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Fast Fourier Transformation of the Entire Low Amplitude Late QRS Potential to Predict Ventricular Tachycardia

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Fast Fourier Transformation of the Entire Low Amplitude Late QRS Potential to Predict Ventricular Tachycardia View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector JACC Vol. 14, No. 7 1731 December 1989:173l-l0 ELECTROPHYSIOLOGIC STUDIES Fast Fourier Transformation of the Entire Low Amplitude Late QRS Potential to Predict Ventricular Tachycardia DAN L. PIERCE, MD, ARTHUR R. EASLEY, JR., MD, FACC, JOHN R. WINDLE, MD, FACC, TOBY R. ENGEL, MD, FACC Omaha, Nebraska Signal-averaged electrocardiograms (X, Y and Z leads) longer high frequency terminal signals <40 PV (p < were acquired from 24 patients with coronary artery dis- 0.0004), but not significantly lower voltage in the last 40 ms. ease and recurrent ventricular tachycardia, 24 control The most useful temporal domain measurement was high patients with coronary artery disease and 23 normal sub- frequency QRS duration (if 2120 ms, odds ratio = 8.2). jects to assess the discriminant value of fast Fourier trans- Patients with ventricular tachycardia had increased high formation of the entire late potential period of the QRS frequency spectral areas (p < 0.0002) in the late potential, complex. Analysis of the vector magnitude in the temporal and the percent high frequency was especially increased domain (25 to 250 Hz bandpass filters) measured high (p = 0.0000; if percent high frequency ?3.1%, odds ratio frequency QRS duration, the duration of terminal signals = 18.4). <40 PV and the root mean square voltage of the last 40 ms. The odds ratio and the area under the receiver operating Late potentials were defined as terminal signals >25 Hz characteristic curve were both greater for percent high that were <40 pV. Analysis in the frequency domain used frequency than for high frequency QRS duration (p < a 120 ms window that encompassed (had onset with) all of 0.03). All patients with ventricular tachycardia had a high the late potential, but the mean value was first subtracted to frequency QRS complex 2107 ms or percent high fre- eliminate a direct current component. High frequency quency 23.1% (sensitivity 100%). For a high frequency spectral areas (60 to 120 Hz) and the percent high fre- QRS complex 2107 ms and percent high frequency quency (100 x [60 to 120 Hz/O to 120 Hz]) were calculated. r3.1%, specificity was 96%. Therefore, high frequencies Results in both temporal and frequency domains were in late potentials, not their duration or reduced voltage, similar in control patients with coronary artery disease and most usefully identify patients with coronary artery disease normal subjects. Patients with ventricular tachycardia had who are prone to ventricular tachycardia. a longer high frequency QRS complex (p < 0.0001) and (J Am Co11Cardiol1989;14:1731-40) The use of fast Fourier transformation of signal-averaged (of interventions or drug effects) and to enable duplication by electrocardiograms (ECGs) to predict ventricular tachycar- other laboratories. Importantly, this involved analyzing the dia is controversial (1-S). One reason for this controversy is entire low amplitude late QRS potential. the differing mathematic treatments of the transformed sig- An additional aim of our study was to analyze normal nals. We present an analysis that can be performed using subjects (as compared with control patients with heart commercially available programs. Discrepant results have disease but not ventricular tachycardia) to learn if coronary also stemmed from analyzing regions of the ECG that are not artery disease itself was a confounding factor that altered our well defined or are difficult to reproduce. We precisely analysis of the frequency content of late potentials. Gomes defined the region of interest to enable comparison studies et al. (6) observed differences in the temporal domain in patients with heart disease (two-thirds with coronary artery disease) who did not have ventricular tachycardia versus From the Department of Internal Medicine, University of Nebraska normal subjects. Worley et al. (3) found the differences College of Medicine, Omaha. Nebraska. This study was presented in part at the North American Society of Pacing and Electrophysiology, Toronto. between normal subjects and patients with myocardial in- Ontario, Canada, May 1989. farction without ventricular tachycardia to be most striking Manuscript received May I, 1989;revised manuscript received June 14. in frequency analysis of a window early in the QRS complex. 1989,accepted June 21. 1989. Add ess for reorints: Toby R. Engel, MD, University of Nebraska In summary, we reviewed signal-averaged ECGs from a MedicalrCenter. 42nd and Dewey Avenue, Omaha, Nebraska 68105. group of normal subjects and patients with coronary artery 01989 by the American College of Cardiology 07351097189/$3.50 1732 PIERCE ET AL. JACC Vol. 14, No. 7 FOURIER TRANSFORM OF ENTIRE LATE POTENTIAL December 1989:1731-40 disease using a fast Fourier transformation routine that could to avoid edge discontinuities. Data were set at the beginning be repeated by others. of the array, and the remaining elements were set to zero (padded with zeros to 600 points). Before fast Fourier transformation, but after zero padding, the mean of all data Methods values was subtracted after windowing (7), and a fast Fourier Study patients. Signal-averaged ECGs were acquired transform was calculated using a “chirp z” algorithm. The from 71 individuals who were not taking antiarrhythmic spectrum was plotted on a voltage scale without normaliza- drugs. Noisy tracings (> 1.O PV zero mean standard devia- tion. The spectral areas between 60 and 120 Hz and between tion of vector sum for the TP interval) from an additional 0 and 120 Hz and the ratio of these areas were calculated for eight patients were excluded from analysis. each ECG lead and then averaged ([X + Y + Z]/3). Percent Patients were selected in three ways. Group I consisted of high frequency was thus considered to be (100 x [60 to 120 normal volunteers (hospital staff) or patients with normal Hz/O to 120 Hz]), there being scant voltage >120 Hz. coronary angiograms and ventricular function at cardiac Contamination with 60 Hz line voltage was not excluded catheterization. Group II included patients with coronary by a 60 Hz filter. However, no patient had spectral peaks artery disease, confirmed by coronary angiography and left reiterated at 60, 120 and 180 Hz, and we never saw a relative ventriculography, without a history of sustained ventricular increase at 60 Hz that was consistent in all three leads. These arrhythmias. These were mostly enlisted in an on-going observations argue against contamination with 60 Hz line study of coronary angioplasty in which the signal-averaged voltage. ECG was recorded before the procedure. None had unstable We chose a window of 120 ms duration. A variable angina pectoris or acute Q wave myocardial infarction within window such as the ST segment was not used because the 3 days of study. Three had electrophysiologic study, which spectrum is composed of harmonics of the fundamental was negative. Group III consisted of patients with coronary frequency that depend on segment length. Kelen et al. (2) artery disease and cardiac arrest, syncope or clinically exemplified the importance of using a window of fixed recurrent sustained ventricular tachycardia. In all but two duration by observing that as little as a 3 ms change resulted patients, ventricular tachycardia was repeatedly induced by in dramatic changes in area ratios. Resolution is a function of one to three twice-threshold extrastimuli (one patient died window duration, but too long a window can attenuate the before the study could be done). Signal-averaged ECGs were contribution of brief late potentials at it onset (when multi- acquired from patients in Groups I and II under protocols plied by the Blackman-Harris window) and tends to involve approved by the Investigational Review Board of the Uni- the T wave. Haberl et al. (5) reported that a 120 ms window versity of Nebraska Medical Center; they were acquired best resolved 60 to 120 Hz in diagnosing ventricular tachy- from patients in Group III for clinical purposes. cardia. In contract to Haberl et al. (5), we did not use QRS Acquisition and analysis of signal-averagedelectrocardio- termination to delineate window onset (they defined the end grams. Recordings were made with a Corazonix ECG sys- of the QRS complex when vector velocity was <5 mV/s and tem and analyzed with a Predictor 4.1 program. The follow- then analyzed a segment that began 20 ms within the QRS ing orthogonal X, Y and Z leads were used: left and right complex). Instead, to ensure that the window encompassed midaxillary lines at the fourth intercostal space (x); proximal all of the late potential without including voltages from the left leg and manubrium (y) and V, and directly posterior (z). QRS complex itself, and at the same time to select a At least 200 beats were averaged, and analog to digital consistently reproducible window onset, we chose onset as conversion with a dynamic range of 16 bits was performed at the time when vector-summed QRS forces last decreased to a sampling frequency of 2,000 Hz. The trigger point was the below 40 PV (at 25 to 250 Hz). This has been used by QRS complex, and 10 correlations were made over a 25 ms Machac et al. (8) in a preliminary report to establish repro- window manually selected by cursor. Beats with a correla- ducible threshold voltage in windowing for frequency anal- tion coefficient (r) co.99 were excluded. ysis. The EC& were analyzed in the temporal domain using Late potentials were thus defined as high frequency (that bidirectional (Butterworth) fourth order 25 to 250 Hz band- is, 25 to 250 Hz) terminal signals ~40 pV, ~40 PV being pass filters. Noise levels were determined from the TP taken as the onset of late potentials in most reports (2- segment, and QRS end oints were identified when the 4,9,10) evaluating the temporal domain with various high d+ vector magnitude ( x + y + z ) was >3 standard devia- pass filters.
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