Statistical accuracy of a moving equivalent dipole method to identify sites of origin of cardiac electrical activation

Abstract

While radio frequency (RF) catheter ablation (RCA) procedures for treating ventricular arrhythmias have evolved significantly over the past several years, the use of RCA has been limited to treating slow ventricular tachycardias (VTs). In this paper, we present preliminary results from computer and animal studies to evaluate the accuracy of an algorithm that uses the single equivalent moving dipole (SEMD) model in an infinite homogeneous volume conductor to guide the RF catheter to the site of origin of the arrhythmia. Our method involves measuring body surface electrocardiographic (ECG) signals generated by arrhythmic activity and by bipolar current pulses emanating from a catheter tip, and representing each of them by a SEMD model source at each instant of the cardiac cycle, thus enabling rapid repositioning of the catheter tip requiring only a few cycles of the arrhythmia. We found that the SEMD model accurately reproduced body surface ECG signals with a correlation coefficients > 0.95. We used a variety of methods to estimate the uncertainty of the SEMD parameters due to measurement noise and found that at the time when the arrhythmia is mostly localized during the cardiac cycle, the estimates of the uncertainty of the spatial SEMD parameters (from ECG signals) are between 1 and 3 mm. We used pacing data from spatially separated epicardial sites in a swine model as surrogates for focal ventricular arrhythmic sources and found that the spatial SEMD estimates of the two pacing sites agreed with both their physical separation and orientation with respect to each other. In conclusion, our algorithm to estimate the SEMD parameters from body surface ECG can potentially be a useful method for rapidly positioning the catheter tip to the arrhythmic focus during an RCA procedure.

Method for guiding the ablation catheter to the ablation site: a simulation and experimental study

“…where / i m was the body surface potential at location i due to the point dipole estimated in the forward-problem with the BEM simulator, after it has been corrupted with zeromean Gaussian noise of standard deviation r i , where r i was an estimate of the electrode noise, dof stands for degrees of freedom, and I was the number of electrodes [2,3].…”

“…A spatial criterion was imposed to force solutions to land inside a predefined volume that includes the heart [1][2][3]8]. In prior studies [2,3] we have learned that the algorithm is convergent to a global minimum and hence is terminated, if two solutions that start from two different seeds are found to be closer than 0.5 cm. Therefore, if after ten seeds there were no solutions that satisfied the spatial criterion and were closer than 0.5 cm apart, the algorithm outcome was considered to be non-convergent (indicated in the Tables).…”

“…In order to satisfy this need, we have introduced a new method to guide the ablative therapy of cardiac arrhythmias, in which we model both cardiac electrical activity (at the exit site of the scar tissue or the site of abnormal automaticity which for purposes of this application we call ''target site'') and current pulses generated from a pair of electrodes at the tip of the ablation catheter with a single equivalent moving dipole (SEMD) [1][2][3]8]. The method employs a three-step approach: (1) models cardiac electrical activity during VT with a SEMD, and from the analysis of the SEMD trajectory identifies the SEMD location corresponding to the target site; (2) models nonexcitatory (high frequency, e.g.…”

“…We have previously demonstrated that in addition to rapid identification of the SEMD parameters, we can also use these SEMD parameters to accurately reconstruct potentials on the body surface that are highly correlated with the original values and identify spatially separated epicardial sources [2,3]. In order to avoid the use of complex individual volume conductor models for which for example magnetic resonance images need to be obtained, the inverse algorithm used to estimate the SEMD parameters ignores boundary effects caused by tissue inhomogeneities and realistic torso geometry.…”

“…with a few seconds delay). Furthermore, the advantages of using the SEMD model are (1) it is a good model when cardiac electrical activity is localized [2,3], as is the case at the initial activation site one wants to ablate; (2) it permits quantification of the strength of the source in biophysical terms that are independent of volume conductor size (in clinical terms this means that one can obtain an estimate of the initial depolarization wavefront from the strength of the SEMD, that is independent of the size of the heart); and (3) the active equivalent source can be assigned a location [1]. Then, the six dipole parameters (three location and three moment parameters) can be obtained easily [2,3] for a real-time application, such as RCA, from ECG signals recorded e.g.…”