PubMed Health⌕ Search

Biomedical subjects

D Bluestein

Publications and source records attributed to D Bluestein.

At least 19 recordsLinked to original sources
PubMed2002

Free emboli formation in the wake of bi-leaflet mechanical heart valves and the effects of implantation techniques.

The high incidence of thromboembolic complications of mechanical heart valves (MHV), primarily due to platelet activation by contact with foreign surfaces and by non-physiological flow patterns past the valve, still limits their success as permanent implants. The latter include elevated shear and turbulent stresses and shed vortices formed in the wake of the valve's leaflets during the deceleration phase, potentially entrapping activated and aggregated platelets. It is hypothesized that these flow patterns induce the formation of free emboli which are the source of cerebrovascular microemboli associated with MHV. Implicit to this hypothesis is that free emboli formation will be affected by the implantation technique employed and the valve orientation, as those will alter the flow characteristics past the valve and the interaction of the platelets with the flow. In this study, numerical simulations of turbulent pulsatile flow past a St. Jude Medical bi-leaflet MHV were conducted. Platelet shear histories were calculated along pertinent turbulent platelet trajectories, and the effect of a misaligned valve on platelet activation was quantified and compared to that of an aligned valve. It demonstrated that the combination of a tilted valve and subannularly sutured pledgets had an explicit detrimental effect on platelet activation, with the following entrapment of the platelets within the shed vortices of the wake leading to a significant increase of the thromboembolic potential of the valve. This numerical model depicted a viable course for free emboli formation, and indicated how the implantation technique may enhance the risk of cardioembolism.

Aortic Valve↗
PubMed2000

Vortex shedding as a mechanism for free emboli formation in mechanical heart valves.

The high incidence of thromboembolic complications of mechanical heart valves (MHV) limits their success as permanent implants. The thrombogenicity of all MHV is primarily due to platelet activation by contact with foreign surfaces and by nonphysiological flow patterns. The latter include elevated flow stresses and regions of recirculation of blood that are induced by valve design characteristics. A numerical simulation of unsteady turbulent flow through a bileaflet MHV was conducted, using the Wilcox k-omega turbulence model for internal low-Reynolds-number flows, and compared to quantitative flow visualization performed in a pulse duplicator system using Digital Particle Image Velocimetry (DPIV). The wake of the valve leaflet during the deceleration phase revealed an intricate pattern of interacting shed vortices. Particle paths showed that platelets that were exposed to the highest flow stresses around the leaflets were entrapped within the shed vortices. Potentially activated, such platelets may tend to aggregate and form free emboli. Once formed, such free emboli would be convected downstream by the shed vortices, increasing the risk of systemic emboli.

Biomedical Engineering↗
PubMed1999

Acetylated prothrombin as a substrate in the measurement of the procoagulant activity of platelets: elimination of the feedback activation of platelets by thrombin.

Human prothrombin was acetylated to produce a modified prothrombin that upon activation by platelet-bound prothrombinase generates a form of thrombin that does not activate platelets but retains its amidolytic activity on a chromogenic peptide substrate. If normal prothrombin is used in such an assay, the thrombin that is generated activates the platelets in a feedback manner, accelerating the rate of thrombin generation and thereby preventing accurate measurement of the initial platelet procoagulant activity. Acetylation of prothrombin was carried out over a range of concentrations of sulfo-N-succinimidyl acetate (SNSA). Acetylation by 3 mM SNSA at room temperature for 30 min at pH 8.2 in the absence of metal ions produced a modified prothrombin that has <0.1% clotting activity (by specific prothrombin clotting assay), but it is activated by factor Xa (in the presence of either activated platelets or factor Va + anionic phospholipid) to produce thrombin activity that is measurable with a chromogenic substrate. Because the feedback action on the platelets is blocked, thrombin generation is linear, allowing quantitative measurement of the initial platelet activation state.

Acetylation↗
PubMed1996

Steady flow in an aneurysm model: correlation between fluid dynamics and blood platelet deposition.

Laminar and turbulent numerical simulations of steady flow in an aneurysm model were carried out over Reynolds numbers ranging from 300 to 3600. The numerical simulations are validated with Digital particle Image Velocimetry (DPIV) measurements, and used to study the fluid dynamic mechanisms that characterize aneurysm deterioration, by correlating them to in vitro blood platelet deposition results. It is shown that the recirculation zone formed inside the aneurysm cavity creates conditions that promote thrombus formation and the viability of rupture. Wall shear stress values in the recirculation zone are around one order of magnitude less than in the entrance zone. The point of reattachment at the distal end of the aneurysm is characterized by a pronounced wall shear stress peak. As the Reynolds number increases in laminar flow, the center of the recirculation region migrates toward the distal end of the aneurysm, increasing the pressure at the reattachment point. Under fully turbulent flow conditions (Re = 3600) the recirculation zone inside the aneurysm shrinks considerably. The wall shear stress values are almost one order of magnitude larger than those for the laminar cases. The fluid dynamics mechanisms inferred from the numerical simulation were correlated with measurements of blood platelet deposition, offering useful explanations for the different morphologies of the platelet deposition curves.

Aortic Aneurysm, Abdominal↗
PubMed1995

The effect of varying degrees of stenosis on the characteristics of turbulent pulsatile flow through heart valves.

Many problems and complications associated with heart valves are related to the dynamic behavior of the valve and the resultant unsteady flow patterns. An accurate depiction of the spatial and temporal velocity and rms distributions imparts better understanding of flow related valve complications, and may be used as a guideline in valve design. While the generalized correlation between increased turbulence level and the severity of the stenosis is well established, few studies addressed the issue of the intermittent nature of turbulence and its timing in the cardiac cycle, and almost none assessed the effect of a progressive stenosis on the flow characteristics through heart valves. In this experimental work we simulated the type of flow which is present in normal and stenosed valves and conducted a comprehensive investigation of valve hemodynamics, valvular turbulence and morphology under varying degrees of stenosis. The characteristics of valves and stenoses were simulated closely, to achieve the flow conditions that initiate turbulent flow conditions. Laser Doppler anemometry (LDA) measurements were carried out in a pulse duplicator system distal to trileaflet polyurethane prosthetic heart valves, installed at mitral and aortic positions. The effect of the degree of the stenosis was comparatively studied through the structure of the turbulent jets emerging from normal and stenotic heart valves. Maximum turbulence level was achieved during the decelerating phase and correlated to the severity of the stenosis, followed by relaminarization of the flow during the acceleration phase. The intermittent nature of the turbulence emphasized the importance of realizing the timing of the turbulence production and its spatial location for optimizing current valve designs. The plug flow through the normal aortic valve prosthesis was replaced by jet like behavior for a 65% stenosis, with the jet becoming narrower and stronger for a 90% stenosis. The morphology of the velocity and turbulence waveforms was found to be governed by the stenosis geometry and the valve position (aortic, mitral).

Aortic Valve↗
PubMed1994

A squeeze flow phenomenon at the closing of a bileaflet mechanical heart valve prosthesis.

In vivo cavitation in cardiovascular flow fields may occur under very unusual circumstances as a localized transient phenomenon which are confined to very small regions in the vicinity of the valve body or leaflet surface. The violent collapse of cavitation bubbles induces local erosion that may lead to structural damage. The fluid mechanical factors that may cause in vivo cavitation inception in mechanical heart valve (MHV) prostheses are investigated. It is established that the closing velocity of the leaflet holds the key to MHV cavitation. During the final phase of valve closing, the fluid mass in the gap space between the closing occluder and the valve's body is squeezed into motion by the rapidly approaching boundaries. The flow pattern created by this motion (termed 'squeeze flow'), is found to be related to the valve geometry, and the impact velocity of the closing leaflet. Given the closing velocity of the leaflet and the geometry of the MHV, computational flow dynamics (CFD) are made to determine the velocity distributions in the gap flow field of a bileaflet MHV in the mitral position. A two dimensional, time dependent model of the gap space show that flow velocity in the gap space can reach values as high as 30 ms-1 in regions near the edge of the inflow surface of the Edwards Duromedics (ED) MHV leaflet. This high speed stream ejected from the gap channel can create the conditions that characterize cavitation. The location of the isolated high speed region corresponds to the surface erosion that was observed in a number of damaged ED-MHV explants.

Acceleration↗
PubMed1994

Transition to turbulence in pulsatile flow through heart valves--a modified stability approach.

The presence of turbulence in the cardiovascular system is generally an indication of some type of abnormality. Most cardiologists agree that turbulence near a valve indicates either valvular stenosis or regurgitation, depending on the phase of its occurrence during the cardiac cycle. As no satisfying analytical solutions of the stability of turbulent pulsatile flow exist, accurate, unbiased flow stability criteria are needed for the identification of turbulence initiation. The traditional approach uses a stability diagram based upon the stability of a plane Stokes layer where alpha (the Womersley parameter) is defined by the fundamental heart rate. We suggest a modified approach that involves the decomposition of alpha into its frequency components, where alpha is derived from the preferred modes induced on the flow by interaction between flow pulsation and the valve. Transition to turbulence in pulsatile flow through heart values was investigated in a pulse duplicator system using three polymer aortic valve models representing a normal aortic valve, a 65 percent stenosed valve and a 90 percent severely stenosed valve, and two mitral valve models representing a normal mitral valve and a 65 percent stenosed valve. Valve characteristics were closely simulated as to mimic the conditions that alter flow stability and initiate turbulent flow conditions. Valvular velocity waveforms were measured by laser Doppler anemometry (LDA). Spectral analysis was performed on velocity signals at selected spatial and temporal points to produce the power density spectra, in which the preferred frequency modes were identified. The spectra obtained during the rapid closure stage of the valves were found to be governed by the stenosis geometry. A shift toward higher dominant frequencies was correlated with the severity of the stenosis. According to the modified approach, stability of the flow is represented by a cluster of points, each corresponding to a specific dominant mode apparent in the flow. In order to compare our results with those obtained by the traditional approach, the cluster of points was averaged to collapse into a single point that represents the flow stability. The comparison demonstrates the bias of the traditional stability diagram that leads to unreliable stability criteria. Our approach derives the stability information from measured flow phenomena known to initiate flow instabilities. It differentiates between stabilizing and destabilizing modes and depicts an unbiased and explicit stability diagram of the flow, thus offering a more reliable stability criteria.

Aortic Valve↗
PubMed1994

Helping pregnant teenagers.

Teenagers who are pregnant face many difficult issues, and counseling by physicians can be an important source of help. We suggest guidelines for this counseling, beginning with a review of the scope and consequences of adolescent pregnancy. Communication strategies should be aimed at building rapport with techniques such as maintaining confidentiality, avoiding judgmental stances, and gearing communication to cognitive maturity. Techniques for exploring family relationships are useful because these relationships are key influences on subsequent decisions and behaviors. We discuss topics related to abortion and childbearing, such as safety, facilitation of balanced decision making, the use of prenatal care, and the formulation of long-term plans. Physicians who can effectively discuss these topics can help pregnant teenagers make informed decisions and improve their prospects for the future.

Abortion, Legal↗
PubMed1994

Knowledge of pregnancy symptoms among abortion patients: is race a predictor?

This cross-sectional study characterizes first-trimester abortion patients who perceived inadequate knowledge of pregnancy symptoms and identifies net predictors of inadequate symptom knowledge. Data were collected at an abortion facility in Hampton Roads, Virginia. Study subjects were women surveyed on the day of their abortions, prior to termination procedures. Self-reported knowledge of pregnancy symptoms was the study's dependent variable. Of 342 women, 120 (35%) perceived inadequate symptom knowledge. These women more often were young, black, single, and poorly educated. Only black race was a net predictor of inadequate symptom knowledge when study variables were entered into a multiple logistic regression. Black race was the only net predictor of inadequate symptom knowledge among first-trimester abortion patients. This racial difference was not explained by socioeconomic or access factors. Future research should consider an alternative hypothesis, the possibility that more effective communications with black abortion patients are needed. Additionally, health-care providers should not presume that first-trimester abortion patients are familiar with pregnancy symptoms and should not stereotype patients who perceive knowledge limitations with regard to socioeconomic status.

Abortion, Legal↗
PubMed1993

Psychosocial determinants of late prenatal care: the Health Belief Model.

This paper presents a theoretical framework for investigating psychosocial determinants of delayed prenatal care among disadvantaged women. This framework is based on the Health Belief Model (HBM), which postulates that beliefs concerning prenatal care are modified by psychological and social attributes and may predispose one to delay prenatal care enrollment. The HBM has a number of advantages for family medicine researchers in comparison to other models; its use in future investigations can provide the understanding necessary for overcoming psychosocial deterrents to entry into prenatal care.

Attitude to Health↗
PubMed1993

Family relationships and depressive symptoms preceding induced abortion.

This study examines the association between depressive symptoms preceding induced abortion and dissatisfaction with family relationships. In a cross-sectional survey, 304 women undergoing a first-trimester abortion completed a short version of the Center for Epidemiologic Studies Depression Scale, the Family APGAR, and a researcher-designed questionnaire. Bivariate analyses revealed that depressive symptoms decreased as measures of age, educational attainment, Family APGAR scores, marriage, and subjective health increased. Depressive symptoms increased as measures of denial, difficulties communicating with male partners, pregnancy symptoms, contraceptive use, and dissatisfaction with choosing abortion increased. Controlling for the effects of these significant bivariate associations, increased depressive symptoms were independently predicted by Family APGAR scores, age, communications difficulties, pregnancy symptoms, contraceptive use, and denial. Low Family APGAR scores exhibited the strongest overall effect. Clinicians who encounter women experiencing depressive symptoms preceding abortion may wish to explore family relationships and the possibility of underlying family dysfunction.

Abortion, Induced↗
PubMed1992

Effects of multiple injections of HCG on testis blood flow.

In attempting to determine whether or not multiple injections of human chorionic gonadotropin (hCG) augment testis blood flow, adult male rats were injected with three doses of 10 IU of hCG every other day and testis blood flow was determined on day 5, the day of the final injection. Testis blood flow (mL/100 g testis tissue/min +/- SEM) as measured by the 133Xe washout method increased from 10.8 +/- 1.3 to 20.4 +/- 4.5 (p less than 0.05) after the three doses of hCG. These observations suggest that multiple injections of hCG appear to have the same effect as a single dose of hCG in increasing testis blood flow. This supports the hypothesis that hCG should be administered to all patients undergoing orchiopexy in the hope that the increased perfusion of the gonad will make it less susceptible to ischemia during the surgical procedure.

Animals↗
PubMed1992

Determinants of delayed pregnancy testing among adolescents.

BACKGROUND: Pregnant teenagers often prolong the interval between suspecting and confirming that they are pregnant. Prior studies suggest a number of potential determinants for this delay but do not specify which ones are most salient. METHODS: In a cross-sectional survey, 123 pregnant teenagers, 64 of whom maintained their pregnancies and 59 of whom had abortions, completed a short version of the Center for Epidemiologic Studies Depression Scale, the Family APGAR test, and a study-specific questionnaire. RESULTS: Significant bivariate determinants of delayed pregnancy testing included young maternal age, black race, lower educational attainment, lack of pregnancy symptoms, continuing the pregnancy, and denial. Only denial, however, retained a significant net effect on delayed testing (P < .05) when the effects of these six variables were modeled using multiple linear regression. CONCLUSIONS: These results suggest that psychological barriers are the most salient determinants of delayed pregnancy testing among the teenagers surveyed in this study. Some teenagers may not volunteer information about a suspected pregnancy. Providers, therefore, should directly question teenagers about sexual activity and discuss the importance of early testing when pregnancy is suspected. Findings also suggest further research that would increase understanding of adolescent health behavior in pregnancy and identify effective clinical and educational interventions.

Adolescent↗
PubMed1992

Ethnic differences in patient requests for pregnancy testing.

This study identifies a black-white difference in pregnancy test requests and in factors predicting such requests among 324 women tested at an academic family practice in 1986. Data were obtained from encounter sheets filled out by clinicians at the time tests were ordered. Analysis of variance revealed that blacks requested fewer pregnancy tests than whites, and that this finding remained significant (P less than .01) after controlling for the effects of gestational status and other clinical and sociodemographic factors. Blacks were less likely to be married or possess health insurance, but more likely to have been pregnant before. Logistic regressions indicated that factors predicting test requests differed by ethnicity, with symptoms and age predicting test requests among whites, and pregnancy the only significant predictor among blacks. Explanations consistent with these findings include possible ethnic differences in reactions to symptoms and economic factors. Further refinement of these hypotheses and consideration of other alternatives will advance understanding of ethnicity as a factor in test-requesting behavior, and enable clinicians to communicate with and care for black women more effectively.

Black or African American↗
PubMed1991

Effect of the antiandrogen, WIN 49,596, on the descent of the testis in the male rat.

Testicular descent in the male rat is believed to be an androgen dependent event with dihydrotestosterone the most likely active androgen. To provide further insight into the endocrinology of this important physiological event, we treated male rats with the antiandrogen, WIN 49596 (50 mg./kg./day), from day 1 to day 27 of age and evaluated its effect on the post-natal androgen-dependent events in this animal model. It was determined that while treatment with WIN inhibited the weights of the ventral prostate, seminal vesicles and penis when compared to those seen in castrate animals, the drug only caused a 19% (3/16) inhibition in the descent of the testes when compared to the control group (0/16; p = 0.112). These data together with those previously obtained in animals exposed to selective inhibitors of the 5-alpha reductase enzyme suggest that other factors possibly working in tandem with androgens play a predominant role in testicular descent in the rat.

5-alpha Reductase Inhibitors↗
PubMed1991

Predicting the occurrence of antibiotic-induced candidal vaginitis (AICV).

This paper reports the sensitivity, specificity, and predictive values of symptoms in the diagnosis of antibiotic-induced candidal vaginitis (AICV) among 74 women recruited from three primary care practices. All subjects, who were examined both pre- and post-antibiotic treatment for acute respiratory, urinary tract, or skin infections, were initially free of vaginitis. Twenty-four subjects developed candidal vaginitis (CV), indicated by vaginitis symptoms or signs and a positive candidal culture or KOH preparation; there were no mixed infections. Fifty women did not develop AICV and, of this group, four developed a nonyeast vaginitis. Aggregate symptoms (pruritus and/or discharge) had 87.5% sensitivity, 95.8% specificity, and positive and negative predictive values of 91.3% and 93.9%, respectively. These values are much higher than those reported in studies of CV that excluded women on antibiotics. We conclude that women who develop vaginitis symptoms while on short courses of antibiotics may be treated as AICV without confirmatory examination.

Adult↗
PubMed1990

Should I trust office pregnancy tests?

Pregnancy tests have become quite reliable since the introduction of monoclonal antibody procedures. Even home kits use this technology and are greatly improved over earlier versions. However, as Dr Bluestein explains, use of these tests must be accompanied by an understanding of factors that can cause false results. He describes these factors and points out the need for further testing and obstetric consultation in some situations.

Antibodies, Monoclonal↗