Haris J. Sih

A frequency domain analysis of spatial organization of epicardial maps

Mapping of organized rhythms like sinus rhythm uses activation times from individual electrograms, and often assumes that the map for a single activation is similar to maps for subsequent activations. However, during fibrillation, activation times and electrograms are not easy to define, and maps change from activation to activation. Volume and complexity of data make analysis of more than a few seconds of fibrillation difficult. Magnitude squared coherence (MSC), a frequency domain measure of the phase consistency between two signals, can be used to help interpret longer data segments without defining activation times or electrograms. Sinus rhythm, flutter, and fibrillation in humans and swine were mapped with an array of unipolar electrodes (2.5 mm apart) at 240 sites on the atrial or ventricular epicardium. Four-second data segments were analyzed. One site near the center of the array was chosen ad hoc as a reference. MSC maps were made by measuring mean MSC from 0-50 Hz between every point in the array relative to the reference. Isocoherence contours were drawn. The effects of bias in the coherence estimate due to misalignment were investigated. Average MSC versus distance from the reference was measured for all rhythms. Results indicate that in a 4-s segment of fibrillation, there can exist some phase consistency between one site and the reference and little or none between a second site and the reference even when both sites are equidistant from the reference. In fibrillation, isocoherence contours are elongated and irregularly shaped, reflecting long-term, but nonuniform, spatial organization. That is, activation during fibrillation cannot be considered as random over a 4-s interval, Bias in the coherence estimate due to misalignment is significant for sinus rhythm and flutter, but can be corrected by manual realignment. Average MSC drops with distance for all rhythms, being most pronounced for fibrillation, MSC maps may provide insights into long-term spatial organization of rhythms that would otherwise be cumbersome and difficult to interpret with standard time domain analysis.<<ETX>>

Observations From Intraatrial Recordings on the Termination of Electrically Induced Atrial Fibrillation in Humans

Background: The circulating wavelet hypothesis suggests that atrial fibrillation could terminate by either progressive fusion or simultaneous block of all wavelets. Methods: Intraatrial recordings from the right atrial free wall were made during procainamide induced (n = 8) or spontaneous (n = 7) termination of electrically induced atrial fibrillation in 14 patients. Atrial rate, mean magnitude squared coherence, and direction of activation during sequential electrograms were measured. Rate and coherence were calculated from the earliest point within 5 minutes prior to termination as well as from the 4‐second interval just prior to termination. Results: Termination was directly to sinus rhythm (13 episodes) or to atrial flutter (2 episodes). For the eight procainamide induced terminations, rate decreased between the first measurement and the measurement just prior to termination, from 443 ±127 beats/ min to 322 ± 119 beats/min. For the seven spontaneous terminations, rate also decreased from 373 ± 119 beats/min to 323 ± 88 beats/min; however, a slight increase in atrial rate prior to termination was observed in three episodes. No specific patterns of atrial cycle lengths were seen during the final few seconds of fibrillation. No increase in coherence was observed. In seven episodes, recordings were made using orthogonal bipoles in the x, y, and z directions, allowing direction of activation of wavefronts to be measured. Three episodes showed multiple instances where direction of activation remained similar over several electrograms as we have previously reported for chronic fibrillation. However, no such instances precipitated termination in any of the seven episodes. Conclusions: Atrial fibrillation usually terminates directly to sinus rhythm and does so abruptly and without forewarning. While we and others have previously reported that the rate of atrial fibrillation decreases with procainamide infusion, a decrease in the rate of atrial fibrillation is not required for the rhythm to terminate and consequently may not be a part of the termination process at all. Coherence does not demonstrate a progressive increase in the organization of atrial fibrillation prior to termination. Lack of stabilization in the direction of activation of wavefronts in the final few seconds also fails to support fusion of wavefronts as the mechanism of termination of atrial fibrillation. Simultaneous block of all wavelets is consistent with, but not proven by our observations.

Further observations of 'linking' of atrial excitation during clinical atrial fibrillation

The objective of this article was to look for evidence of nonrandom behavior during atrial fibrillation hy examining long (> 15 minutes) recordings. We have previously reported transient “linking“ of atrial activation during atrial fibrillation, and showed that activation was not entirely randotn. Over the few episodes of linking seen during 1 minute, activation directions apparently repeated, indicating a possible anatomical or physiological constraint. In the present study, we examined atrial fibrillation over longer time periods to see if this constancy of direction was stable. Endocardial recordings were made from 12 patients with atrial fibrillation using a catheter with three orthogonal bipoies, aHowing measurements of local activation directions in three dimensions. The direction was calculated using Pipbergers half‐area method, and episodes of transient linking were identified. An average direction for each episode of linking was calculated and plotted in two dimensions using spherical coordinates (altitude and azimuth). In addition, the nature of initiation and termination of linking was examined. Of the twelve patients, 611 episodes of linking (range 1 to 169 per patient, mean 51) were identified. The episodes for most patients clustered closely in direction. In contrast, directions measured for all activations (i.e., linked and not linked) filled up the entire available range. Linking in most cases subjectively appeared to initiate and terminate suddenly. The results indicate that the local anatomy, pathology, or physiology of the atrium has a strong constraining effect on the electrical activations occurring during atrial fibrillation, and revises our perception of activation during atrial fibrillation as “random.“ The demonstration that local properties greatly influence conduction during fibrillation has important implications for ablation or pacing therapy.

Measurement of Electrical Coupling Between Cardiac Ablation Catheters and Tissue

Managing cardiac arrhythmias with catheter ablation requires positioning electrodes in contact with myocardial tissue. Objective measures to assess contact and effective coupling of ablation energy are sought. An electrical coupling index (ECI) was devised using complex impedance at 20 kHz to perform in the presence of RF ablation and deliver information about electrical interactions between the tip electrode and its adjacent environment. ECI was derived and compared with clinical judgment, pacing threshold, electrogram amplitude, and ablation lesion depth and transmurality in a porcine model. ECI was also compared with force and displacement using ex vivo bovine myocardial muscle. Mean noncontact ECI was 97.2 ± 14.3 and increased to 145.2 ± 33.6 (p <; 0.001) in clinician assessed (CLIN) moderate contact. ECI significantly improved CLINs prediction of the variance in pacing threshold from 48.7% to 56.8% ( ). ECI was indicative of contact force under conditions of smooth myocardium. Transmural lesions were associated with higher pre-RF (109 ± 17 versus 149 ± 25, ) and during-RF (82 ± 9 versus 101 ± 17, ) ECI levels. ECI is a tip specific, robust, correlate with contact and ablation efficacy, and can potentially add to clinical interpretation of electrical coupling during electrophysiology procedures.

Detection of low level ST segment changes from the ambulatory ECG and their correlation with ventricular premature beats

The role of ischemia in arrhythmogenesis has not been well established. From the ambulatory ECG the most common measure of ischemia is the ST segment using a 100 /spl mu/Volt threshold. The premature ventricular beat (PVB) was the focus of this study and a contextual analysis of the ST was performed to see if quantifiable, subclinical (<100 /spl mu/V) changes in the ST segment presage these PVCs.

Multielectrode Mapping of the Heart

Multielectrode cardiac mapping has at least a 50-year history in cardiac research, and the development of this methodology has closely followed the technological advances in instrumentation and computing. The methodology has proven to be quite effective in characterizing potential distributions on both the body surface and the epicardial surface of the heart. However, the more challenging problem for multielectrode systems is the identification and display of cardiac activation or isochronal maps. In the earlier era of cardiac mapping, hardware limitations, particularly the speed of computer processing and digital data acquisition, were the major challenges for obtaining continuous data from a high number of recording channels. For the current generation of digital electronics and computers this is no longer a significant challenge. The analysis and interpretation of the data still pose a number of challenges, since in many cases, such as diseased myocardium or during complex tachyarrhythmias, the biophysical basis of conduction is not fully developed. For example, the use of contour-generation software often does not consider the actual nature of the underlying pathophysiology. Many standard interpolation algorithms will indeed create contours overlying scar tissue within infarcted regions. This is an inherent error. A number of newer mapping approaches rely on mathematical models to create images based on data at some distance from the actual sources. In some cases these systems are proprietary and may have indeed conquered some long-standing problems. In other cases, because the systems produce “good looking” images that fit a preconceived model of activation, their underlying models are not challenged. This chapter focuses on the issues surrounding direct contact, multielectrode mapping approaches and will concentrate on the problems associated with producing activation maps, especially from regions surrounding and within infarct regions.

Comparison of time-frequency methods using spectral turbulence to identify late potentials in patients with ventricular conduction defects

The link between ventricular late potentials recorded in the signal-averaged electrocardiogram and sustained ventricular tachycardia (VT) has been well established. Patients with ventricular conduction defects are excluded in conventional time-domain analysis. This study aimed to detect the late potentials or abnormal intra-QRS potentials in patients with ventricular conduction defects using spectral turbulence analysis in the time-frequency domain. Patients with either (1) left bundle branch block, (2) right bundle branch block, and (3) intraventricular conduction defects were studied. The time-frequency representation was formed from the signal averaged XYZ leads using the short-time Fourier transform, the wavelet transform, and Choi-Williams distributions. Spectral turbulence analysis of these representations was comprised of four parameters, (1) interslice correlation mean, (2) interslice correlation standard deviation, (3) low-slice correlation ratio, and (4) spectral entropy. Separation of VT from non-VT patients was primarily observed in the X-lead using the short time FFT. Neither the wavelet transform nor the Choi-Williams distribution performed as well as the short time FFT.

A method for determining high-resolution activation time delays in unipolar cardiac mapping

A method is presented for determining activation time delays in unipolar cardiac mapping data to resolutions considerably smaller than the sample interval. The method involves taking two filtered, differentiated electrograms and computing the Hilbert transform of their cross correlation, which exhibits a negative-to-positive zero crossing at the delay time between the signals. We applied this method to estimate transmural activation time delays in the swine right atrium during sinus rhythm. We were able to calculate very precise time delay measurements, which exhibited much less variability than time delays calculated with more traditional methods. This method was also used to create an activation map of the swine right atrial epicardium during sinus rhythm, which was compared to an activation map calculated in the traditional manner.<<ETX>>

Effects of uniform anisotropy on wavelet fractionation and electrogram simulations in a computer model of fibrillation

Using a 10000-element modified Moe computer model of a fibrillating cardiac tissue sheet, cases of isotropic and uniformly anisotropic conduction were implemented and compared to determine how uniform anisotropy contributes to wavelet fractionation. In this model, a bipolar electrogram calculation based on the Ploney volume-conductor equations is implemented in several simple test cases and in a fibrillating, isotropic model sheet. It is shown that in this computer model, the role of uniform anisotropy in wavelet fractionation is less significant than effects due to overall velocity changes. Bipolar recordings in this model can approximate many characteristics in both the time and frequency domains of fibrillation cardiac tissue.<<ETX>>