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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5943-5949 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientation and extension of a finitely extensible nonlinearly elastic (FENE) dumbbell in an electric field is analyzed to model the nonlinear phenomena observed during orientation and relaxation of flexible polymer molecules in transient electric birefringence measurements. In the model the dumbbell interacts with the electric field via an induced dipole with a dipole moment proportional to the end-to-end separation of the dumbbell. The electric field produces a strong "elongational'' deformation of the molecule which has analogies with the deformations produced by elongational flow. The steady state birefringence, as a function of field strength, is calculated in two ways: first using matched expansions around low and high field conditions, and then using Peterlin's approximation of preaveraging the connector force law. Both techniques yield similar results: Kerr law behavior is observed at low field strengths and saturation of birefringence at high field strengths. Birefringence relaxation after the field is removed shows a strong field dependence on the initial rate of relaxation. Previously proposed means of analyzing birefringence relaxation data for flexible molecules are discussed in light of this observation.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Criminology 6 (1969), S. 0 
    ISSN: 1745-9125
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Law
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Biotechnology progress 11 (1995), S. 99-103 
    ISSN: 1520-6033
    Source: ACS Legacy Archives
    Topics: Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 3212-3219 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientation and relaxation dynamics of flexible polymers in an electric field is analyzed by following the change in electric birefringence with time. The polymer chain is represented by two beads (dipole) connected by a Hookean spring and interacts with the electric field through a local induced dipole which is proportional to the end-to-end distance between the beads. Equations of motion are written for the beads taking into account (a) the hydrodynamic drag force, (b) the Brownian force, (c) the spring force, and (d) the electric force acting on the molecule. The electric field produces a strong anisotropic orientation of the polymer chain. Thus, equations are derived and analytically solved to yield the time dependence of the rise of birefringence in the electric field, the relaxation of the birefringence from a nonequilibrium state, and the change in the mean-squared end-to-end distance of the molecule with time. The dynamics are found to be governed by the dimensionless number μ0E2/H, where μ0 is a constant related to the polarizability of the molecule, E is the electric field strength, and H the Hookean spring constant. A merit of this analysis lies in the fact that expressions for the time course of the birefringence rise in the presence of an orienting field of strength E as well as relaxation in the presence of a reduced electric field, Er (Er〈E) can be readily derived; hence cases other than the field-free decay (Er=0) can be interpreted. Moreover, for a biological system, a state of equilibrium with Er=0 may never be reached. Finally, the field dependence of the birefringence and the orientation and relaxation time constants has been obtained using the model and comparison of the steady state and relaxation behavior of Hookean and finitely extensible nonlinear elastic (FENE) polymer chains has been carried out. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of interventional cardiology 8 (1995), S. 0 
    ISSN: 1540-8183
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Catheter ablation has evolved into the dominant therapeutic modality in the treatment of a variety of arrhythmias, particularly supraventricular arrhythmias with the mechanisms of atrioventricular (AV) nodal reentry and AV reciprocating tachycardia via an accessory pathway. The mode of catheter ablation used in the great majority of cases is radiofrequency (RF) catheter ablation. This technology is well-suited for the above arrhythmias because the targets and the RF lesions are both small and discrete. Using temperature monitoring may improve the outcome of these procedures by decreasing procedure time and incidence of coagulum formation on the catheter after a sudden rise in electrical impedance. New RF catheter designs and new modalities of creating catheter-induced focal myocardial injury will allow operators to have improved success with the ablation of less approachable arrhythmias, including atrial flutter and reentrant ventricular tachycardia. Studies are currently underway to create a catheter based “maze” procedure for the treatment of atrial fibrillation. As techniques and technologies evolve, a greater proportion of patients with symptomatic or threatening arrhythmias may be approached with catheter ablation as a curative or palliative procedure.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Visually Directed Subendocardial Resection. introduction: in prior studies, 20% to 40% of patients undergoing subendocardial reaction (SER) for ventricular tachycardia (VT) could not he mapped intraoperatively because the VT was either noninducible or nonmappable following the ventriculotomy. The optimal surgical approach to such patients is not known. Methods and Results: In this study, we retrospectively compared the long-term survival and functional outcome of 29 patients with VT and prior myocardial infarction who were either noninducible or nonmappable intraoperatively and underwent a visually directed extended SER These results were then compared to 85 patients who had inducible VT intraoperatively and underwent a map-guided sequential SER. The two patient groups had different clinical characteristics. The visually directed cohort was more likely to he male, experienced fewer VT episodes before surgery, and underwent fewer antiarrhythmic drug trials prior to reaction. In addition, the visually directed group had slower VT induced at a preoperative electrophysiologic study and was less likely to present to the operating room in stock or incessant VT than the map-guided group. The postoperative VT clinical recurrence or inducibility rate was 14% in both the visually directed and map-guided groups. The long-term actuarial survival at 1, 3, and 5 years was 93%, 86%, and 75%, respectively, in the visually directed group, compared to 77%, 58%, and 58%, respectively, in the map-guided group (P = 0.06). There were no documented nonfatal recurrences of VT in either group. At 24 months following surgery, 77% of patients who had a visually directed SER were in New York Heart Association Functional Class I or II, compared to 46% of patients who underwent a map-guided SER (P 〈 0.05). Conclusion: In patients with VT and prior myocardial infarction, the inability to induce or map the VT in the operating room does not preclude a favorable long-term outcome if a visually directed extended SER technique is used.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiovascular electrophysiology 5 (1994), S. 0 
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Ultrastructural Findings After RF Ablation. Introduction: We hypothesized that myocardial injury following radiofrequency (KF) catheter ablation may extend beyond the region of acute coagulation necrosis as defined by histochemical staining. Methods and Results: Five RF lesions were created in vivo in the left ventricle of two dogs using a 4-inm tipped ablation electrode in which RF power was adjusted to maintain an electrode-tissue interface temperature of 85°C for 60 seconds. The lesions were bisected; one half of the lesions were stained with nitroblue tetrazolium (NBT) and the other half processed for electron microscopy. Three zones of interest were identified extending 0–3 mm, 3–6 mm, and 〉 6 mm from the visible pathologic lesion border. The degree of Ultrastructural injury to the myocardium was scored for each zone. Electron microscopy demonstrated the presence of significant abnormalities of the plasma membrane, mitochondria, sarcomeres, sarcoplasmic reticuluni, and gap junctions of myocytes, as well as damage to the microvasculature extending up to 6 mm beyond the pathologic lesion edge. The plasma membrane and gap junctions of myocytes and the microvasculalure appeared particularly sensitive to thermal injury, whereas the intercalated discs were relatively thermally resistant. Conclusion: RF catheter ablation results in Ultrastructural damage to the myocardium extending up to 6 mm beyond the acute pathologic RF lesion border as denned by NBT histochemical staining.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Pacing and clinical electrophysiology 20 (1997), S. 0 
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Inc
    Journal of cardiovascular electrophysiology 12 (2001), S. 0 
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Hyperthermic Myocardial Injury. Introduction: We hypothesized that intracellular calcium overload may play an important role in heat-induced myocardial injury. This postulate was investigated using a model of isolated guinea pig papillary muscle in which resting tension was measured as an indirect indicator of cytosolic free-calcium concentration and the fluorescence changes of Fluo-3 AM dye was measured as a direct indicator of cytosolic free-calcium concentration. Methods and Results: Excised guinea pig right ventricular papillary muscles were attached to a force transducer in a high-flow tissue bath and superfused with Tyrode's solution at 37°± 0.5°C. The temperature was rapidly changed to between 38.0° and 56.0°C for 60 seconds and then returned to 37.0°C. Hyperthermia caused a reversible increase in resting tension at temperatures between 45° and 50°C and irreversible contracture at ≥50°C. Rapid cooling contracture experiments and experiments measuring fluorescence of myocytes loaded with 5 μ M Fluo-3 AM dye demonstrated that the hyperthermia-induced rise in resting tension was likely due to an increase in intracellular calcium content. Inhibition of the sarcoplasmic reticulum calcium pump with 20 μ M thapsigargin resulted in irreversible contracture of the papillary muscles at temperatures between 45° and 50°C and significant increases in Fluo-3 fluorescence at 48°C. Blockade of sarcolemmal calcium channels with 0.5 mM cadmium or 40 μ M verapamil did not attenuate the heat-induced increase in resting tension and Fluo-3 fluorescence. Conclusion: Hyperthermia causes an increase in resting tension of cardiac muscle that most likely is mediated by a calcium channel-independent increase in calcium permeability of the sarcolemmal membrane and/or release of stored intracellular calcium.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiovascular electrophysiology 6 (1995), S. 0 
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: RF Ablation and Creatine Kinase. Introduction: The primary mechanism of myocardial injury during radiofrequency (RF) catheter ablation in the heart is presumed to be thermal. Creatine Kinase has been measured in serum to assess the volume of myocardial injury after ablation. However, its thermal inactivation by RF ablation could lead to underestimation of the true volume of injury. Methods and Results: Serial RF lesions were created in 10 canine left ventricles in vivo, and serial serum Creatine Kinase activities were measured and compared to lesion volume. To assess the stability of myocardial Creatine Kinase during RF catheter ablation, 29 RF ablations were made on the epicardial surface of porcine left ventricle in vivo and a 2-mm core biopsy was rapidly removed. The cores were rapidly frozen, sectioned longitudinally in 1-mm slices, and homogenized in 0.3 M Tris buffer solution containing EDTA and dithiothreitol for subsequent analysis of Creatine Kinase activity. An additional 19 tissue cores from RF lesions were stained and used to determine mean lesion depth. Normal tissue biopsies were exposed to 60 seconds of hyperthermia (37° to 85°C, n = 190), or high-density RF current at 50X (0 to 100 mA/mm2, n = 50), and tissue Creatine Kinase activity was measured. There was no evidence of Creatine Kinase washout within the first 2 hours, and peak values were measured 5 to 7 hours postablation. Tissue Creatine Kinase activity in the first mm depth of RF lesions averaged 10% of control values and increased over the first 5 mm of lesion depth. The mean Creatine Kinase activity within the hemisphere of ablated myocardium was calculated to be 31% of control. Creatine Kinase activity declined significantly at temperatures above 65°C, but no difference in tissue Creatine Kinase activity was observed among differing levels of RF current exposure in the absence of significant heating. Conclusions: Creatine Kinase activity in myocardial tissue is significantly diminished within the RF lesion. Creatine Kinase activity is not stable at temperatures above 65°C, which are routinely achieved within the central zone of RF ablation, and is unaffected by RF current in the absence of hyperthermia. Measurements of serum Creatine Kinase activity after RF catheter ablation may significantly underestimate the volume of myocardial injury.
    Type of Medium: Electronic Resource
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