Kristina M. Ropella

The coherence spectrum. A quantitative discriminator of fibrillatory and nonfibrillatory cardiac rhythms.

Previous work has suggested that a comparison of electrograms from two or more sites may best differentiate fibrillatory from nonfibrillatory rhythms. The coherence spectrum is a measure by which two signals may be compared quantitatively in the frequency domain. In the present study, the coherence spectrum was used to quantify the relation between spectral components of electrograms from two sites in either the atrium or ventricle during both fibrillatory and nonfibrillatory rhythms. Bipolar recordings of 35 rhythms from 20 patients were analyzed for coherence in the 1-59 Hz band. The 17 nonfibrillatory rhythms were sinus rhythm (six), paroxysmal supraventricular tachycardia (two), atrial flutter (four), and monomorphic ventricular tachycardia (five). The 18 fibrillatory rhythms were atrial fibrillation (12) and ventricular fibrillation (six). Nonfibrillatory rhythms exhibited moderate-to-high levels of coherence throughout the 1-59 Hz band, with peaks concentrated at the rhythms fundamental frequency and its harmonics. Fibrillatory rhythms exhibited little coherence throughout the 1-59 Hz band, and harmonics were not evident. The mean magnitude-squared coherence (scale of 0 to 1) for the 1-59 Hz band ranged from 0.22 to 0.86 (mean +/- SD, 0.52 +/- 0.19) for nonfibrillatory rhythms and from 0.042 to 0.12 (0.067 +/- 0.021) for fibrillatory rhythms. Separation of fibrillatory and nonfibrillatory rhythms was possible whether signals were recorded by floating or fixed-electrode configurations. These findings indicate that comparison of two electrograms with magnitude-squared coherence measurements differentiates fibrillatory from nonfibrillatory rhythms. A recognition algorithm based on coherence spectra may provide a major variations in lead configuration.(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation of Ventricular Tachyarrhythmias

Implantable devices capable of several modes of therapy will require differentiation of various ventricular tachyarrhythmias. Three methods of arrhythmia analysis, magnitude-squared coherence, ventricular rate, and irregularity of cycle length were performed for 45 episodes of induced ventricular tachyarrhythmia in 15 patients. Differentiation of monomorphic ventricular tachycardia from polymorphic ventricular tachycardia and ventricular fibrillation was possible by mean magnitude-squared coherence, less possible by rate, and not possible by beat-to-beat irregularity. Faster monomorphic ventricular tachycardia overlapped with rates of polymorphic ventricular tachycardia and ventricular fibrillation. Differentiation of polymorphic ventricular tachycardia and ventricular fibrillation was not possible by rate or irregularity. A progressive decrease in mean magnitude-squared coherence from monomorphic ventricular tachycardia to polymorphic ventricular tachycardia to ventricular fibrillation strengthens previous observations that coherence is a measure of rhythm organization.

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.

Effects of procainamide on intra-atrial [corrected] electrograms during atrial fibrillation: implications [corrected] for detection algorithms.

The effects of antiarrhythmic drugs on electrograms have implications for arrhythmia-detection algorithms in implantable antitachycardia devices. Filtered and unfiltered intra-atrial electrograms were analyzed in eight patients who received procainamide (50 mg/min iv, up to 1000 mg) during 11 episodes of atrial fibrillation. Continuous recordings were made before, during, and after the infusion. The recordings were digitized, divided into 4.27 sec segments, and analyzed for atrial rate, median frequency and amplitude probability density function. Significant differences were noted before and after infusion of procainamide for atrial rate (498 +/- 97 vs 356 +/- 146 beats/min; p less than .005), median frequency (5.50 +/- 1.22 vs 4.24 +/- 0.99 Hz; p less than .0005), and density (58.3 +/- 13.9% vs 69.1 +/- 15.0%; p less than .005). Pre- and postprocainamide values were compared with published criteria for detection of atrial fibrillation. Before procainamide, only 2.3%, 5.7%, and 3.4% of the data segments failed to meet criteria for atrial fibrillation by rate, frequency content, and density, respectively. In contrast, after procainamide, 50%, 36.4%, and 28.4% of the data segments failed to meet these same criteria, despite electrograms still meeting morphologic criteria for atrial fibrillation. Thus procainamide resulted in changes sufficient to cause failure of published criteria for detection of atrial fibrillation. These findings have broad implications for the function of antitachycardia devices in patients receiving antiarrhythmic drug therapy.

Effect of Bipole Configuration on Atrial Electrograms During Atrial Fibrillation

Despite an increasing body of work on the nature of fibrillatory rhythms, and the application of different bipole configurations in antifibrillatory devices, little published work has assessed the effect of bipole configuration on the endocardial recordings of fibrillatory rhythms. To address this issue, a specially designed 6 Fr decapolar catheter was used to record intra‐atrial electrograms during sustained atrial fibrillation in 15 patients. Simultaneous filtered (30–500 Hz) and unfilfered (0,05–5,000 Hz) recordings of atrial fibrillation were performed of four different bipole configurations: (a) 1‐mm interelectrode spacing adjacent to the atrial wall; (b) 10‐mm interelectrode spacing adjacent to the atrial wall; (c) 10‐mm inter‐electrode spacing 24 mm from the distal catheter tip; (d) 1‐mm interelectrode spacing 24 mm from the distal catheter tip. One minute of such data was recorded, and each 4.27‐second segment (X 14 segments) was analyzed for atrial rate, electrogram amplitude, amplitude probability density function (apdf), median frequency in the 2–9 Hz band, and elecfrogram morphology. Changes in bipole configuration resulted in profound changes in calculated afrial rate, amplitude, and apdf (P < 0.001 by two‐way ANOVA in each instance). Specifically, closer interbipole spacing and closer proximity to the atrial wall resulted in lower calculated atrial rates, higher electrogram amplitudes, and higher apdf values. In contrast, median frequency proved to be a more robust measure despite multiple configurations (P> 0.10 by two‐way ANOVA). These changes significantly affected the predictive value of previously published detection criteria for rate (P < 0.01) and apdf (P < 0.000001). Bipole location also affected morphology, with locations adjacent to the atrial wall and with closer interbipole spacing having more discrete electrograms and greater apparent organization (P < 0.0001). Further, when data segments from all patients and bipole configurations were grouped, rate and apdf were found fo be strongly inversely correlated (r = ‐0.808). In conclusion: (1) Bipole configuration has important effects on calculated atrial rate, electrogram amplitude, and apdf during atrial fibrillation; (2) Median frequency and frequency domain analysis may be a more robust way of characterizing atrial fibrillation despite the use of different bipole configurations; (3) Changes in bipole configuration affect the efficacy of detection criteria, and considerations about the level of organization of a cardiac rhythm; (4) Rate and apdf may be largely redundant measures of fibrillatory rhythms; and (5) Traditional estimates of atrial rates up to 700/min during atrial fibrillation, based on the unipolar or widely spaced bipolar leads of the surface electrocardiogram, reflect the effects of their recording methods. and are an overesfimation of the true atrial rate.

Detection of changes in atrial endocardial activation with use of an orthogonal catheter

The ability of a catheter with an orthogonal electrode configuration to sense differences in the direction of local atrial endocardial activation was tested in 18 consecutive patients with intact retrograde conduction. In all 18, discrimination of anterograde from retrograde conduction at a single atrial site was examined; in 5 of the 18, multiple sites were examined to determine if the discriminatory ability of the catheter was site dependent. The catheter was specially designed with bipoles in the x, y and z directions. A vector was computed for each electrogram during anterograde and retrograde conduction. Electrogram amplitude along the standard bipole was also compared for anterograde and retrograde conduction. Mean electrogram amplitude for the standard bipole was significantly different for anterograde than for retrograde conduction in 17 of 18 patients (mean +/- SD 4 +/- 1.9 vs. 2.7 +/- 1.3 mV; p less than 0.005), with complete separation of amplitude distributions in 4 patients. The electrogram vector during anterograde conduction was significantly different from that during retrograde conduction in all 18 patients (p less than 0.0001), with complete separation of vector distributions in 14. In some patients with multiple site recordings, the choice of site greatly affected separation based on electrogram amplitude or vector, or both. The orthogonal catheter can be used to sense directional differences in local endocardial activation. The catheter shows promise for discriminating anterograde from retrograde conduction and examining the direction of endocardial activation in the heart during an electrophysiologic examination.

Measuring the organization of cardiac rhythms using the magnitude-squared coherence function

The application of the magnitude-squared coherence (MSC) spectrum as a measure of the degree of organization of the cardiac electrical activity is explored. The MSC spectrum is a frequency-domain measure of the linear relationship between two signals. In the work described the two signals are two bipolar electrograms from either acutely placed catheter(s) or automatic implantable cardioverter/defibrillator electrodes. It is shown that the MSC is a dimensionless (no units), real-valued spectrum that is always in the range of zero to unity. The case of zero is found at frequencies where there is no linear relationship between the signals, and the case of unity implies a linear, noise-free relationship. The way the MSC spectrum is normalized makes it insensitive to gain or gain differences between the two signals. Example MSC spectra are presented and discussed. Striking differences in the spectra for fibrillatory and nonfibrillatory rhythms are seen.<<ETX>>

Adaptive coherence estimation on brief intracardiac recordings

Adaptive techniques are considered for the estimation of the magnitude-squared coherence spectrum of two simultaneous intracardiac signals. Estimators based on single- and multiple-pass algorithms are proposed. Adaptive estimates of the magnitude-squared coherence spectrum are shown to effectively separate fibrillatory from several nonfibrillatory rhythms on the basis of 5 s of data. The significance of this work for sophisticated antitachyarrhythmia devices is indicated.<<ETX>>

Coherence measures of cardiac arrhythmias from intra-cardiac and epicardial leads

The magnitude-squared coherence (MSC) spectrum is introduced as a dimensionless frequency-domain measure of rhythm organization from pairs of intracardiac and/or epicardial leads. The MS spectrum is simply defined as the magnitude of the complex cross-power-spectrum normalized to the product of the two individual auto-power-spectra. In this way, two linearly related signals (in the absence of uncorrelated additive noise) will have MSC values of unity at all signal frequencies present in both signals in any amount. Mean MSC values are shown to discriminate fibrillatory from nonfibrillatory rhythms. Speed/accuracy tradeoffs in estimation of the MSC are considered.<<ETX>>

Effect of data segmentation on coherence estimates of cardiac rhythms

Both fibrillatory and nonfibrillatory cardiac rhythms were analyzed for coherence spectra while varying the segmentation of finite records of data. Results indicate that although the magnitude and mean of the magnitude-squared coherence function do not change significantly for nonfibrillatory rhythms, these same parameters show a marked decrease for fibrillatory rhythms. These findings are explained in terms of the bias and variance of the coherence estimate.<<ETX>>