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At least 73 records · Page 4Linked to original sources
PubMed1991

Multiparameter optimizations in micellar liquid chromatography using the iterative regression optimization strategy.

An extension of the iterative regression optimization strategy to multi-parameter optimizations is described and applied to the separation of ionic compounds (amino acids and peptides) by means of micellar liquid chromatography. The parameters examined are the concentration of surfactant, the concentration of 2-propanol and pH. Fairly regular (linear, weakly curved) retention behaviour of the compounds as a function of the parameters results in an efficient optimization using a relatively small number of initial experiments.

Amino Acids↗
PubMed1990

Electric field-mediated gene transfer into K562 cells: optimization of parameters affecting efficiency.

Since hemopoietic cells are refractory to transfection by conventional chemical means, we have developed a reliable and efficient gene transfer system for K562 cells which uses electric field-mediated gene transfer (EFMGT). EFMGT involves the exposure of cells in suspension to an electric field which transiently allows the entry of DNA into the cell and its subsequent integration and expression. Plasmids bearing the neo gene were used to identify and select transfected clonogenic cells manifested by geneticin resistance in semisolid medium. Transfection efficiency is significantly affected by the following variables: voltage, capacitance, time constant, number of pulses, buffer type and temperature, DNA concentration, configuration, and promoter type. Cell cycle status also appears to be critical as shown in studies employing aphidicolin synchronization. Using optimal conditions, we have consistently achieved a transfection efficiency of 0.3-0.4% of clonogenic cells per microgram DNA. Stability of neo gene expression was also demonstrated after 4 months in nonselective culture conditions. This level of efficiency compares favorably with other reports of gene transfer into human hemopoietic progenitor cells.

Blast Crisis↗
PubMed1992

[Contact cyclophotocoagulation with the continuous wave Nd:YAG laser with quartz fiber. Optimizing coagulation parameters].

Transscleral cyclo-photocoagulation with a cw-Nd:YAG-laser was studied in enucleated porcine eyes with application times of between 10 ms and 1.5 s. Contact coagulation via a quartz fiber (core 600 microns) required about 32% less power to create visible ciliary body coagulation compared to non-contact coagulation via a focusing handpiece. Application of focusing fiber tips led to a further reduction in the coagulation threshold by a factor of 0.6. The energy per laser application required for ciliary body coagulation increased with application time. During contact coagulation using plane fiber tips a pronounced temperature rise was sometimes observed at the fiber tip due to deposits of carbonized tissue with the subsequent risk of scleral damage. In a pilot study five eyes with secondary glaucoma were treated under coagulation conditions found to be optimal (8 W at 0.2 s).

Anesthesia, Local↗
PubMed1976

Augmented immunogenicity of tumor cell membranes produced by surface budding viruses: parameters of optimal immunization.

Membranes prepared from tumor cells infected with surface budding viruses are much more immunogenic than membranes from uninfected tumor cells. Factors affecting immunization with membranes from virus-infected tumor cells were studied. Preparations made with influenza virus were clearly superior to those prepared with vesicular stomatitis virus (VSV). Membranes infected with VSV were maximally immunogenic at a dose equivalent to a 10% cell pack whereas influenza-virus-infected membranes were immunogenic at 1/100th of this dose. Subcutaneous inoculation was better than other routes of administration. Maximum protection against challenge with viable tumor cells was afforded by two inoculations of VSV-infected membranes spaced 3 days apart or a single inoculation with influenza-virus-infected membranes. Administration of membranes in complete Freund's adjuvant either had no effect of induced a slight degree of tumor enhancement. Immunization with influenza-virus-infected membranes significantly reduced tumor size and incidence even at a challenge dose of tumor cells which was 50 times the LD100.

Animals↗
PubMed1989

Computational issues of importance to the inverse recovery of epicardial potentials in a realistic heart-torso geometry.

In vitro data from a realistic-geometry electrolytic tank were used to demonstrate the consequences of computational issues critical to the ill-posed inverse problem in electrocardiography. The boundary element method was used to discretize the relationship between the body surface potentials and epicardial cage potentials. Variants of Tikhonov regularization were used to stabilize the inversion of the body surface potentials in order to reconstruct the epicardial surface potentials. The computational issues investigated were (1) computation of the regularization parameter; (2) effects of inaccuracy in locating the position of the heart; and (3) incorporation of a priori information on the properties of epicardial potentials into the regularization methodology. Two methods were suggested by which a priori information could be incorporated into the regularization formulation: (1) use of an estimate of the epicardial potential distribution everywhere on the surface and (2) use of regional bounds on the excursion of the potential. Results indicate that the a posteriori technique called CRESO, developed by Colli Franzone and coworkers, most consistently derives the regularization parameter closest to the optimal parameter for this experimental situation. The sensitivity of the inverse computation in a realistic-geometry torso to inaccuracies in estimating heart position are consistent with results from the eccentric spheres model; errors of 1 cm are well tolerated, but errors of 2 cm or greater result in a loss of position and amplitude information. Finally, estimates and bounds based on accurate, known information successfully lower the relative error associated with the inverse and have the potential to significantly enhance the amplitude and feature position information obtainable from the inverse-reconstructed epicardial potential map.

Animals↗
PubMed1990

Perioperative assessment of cardiac energy metabolism by means of arterio-coronary venous difference in lactate concentration (acDL). A parameter for optimizing ventricular function of the postcardioplegic myocardium.

Fourty-three patients undergoing open heart surgery were subjected to extended hemodynamic and metabolic monitoring (arterial and coronary sinus (CS) lactate concentration). In 95% of patients investigated, a significant increase in CS lactate concentration above the arterial lactate concentration was established at the beginning of reperfusion (RPT). In these cases, a decrease in the CS lactate below the arterial concentration occurs within the RPT creating a cross-over point (COP) from negative to positive lactate extraction values. Whereas the initial rise in CS lactate is an expression of the expected ischemia-induced acceleration of anaerobic glycolysis, the onset of a positive lactate extraction can be related to reconsolidation of mitrochondrial oxidative phosphorylation. We found two groups with an early and late COP at 7.2 +/- 0.8 min (ECOP) and 42.5 +/- 5.2 min (LCOP) (P less than 0.005). Whereas a short cross-over time was associated with a normal postoperative outcome, cardiac recovery was delayed in the LCOP group. With the aid of metabolic monitoring it is possible to identify the COP and cardiac overload situations and so to optimize the perioperative outcome, especially in selected patients (NYHA stage III/IV, re-operation, high-risk operation).

Adenosine Triphosphate↗
PubMed1986

Studies on the optimization of parameters of preparative liquid chromatographic columns for production of cardiac glycosides.

Investigations aimed at maximization of the output of the production of cardiac glycosides from their mixtures using liquid chromatography are reported. The dependences of the yield of the process on column length, particle size and surface area of the packing and linear velocity of the mobile phase were examined. It has been established that a simultaneous decrease in the particle size and an increase in the column length and mobile phase flow-rate results in an improvement in the yield. However, such a procedure is limited by technological considerations. Optimum production conditions have been characterized.

Cardiac Glycosides↗
PubMed1979

Glucose requirements following burn injury. Parameters of optimal glucose infusion and possible hepatic and respiratory abnormalities following excessive glucose intake.

Glucose and leucine metabolism in 18 severely burned patients were studied using the primed constant infusion of U-13C-glucose and 1-13C-leucine, respectively. The leucine data were used to calculate rates of whole-body protein synthesis. In four additional burn patients and seven normal controls, the effects of exogenously infused insulin on the metabolism of infused glucose were evaluated. Also, the effect on leucine metabolism of adding insulin to infused glucose was tested and rates of protein synthesis were calculated. The protein studies were divided into two groups depending on the rate of glucose infusion. Protein synthesis was 4.3 + 0.54 g protein/kg/day during the lower infusion rates (1.4--4.5 mg/kg/min) and 5.17 + 0.19 g protein/kg/day during the higher infusion rates (4.7--9.3 mg/kg/min) (statistically different, p less than 0.05). However, when the high infusion rate group was divided into two subgroups (high, 4.7--6.8 mg/kg/min, and very high, 7.03--9.31 mg/kg/min), there was no difference in the rate of protein synthesis. When U-13C-glucose was infused during varying rates of unlabeled glucose infusion, we found that the per cent of CO2 coming from the direct oxidation of glucose rose rapidly at the lower infusion rates but reached a plateau at approximately 55% as the infusion rates exceeded 5 mg/kg/min. Addition of insulin did not affect the rate of glucose oxidation but did seem to exert a stimulatory effect on protein synthesis. It was concluded that there appears to be a maximal rate of glucose infusion, beyond which physiologically significant increases in protein synthesis and direct oxidation of glucose cannot be expected. Furthermore, there appears to be a physiological cost of exceeding the optimal glucose infusion rate, as indicated by increased rates of CO2 production during infusion as well as large fat deposits in the liver at autopsy in patients infused with large amounts of glucose.

Adolescent↗
PubMed1975

Optimal stimulus parameters for minimum pain in the chronic stimulation of innervated muscle.

The degree of pain reported by ten volunteers upon electrical stimulation of their peroneal nerves was measured while stimulus parameters were varied. The responses were compared for stimulations, all of which produced the same amount of ankle torque. The stimulation parameters examined were pulse duration, 0.3 and 1.0 msec; repetition frequence, 20, 50 and 100 hertz; pulse shape, square wave and exponential wave; impedance of stimulator output, constant voltage and constant current; direct current bias, positive or negative to the stimulus; and wave phase, uniphasic and biphasic. Pulse duration of 0.3 msec and constant voltage output were the only two parameters significantly associated with minimum pain response. Analysis of voltage and current wave forms for the different stimulus parameters indicated that the sensation of pain was dependent upon the total amount of electrical charge delivered to the tissue with the pulse.

Ankle↗
PubMed1990

Effect of tumor necrosis factor on the antitumor efficacy and toxicity of aminopterin-monoclonal antibody conjugates: parameters for optimization of therapy.

Immunoconjugates of monoclonal antibodies with drugs, isotopes, or toxins are currently being investigated for their therapeutic effect on tumors. However, all have problems of access of the immunoconjugate to the tumor, particularly with solid tumors. To address this problem, we have used aminopterin-monoclonal antibody (AMN-mAb) conjugates combines with murine tumor necrosis factor (mTNF-alpha), which is known to have specific effects on tumor vasculature. In a murine model, well-established tumors (measuring 1.0-1.4 cm in diameter) were either totally eradicated or considerably reduced in size with combined therapy--a greater effect than with either mTNF-alpha or AMN-mAb used alone. The mechanisms involved in the improved antitumor effect were investigated using in vitro assays, autoradiography, and biodistribution experiments. mTNF-alpha was found both to increase the cytotoxic activity of the conjugate in vitro and to increase in vivo tumor localization of mAb up to 5-fold. The timing of mTNF-alpha administration was crucial to effects on tumor localization; mTNF-alpha given with mAb caused the greatest increase in localization and mTNF-alpha given well before mAb decreased localization. mTNF-alpha also reduced the toxicity to mice of AMN-mAb depending on the timing of injection. These results indicate that mTNF-alpha has a useful role in potentiation of immunoconjugate therapy but shows the need for careful planning of the dose regimen.

Aminopterin↗
PubMed1985

Pressure volume parameters for optimal function of an intraaortic balloon without a pump.

An air filled intraaortic balloon connected to an extravascular air chamber (compression chamber system-CCS) was used in 10 anesthetized dogs to increase its stroke volume (SV). Highest SV increases (up to over 200%) were observed at low SV states, with a high CCS volume and with high increases of the systolic pressure. The CCS pressure--systolic pressure relationship was a sigmoid-like curve. In conclusion, highest SV's may be achieved at low SV states, using CCS volumes about 400 ml and the highest CCS pressure that may cause a linear aortic pressure increase.

Animals↗
PubMed1990

Optimization of a polypyrrole glucose oxidase biosensor.

An amperometric glucose biosensor was fabricated by the electrochemical polymerization of pyrrole onto a platinum electrode in the presence of the enzyme glucose oxidase in a KCl solution at a potential of +0.65 V versus SCE. The enzyme was entrapped into the polypyrrole film during the electropolymerization process. Glucose responses were measured by potentiostating the enzyme electrode at a potential of +0.7 V versus SCE in order to oxidize the hydrogen generated by the oxidation of glucose by the enzyme in the presence of oxygen. Experiments were performed to determine the optimal conditions of the polypyrrole glucose oxidase film preparation (pyrrole and glucose oxidase concentrations in the plating solution) and the response to glucose from such electrodes was evaluated as a function of film thickness, pH and temperature. It was found that a concentration of 0.3 M pyrrole in the presence of 65 U/ml of glucose oxidase in 0.01 M KCl were the optimal parameters for the fabrication of the biosensor. The optimal response was obtained for a film thickness of 0.17 microns (75 mC/cm2) at pH 6 and at a temperature of 313 K. The temperature dependence of the amperometric response indicated an activation energy of 41 kJ/mole. The linearity of the enzyme electrode response ranged from 1.0 mM to 7.5 mM glucose and kinetic parameters determined for the optimized biosensors were 33.4 mM for the Km and 7.2 microA for the Imax. It was demonstrated that the internal diffusion of hydrogen peroxide through the polypyrrole layer to the platinum surface was the main limiting factor controlling the magnitude of the response of the biosensor to glucose. The response was directly related to the enzyme loading in the polypyrrole film. The shelf life and the operational stability of the optimized biosensor exceed 500 days and 175 assays, respectively. The substrate specificity of the entrapped glucose oxidase was not altered by the immobilization procedure.

Biosensing Techniques↗
PubMed1975

Application of modified Rosenbrock's method for optimization of nutrient media used in microorganism culturing.

The Rosenbrock's procedure has been modified for optimization of nutrient medium composition and has been found to be less tedious than the Box-Wilson method, especially for larger numbers of optimized parameters. Its merits are particularly obvious with multiparameter optimization where the gradient method, so far the only one employed in microbiology from a variety of optimization methods (e.g., refs, 9 and 10), becomes impractical because of the excessive number of experiments required. The method suggested is also more stable during optimization than the gradient methods which are very sensitive to the selection of steps in the direction of the gradient and may thus easily shoot out of the optimized region. It is also anticipated that other direct search methods, particularly simplex design, may be easily adapted for optimization of medium composition. It is obvious that direct search methods may find an application in process improvement in antibiotic and related industries.

Culture Media↗
PubMed1989

The evolution strategy--a search strategy used in individual optimization of electrical parameters for therapeutic carotid sinus nerve stimulation.

Optimization problems, arising in the search for parameters and/or techniques of functional electrostimulation (FES), disproportionally increase when multiple electrodes, electrode configurations, electrical parameters, and stimulation modes may be applied. When computational or investigational effort precludes systematic studies in FES, we propose to apply and evaluate Rechenberg's evolution strategy, which in technical use and numerical optimization has been valid in comparison to more traditional methods. This strategy implements mutation and selection processes in analogy to biological evolution. The effect of combined multiple input variables on a quality function (Q) is experimentally evaluated. The actual computed value of Q serves as a selection criterion for those input variable combinations which lead Q to approach a target value (maximization), similar to a hill-climbing procedure. In radiofrequency controlled, therapeutic electrical carotid sinus nerve stimulation (CSNS), we varied (mutated) combinations of pulse frequency and pulse amplitude parameters, according to the evolution strategy, in individual patients. CSNS lowers blood pressure and decreases heart rate. Q was computed from blood pressure and heart rate responses to CSNS. The strategy individually optimized electrical parameters to achieve large depressor responses upon CSNS. Although, in contrast to technical usage, only two input variables were investigated, and biomedical experience with the evolution strategy is limited so far, its potential use in other fields of FES, especially when more input variables are to be optimized, is discussed and encouraged.

Algorithms↗