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Biomedical subjects

B Taccardi

Publications and source records attributed to B Taccardi.

At least 19 recordsLinked to original sources
PubMed1998

Electrocardiographic imaging: Noninvasive characterization of intramural myocardial activation from inverse-reconstructed epicardial potentials and electrograms.

BACKGROUND: A recent study demonstrated the ability of electrocardiographic imaging (ECGI) to reconstruct, noninvasively, epicardial potentials, electrograms, and activation sequences (isochrones) generated by epicardial activation. The current study expands the earlier work to the three-dimensional myocardium and investigates the ability of ECGI to characterize intramural myocardial activation noninvasively and to relate it to the underlying fiber structure of the myocardium. This objective is motivated by the fact that cardiac excitation and arrhythmogenesis involve the three-dimensional ventricular wall and its anisotropic structure. METHODS AND RESULTS: Intramural activation was initiated by pacing a dog heart in a human torso tank. Body surface potentials (384 electrodes) were used to compute epicardial potentials noninvasively. Accuracy of reconstructed epicardial potentials was evaluated by direct comparison to measured ones (134 electrodes). Protocols included pacing from five intramural depths. Epicardial potentials showed characteristic patterns (1) early in activation, central negative region with two flanking maxima aligned with the orientation of fibers at the depth of pacing; (2) counterclockwise rotation of positive potentials with time for epicardial pacing, clockwise rotation for subendocardial pacing, and dual rotation for midmyocardial pacing; and (3) central positive region for endocardial pacing. Noninvasively reconstructed potentials closely approximated these patterns. Reconstructed epicardial electrograms and epicardial breakthrough times closely resembled measured ones, demonstrating progressively later epicardial activation with deeper pacing. CONCLUSIONS: ECGI can noninvasively estimate the depth of intramyocardial electrophysiological events and provides information on the spread of excitation in the three-dimensional anisotropic myocardium on a beat-by-beat basis.

Animals↗
PubMed1998

Spread of excitation in 3-D models of the anisotropic cardiac tissue. II. Effects of fiber architecture and ventricular geometry.

We investigate a three-dimensional macroscopic model of wave-front propagation related to the excitation process in the left ventricular wall represented by an anisotropic bidomain. The whole left ventricle is modeled, whereas, in a previous paper, only a flat slab of myocardial tissue was considered. The direction of cardiac fibers, which affects the anisotropic conductivity of the myocardium, rotates from the epi- to the endocardium. If the ventricular wall is conceived as a set of packed surfaces, the fibers may be tangent to them or more generally may cross them obliquely; the latter case is described by an "imbrication angle." The effect of a simplified Purkinje network also is investigated. The cardiac excitation process, more particularly the depolarization phase, is modeled by a nonlinear elliptic equation, called an eikonal equation, in the activation time. The numerical solution of this equation is obtained by means of the finite element method, which includes an upwind treatment of the Hamiltonian part of the equation. By means of numerical simulations in an idealized model of the left ventricle, we try to establish whether the eikonal approach contains the essential basic elements for predicting the features of the activation patterns experimentally observed. We discuss and compare these results with those obtained in our previous papers for a flat part of myocardium. The general rules governing the spread of excitation after local stimulations, previously delineated for the flat geometry, are extended to the present, more realistic monoventricular model.

Animals↗
PubMed1998

A possible mechanism for electrocardiographically silent changes in cardiac repolarization.

Despite the widespread use of electrocardiogram (ECG), changes in cardiac activity resulting from ischemia or altered recovery characteristics sometimes remain electrocardiographically "silent" or are first detectable by techniques that measure ventricular contractility, such as ultrasound or blood pressure. Especially local changes in repolarization can go undetected when ECG electrodes do not lie close to the area of the heart affected. Experiments were performed on an isolated, perfused canine heart suspended in a realistically shaped, instrumented, electrolytic torso tank with the goal of determining some mechanisms for these ambiguities. By recording simultaneously both epicardial and torso tank surface potentials, complete descriptions were obtained of the electrical response to interventions such as coronary occlusions and alterations in pacing site and frequency. One hypothesis was that some interventions produce highly variable ECG responses primarily because of differences in their location within the heart. To test this, the effect was measured of repeating the same intervention as the heart's location and orientation in the tank were varied. A numerical forward solution was also used to investigate variation of torso tank potentials with heart location. The resulting changes in tank surface potentials illustrate how, for example, precordial ST-segment shifts following occlusion change from elevation to depression to become almost undetectable as the heart rotates in the tank. The results suggest that some events are electrocardiographically silent because of the complex geometric relationship of the heart, torso, and site of the lesion, as well as the spatial sampling and analysis techniques used in detection.

Animals↗
PubMed1998

QT interval dispersion: dispersion of ventricular repolarization or dispersion of QT interval?

The QT interval (QTI) has long been useful as a clinical index of the duration of ventricular repolarization, particularly as a marker of prolonged repolarization and its well-established association with arrhythmogenic cardiac states. Likewise, inhomogeneity (dispersion) of repolarization has been linked definitively to increased susceptibility to reentrant arrhythmias. Recent studies have reported the use of QTI dispersion as a meaningful clinical index to identify patients at risk, but the interpretation of the measurement has been controversial. A Langendorff-perfused, isolated canine heart suspended in a torso-shaped, electrolytic tank filled with NaCl-sucrose solution was used to investigate the relationship between body surface QTIs and ventricular repolarization measured directly from the cardiac surface by using activation-recovery intervals, which have been documented to reflect the duration of local action potentials as well as local refractory periods. The data showed poor correlation between cardiac surface activation-recovery intervals and QTIs, as well as the insensitivity of QTIs to regional repolarization shortening in the presence of prolonged repolarization elsewhere. Furthermore, the data confirmed that torso tank QTI dispersion does not reflect directly the full range of measured ventricular repolarization inhomogeneity. It is concluded that body surface QTI dispersion is not a reliable index of repolarization dispersion.

Action Potentials↗
PubMed1997

Noninvasive electrocardiographic imaging: reconstruction of epicardial potentials, electrograms, and isochrones and localization of single and multiple electrocardiac events.

BACKGROUND: The goal of noninvasive electrocardiographic imaging (ECGI) is to determine electric activity of the heart by reconstructing maps of epicardial potentials, excitation times (isochrones), and electrograms from data measured on the body surface. METHODS AND RESULTS: Local electrocardiac events were initiated by pacing a dog heart in a human torso-shaped tank. Body surface potential measurements (384 electrodes) were used to compute epicardial potentials noninvasively. The accuracy of reconstructed epicardial potentials was evaluated by direct comparison to measured ones (134 electrodes). Protocols included pacing from single sites and simultaneously from two sites with various intersite distances. Body surface potentials showed a single minimum for both single- and double-site pacing (intersite distances of 52, 35, and 17 mm). Noninvasively reconstructed epicardial electrograms, potentials, and isochrones closely approximated the measured ones. Single pacing sites were reconstructed to within < or = 10 mm of their measured positions. Dual sites were located accurately and resolved for the above intersite distances. Regions of sparse and crowded isochrones, indicating spatial nonuniformities of epicardial activation spread, were also reconstructed. CONCLUSIONS: The study demonstrates that ECGI can reconstruct epicardial potentials, electrograms, and isochrones over the entire epicardial surface during the cardiac cycle. It can provide detailed information on local activation of the heart noninvasively. Its uses could include localization of cardiac electric events (eg, ectopic foci), characterization of nonuniformities of conduction, characterization of repolarization properties (eg, dispersion), and mapping of dynamically changing arrhythmias (eg, polymorphic VT) on a beat-by-beat basis.

Animals↗
PubMed1997

Noncontact endocardial mapping: reconstruction of electrograms and isochrones from intracavitary probe potentials.

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.

Animals↗
PubMed1997

Anatomical architecture and electrical activity of the heart.

In most early studies of cardiac electrophysiology, the correlation between propagation of excitation and the architecture of cardiac fibers was not addressed. More recently, it has become apparent that the spread of excitation, the sequence of recovery, the associated time-varying potential distributions and the intra- and extracardiac electrocardiograms are strongly affected by the complex orientation of myocardial fibers. This article is a review of older and very recent, partly unpublished, mathematical simulations and experimental findings that document the relationships between cardiac electrophysiology and fiber structure. Important anatomical factors that affect propagation and recovery are: the elongated shape of myocardial fibers which is the basis for electrical anisotropy; the epi-endocardial rotation of fiber direction in the ventricular walls; the epi-endocardial obliqueness of the fibers ("imbrication angle"), and the conduction system. Due to the complex architecture of the fibers, many different pathways are available to an excitation wavefront as it spreads from a pacing site: the straight line; the multiple, bent pathways resulting from the epi-endocardial rotation of fiber direction; the coiling intramural pathways associated with the "imbrication" angles (Streeter) and the pathways involving the Purkinje network. Only in a few cases is the straight line the fastest pathway. The shape of an excitation wavefront at a given time instant results from the competition between all possible pathways. To compute the potential distributions and ECG waveforms generated by a spreading excitation wave we must know the successive shapes and positions of the wavefront, the architecture of the fibers through which it propagates and the spatial distribution of their anisotropic electrical properties.

Electrophysiology↗
PubMed1997

Anatomical and microstructural factors affecting the electrical activity of the heart. Experimental findings and mathematical simulations.

We developed an experimental preparation in which we record 1500 intramural electrograms from the ventricular walls of an isolated dog heart. The heart is plunged into an electrolytic tank shaped as a human thorax. The tank carries 1300 electrodes uniformly distributed in the conducting volume and on its surface. We obtain 2800 electric signals which enable us to define the spread of excitation and the sequence of repolarization in the heart, and to map the spatial distribution of potentials and currents in the heart, in the chest and on the body surface during a heart beat. Experimental results and mathematical simulations show that excitation, repolarization, the spatial distribution of heart potentials and the shape of the electrocardiograms are dominated by a small number of anatomical factors, namely: the geometry of the ventricles, the epi-endocardial rotation of myocardial fibers orientation in the ventricular walls, the obliqueness of the fibers relative to the epicardial surface and structure of the conduction system.

Animals↗
PubMed1996

Measuring spatial waves of repolarization in canine ventricles using high-resolution epicardial mapping.

The importance of the role of ventricular repolarization in arrhythmogenesis and defibrillation prompted the exploration of new methods for observing and measuring repolarization. Specifically, the authors' goal was to establish independent procedures for assessing activation-recovery intervals. Canine epicardial electrograms from high-resolution arrays (2-mm spacing, 25 x 21 electrodes) were recorded-during pacing from a variety of single or simultaneously paced epicardial locations in canine hearts. For each activation sequence, the activation and repolarization times were measured using timing of intrinsic QRS and T wave deflections (activation-recovery interval method) and timing of the peak magnitude of spatial derivatives (gradient method). Both methods should, theoretically, provide estimates of local activation and repolarization times, which reflect timing of local action potential upstrokes and downstrokes. Scattergrams comparing activation and recovery times for the two methods showed high correlation, slopes close to 1.0, and intercepts near the origin. For most activation sequences, observation of the potential and gradient distributions as dynamic, three-dimensional perspective displays, revealed a well-defined, rapidly propagating repolarization wave, superimposed on a slowly varying, high-amplitude distribution occurring during the T wave. These data suggest that repolarization times measured using temporal or spatial derivatives are consistent with theoretical predictions and reflect timing of local action potential downstrokes. They also suggest potential utility of combining spatial and temporal approaches for improving reliability in the measurements.

Action Potentials↗
PubMed1996

Evaluation of novel measurement methods for detecting heterogeneous repolarization.

There exists a well-documented link between heterogeneity of cardiac recovery characteristics and vulnerability to arrhythmia; however, electrocardiographic detection of this heterogeneity remains problematic. The only modalities suitable for measuring variation of repolarization are electrophysiologic in nature, with action potential duration in single cells the most direct method and QT intervals from the body surface electrocardiogram the most common clinical approach. The authors have shown previously, however, that the QT interval is a poor measure of regional change in repolarization, especially when shortening occurs. Here, the authors discuss an experimental preparation based on an isolated canine heart suspended in a human-shaped, instrumented, electrolytic tank and describe a method of applying cold to create local, transient changes in recovery characteristics. The authors have simultaneously recorded epicardial and torso tank surface potentials before, during, and after intervention, and from them have generated isopotential and isointegral maps and computed activation-recovery intervals (ARIs). In all cases, epicardial potentials revealed changes in recovery associated with localized heating and cooling. The changes were visible from tank surface potential distributions in some, but not all, cases. The results also suggest that epicardial ARIs are sensitive to changes in recovery and that, at least for a subset of tank surface leads, ARIs can be used to create noninvasive indices of disparity of repolarization characteristics.

Animals↗
PubMed1995

Reconstruction of endocardial potentials and activation sequences from intracavitary probe measurements. Localization of pacing sites and effects of myocardial structure.

BACKGROUND: Mapping of endocardial activation is an important procedure for diagnosing cardiac arrhythmias and locating the arrhythmogenic site before treatment. The objective of the present study was to develop and test a mathematical method to reconstruct the endocardial potentials and activation sequences (isochrones) from potential data measured with a noncontact, intracavitary multielectrode probe (the "inverse problem"). METHODS AND RESULTS: A boundary element based mathematical method, combined with a numeric regularization technique, was developed for computing the inverse solution. Endocardial potentials were computed from intracavitary potentials measured with a multielectrode probe placed in the cavity of an isolated, perfused canine left ventricle. Data were acquired during rhythms induced by electrical stimuli applied at different locations and varying depths within the myocardium. Endocardial potentials were measured using intramural needles to evaluate the accuracy of the inverse solutions by direct comparison. Inversely computed endocardial potentials, from measured probe potentials, reconstruct with good accuracy the major features (potential maxima and minima, regions of negative and positive potentials) compared with the measured endocardial potentials. During early activation, the computed endocardial potentials exhibit a potential minimum in close proximity to the pacing site, determining the location of the stimulus with good accuracy (within 10-mm error). Multiple stimuli, as close as 10 to 20 mm to each other, can be distinguished and localized to their sites of origin by the inverse reconstruction. Similar to the measured endocardial potentials, the spatial distribution of the computed endocardial potentials reflects the underlying cardiac fiber direction, and dynamic changes of the computed endocardial potentials reflect the rotation of fibers with intramural depth. Maps of isochrones show good correspondence between the isochrones determined from the computed endocardial potentials and those determined directly from the measured endocardial potentials. CONCLUSIONS: Compared with actual, measured endocardial potentials and activation sequences, endocardial potential patterns and activation sequences can be reconstructed on a beat-by-beat basis from cavitary potentials measured with a multielectrode, noncontact probe. The approach presented here is shown to reconstruct, with 10-mm accuracy and resolution of 10 to 20 mm, local events of cardiac excitation (eg, pacing sites). In addition, the reconstructed endocardial potentials correctly reflect the underlying fibrous structure of the myocardium. These results demonstrate the feasibility of the approach. In the experiments, the probe position and endocardial geometry were determined invasively. To be clinically applicable, the reconstruction method should be combined with a noninvasive method for determining the probe-cavity geometry in the catheterization laboratory. It could then be developed into a catheter-based technique for locating arrhythmogenic sites and for studying and diagnosing conduction abnormalities, reentrant activity, and the effects of drugs and other interventions on cardiac activation and arrhythmias.

Animals↗
PubMed1994

Assessment of spatial and temporal characteristics of ventricular repolarization.

Measurement of dynamics and spatial characteristics of ventricular repolarization is of interest in assessing patients with ischemic heart disease, particularly in relation to the detection and characterization of ischemic events, identification of patients at risk of ventricular arrhythmias, or determination of the efficacy of drugs intended to alter repolarization. The QT interval (QTI) has been the index of choice for assessing repolarization abnormalities. It is a general measure of repolarization duration but lacks the power to assess the spatial aspects of repolarization and the ability to detect localized shortening in the setting of global prolongation. For direct cardiac surface measurement, QRST integrals and activation recovery intervals (ARIs) were used to assess repolarization and its disparity. The use of similar measurements from the body surface was proposed to provide better characterization of repolarization, its disparity, and its dynamics than is possible using the QTI. In one open-chest experiment using an intact canine heart and two experiments using isolated canine hearts suspended in a torso-shaped electrolytic tank, 64 epicardial electrograms and 192 torso surface electrocardiograms were measured simultaneously. Ventricular repolarization was globally altered by varying pacing cycle lengths or tank temperature. Atrial and ventricular pacing were used to assess sensitivity of repolarization indices to activation sequence. At the cardiac surface, (1) QTI tracks global repolarization changes but is affected by activation sequence and insensitive to localized shortening of repolarization; (2) distribution of QRST integrals reflects disparity of repolarization and is largely independent of activation sequence; and (3) ARI measures local repolarization duration and is only weakly affected by activation sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Effect of myocardial fiber direction on epicardial potentials.

BACKGROUND: Understanding the relations between the architecture of myocardial fibers, the spread of excitation, and the associated ECG signals is necessary for addressing the forward problem of electrocardiography, that is, predicting intracardiac and extracardiac ECGs from known intracardiac activity. So far, these relations have been studied experimentally only in small myocardial areas. In this study, we tested the hypothesis that potential distributions measured over extensive epicardial regions during paced beats reflect the direction of superficial and intramural fibers through which excitation is spreading in both the initial and later stages of ventricular excitation. We also tried to establish whether the features of the epicardial potential distribution that correlate with fiber direction vary as a function of pacing site, intramural pacing depth, and time elapsed after the stimulus. An additional purpose was to compare measured epicardial potentials with recently published numerical simulations depicting the three-dimensional spread of excitation in the heart muscle and the associated potential fields. METHODS AND RESULTS: The hearts of 18 mongrel dogs were exposed and 182 to 744 unipolar electrograms were recorded from epicardial electrode arrays (2.3 x 3.0 to 6.5 x 6.5 cm). Hearts were paced at various intramural depths through an intramural needle. The overall number of pacing sites in 18 dogs was 241. Epicardial potential distributions, electrographic waveforms, and excitation time maps were displayed, and fiber directions in the ventricular wall underlying the electrodes were determined histologically. During the early stages of ventricular excitation, the position of the epicardial maxima and minima revealed the orientation of myocardial fibers near the pacing site in all cases of epicardial and intramural pacing and in 60% of cases of endocardial or subendocardial pacing. During later stages of propagation, the rotation and expansion of the positive areas correlated with the helical spread of excitation through intramurally rotating fibers. Marked asymmetry of potential patterns probably reflected epicardial-endocardial obliqueness of intramural fibers. Multiple maxima appeared in the expanding positive areas. CONCLUSIONS: For 93% of pacing sites, results verified our hypothesis that epicardial potential patterns elicited by ventricular pacing reflect the direction of fibers through which excitation is spreading during both the initial and later stages of propagation. Epicardial potential distributions provided information on the site of origin and subsequent helical spread of excitation in an epicardial-endocardial, endocardial-epicardial, or double direction. Results were in agreement with previously published numerical simulations except for the asymmetry and fragmentation of the positive areas.

Animals↗
PubMed1993

Spread of excitation in a myocardial volume: simulation studies in a model of anisotropic ventricular muscle activated by point stimulation.

INTRODUCTION: The purpose of this study was to present simulations of excitation wavefronts spreading through a parallelepipedal slab of ventricular tissue measuring 6.5 x 6.5 x 1.0 cm. METHODS AND RESULTS: The slab incorporates the anisotropic properties of the myocardium including the transmural counterclockwise fiber rotation from epicardium to endocardium. Simulations were based on an eikonal model that determines excitation times throughout the ventricular wall, which is represented as an anisotropic bidomain. Excitation was initiated by delivering ectopic stimuli at various intramural depths. We also investigated the effect of a simplified Purkinje network on excitation patterns. Excitation wavefronts in the plane of pacing, parallel to epicardial-endocardial surfaces, were oblong with the major axis approximately oriented along the local fiber direction, with bulges and deformations due to attraction from rotating fibers in adjacent planes. The oblong intersections of the wavefront with planes at increasing distance from pacing plane rotated clockwise or counterclockwise, depending on pacing depth, but wavefront rotation was always less than fiber rotation in the same plane. For all pacing depths, excitation returned toward the plane of pacing. Return occurred in multiple, varying sectors of the slab depending on pacing depth, and was observed as close as 6 mm to the pacing site. CONCLUSION: Curvature of wavefronts and collision with boundaries of slab markedly affected local velocities. Shape and separation of epicardial isochrones and spatial distribution of epicardial velocities varied as a function of site and depth of pacing. When the Purkinje network was added to the model, epicardial velocities revealed the subendocardial location of the Purkinje-myocardial junctions. Considerable insight into intramural events could be obtained from epicardial isochrones. If validated experimentally, results may be applicable to epicardial isochrones recorded at surgery.

Anisotropy↗
PubMed1993

Potential distributions generated by point stimulation in a myocardial volume: simulation studies in a model of anisotropic ventricular muscle.

INTRODUCTION: We present simulations of extracellular potential patterns elicited by delivering ectopic stimuli to a parallelepipedal slab of ventricular tissue represented as an anisotropic bidomain incorporating epi-endocardial fiber rotation. METHODS AND RESULTS: Simulations were based on an eikonal model that determines wavefront shapes throughout the slab at every time instant during the depolarization phase, coupled with an approximate model of the action potential profile. The endocardial face of the slab was in contact with blood and the composite volume was surrounded by an insulating medium. The effect of a simplified Purkinje network was also studied. RESULTS: (1) For all pacing depths, except endocardial pacing, a central negative area and two potential maxima were observed at QRS onset in all intramural planes parallel to the epicardium. In all planes, the axis joining the two maxima was approximately aligned with the direction of fibers in the plane of pacing. Endocardial pacing generated a different pattern, but only when blood was present; (2) During later stages of excitation, outflowing currents (from the wavefront toward the resting tissue) were always emitted, at all intramural depths, only from those portions of the wavefront that spread along fibers. At any given instant, the position of the two potential maxima in a series of planes parallel to the epicardium and intersecting the wavefront rotated as a function of depth, following the rotating direction of intramural fibers. Purkinje involvement modified the above patterns. CONCLUSION: Epicardial and endocardial potential maps provided information on pacing site and depth and on subsequent intramural propagation by reflecting the clockwise or counter-clockwise rotation of the deep positivity. Results may be applicable to epicardial and endocardial potential maps recorded at surgery or from endocavitary probes.

Action Potentials↗