Interactions of thrombin and antithrombin III with artificial surfaces.
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Adsorption↗
Biomedical subjects
S F Mohammad
Publications and source records attributed to S F Mohammad.
Quantitative removal of heparin from plasma and other aqueous solutions by affinity adsorption on poly(L-lysine)-sepharose 4B.
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Adsorption↗
Prostacyclin (PGI2) inhibits the enhancement of granulocyte adhesion to Cuprophane induced by immunoglobulin G.
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Cell Adhesion↗
A comparison of the antiplatelet effect of aspirin on human and bovine platelets.
The risk of thromboembolism in human patients or animal models with blood contacting prosthetic devices is well documented. Aspirin is used frequently as an antiplatelet agent to minimize this risk. Although the inhibitory effect of aspirin on human platelets has been clearly established, preliminary studies to examine this effect on bovine platelets revealed a minimal inhibition of platelet function in vivo as well as in vitro. Because a considerable amount of implant research is carried out in bovine models, it was considered important to evaluate carefully the antiplatelet effect of aspirin on bovine platelets. To evaluate the effect of aspirin, experiments were conducted on human and bovine blood in vitro as well as after the administration of aspirin up to 4,000 mg/day (p.o.) in calves and 2,500 mg/day in humans. Appropriate amounts of buffered aspirin were added to heparinized or citrated blood incubated for 25 min and centrifuged to obtain platelet rich plasma (PRP). An aliquot of PRP was then challenged by a predetermined concentration of ADP (an aggregating agent) or collagen to evaluate platelet aggregation and release reaction. After aspirin was administered in vivo, blood was drawn from the animal at predetermined intervals to evaluate platelet function. In human blood, the inhibitory effect of aspirin was discernable at 80 mg/L in vitro and 2,500 mg/day when ingested orally. Under identical experimental conditions, no inhibition of bovine platelet aggregation was observed using dosages of aspirin up to 1,000 mg/L in vitro or 4,000 mg/day in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)
Adenosine Triphosphate↗
Characterization of blood microemboli associated with ex vivo left ventricular assist devices in a bovine model.
An ex vivo left ventricular assist device (LVAD) model was used in calves to study the production of blood microemboli (BME) and to evaluate possible correlations between constant-pressure filtration (CPF) measurements of flow-resistant BME, light-scattering microemboli detection (LSMD), observable thrombus accumulation, and hematologic markers. Aortic LVAD cannulae were implanted through the chest wall in two calves, and each calf received a series of LVADs, each operating 1-4 days. Blood samples from the LVADs underwent CPF through 20 mu pore filters at 20 mm Hg to produce estimates of occlusive BME concentration. Laser (He-Ne) light was directed through the outflow cannula, and the differentially-scattered light was detected for computer estimation of microemboli size, volume, and frequency. Blood chemistry and coagulation parameters were also analyzed. Removed LVADs were examined with polar coordinate mapping of accumulated perivalvular thrombi. One ventricle produced significantly greater CPF results, LSMD results, and Factor XII levels. During the use of that ventricle, the CPF results increased slowly with time after LVAD connection, while the LSMD results and Factor XII levels increased immediately after connection, followed by a later decrease. This contrast is explainable in terms of the dynamic development of BME strength. The calf model appears useful for the study of thrombogenesis, and the CPF and LSMD methods of BME analysis complement each other, and other measurements.
Animals↗
Immobilized albumin-immunoglobulin G for improved hemocompatibility of biopolymers.
Efforts to render blood contacting surfaces less thrombogenic have included chemical modification of the polymer, as well as surface modifications with proteins, anticoagulants, or antiplatelet agents. Studies reported from this laboratory in the past have shown that certain surfaces pretreated with albumin-Immunoglobulin G (alb-IgG) become relatively more hemocompatible (reduced platelet adhesion, minimal thrombus). However, absorbed proteins, including alb-IgG, desorb rapidly when exposed to circulating blood. Therefore, efforts were made to immobilize alb-IgG on glass or Biomer as follows: a 2 mg/ml solution of IgG was crosslinked with glutaraldehyde on the test surface. Crosslinked IgG was then treated with mercaptoethanol to reduce the disulfide bonds, followed by incubation with albumin previously reduced with mercaptoethanol under conditions that allowed reassociation of disulfide bridges between albumin and crosslinked IgG. Alb-IgG immobilized at the test surface following this method was found to remain at the interface when the surface was exposed to blood in vitro for up to 7 days under static conditions. Glass or Biomer surfaces with crosslinked Alb-IgG showed considerable reduction in the adhesion of 111In-platelets and adsorption of 125I-fibrinogen when test surfaces were analyzed in a flow chamber. Under identical experimental conditions, surfaces with crosslinked albumin alone were not effective in reducing the adhesion of platelets. Crosslinked IgG or immobilization of IgG on crosslinked albumin following the above methods rendered the surfaces more thrombogenic. It was noted, however, that crosslinked proteins on Biomer were dislodged rapidly in areas with turbulent flow.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
An in vitro test model to study the performance and thrombogenicity of cardiovascular devices.
During the development of cardiovascular devices, it is necessary to evaluate their performance in vitro under experimental conditions that closely resemble their use in vivo. Therefore, an in vitro test model was developed to evaluate the performance of cardiovascular devices with blood. The in vitro test model consists of a pneumatically driven 50 cc polyurethane ventricle with either mechanical or experimental polyurethane tricuspid semilunar valves, connected to a horseshoe-shaped blood reservoir. A sampling port permits frequent removal of blood for hematologic evaluation. All the couplings are of the quick connect type to facilitate assembly and exchange of parts. The total volume of blood in the test model is approximately 1.4 L, and experiments are conducted in the absence of an air-blood interface. Except for mechanical valves and the chosen test interface, the entire blood contacting surface consists of polyurethane. Fresh bovine heparinized blood is used for the experiments. After filling the reservoir and the ventricle with blood, the concentration of heparin is adjusted with protamine to an APTT level of three times normal (80-100 sec). Once this level of anticoagulation is achieved, the blood is circulated for the next 2-3 hr at the desired test parameters (beats/min, cardiac output, etc.). A number of hematologic parameters are monitored during the test, including APTT, PT, plasma free hemoglobin and ADP-induced platelet aggregation. With the help of this model a number of devices, including experimental polyurethane tricuspid semilunar valves, Silastic tube valves, valves with heparin coated leaflets, and small (50 cc) experimental ventricles have been studied.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Microbiologic survey of prosthetic blood pumps presterilization and poststerilization and at explant retrieval.
Device-associated infection remains a major complication of implanted total artificial hearts (TAH). The possibility of microbes being introduced on the device was investigated by conducting a gross microbial assay, pre- and poststerilization, and following explant retrieval. Culture samples were obtained from the housing, base, and blood-contacting diaphragm of Utah-100 artificial ventricles. Additional samples were obtained from atrial sewing cuffs, outflow grafts, drive lines, and percutaneous leads, along with reference control samples prior to ethylene oxide sterilization (ETO). Culturing was repeated poststerilization and at device explant retrieval. Positive bacterial and fungal cultures were found in 24% of the presterilization samples; in the poststerilization samples, positive cultures were found in 6%. Following device explant retrieval, 84% of the cultures were positive. The reference control samples were positive in a limited number of the poststerilization samples. There was no correspondence of the species of micro-organisms found at the same location for each sampling condition. These data demonstrate that the surfaces of the TAH can become contaminated during fabrication. The presence of microbial activity poststerilization raises the possibility of inadequacy of the ETO protocol used with these devices, or contamination of the surgical field. Hearts at explant retrieval had cultures positive for microbes differing from those identified prior to implantation. This finding suggests that device-associated micro-organism colonization occurs through a source other than manufacturing or surgical contamination.
Animals↗
Anaerobic threshold in total artificial heart animals.
The anaerobic threshold represents an objective measure of functional capacity and is useful in assessment of pulmonary and cardiovascular dysfunction. This study determined the anaerobic threshold in total artificial heart animals and evaluated the performance of the total artificial heart system. Five animals with total artificial hearts were put under incremental exercise testing after exercise training. The intensity of exercise ranged from 2.0 to 4.5 km/hr, with an increment of 0.5 km/hr every 3 min. The anaerobic threshold was 6.72 +/- 0.84 ml/kg/min as detected by the lactate method, and 6.48 +/- 0.79 by the CO2 method. The value of the anaerobic threshold in total artificial heart animals implies that the performance capacity of a total artificial heart is not sufficient to meet the oxygen requirements of vigorously exercising skeletal muscle. The protocol does not allow for driving parameter changes during exercise, and this situation, combined with the manual mode of the control system used, was inadequate to allow the total artificial heart animals to exercise more vigorously. Using an automatic control mode might be helpful, as well as considering the relationship between indices of oxygen metabolism, such as oxygen delivery, oxygen consumption, and oxygen extraction rate, in the control algorithms in total artificial heart control systems.
Anaerobic Threshold↗
Magnetic field enhancement of antibiotic activity in biofilm forming Pseudomonas aeruginosa.
Device related infection initiated by biofilm bacteria are often difficult to resolve with antimicrobial therapy. Study results indicate that application of static magnetic fields may enhance the activity of gentamicin against biofilm forming Pseudomonas aeruginosa adherent to a polymer substrate. Results indicate a maximal reduction of 86.5 +/- 7.2% (n = 6) in the number of adherent viable bacteria compared with a control for samples exposed to a 5 gauss (G) magnetic field and gentamicin. The effect appears to be limited to magnetic fields between 5 and 20 G. Experiments using glass, Chronoflex (Polymedica, Golden, CO), Biomer (Ethicon, Somerville, NJ), and polystyrene substrate showed that the effect was independent of substrate surface. Autoradiograms from In111 uptake experiments showed that bacteria colonizing the substrate surface were significantly reduced in samples subjected to a magnetic field and gentamicin.
Anti-Bacterial Agents↗
Oxygen metabolism in animals with total artificial hearts.
The relationship between indices of oxygen metabolism has been widely used in clinical practice to evaluate the adequacy of tissue perfusion, to predict the outcome of the critically ill patient, and to evaluate the effectiveness of therapies. This study quantitated and correlated the relationship between oxygen delivery (DO2), oxygen consumption (VO2), and oxygen extraction rate (EO2) in 14 animals with total artificial hearts (TAH) to investigate the oxygen metabolism in animals with TAH during different physiologic and pathologic conditions. These 14 animals were subdivided into healthy, critical, and exercise groups. There was a physiologic dependence of DO2 to VO2 in animals in the healthy and exercise groups, whereas a pathologic dependence of VO2 to DO2 appeared to occur in animals in the critical group. Reduced or inadequate VO2 leads to organ dysfunction, shock syndrome, multiple organ failure, and finally, mortality. Providing a higher level of DO2 by restoring circulating blood volume, increasing cardiac output, raising hematocrit levels, and improving pulmonary function to achieve a higher level of oxygen extract efficiency and oxygen consumption in animals with TAH that are in a critical condition might be helpful for the treatment of complications and result in decreasing mortality. Using the relationship between indices of oxygen metabolism as a physiologic modifier for TAH control algorithms also might improve the physiologic performance and quality of life of TAH recipients.
Animals↗
Markers of thromboembolization in a bovine ex vivo left ventricular assist device model.
The production of blood microemboli (BME) was studied using an ex vivo exteriorized left ventricular assist device (LVAD) model in calves. Each of eight calves received a series of three LVADs, each operating for 24 hr. Blood microemboli were measured directly by a laser (624 nm and 828 nm) light scattering microemboli detection (LSMD) system through the LVAD outflow cannula and by constant pressure filtration (CPF) of blood samples from the LVAD outflow cannula. Hematologic parameters were also measured. After LVAD removal, perivalvular thrombi were evaluated using polar coordinate mapping. The average LSMD and CPF results correlated. For example, in one series of three calves, one ventricle exhibited significantly greater thrombogenesis than did the other ventricles, as indicated by both the LSMD and CPF results. In a series of five calves, one calf developed an abnormally high activated thromboplastin time (APTT), even in the absence of heparin. For two of the three ventricles tested in that calf, microemboli concentration (CPF), Factor XII activity, level of fibrin degradation products (FDP), and accumulated thrombus were significantly lower than for the other calves. The whole blood viscosity (WBV, at 230 s-1) in this calf also decreased to lower values than were seen with the other calves.
Animals↗
Effects of plasma on adhesion of biofilm forming Pseudomonas aeruginosa and Staphylococcus epidermidis to fibrin substrate.
Bacterial adhesion has been identified as the critical initial step in the pathogenesis of foreign body related infection. Recent investigations have shown microbial binding to implanted polymeric materials using specific adhesion of bacteria to immobilized plasma proteins, such as fibrin. These proteins are though to function as bridging molecules to facilitate bacterial colonization of the surface. The authors' results indicated a significant reduction in adhesion of biofilm forming Pseudomonas aeruginosa and coagulase negative Staphylococcus epidermidis to immobilized fibrin strands in the presence of platelet poor plasma (PPP) as compared to studies performed with phosphate buffered saline and Hank's balanced salt solution. A 10-fold decrease in the number of adherent bacteria was noted for samples exposed to PPP as compared to control samples. The effective range of PPP concentrations capable of producing the marked decrease in binding to fibrin strands was determined to be 1-100% for P. aeruginosa and 4-100% for S. epidermidis.
Bacterial Adhesion↗
Evidence that bacteria prefer to adhere to thrombus.
This study was undertaken to investigate the possible association between thrombosis and infection using an in vitro test model in which fresh bovine blood was recirculated through test conduits (3.5 mm inner diameter) containing stent-like devices. Anticoagulation was adjusted so that the recirculating blood deposited thrombi on the stent to cause gradual occlusion, thus impeding the flow. Four stent-like devices were placed in separate conduits in each experiment, and blood was recirculated with the help of pneumatically driven ventricles. Flow through these conduits was monitored by ultrasonic flow detection. To quantitate bacterial interaction with thrombi, Staphylococcus epidermidis (15E10(9)) was labeled with 111Indium-oxine and added to the blood. Experiments lasted until the flow in the test conduits dropped to 10% of the starting flow. During this recirculation, as flow gradually decreased, one stent was taken out when flow was still at 100%, the second at 75%, the third at 50%, and the fourth at 10% of the starting flow. The number of bacteria associated with the thrombus was measured by gamma counting. The following observations were made: 1) the amount of thrombus increased with time in all experiments (this was confirmed in separate experiments by using autologous 111Indium labeled platelets); 2) bacterial adhesion showed a concomitant increase as thrombus size increased (this was confirmed by using 111Indium labeled bacteria), and 3) bacterial incorporation into the thrombus occurred regardless of whether they were viable or pretreated with the antibiotic rifampin. These observations suggest that as thrombi develop, they may preferentially attract micro-organisms. This suggests that devices with adherent thrombi may have greater susceptibility for infection.
Animals↗
Device induced thromboembolism in a bovine in vitro coronary stent model.
The potential of a new bovine in vitro model to evaluate various aspects of device induced thromboembolism was studied using two test modes. First, the effect of an antithrombotic drug on stent induced thromboembolism was assessed. The antithrombotic potential of an antiplatelet agent was compared with that of the other conventional antithrombotic agents (aspirin, dipyridamole) used in the past with this in vitro model. Stent associated thrombus was assessed gravimetrically at the end of the experiment. Emboli were assessed continuously using a light scattering microemboli detection system. Second, the sensitivity of the model to flow induced thromboembolism was studied using a combination of surface roughness and stenosis. Thrombus was assessed visually, and emboli were assessed as described earlier. The results show that 1) this in vitro model is sensitive to the action of antithrombotic drugs, and to the effect of hemodynamics on thromboembolism; 2) the antiplatelet drug used in this study was effective in attenuating thromboembolism; 3) a stenosis in combination with roughness produced more emboli than roughness alone; and 4) the model was useful for the study of physical and biochemical aspects of thromboembolism.
Animals↗
Thromboembolization associated with sudden increases in flow in a coronary stent ex vivo shunt model.
To observe the dynamics of thromboembolism (TE) in an animal model, a carotid-carotid arterial ex vivo shunt was developed. A coronary stent deployed in a 3.5 mm polyvinylchloride (PVC) tubing segment was used as a model device in the shunt. A light-scattering microemboli detector monitored the embolic content of the blood flowing through the shunt at 50-150 ml/min as determined by a clamp-on ultrasound flow probe. The stent was found to actively develop thrombi and release emboli for 1-3 hours when the activated clotting time (ACT) was maintained between 125 and 150 seconds. The shunt flow rate fluctuated considerably (from 50 to 150 ml/min) depending on the animal's activity. When the time profile of this fluctuating flow rate was super imposed on the time profile of embolization, it was noted that sudden increases in flow rate were associated with incidents of embolization. Statistically, sudden flow rate increases of 100% or more were accompanied by embolic events 95% of the time (p < 0.01). Based on the results of this study, it was postulated that the increased embolization may be due to the fluid forces associated with accelerating flow. To explore this postulate, in vitro studies were conducted to compare the effects of pulsatile flow with steady flow on stent induced TE. Results of this study suggested a significant increase (100%) in both stent thrombosis and embolism during pulsatile flow compared with steady flow.
Animals↗
Formation of occlusive platelet aggregates in whole blood caused by low concentrations of ADP.
Minute concentrations of ADP are released when platelets are exposed to shear stress during extracorporeal flow. However, based on current methods, these low concentrations have not been shown to have a significant impact on platelet function. We report here the formation of rigid microaggregates (MA) in response to low concentrations of ADP. A newly developed light scattering whole blood aggregometer (LSWBA) was used to detect an aggregation dose response to ADP (0-2 microM) in heparinized (1.5 U/ml) human blood. Although the LSWBA showed that ADP induced MA were reversible, evidence provided by constant pressure filtration (50 mm Hg) suggested that aggregates existed as rigid particles in the blood for up to 6 minutes. The possible implications of these findings to extracorporeal circulation are discussed.
Adenosine Diphosphate↗
Enhanced risk of infection with device-associated thrombi.
Thrombosis and infection are two well-recognized risks with prosthetic devices that contact blood. Many of the currently used biomaterials may present an attractive surface for thrombus development as well as bacterial adhesion and colonization. Clinical experience with vegetative endocarditis patients has suggested that thrombosis may lead to enhanced risk of infection, and the possibility that adherent bacteria may enhance the risk of thrombosis has been noted by several investigators. To investigate the correlation between thrombosis and infection, a series of tests were conducted to assess the affinity of pathogen with surfaces in the absence and presence of blood components. Coronary stents were used as a model device to attract thrombi in a recirculating loop in vitro. Fresh heparinized blood was used to investigate thrombus development and bacterial interaction. (111)Indium-labeled Staphylococcus epidermidis and (111)Indium-labeled platelets were utilized to quantify bacterial interaction with thrombi under various test conditions. Anticoagulants, antiplatelet agents, and antibiotics were utilized in attempts to selectively influence bacteria, platelets, or thrombosis. The results suggest that under appropriate conditions, bacteria may preferentially adhere to platelet rich thrombus. These observations also suggest that by reducing the risk of thrombosis, the risk of device-associated infection may also be reduced.
Bacterial Adhesion↗