically integrated to determine concentration profiles in the developing region of a tube flow. With the approximate drift function and typical values of augmented diffusion constant, the calculated concentration profiles have near-wall excesses that mimic experimental results, thus implying the extended equation is a valid description of rheological events. Stochastic differential equations that are equivalent to the convective diffusion transport equation are shown, and simulations with them are used to illustrate the impact of the drift term on platelet concentration profiles during deposition in a tube flow.

Animals↗
PubMed1990

Concentration profiles of platelet-sized latex beads for conditions relevant to hollow-fiber hemodialyzers.

A freeze-capture method was used to obtain the concentration profiles of platelet-sized latex beads (2.5 microns diameter) in saline-based red cell suspensions flowing through tubes with inner diameters of 135 and 200 microns. Without red cells the concentration profile exhibited a slight deficit near the wall. For hematocrits from 15 through 52%, the concentration profile exhibited a near-wall excess of beads, from two to eight times the central concentration. Near-wall excesses were small or nonexistent in the first half centimeter of the tube. For hematocrits of 15 and 30%, the profile and near-wall excess were equal at stations 2 cm or more from the entrance. These results suggest that there is a flow-driven mechanism that moves platelet-sized beads toward, but not to, the wall.

Blood Platelets↗
PubMed1989

A freeze-capture method for the study of platelet-sized particle distributions.

A rapid freezing method was developed to study the distributions of fluorescent platelet-sized particles in flows of blood suspensions through thin-walled capillary tubes. Segments of frozen tubes were mounted in a refrigerated microtome on the stage of an epifluorescence microscope. Sections of tube were cut away, images of newly exposed cross-sections were recorded on video tape, and distances of the particles from the wall were measured from recorded images. The distance data were used to construct histograms that were proportional to the local concentration. Results indicated that this method is suitable for the study of the distribution of platelet-sized particles over a wide range of hematocrit, that the basic profile is reproducible to within 15%, and that the non-uniform profile is not a result of events at the tube entrance.

Blood Platelets↗
PubMed1988

Conditions for the occurrence of large near-wall excesses of small particles during blood flow.

Prior work showed that the near-wall concentration of platelet-sized latex beads (2.38 microns diam) in flowing blood suspensions can be greater than three times the concentration in the central region of the flow. Similar methods were used to explore the dependence of the near-wall excess (NWE) of beads on the channel height and suspension composition. The bead diameter, suspending fluid viscosity, and red blood cell deformability were varied; the hematocrit was fixed at 15%. Results showed that NWEs greater than or equal to three times the central concentration were associated with shear stress, rather than with strain rate, required red cell deformability, and occurred with bead diameters of 2.2 microns or larger. The amplitude of NWEs observed in the 30- and 50-microns channels changed sharply from small to large as the wall shear rate (WSR) was increased, while those observed in 100-microns channels exhibited a more gradual dependence on WSR.

Blood Circulation↗
PubMed1987

The near-wall excess of platelet-sized particles in blood flow: its dependence on hematocrit and wall shear rate.

Methods involving microscopy were used to obtain concentration profiles of platelet-sized beads during flow through glass channels. Suspensions of fluorescent latex beads (2.38 microns diam) and washed red blood cells were made from an isotonic albumin-dextrose solution. A syringe pump regulated the suspension flow through glass channels, which were either 50 or 100 microns wide; most experiments used a wall shear rate of 1630 sec-1. Via stroboscopic epifluorescence microscopy, photographs were collected on image planes parallel to the channel wall. Profiles of the bead concentration in the narrow channel direction were made by assembling counts of the focused bead images in the photographs. The results showed that a near-wall excess of the beads occurred when the suspension contained a significant fraction of red cells (over 7%). For hematocrits of 15 to 45% (the highest studied), the excess was above five times the concentration in the central region. Experiments with channels of both widths showed the region of excess beads was 5 to 8 micron thick. A series of experiments with 50-micron channels, with a suspension hematocrit of 15%, and with wall shear rates from 50 to 3180 sec-1 showed that near-wall excesses were large only for wall shear rates of 430 sec-1 and above. This work demonstrated the effects of wall shear rate (flow rate) and hematocrit on the number of platelet-sized beads near a surface and hence illustrated physical (rheological) factors that act in blood-surface interaction.

Blood Flow Velocity↗
PubMed1987

Transport of platelets in flowing blood.

Distribution and transport of platelets in flowing blood were studied experimentally using suspensions of washed red cells and fluorescent latex beads as platelet analogues. Distributions of the platelet analogues were obtained from stroboscopic epifluorescence photomicrographs of flow in 50-micron channels and from images of the cut cross sections of cryogenically frozen thin-walled 200-micron tubes. Concentration profiles of platelet analogues had a substantial near-wall excess for situations with a substantial hematocrit (greater than 10%) and a substantial wall shear rate (greater than 400 s-1). The viscosity of the suspending fluid was found to affect the size of the near-wall excess and its shear-dependent onset. Additionally, the shear-rate dependence of the near-wall excess did not occur with suspensions of hardened red cells. The excess extended a substantial distance from the wall in the 200-micron tubes and a portion of the profile could be fitted to an exponential curve. The random walk model that is used to describe enhanced platelet diffusion is envisioned as a walk (lateral platelet motion) caused by shear-induced collisions with red cells. A more comprehensive random walk model that includes biased collisions produces an effective lateral motion of convective nature in addition to a diffusional motion; it is used to explain the observed nonuniform distributions of platelet analogues.

Animals↗
PubMed1986

Regional platelet concentration in blood flow through capillary tubes.

Platelet concentration was measured in samples from the various components of a bloodflow circuit, including the reservoir, the tube (with i.d. between 50 and 210 micron), and the discharge. The tube sample was collected by halting the flow and then flushing out a length of tube; thus, this sample collected equally from all radial locations. As the discharge sample was well mixed, it reflected the velocity field in the tube. Each reservoir sample was a traditional bulk collection. To ensure that the results represented the physical effects of flow on regional platelet concentration and could be interpreted with simple mass balance relationships, strong anticoagulation (sodium citrate and heparin) and platelet inhibition (prostaglandin E1) were used. Results for all tube diameters and for reservoir hematocrits from 5.5 to 77% and wall shear rates from 80 to 8000 sec-1 show that tubular platelet concentration is greater than reservoir or discharge platelet concentrations, which are equal. For platelet-rich plasma the tubular platelet concentration is decreased compared to the reservoir or discharge values. Mass balances show that the elevated tubular platelet concentration is due to an excess of platelets in radial locations with below average speeds; coupled with the need for red cells, this suggests that excess platelets have a near-wall location. Nonparametric statistical tests show that wall shear rate is a significant variable at a 0.05 confidence level; inner diameter is not found to be a significant variable, probably because of the limited diameter range studied and the experimental errors involved in determining platelet concentrations.

Blood Flow Velocity↗
PubMed1984

Erythrocyte deformability and size measured in a multiparameter system that includes impedance sizing.

A Coulter Counter was modified to allow the photographing of erythrocytes as they exist the sizing orifice. These photographs provide the necessary information to determine the shape of individual cells. This data was combined on a cell-by-cell basis with the corresponding impedance signal to determine accurately both the cell size and deformed state of each cell. The particle shape is required if the size is to be determined accurately. Normal, diabetic and sickle cell samples were used in this study. The cell size was corrected according to theory that relates the impedance signal with cell shape. Although there was no objective test to determine the efficacy of this calculation in terms of improving the size measured, the reduction in the associated distribution width is taken as indicative of an improvement in the measuring process. The data presented represent the first attempt to relate cell size with deformed state on a cell-by-cell basis and indicates the potential usefulness of multiparameter deformability and size measurements.

Anemia, Sickle Cell↗