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  • 1
    Electronic Resource
    Electronic Resource
    Mechanisms of the Spatial Distribution of QT Intervals on the Epicardial and Body Surfaces (1999)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiovascular electrophysiology 10 (1999), S. 0 
    Details
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Spatial Distribution of the QT Interval. Introduction: The role of QT dispersion as a predictor of arrhythmia vulnerability has not been consistently confirmed in the literature. Therefore, it is important to identify the electrophysiologic mechanisms that affect QT duration and distribution. We compared the spatial distributions of QT intervals (QTI) with potential distributions on cardiac and body surfaces and with recovery times on the cardiac surface. We hypothesized that the measure of QTI is affected by the presence of the zero potential line in the potential distribution, as well as the sequence of recovery. We also investigated use of the STT area as a possible indicator of recovery times on the cardiac surface. Methods and Results: High-resolution spatial distributions of QTI and potentials were determined on the body surface of human subjects and on the surface of a torso-shaped tank containing an isolated canine heart. Additionally, spatial distributions of QTI, recovery times, and STT areas were determined on the surface of exposed canine hearts. Unipolar electrograms were recorded during atrial and ventricular pacing for normal hearts and cases of myocardial infarction. Regions of shortest QTI always coincided with the location of the zero potential line on the cardiac and body surfaces. On the cardiac surface, in regions away from the zero line, similarities were observed between the patterns of QTI and the sequence of recovery. STT areas and recovery times were highly correlated on the cardiac surface. Conclusion: QTI is not a robust index of local recovery time on the cardiac surface. QTI distributions were affected by the position of the zero potential line, which is unrelated to local recovery times. However, similarities in the patterns of QTI and recovery times in some regions may help explain the frequently reported predictive value of QT dispersion. Preliminary results indicate STT area may be a better index of recovery time and recovery time dispersion on the epicardium than QTI.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8167.1999.tb00225.x
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  • 2
    Electronic Resource
    Electronic Resource
    Mechanisms in Simulated Torsade de Pointes (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiovascular electrophysiology 4 (1993), S. 0 
    Details
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Torsade de Pointes. A mechanism of torsade de pointes consisting of moving sites of reentrant excitation has been proposed on the basis of findings with a computer model. Substantialadditions to that mechanism are now proposed based on further studies with the same model.The model simulated propagation, cycle length-dependent recovery of excitability, and slowpropagation during incomplete recovery. Regions of relatively short and long recovery wereassigned because of evidence of regional prolongation of recovery in long QT syndromes inwhich torsade dc pointes is frequent. As previously reported, premature excitation in the shortrecovery region initially propagated independently, then entered the long recovery region andreentered the short recovery region distal to the site of origin. Reentrant excitation initiated asimilar series of events, and serial reentry at systematically changing locations resulted inchanging patterns of excitation compatible with the changing QRS waveform in torsade depointes. Episodes terminated when reentrant excitation reached the end of unclosed shortrecovery paths, collided in closed paths, or encountered refractoriness in the presence ofnonuniform short recovery. In this study, it was shown that excitation preceding reentry hadimportant effects on the mechanism. These included reversal of the direction of serial reentry, bidirectional serial reentry, reentry at multiple sites from the same parent conditions, andoccurrence of reentry without the requirement of slow propagation. Evidence for a Dopplershift of cycle lengths in regions from which serial reentry was receding or approaching wasobtained. Sustained serial reentry was also demonstrated and is a possible mechanism for polymorphic ventricular tachycardia. Findings further define a possible mechanism of torsade depointes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8167.1993.tb01243.x
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  • 3
    Electronic Resource
    Electronic Resource
    Three-Dimensional Distribution of ST-T Wave Alternans During Acute Ischemia (1997)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiovascular electrophysiology 8 (1997), S. 0 
    Details
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Repolarization Alternans in Ischemia. Introduction: A canine model of reversible ischemia was used to measure the magnitude and transmural distribution of repolarization alternans. Methods and Results: Twenty-four multielectrode needles were inserted into a reversibly ischemic region created by 8 minutes of coronary occlusion. One hundred ninety-two unipolar electrograms were simultaneously recorded at 1-minute intervals for 8 minutes of ischemia and 3 minutes of reflow recovery. Beat-to-beat repolarization alternans was quantified for all electrograms using the standard deviation of QRST integrals. When alternans from animals that fibrillated was compared with alternans from animals that did not, the magnitude of alternans in the fibrillation group was an average standard deviation of 1125 ± 99.7 mV-msec at the time of fibrillation and 409 ± 183 mV-msec at 8 minutes of ischemia in the animals that did not fibrillate. The increase in alternans occurred mainly in the mid-myocardial and epicardial regions in the animals that fibrillated. QRS morphology of sequential electrograms did not differ in beat-to-beat comparison, suggesting that repolarization alternans measured was not due to alternating conduction block in the region of reversible ischemia. Conclusion: During acute ischemia, the magnitude and distribution of repolarization alternans are greater and differ in hearts that experience ventricular fibrillation. This observation may have clinical utility in arrhythmia prediction. It also is consistent with the possibility there may be multiple mechanisms for repolarization alternans.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8167.1997.tb01038.x
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  • 4
    Electronic Resource
    Electronic Resource
    Value of Magnetocardiographic QRST Integral Maps in the Identification of Patients at Risk of Ventricular Arrhythmias (1999)
    Oxford, UK : Blackwell Publishing Ltd
    Pacing and clinical electrophysiology 22 (1999), S. 0 
    Details
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: It has been shown that regional ventricular repolarization properties can be reflected in body surface distributions of electrocardiographic QRST deflection areas (integrals). We hypothesize that these properties can be reflected also in the magnetocardiographic QRST areas and that this may be useful for predicting vulnerability to ventricular tachyarrhythmias. Magnetic field maps were obtained during sinus rhythm from 49 leads above the anterior chest in 22 healthy (asymptomatic) control subjects (group A) and in 29 patients with ventricular arrhythmias (group B). In each subject, the QRST deflection area was calculated for each lead and displayed as an integral map. The mean value of maximum was significantly larger in the control group A than in the patient group B (1,626 ± 694 pTms vs 582 ± 547 pTms, P 〈 0.0001). To quantitatively assess intragroup variability in the control group A and intergroup variability of the control and patient groups, we used the correlation coefficient r and covariance σ. These indices showed significantly less intragroup than intergroup variation (e.g., in terms of σ, 28.0 · 10−6± 12.3 · 10−6 vs 3.4 · 10−6± 12.5 · 10−6, P 〈 0.0001). Each QRST integral map was also represented as a weighted sum of 24 basis functions (eigenvectors) by means of Karhunen-Loeve transformation to calculate the contribution of the nondipolar eigenvectors (all eigenvectors beyond the third). This percentage nondipolar content of magnetocardiographic QRST integral maps was significantly higher in the patient group B than in the control group A (13.0%± 9.1% vs 2.6%± 2.0%, P 〈 0.0001). Discriminations between control subjects and patients with ventricular arrhythmias based on magnitude of the maximum, covariance σ, and nondipolar content were 90.2%, 90.2%, and 86.3% accurate, with a sensitivity of 89.7%, 93.1%, and 75.9%, and a specificity of 90.9%, 86.4%, and 100%. We have shown that magnitude of the maximum and indices of variability and nondipolarity of the magnetocardiographic QRST integral maps may predict arrhythmia vulnerability. This finding is in agreement with earlier studies that used body surface potential mapping and suggests that magnetic field mapping may also be a useful diagnostic tool for risk analysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8159.1999.tb00622.x
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  • 5
    Electronic Resource
    Electronic Resource
    Mechanisms in Adrenergic Dependent Onset of Torsades de Pointes (1997)
    Oxford, UK : Blackwell Publishing Ltd
    Pacing and clinical electrophysiology 20 (1997), S. 0 
    Details
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Pause dependent onset of torsades de pointes is characteristic in acquired long QT syndromes, and the probable mechanism is reentry facilitated by increased disparity of refractoriness following a long cycle. Adrenergic dependent onset is usual in familial long QT syndromes, and the mechanism is uncertain. In this study with a computer simulation of torsades de pointes, possible mechanisms of adrenergic dependent onset have been identified. Decreased refractory periods facilitated the initiation of torsades de pointes by permitting earlier premature excitation and allowing reentry in the presence of the shorter refractory period that had been further shortened by the earlier excitation. In addition, accelerating rate resulted in responses occurring in the presence of refractory periods set by the prior response so each response was premature with respect to the preceding one. The difference between cycle lengths and refractory period decreased with increasing rate leading to the functional block required for initiation of simulated torsades de pointes. Findings define possible mechanisms in which the adrenergic effects of reduced refractory period duration and increased rate may lead to the initiation of torsades de pointes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8159.1997.tb04816.x
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  • 6
    Electronic Resource
    Electronic Resource
    Useful Lessons from Body Surface Mapping (1998)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiovascular electrophysiology 9 (1998), S. 0 
    Details
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Useful Lessons from Body Surface Mapping. Body surface potential maps (BSMs) depict the time varying distribution of cardiac potentials on the entire surface of the torso. Hundreds of studies have shown that BSMs contain more diagnostic and prognostic information than can he elicited from the 12-lead ECG. Despite these advantages, body surface mapping has not become a routinely used clinical method. One reason is that visual examination and sophisticated analysis of BSMs do not permit inferring the sequence of excitation and repolarization in the heart with a sufficient degree of certainty and detail. These limitations can be partially overcome by implementing inverse procedures that reconstruct epicardial potentials, isochrones, and ECGs from body surface measurements. Furthermore, ongoing experimental work and simulation studies show that a great deal of information about intramural events can he elicited from measured or reconstructed epicardial potential distributions. Interpreting epicardial data in terms of deep activity requires extensive knowledge of the architecture of myocardial fibers, their anisotropic properties, and the role of rotational anisotropy in affecting propagation and the associated potential fields.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8167.1998.tb00965.x
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  • 7
    Electronic Resource
    Electronic Resource
    Cardiac Resynchronization Therapy and the Arrhythmogenic Substrate (2005)
    350 Main Street , Malden , MA 02148-5018 , USA , and 9600 Garsington Road , Oxford OX4 2DQ , UK . : Blackwell Science Inc
    Journal of cardiovascular electrophysiology 16 (2005), S. 0 
    Details
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8167.2005.50031.x
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  • 8
    Electronic Resource
    Electronic Resource
    The Mechanism of Simulated Torsade de Pointes in a Computer Model of Propagated Excitation (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiovascular electrophysiology 2 (1991), S. 0 
    Details
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Mechanism of Torsade de Pointes. A computer model that simulated propagation, nonuniform cycle length dependent recovery of excitability, and slow propagation during incomplete recovery in cardiac muscle has been shown to exhibit behavior similar to torsade de pointes. The conditions required in the model resembled those in long QT syndromes with regionally prolonged ventricular repolarization. In the model, premature excitation of a path with relatively short refractory periods resulted in sequential reentry of that path from a region with longer refractory periods. Sites of reentry become progressively more distal to the site of initiation. The episodes terminated spontaneously when reentrant excitation reached the end of unclosed paths or collided in closed paths. Each reentry site moved away from its initial position in the short recovery path because the distance traversed by propagation in that path exceeded the distance at which reentry occurred via the longer recovery region. The migrating sites of reentry resulted in changing QRS waveform in calculated electrocardiograms including changing QRS polarity when calculated from appropriate sites. Variations of the basic behavior occurred with nonuniform distribution of refractory periods within relatively short and long refractory period regions and included unidirectional progression of reentry sites and multiple circuits of reentry in closed paths. Termination of serial reentry at times other than those when activation reached the end of unclosed paths or collided in closed paths also occurred with nonuniform distribution of refractory periods. Findings provide a plausible explanation for the changing QRS waveform, spontaneous termination, and association of torsade de pointes with long QT syndromes but do not elucidate the mechanism of initiation of the tachyarrhythmia.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8167.1991.tb01321.x
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  • 9
    Electronic Resource
    Electronic Resource
    Noncontact Endocardial Mapping: Reconstruction of Electrograms and Isochrones From Intracavitary Probe Potentials (1997)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiovascular electrophysiology 8 (1997), S. 0 
    Details
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Noncontact Endocardial Mapping. Introduction: Mapping endocardial activation and repolarization processes is critical to the study of arrhythmias and selection of therapeutic procedures. Previously, we developed methodology for reconstructing endocardial potentials from potentials measured with a noncontact, intracavitary probe. This study further develops and evaluates the ability of the approach to provide detailed information on the spatiotemporal characteristics of the activation process. Specifically, we reconstructed endocardial electrograms and isochrones throughout the activation process over the entire endocardium during a single beat. Methods and Results: Cavity potentials were measured with a 65-electrode probe placed inside an isolated canine left ventricle. Endocardial potentials were measured simultaneously using 52 electrodes. Potentials were acquired during subendocardial pacing from different locations. Computed electrograms at various sites closely resemble the measured electrograms (correlation coefficient 〉 0.9 at 60% of the electrodes). Computed isochrones locate subendocardial pacing sites with 10-mm accuracy. Two pacing sites, 17 mm apart, were resolved. Critical regions, such as areas of isochrone crowding, were accurately reconstructed. Conclusions: Results indicate the applicability of the approach to mapping the cardiac excitation process on a beat-by-beat basis without occluding the ventricle. The ability of locating electrical events (e.g., single or multiple initiation sites) is demonstrated. Importantly, the method is shown to be capable of reconstructing electrograms over the entire endocardium and determining nonuniformities of activation spread (e.g., areas of slow conduction). These capabilities are important to clinical application in the electrophysiology laboratory and experimental studies of arrhythmias in the intact animal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8167.1997.tb00807.x
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  • 10
    Electronic Resource
    Electronic Resource
    Three-Dimensional Activation Mapping in Ventricular Muscle: Interpolation and Approximation of Activation Times (1999)
    Springer
    Annals of biomedical engineering 27 (1999), S. 617-626 
    Details
    ISSN: 1573-9686
    Keywords: Activation ; Cardiac mapping ; Interpolation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract Interpolation plays an important role in analyzing or visualizing any scalar field because it provides a means to estimate field values between measured sites. A specific example is the measurement of the electrical activity of the heart, either on its surface or within the muscle, a technique known as cardiac mapping, which is widely used in research. While three-dimensional measurement of cardiac fields by means of multielectrode needles is relatively common, the interpolation methods used to analyze these measurements have rarely been studied systematically. The present study addressed this need by applying three trivariate techniques to cardiac mapping and evaluating their accuracy in estimating activation times at unmeasured locations. The techniques were tetrahedron-based linear interpolation, Hardy's interpolation, and least-square quadratic approximation. The test conditions included activation times from both high-resolution simulations and measurements from canine experiments. All three techniques performed satisfactorily at measurement spacing ⩽ 2mm. At the larger interelectrode spacings typical in cardiac mapping (1 cm), Hardy's interpolation proved superior both in terms of statistical measures and qualitative reconstruction of field details. This paper provides extensive comparisons among the methods and descriptions of expected errors for each method at a variety of sampling intervals and conditions. © 1999 Biomedical Engineering Society. PAC99: 8719Nn, 0260Ed, 8719Ff
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1114/1.211
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