Edward V. Simpson

The Assumptions of Isochronal Cardiac Mapping

Isochronal maps of cardiac activation are commonly used to study the mechanisms and to guide the ablative therapies of arrhythmias. Little has been written about the assumptions implicit in the construction and use of isochronal cardiac maps. These assumptions include the following: (1) the location of the recording electrodes is known with sufficient accuracy to determine the mechanism of an arrhythmia or to guide therapy; (2) a single, discrete activation time can be assigned to each recording electrode location; (3) the presence or absence of activation at an electrode site can be reliably ascertained, and when activation is present, the time of activation can be determined with sufficient accuracy to specify the mechanism of an arrhythmia or to guide therapy; and (4) the recording electrodes are close enough together that the activation sequence can be estimated with sufficient accuracy to determine the mechanism of an arrhythmia or to guide therapy. The manuscript reviews evidence that these assumptions may not always be true, and when they are not, the isochronal map may be misleading.

A graphical display system for animating mapped cardiac potentials

A graphical display system for animating mapped cardiac potentials is described. The system displays recorded voltages from 121 epicardial electrodes simultaneously on a computer monitor using color to indicate the magnitude of the voltage. Numerous options accessible through the mouse-driven user interface provide the user with a high degree of interaction with the data. The user can observe the changes in the recorded voltages over time by defining a portion of the data over which to animate the voltages. Animation speeds of up to 2 1/4 frames per second can be achieved, depending on the workstation used. Use of the graphical display system provides a new tool for studying the electrical conduction properties of the heart. Through its use, a more complete understanding (and potentially a more effective treatment) of arrhythmias might be achieved.<<ETX>>

Alteration of Ventricular Fibrillation by Propranolol and Isoproterenol Detected by Epicardial Mapping with 506 Electrodes

Adrenergic Effects on VF. Introduction: We hypothesized that drugs which alter ventricular refractoriness or excitability produce quantifiable changes in ventricular fibrillation.

Three-dimensional visualization of electrical variables in the ventricular wall of the heart

A set of programs developed to display the potentials and the magnitudes of the estimated potential gradients generated by defibrillation shocks are discussed. There are three types of displays. The first type displays three-dimensional surfaces constructed from electrode locations. Potentials or gradient magnitudes are interpolated over each surface and are displayed as spatial changes in intensity or hue. The second type of display is a three-dimensional surface reconstruction of the heart and electrodes. This display is used to verify the three-dimensional electrode coordinates that are obtained from digitized images of the heart. The third type of display is a volume reconstruction of the myocardium. In this display, spatial changes in potentials or gradient magnitudes in the myocardial volume are represented by spatially varying the hues of the volume elements. The electrical values of most volume elements are interpolated by means of a three-dimensional method based on discrete smooth interpolating.<<ETX>>

Evaluation of an automatic cardiac activation detector for bipolar electrograms

The identification of local activation events in bipolar cardiac electrograms, the first step of isochronal map construction, is a time-consuming and difficult process. Owing to the variability among bipolar activation complexes and the lack of practical knowledge concerning the relationship of the bipolar waveform to action potential characteristics, a set of empirical rules to guide the assignment of local activation times have been adopted. A computer program, called AP, has been designed, which implements these rules in the form of a syntactic analyser. Canine epicardial recordings were used to evaluate AP by comparing local activation times, assigned by AP, with times assigned independently by three investigators. The Hermes-Cox model for detector evaluation and a bootstrap statistical method were used in conjunction with ROC analysis to evaluate the ability of AP to detect events. Analysis of discrepancies among investigator-assigned times showed that the reliabilities of AP event detection and AP-assigned times were comparable to those of the investigators. The methods used in system design and evaluation are applicable to a broad range of problems in the detection and localisation of waveform components.

An automatic activation detector for bipolar cardiac electrograms

The authors have developed a computer program, called AP, that automatically detects and locates activation events in bipolar electrograms. AP uses a slope threshold to detect events and a pattern-matching technique to locate the time of activation within each activation waveform. For sinus rhythm and paced and ectopic contractions AP has a detection sensitivity of 0.942 and mislocates 16.4% of the true detections by a mean of 1.59 ms. For ventricular fibrillation the detection sensitivity is 0.882 with 27.1% of the true detections being mislocated by a mean of 3.11 ms.<<ETX>>

Computing ROC curve confidence intervals for cardiac activation detectors

It is shown how to use the bootstrap to construct a nonparametric confidence interval for a receiver operating characteristic (ROC) curve. The method is used to perform a statistical comparison of a detectors curve with a particular point in ROC space or to compare curves derived from two similar detectors. The method is illustrated with a specific example in which a computer algorithm (AP) that was designed to detect cardiac activations in recordings taken from extracellular bipolar electrodes is evaluated. Twelve recordings of ventricular fibrillation were made on the epicardial surface of the heart in anesthetized mongrel dogs. Each recording contained between 115 and 237 activations. A cardiologist examined each recording, identified all activation events, and assigned a time of local activation (lat/sub C/) to each event. Next, each recording was analyzed by the computer program, AP, which produced its own list of local activation times (lat/sub AP/). The total numbers of true detections (N/sub TD/), false detections (N/sub FD/), and false negatives (N/sub FN/) were calculated for each recording. A true detection was defined as a lat/sub AP/ that fell within +or-6 ms of a lat/sub C/.<<ETX>>

A language for the classification and shading of multivariate volume data

A novel, language-based interface to the specification of multivariate volume classification and shading algorithms has been implemented. The system facilitates experimentation by providing access to data relevant to volume classification and shading (scalars, gradients, and gradient magnitudes) in a C-like language environment. The user writes code to calculate opacity and colour on a per voxel basis. The code is interpreted and compiled in a transparent fashion and then executed on a volume data-set. The output is a volume primitive suitable for input to standard volume rendering algorithms.

A Cellular Automata Three-dimensional Model Of Ventricular Cardiac Activation

A three-dimensional cellular automata model of activation wavefront propagation in ventricular cardiac tissue was created. The model geometry was obtained from magnetic resonance (MR) images of a canine heart which was used in a ventricular fibrillation (VF) study. Propagation within the simulated and real hearts was compared. Activation times in the simulated heart had a moderately high correlation when compared to the activation times recorded in the actual heart study. The model was run with and without fiber orientation included. Inclusion of fiber orientation did not greatly increase the correlation.

Use of coherence in activation detection during ventricular fibrillation

The use of cardiac activation mapping to study ventricular fibrillation (VF) is discussed. The locations of isochrones in maps are based on decisions on whether and when segments of the electrogram have local electrical activation. The authors compare two quantitative criteria for making decisions about local activity in electrograms. The criteria are based on the comparison of electrograms recorded in those excitable zones with electrograms recorded in normal myocardium. One uses the first derivative of the electrogram (only one electrogram affects the decision); the second uses the first derivative of the primarily local component of the waveform.<<ETX>>