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Spatial Dispersion (spatial + dispersion)

Distribution by Scientific Domains

Business, Economics, Finance and Accounting	13%

Selected Abstracts

Effect of Action Potential Duration and Conduction Velocity Restitution and Their Spatial Dispersion on Alternans and the Stability of Arrhythmias

JOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 11 2002
ISABELLE BANVILLE Ph.D.
Restitution and Spatial Heterogeneities vs Arrhythmias.Introduction: The slope of the action potential duration (APD) restitution curve has been used to explain wavebreaks during arrhythmia initiation and maintenance. This hypothesis remains incomplete to fully describe the experimental data. Other factors contributing to wavebreaks must be studied to further understand arrhythmia dynamics. Methods and Results: Control APDs were measured from isolated rabbit hearts using a monophasic action potential probe. APD and conduction velocity (CV) restitution were quantified over the heart surface for two drugs, diacetyl monoxime (DAM) and cytochalasin D (CytoD), using a dual camera video imaging system. For all pacing intervals: (1) control APDs were shorter than for CytoD but longer than for DAM; and (2) CV was greater for CytoD compared with DAM. APD dispersion increased as pacing interval decreased for both drugs. For DAM, increased dispersion was due to a difference in APD restitution between the right and left ventricle. For CytoD, increased dispersion was due to discordant alternans, with no significant spatial variation in restitution. Fibrillation was sustained only in the control hearts; with DAM, stable reentry was sustained with shorter APD and cycle length compared with CytoD for which only nonsustained unstable reentry occurred. Conclusion: Alternans and arrhythmia dynamics are affected by the spatial dispersion of APD restitution as well as CV restitution, not simply the slope of APD restitution. Therefore, a direct link of the APD restitution slope to alternans and arrhythmia dynamics in rabbit heart does not exist. Designing antiarrhythmic drugs to alter only the restitution slope may not be appropriate. [source]

Mechanism of Propensity to Atrial Fibrillation in Patients Undergoing Isthmus Ablation for Typical Atrial Flutter

JOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 2 2005
HEMANTH RAMANNA M.D.
Background: Patients undergoing isthmus ablation for atrial flutter (AFL) may reveal postablation atrial fibrillation (AF). The electrophysiological mechanism is unclear. In patients with idiopathic AF, enhanced spatial dispersion of right atrial refractoriness was the substrate for the initiation of AF. We hypothesize that dispersion of right atrial refractoriness in patients undergoing AFL ablation is the major cause of postablation AF. Methods: Consecutive patients (n = 42) undergoing isthmus ablation for typical AFL were included. Twelve right atrial unipolar electrograms were recorded. Inducibility of AF was assessed by a pacing protocol, starting with one extrastimulus, followed by more aggressive pacing until AF was induced. Mean fibrillatory intervals were used to assess local refractoriness of each recording site. Spatial dispersion of right atrial refractoriness was calculated as the coefficient of dispersion (CD-value: standard deviation of the mean of all local mean fibrillatory intervals as a percentage of the overall mean fibrillatory interval). A CD-value of 3.0 or less was defined as normal, whereas CD-value greater than 3.0 was considered enhanced dispersion. PES and refractoriness analysis were followed by isthmus ablation. Results: Of the 42 patients, 29 had CD-value of 3.0 or less. In these 29 patients, AF was induced with 1 extrastimulus in only 1 patient, with 2 extrastimuli in 4 patients and burst pacing was required to induce AF in 24 of these 29 patients. Prior to the procedure, 5 of 29 patients had AF episodes, after ablation 6 of 29 patients. Of the 42 patients, 13 had CD-value greater than 3.0, AF was induced with a single extrastimulus in 11 patients, with 2 extrastimuli in the remaining 2 patients. Of the 13 patients, 11 had AF episodes both before and after ablation (P < 0.001). Conclusion: Enhanced spatial dispersion of right atrial refractoriness may be the substrate for propensity to AF in patients with AFL. The substrate was associated with enhanced inducibility of atrial fibrillation. [source]

On the Atrial Response to Focal Discharges in Man

JOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 6 2004
HEMANTH RAMANNA M.D.
Introduction: Triggers and vulnerability are key factors for the occurrence of atrial fibrillation (AF). The aim of this study was to assess spatial dispersion of atrial refractoriness and vulnerability in response to both focal discharges as well as programmed electrical stimulation in patients undergoing ablation of atrial arrhythmogenic foci. Methods and Results: Twenty-nine patients were studied, and 12 right atrial unipolar electrograms were recorded. Inducibility of AF was assessed by a pacing protocol that started with one extrastimulus, followed by more aggressive pacing until AF was obtained. Mean fibrillatory intervals were used to assess the local refractoriness of each recording site. Spatial dispersion of refractoriness was calculated as the coefficient of dispersion (CD value: standard deviation of the mean of all local mean fibrillatory intervals as a percentage of the overall mean fibrillatory interval). Based on our previous study, a CD value , 3.0 was defined as normal, whereas a CD value >3.0 was considered enhanced spatial dispersion of refractoriness. Fifteen of 29 patients had normal dispersion of refractoriness (mean CD value 1.65 ± 0.43), and AF was inducible with burst pacing only. These patients had focal discharges causing rapid atrial tachycardia with a focal activation pattern. Activation mapping of focal activity was possible in 14 of 15 patients. Focal triggering of AF occurred in only 1 of 15 patients. Fourteen of 29 patients had enhanced dispersion (mean CD value 4.2 ± 0.72). AF was inducible with a single extrastimulus in 11 of 14 patients (P < 0.001). Focal triggering of AF occurred in all 14 patients. Conclusion: Spatial dispersion of atrial refractoriness determines whether focal atrial discharges trigger AF with disorganized activity or, alternatively, only rapid atrial tachycardia. (J Cardiovasc Electrophysiol, Vol. 15, pp. 1-8, June 2004) [source]

The effect of information and communication technologies on urban structure

ECONOMIC POLICY, Issue 54 2008
Yannis M. Ioannides
SUMMARY Cities This paper examines the effects of information and communication technologies (ICT) on urban structure. Improvements in ICT may lead to changes in urban structure, for example, because they reduce the costs of communicating ideas from a distance. Hence, they may weaken local agglomeration forces and thus provide incentives for economic activity to relocate to smaller urban centres. We use international data on city size distributions in different countries and on country-level characteristics to test the effect of ICT. We find robust evidence that increases in the number of telephone lines per capita encourage the spatial dispersion of population in that they lead to a more concentrated distribution of city sizes. So far the evidence on internet usage is more speculative, although it goes in the same direction. We argue that the internet is likely to have similar, or even larger, effects on urban structures once its use has spread more thoroughly through different economies. , Yannis M. Ioannides, Henry G. Overman, Esteban Rossi-Hansberg and Kurt Schmidheiny [source]

Sexually selected behaviour: red squirrel males search for reproductive success

JOURNAL OF ANIMAL ECOLOGY, Issue 2 2009
Jeffrey E. Lane
Summary 1Differential male reproductive success is commonplace in mammals and frequently attributed to variation in morphological traits that provide individuals with a competitive advantage in female defence mating systems. Other mammalian mating systems, however, have received comparatively little attention and correlates of male reproductive success in them are less well understood. 2We studied a free-ranging population of North American red squirrels (Tamiasciurus hudsonicus Erxleben) exhibiting year-round individual territoriality. Males must temporarily vacate their territories to locate spatially dispersed receptive females, thereby setting the stage for a scramble competition mating system. 3We predicted that both male annual mating success (measured as the number of females copulated with) and annual reproductive success (measured as the number of offspring sired) would be positively correlated with both search ability (measured as the number of oestrous females located over the mating season) and effort (measured as mating season home range size), generating directional sexual selection on these two metrics. 4Mating season home ranges of males showed, on average, an almost 10-fold increase relative to those measured during the nonmating season, while those of females showed a more moderate twofold increase and both annual mating and reproductive success of males was positively correlated with search ability and search effort. 5The spatial dispersion of females, resulting from the strict territorial social structure of red squirrels, gave rise to a predicted scramble competition mating system. Furthermore, the strength of sexual selection on behavioural traits in this mating system equalled previous estimates for morphological traits in female defence mating systems. [source]

Mechanism of Propensity to Atrial Fibrillation in Patients Undergoing Isthmus Ablation for Typical Atrial Flutter

On the Atrial Response to Focal Discharges in Man

Improvement of Defibrillation Efficacy with Preshock Synchronized Pacing

JOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 5 2004
HUI-NAM PAK M.D., Ph.D.
Introduction: We previously demonstrated that wavefront synchronization by spatiotemporal excitable gap pacing (Sync P) is effective at facilitating spontaneous termination of ventricular fibrillation (VF). Therefore, we hypothesized that a spatiotemporally controlled defibrillation (STCD) strategy using defibrillation shocks preceded by Sync P can improve defibrillation efficacy. Method and Results: We explored the STCD effects in 13 isolated rabbit hearts. During VF, a low-voltage gradient (LVG) area was synchronized by Sync P for 0.92 second. For Sync P, optical action potentials (OAPs) adjacent to four pacing electrodes (10 mm apart) were monitored. When one of the electrodes was in the excitable gap, a 5-mA current was administered from all electrodes. A shock was delivered 23 ms after the excitable gap when the LVG area was unexcitable. The effects of STCD was compared to random shocks (C) by evaluating the defibrillation threshold 50% (DFT50; n = 35 for each) and preshock coupling intervals (n = 208 for STCD, n = 172 for C). Results were as follows. (1) Sync P caused wavefront synchronization as indicated by a decreased number of phase singularity points (P < 0.0001) and reduced spatial dispersion of VF cycle length (P < 0.01). (2) STCD decreased DFT50 by 10.3% (P < 0.05). (3) The successful shocks showed shorter preshock coupling intervals (CI; P < 0.05) and a higher proportion of unexcitable shock at the LVG area (P < 0.001) than failed shocks. STCD showed shorter CIs (P < 0.05) and a higher unexcitable shock rate at LVG area (P < 0.05) than C. Conclusion: STCD improves defibrillation efficacy by synchronizing VF activations and increasing probability of shock delivery to the unexcitable LVG area. (J Cardiovasc Electrophysiol, Vol. 15, pp. 581-587, May 2004) [source]

Effect of Action Potential Duration and Conduction Velocity Restitution and Their Spatial Dispersion on Alternans and the Stability of Arrhythmias

Arrhythmogenesis of T Wave Alternans Associated with Surface QRS Complex Alternans and the Role of Ventricular Prematurity: Observations from a Canine Model of LQT3 Syndrome

JOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 6 2002
MASAOMI CHINUSHI M.D.
Intramural TWA and Its Arrhythmogenesis.Introduction: T wave alternans (TWA) is characterized by cycle-to-cycle changes in the QT interval and/or T wave morphology. It is believed to amplify the underlying dispersion of ventricular repolarization. The aim of this study was to examine the mechanisms and arrhythmogenesis of TWA accompanied by QRS complex and/or blood pressure (BP) waveform alternans, using transmural ventricular electrogram recordings in an anthopleurin-A model of long QT syndrome. Methods and Results: The cardiac cycle length was gradually shortened by interruption of vagal stimulation, and TWA was induced in six canine hearts. Transmural unipolar electrograms were recorded with plunge needle electrodes from endocardial (Endo), mid-myocardial (Mid), and epicardial (Epi) sites, along with the surface ECG and BP. The activation-recovery interval (ARI) was measured to estimate local refractoriness. During TWA, ARI alternans was greater at the Mid than the Epi/Endo sites, and it was associated with the development of marked spatial dispersion of ventricular repolarization. As TWA increased, ventricular activation of the cycles associated with shorter QT intervals displayed delayed conduction at the Mid sites as a result of a critically longer ARI of the preceding cycle and longer QT interval, while normal conduction was preserved at the Epi site. Delayed conduction at the Mid sites manifested as surface ECG QRS and BP waveform alternans, and spontaneous ventricular tachyarrhythmias developed in absence of ventricular prematurity. In other instances, in absence of delayed conduction during TWA, ventricular premature complexes infringed on a prominent spatial dispersion of ventricular repolarization of cycles with long QT intervals and initiated ventricular tachyarrhythmia. Conclusion: TWA accompanied by QRS alternans may signal a greater ventricular electrical instability, since it is associated with intramural delayed conduction, which can initiate ventricular tachyarrhythmia without ventricular premature complexes. [source]

Thermalization and recombination in exponential band tail states

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006
M. Niehus
Abstract We present an analytical model that combines the complementary experimental evidence of spatial dispersion (DAP recombination) and energetic dispersion (band tails). The model describes the competition between thermalization and recombination of excess carriers trapped in exponentially distributed (in energy), discrete localized (in space) states. We use the energy dependence of the relaxation rates to derive the energy and time dependence of sub gap photoluminescence. The model predicts that the yellow luminescence band (YLB) and blue luminescence band (BLB) commonly observed in GaN are not separate entities, but reflect the competition of thermalization and recombination. A distinct kink is observed in transient PL in the microsecond range, in the limiting cases of strong tailing and/or low temperatures, indicating the transition from thermalization-limited to (radiative) recombination-limited excess carrier relaxation. Both prediction are in line with experiment, and able to resolve interpretational difficulties. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Services for immigrant women: an evaluation of locations

THE CANADIAN GEOGRAPHER/LE GEOGRAPHE CANADIEN, Issue 2 2000
MARIE TRUELOVE
The Toronto region receives one-quarter of new immigrants to Canada and they become widely dispersed throughout the metropolitan area. Most immigrants arrive with language, social and cultural needs, creating demand for social services from existing agencies. ,How can agencies choose locations that meet the needs of new immigrants?' is the central focus. The results of a study in Metropolitan Toronto of 68 nonprofit agencies that provide a variety of settlement services for immigrant and refugee women are discussed. Immigrant and language groups and the agencies serving them are mapped; the locations of agencies are evaluated. While service agencies are responding to the arrival of new groups and the spatial dispersion of new immigrants, more services in the northern portions of the study area are required. The spatial dispersion of some language groups means that they have poorer access to services than groups that are concentrated in the traditional immigrant reception area. La région de Toronto accueille le quart des immigrants au Canada, et ceux-ci sont dispersés dans l'agglomération torontoise. La plupart d'entre eux ont des exigences linguistiques, sociales et culturelles qui augmentent la demande en services sociaux dispensés par les organismes en place. Ce document porte essentiellement sur la façon dont ces derniers determinent les lieux de prestation de services qui répondront le mieux aux besoins des immigrants. II est également question des résultats d'une étude menée dans la communauté urbaine de Toronto auprés de 68 organismes à but non lucratif offrant un éventail de services d'établissement pour les immigrantes et les réfugiées. Les immigrants et les groupes linguistiques, ainsi que les organismes qui les servent, y sont répertoriés géographiquement. La localisation de ces organismes fait aussi l'objet d'une évaluation. La plupart répondent déjà aux besoins des nouveaux venus et tiennent compte de leur dispersion mais, selon cette étude, il faudrait plus de services dans le nord de l'agglomération torontoise. En raison de cet éparpillement, certains groupes linguistiques ont plus difficilement accès à des services que d'autres qui se trouvent dans les zones d'ancrage habituelles. [source]

The costs and benefits of land fragmentation of rice farms in Japan,

AUSTRALIAN JOURNAL OF AGRICULTURAL & RESOURCE ECONOMICS, Issue 4 2010
Kentaro Kawasaki
Land fragmentation, in which a farm operates multiple, separate plots of land, is a common phenomenon in Japan and many other countries. Usually, land fragmentation is regarded as a harmful phenomenon as it increases production costs and reduces the advantages of scale economies. However, it is also known that fragmentation may have beneficial effects in reducing risk through spatial dispersion of plots. Thus, land fragmentation has both costs and benefits, and whether it is beneficial or harmful is determined by the magnitude of these costs and benefits. This article investigates the costs and benefits of land fragmentation empirically using panel data from Japanese rice farms. The empirical results reveal that fragmentation increases production costs and offsets economies of size, and these impacts strengthen as farm size increases. Moreover, although fragmentation does reduce production risk, its monetary value is far below the cost of land fragmentation. From these findings, we conclude that land fragmentation is an impediment to efficient rice production in Japan. [source]