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At least 343 records · Page 19Linked to original sources
PubMed2005

Evaluation of ablation patterns using a biophysical model of atrial fibrillation.

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia. Surgical/Radiofrequency (RF) ablation is a therapeutic procedure that consists of creating lines of conduction block to interrupt AF. The present study evaluated 13 different ablation patterns by means of a biophysical model of the human atria. In this model, ablation lines were abruptly applied transmurally during simulated sustained AF, and success rate, time to AF termination and average beat-to-beat interval were documented. The gold standard Cox's Maze III procedure was taken as reference. The effectiveness of twelve less invasive patterns was compared to it. In some of these incomplete lines (entailing a gap) were simulated. Finally, the computer simulations were compared to clinical data. The results show that the model reproduces observations made in vivo: (1) the Maze III is the most efficient ablation procedure; (2) less invasive patterns should include lines in both right and left atrium; (3) incomplete ablation lines between the pulmonary veins and the mitral valve annulus lead to uncommon flutter; (4) computer simulations of incomplete lines are consistent with clinical results of non-transumural RF ablation. Biophysical modeling may therefore be considered as a useful tool for understanding the mechanisms underlying AF therapies.

Atrial Fibrillation↗
PubMed2005

Mechanical optimization of an arteriovenous malformation embolization material: a predictive model analysis.

Arteriovenous malformations (AVMs) pose a constant danger of hemorrhages, seizures, and headaches to patients; they also disrupt oxygen-rich blood flow entering capillaries of the brain. We have utilized a linear model to mechanically characterize and optimize a water-borne, reverse emulsion, self-reactive, in situ cross-linking material, which we propose clinical use as an embolization material. The material is formed by cross-linking various acrylate and thiol multifunctional precursors with NaOH supplemented PBS. We compared theoretical elastic modulus values to modulus values observed during compression testing to determine the cross-linking efficiency of the material. Empirically determined elastic moduli for various material compositions ranged from 0.76 to 2.26 MPa, with corresponding cross-link efficiencies averaging 55+/-4%. We predict a reduction in theoretical circumferential stress exerted on AVM vasculature from 4933 to 10.9 Pa after embolization with the optimal material configuration. Theoretical risk of AVM rupture, as defined by Hademenos et al., was reduced below 1.0% for extreme variations of vessel modulus, thickness, and blood pressure after embolization with the optimized material. We will be using this material configuration to embolize swine rete mirabile AVM models and further assess the clinical viability of this potential embolization material.

Acrylates↗
PubMed2006

Near-wall deposition probability of blood elements as a new hemodynamic wall parameter.

The present study was performed to investigate deposition probability of blood particles on the vessel walls. To track dynamics of movement and adhesion of blood particles in the near wall region, two models such as the particle rolling model (PR-model) and the near wall force model (NWF-model) were employed in the present study. Simulations of the present models for the pre-activated platelets in the stagnated point flow chamber and for the pre-activated monocytes in the stenotic perfusion tube resulted in significant correlations with the experimental data. The proposed near wall deposition probability (NWDP) index exhibited good fits with the experimental data of the stagnation point flow chamber for the platelet. As for the monocyte, the NWDP index exhibited the best fit with the experimental data of the stenotic tube. The new hemodynamic index, NWDP, is different from the wall shear stress (WSS)-based hemodynamic parameters, such as MWSS (Mean Wall Shear Stress), AWSS (Amplitude of Wall Shear Stress), and OSI (Oscillatory Shear Index) in that it locates regions of both the high and low WSS. The proposed NWDP index needs to be tested and compared in real geometries for its effectiveness in locating regions of lesion-prone sites.

Animals↗
PubMedDate not supplied

Spectral analysis of event-related hemodynamic responses in functional near infrared spectroscopy.

The goal of this paper is to design experiments that confirm the evidence of cognitive responses in functional near infrared spectroscopy and to establish relevant spectral subbands. Hemodynamic responses of brain during single-event trials in an odd-ball experiment are measured by functional near infrared spectroscopy method. The frequency axis is partitioned into subbands by clustering the time-frequency power spectrum profiles of the brain responses. The predominant subbands are observed to confine the 0-30 mHz, 30-60 mHz, and 60-330 mHz ranges. We identify the group of subbands that shows strong evidence of protocol-induced periodicity as well as the bands where good correlation with an assumed hemodynamic response models is found.

Brain↗
PubMed1984

Limitations of visual assessment of redistribution in thallium images.

Potential limitations of visual assessment of redistribution in thallium (TI) images were studied and results were compared with computer assessment of redistribution. A four-section phantom filled with TI was imaged (300K counts, 128 X 128 matrix) with appropriate background activity and scatter material. Activity in a "defect" section (DS) was varied from 20% to 100% of reference sections (RS). After interpolative background correction, pseudo "initial" and "late" image pairs (N = 35) were photographed on polaroid film and read by three "blinded" observers using an 0-2, 1/2 step, scale (0 = absent and 2 = normal activity). Scan defects were detected by all readers when DS activity was less than or equal to 59% of RS activity. No reader detected a defect when DS activity was greater than or equal to 67% of RS activity. All "initial" defects were detected by computer analysis. Visual assessment of "initial" DS:RS activity ratio did not correlate well with DS:RS activity ratio of the phantom. In contrast, computer assessment of "initial" DS:RS activity ratio correlated well with phantom DS:RS activity ratio (r = 0.96, p less than .0001). Although 22 of 27 scan pairs with partial (N = 26) or full (N = 1) redistribution were correctly identified as showing redistribution by at least two of three observers, the extent of redistribution was not estimated well by visual analysis. Thus, visual assessment of absolute change ("initial"-to-"late") in DS:RS activity ratio showed considerable scatter in relations to actual changes in DS:RS activity ratio of the phantom.(ABSTRACT TRUNCATED AT 250 WORDS)

Computers↗
PubMed1991

The effects of regurgitant orifice size, chamber compliance, and systemic vascular resistance on aortic regurgitant velocity slope and pressure half-time.

The determinants of the aortic regurgitant velocity profile have been investigated using computer and in vitro simulations in which regurgitant orifice area, ventricular and aortic compliance, and systemic vascular resistance could be independently varied. In the study, regurgitant fraction was altered, either by changing the size of the regurgitant orifice or by holding the regurgitant orifice constant and changing chamber compliance or systemic vascular resistance. Upon increasing regurgitant fraction by increasing the size of the regurgitant orifice, the slope got steeper and the pressure half-time shortened, the response anticipated in current clinical practice. However, when the regurgitant orifice was kept constant and regurgitation fraction was increased by increasing the systemic vascular resistance or by increasing the compliance of the left ventricle, slope became less steep and pressure half-time lengthened. Multivariate analysis was used to quantify the relationship of regurgitant fraction to slope and pressure half-time. When orifice area was allowed to vary, slope was related directly (multiple r = 0.78, p less than 0.001) and half-time was related inversely (multiple r = 0.66, p less than 0.001) to regurgitant fraction. With the orifice area fixed, however, directionally opposite responses were seen; slope varied inversely (multiple r = 0.87, p less than 0.001), whereas half-time varied directly (multiple r = 0.88, p less than 0.001) with regurgitant fraction. This study suggests that the utility of the slope and pressure half-time of the regurgitant velocity tracing in clinical practice relates to their ability to discriminate regurgitant orifices of differing sizes.(ABSTRACT TRUNCATED AT 250 WORDS)

Aortic Valve Insufficiency↗
PubMed1994

Ultrasound accelerates urokinase-induced thrombolysis and reperfusion.

We have shown that ultrasound accelerates TPA-induced thrombolysis in vitro as assessed by release of labeled fibrinogen from radioactive labeled clots. Others have shown that ultrasound shortens the time to recanalization of TPA treated thrombi in animal models. The aim of this study was to test the hypothesis that ultrasound enhances thrombolysis and reperfusion by using urokinase in an in vitro flow system. An in vitro flow system of a branching tubing circuit was developed. Flow in one branch was obstructed by a thrombus. Five control clots were exposed to continuous wave ultrasound at a frequency of 1 MHz and intensity of 2.5 W/cm2 only without any thrombolytic agent (group 1). Twenty clots were exposed to a bolus of 80,000 U of urokinase and randomized to either ultrasound exposure (group 2) or to urokinase only without ultrasound (group 3). Flow distal to the clot and the rate of release of radiolabeled fibrin were used as indexes of reperfusion and thrombolysis, respectively. Exposure to ultrasound significantly accelerated urokinase-mediated reperfusion, with 40.6% +/- 11.8% of maximal flow in group 2 versus 1.3% +/- 0.7% in group 3, p < 0.0015 after 25 min. The maximal difference in flow between groups 2 and 3 was achieved at 40 minutes (67.4% +/- 11.1% vs 13.1% +/- 5.6%, p < 0.0009). Thrombolysis was significantly higher after 25 minutes of ultrasound exposure (24.1% +/- 4.6% in the ultrasound-treated group vs 9.7% +/- 3.5% in group 3, p < 0.013). The maximal difference in thrombolysis between groups 2 and 3 was 60 minutes. (52.5% +/- 5.1% vs 18.7% +/- 6.2%, p < 0.00015).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗
PubMed1994

Solid-angle theory and heart rate adjustment of ST-segment depression for the identification and quantification of coronary artery disease.

Determinants of the ST-segment response to exercise can be mathematically modeled by solid-angle theory, and heart rate adjustment of the magnitude of exercise-induced ST-segment depression can remodel the solid-angle relationship to provide a theoretic and practical basis for application of heart rate-adjusted indexes of ST depression in exercise electrocardiography. Solid-angle theory indicates that the magnitude of ST depression recorded at a surface electrode (epsilon) can be described as the product of spatial and nonspatial determinants: epsilon = (omega/4 pi).(delta Vm).K (equation 1), where omega is the solid angle subtending the boundary of the ischemic territory, delta Vm is the difference in transmembrane voltage between the ischemic and adjacent nonischemic regions, and K is a term correcting for differences in intracellular and extracellular conductivity and changes in end-plate conductance. As a consequence, the magnitude of ST depression recorded by a surface electrode will be proportional both to the area of ischemic territory subtended by the recording electrode, which reflects the solid angle, and to the local transmembrane potential difference, which in turn reflects the electric consequences of the metabolic severity of ischemia at the level of the myocardial cell. It follows from equation 1 that the amplitude of ST depression can accurately reflect the area of ischemic boundary only when the severity of ischemia is constant or otherwise controlled, and differences in ST depression will only reflect varying areas of underlying ischemia when similar severity of ischemia is present. During exercise the severity of ischemia is directly proportional to changes in myocardial oxygen demand and coronary blood flow, which in turn are directly related to increasing heart rate (delta HR). Because the change in transmembrane voltage across the ischemic boundary is linearly proportional to delta HR, delta Vm/delta HR remains constant as ischemia develops. Dividing the solid-angle relationship in equation 1 by delta HR and making the appropriate substitution for a constant delta Vm/delta HR then indicates that epsilon/delta HR = (omega/4 pi).(c . K) [equation 2], where c is the new constant. Under conditions where changes in conductance are proportional or small, this simplified relationship reduces to delta ST/delta HR = c'.omega [equation 3], where delta ST reflects the magnitude of ST depression recorded by the surface electrode, delta HR the change in heart rate during developing ischemia, and c' the resulting empiric constant.

Coronary Disease↗
PubMed1989

Univariate genetic analysis of blood pressure in children (the Medical College of Virginia Twin Study).

The relative contributions of genetic, individual environmental and shared environmental effects on resting blood pressure (BP) and heart rate (HR) were studied in prepubescent twins. The study population consisted of 251 caucasian 11-year-old twin pairs. Correlations were higher for all variables in monozygotic twins compared to dizygotic twins; this is consistent with a significant genetic effect. Path analysis revealed that the model of additive genetic and individual environmental effects fit systolic BP, diastolic BP and HR. In boys and girls, sex-specific genetic effects controlled systolic BP. The magnitudes of the sex-specific genetic effects on systolic BP were similar in both boys and girls and accounted for 66% of the variance. In boys, for diastolic BP, genetic effects accounted for 64% of the variance while in girls they accounted for 51%. These results provide no evidence for different genetic effects on HR in boys or girls. No shared environmental effects were detected. The large sample size and design, using different-sex dizygotic twins of the same age, establish that genes play an important role in the influence of resting BP and HR and that there are sex-specific genetic contributions in early pubertal children.

Blood Pressure↗
PubMed1990

Balloon dilatation of the stenosed aortic valve: how does it work? Why does it fail?

The hemodynamic changes that may occur in patients undergoing aortic balloon valvuloplasty were examined in the circulatory model. Four conclusions were reached. (1) Significant transvalvular pressure gradients appear only if the orifice is severely narrowed. (2) The magnitude of this gradient is highly flow dependent. (3) At critical narrowings, minute alterations in orifice size may induce most significant changes in the transvalvular gradient. (4) In low flow states significant gradients appear only if the stenosis is extreme. In patients with aortic stenosis, especially those with failing hearts and low cardiac output, the pressure gradient may be effectively decreased by minimal dilatation of the aortic orifice. These patients, however, remain in jeopardy because recurrent narrowing may cause a gradient incompatible with life.

Aortic Valve↗
PubMed1991

Pitfalls in the determination of absolute dimensions using angiographic catheters as calibration devices in quantitative angiography.

Using catheter outer diameter as a scaling device, quantitative coronary arteriography allows the precise and objective measurement of change in absolute dimensions of coronary arteries after mechanical or pharmacologic intervention. Because of variable density in the wall of the catheter, automated systems might vary in the determination of the outer catheter diameter. To examine this premise, catheters in a variety of French sizes from 6 manufacturers were injected with radiographic contrast and used as scaling devices for arterial phantoms of known geometric dimension. Radiographic diameters of the catheters were determined by applying the quantitative coronary arteriographic algorithm to the catheters using a calibration grid in the same field of view. The varying composition of the catheters resulted in differing x-ray attenuation and, subsequently, automated edge-detection algorithms varied widely in determining the actual catheter diameter to be used as a scaling factor. For instance, a Lucite "artery" with a minimal luminal diameter of 1.50 mm (image calibrated using the micrometer-determined outside diameter of a Baxter 8Fr guiding catheter) resulted in a quantitative angiographic diameter of 2.03 mm (overestimation by 35%). If the diameter of a similar size Shiley catheter was used to calibrate the image, a luminal diameter of 1.60 mm was determined: a difference of 0.43 mm based solely on differences in scaling catheter attenuation. These data suggest that a specific "fingerprint" for each catheter material and catheter French size exists, rendering generalizations about catheter size questionable. These observations are important for quantitative angiography where many brands and sizes of angiographic catheters are being used clinically.(ABSTRACT TRUNCATED AT 250 WORDS)

Calibration↗
PubMed1991

Regional myocardial stress distribution from magnetic resonance image-based mathematical models.

The instantaneous regional stress distribution within the myocardium, which cannot be directly measured, has been estimated using improved numerical methods and nonaxisymmetric biventricular geometry. To do this, we have employed computer-aided solid mathematical modeling to generate a three-dimensional representation for an ex vivo canine biventricular unit using magnetic resonance imaging. A two-dimensional transverse section was isolated from the solid mathematical model for regional stress analysis using p-version finite element analysis. Loading conditions and material property descriptions were taken from published reports. Analyses showed the maximum principal stresses to range from -1.76 X 10(5) to 8.52 X 10(5) dynes/cm2 during systolic loading, and from -3.85 X 10(4) to 1.13 X 10(5) dynes/cm2 during diastolic loading. This study demonstrates that magnetic resonance image-based solid mathematical biventricular models are suitable for regional stress analysis using p-version finite element analysis. p-Version finite element analysis using magnetic resonance image-based cardiac representations facilitates in vivo stress-strain analyses and may allow the clinical estimation of regional myocardial stress.

Animals↗
PubMed1995

The O'Brien-Angell stentless porcine valve: early results with 150 implants.

From August 1991 to June 1994, 150 patients underwent aortic valve replacement with the O'Brien-Angell stentless porcine xenograft (Bravo Cardiovascular Model 300, Cryolife, Atlanta, GA). To establish trends we analyzed three consecutive groups of 50 patients. We found significant differences in low postoperative gradients (mean < or = 10 mm Hg): 24% in group 1, 42% in group 2, and 96% in group 3. Comparing groups 1 and 3, gradients were significantly lower in all valve sizes. The difference is credited to better supraannular positioning of the valve, which is the key to the learning curve. Trivial central regurgitation was present in the three groups at 6%, 12%, and 0%, respectively. Peripheral regurgitation was trivial in 6%, 8%, and 0%, and mild to moderate in 4%, 2% and 0%, respectively. Seventy-eight of 107 patients with an available follow-up exceeding 1 year had noninvasive controls. Two early cases with moderate perivalvular leaks evolved to moderately severe leaks. Two valves were explanted. The O'Brien-Angell stentless valve is easy to handle and correct supraannular positioning provides excellent hemodynamic results.

Adult↗
PubMed1993

Mathematical modeling of time-dependent oxygen transport in rat peripheral nerve.

We modeled time-dependent transport of oxygen in peripheral nerve. Simulation began with a steady-state oxygen tension field determined by capillary diameter and length, intercapillary distance, blood-flow velocity, oxygen consumption rate, and arterial oxygen tension. One of these parameters was assumed to change rapidly to new constant value, producing time-varying oxygen tensions. A monoexponential or biexponential function characterized the oxygen tension time variation. Rate constants of the slower exponential ranged from 0.017 sec-1 to 0.46 sec-1, implying minimal time lag in response of peripheral nerve oxygen tensions to alterations in blood flow, arterial blood oxygenation, or metabolic demands.

Animals↗
PubMedDate not supplied

Polychotomous multivariate models for coronary heart disease simulation. II. Comparisons of risk functions.

This is the second in a series of papers dealing with models of coronary heart disease. Three different types of statistical models are considered as risk functions: the multivariate logistic model, the Cox proportional hazard model and the Neyman exponential risk avoidance model. The types of models differ in the form hypothesized for the probability of occurrence of coronary heart disease outcomes: incident myocardial infarct, cardiac death, and death from other causes. Although the three risk functions are strikingly different, they can all be tested using the CRISPERS chronic disease simulation system. Simulations were performed using data from North Karelia, Finland. The polychotomous multivariate logistic risk function is convenient for studies involving increasing numbers of risk factors. The Cox proportional hazard regression model is shown to be unsuitable for the cohort dataset used as well as for some of the intended uses of the simulation models. The Neyman exponential risk avoidance model involves time in a quite different fashion. It has the inherent advantage of being easier to relate to underlying biological mechanisms because it is the integral of first order rate equations. It is concluded that more than one risk function should be evaluated for simulations of coronary heart disease.

Adult↗
PubMedDate not supplied

Polychotomous multivariate models for coronary heart disease simulation. III. Model sensitivities and risk factor interventions.

This is the third in a series of papers dealing with models of coronary heart disease. Sensitivity analyses of the logistic risk function and the Neyman risk function are reported. The resulting response surfaces are also used to investigate the optimality of the set of values for the risk coefficients. It is shown that the coefficients estimated by maximum likelihood are preferable to the sets from an optimisation procedure. Two different sets of risk coefficients estimated using short periods and entire epochs for the logistic risk function are shown to lead to similar conclusions concerning simulated primary intervention strategies. However, the corresponding risk factor reductions using the Neyman risk function lead to somewhat different effects. Additional information is needed to distinguish between these two assumptions of the risk function used to model coronary heart disease. This underscores the need to understand the effects of the underlying risk function assumed when interpreting simulated outcomes of intervention strategies.

Computer Simulation↗
PubMedDate not supplied

Polychotomous multivariate models for coronary heart disease simulation. IV. The impact of physiological aging.

This is an extension of a series of papers dealing with certain models used in the simulation of coronary heart disease. The current study investigates implications of including age as a risk factor in the models discussed in the preceding papers. The effects of using age as a risk factor were investigated in two ways. In one of these, age is interpreted as age of entry into the study; it is similar to the other risk factors in that it is assumed to be constant throughout the study. In the other, age is interpreted as the actual age; thus it increases during the course of simulations. Two polychotomous, multivariate risk functions developed in previous studies, the logistic risk and the Neyman exponential risk, were used to explore the effects of including age as a risk factor. The estimated risk coefficient for age was found to be statistically significant for both functions. The model performance was evaluated by comparing the observational data with outcomes simulated using Monte Carlo techniques. It was found that the logistic risk function failed to describe the observations either with age as a constant or with aging during the simulations. The models including the Neyman exponential risk avoidance fit the data well. The evaluation of the results indicates that aging during the simulations is better than using only the age as the constant value at entry to the study.

Adult↗