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A new method for measuring high-frequency noise
Poster Session III / Journal of Electrocardiology 38 (2005) 140 – 144 Broken asymmetry of the human heartbeat: loss of time irreversibility in aging and disease Madalena D. Costa a,b, Ary L. Goldberger a, C.-K. Peng a ( aBeth Israel Deaconess Medical Center, Harvard Medical School, Massachusetts 02215, USA; bFaculty of Sciences, University of Lisbon, Campo Grande, 1749-016 Lisbon, Portugal)
Living systems function in conditions far from equilibrium. They use energy to evolve to more hierarchically ordered structural configurations and less entropic states in comparison with the surrounding environment. In extreme cases, presaging death, a state approaching maximum equilibrium, is reached. Time irreversibility, defined as lack of invariance of the statistical properties under the operation of temporal inversion, is a fundamental property of nonlinear nonequilibrium systems. Surprisingly, relatively little work has been published on practical implementation of time reversibility to biologic time series. We introduce a time irreversibility measure and show the results of applying it to the analysis of approximately 24-hour human cardiac interbeat interval time series for groups of healthy young (n = 26) and healthy elderly (n = 46) subjects, those with congestive heart failure in sinus rhythm (n = 43), and those with atrial fibrillation (n = 9) from an open-access database (www.physionet.org). The time asymmetry index is significantly higher for young healthy subjects than for both healthy elderly subjects and subjects with pathology ( P b .005). Furthermore, the time asymmetry index is significantly higher for the elderly healthy group than for the pathological groups ( P b .005). These findings provide, for the first time, evidence that supports the hypotheses that (1) time irreversibility is greatest for healthy physiological systems, which exhibit the most complex dynamics; and 2) time irreversibility decreases with aging or pathology, offering a new marker of loss of nonlinear functionality and adaptability. Our results also suggest that a readily implemented quantitative measure of time irreversibility complement traditional indexes of heart rate variability. doi:10.1016/j.jelectrocard.2005.06.076
Electrocardiograph high-pass frequency response revisited Budimir S. Drakulic, PhD, Zoran Konstantinovic, PhD, Marko Kostic, Branislav Vlajinic, Garett Nakatsuka (Recom Managed Systems, Inc, Studio City, California, USA)
The main purpose of the high-pass filter in electrocardiograph equipment is to suppress baseline wander. Review of high-pass filter settings on various modern electrocardiographs shows that manufacturers provide a number of different options. Furthermore, the analysis of American National Standards Institute/AAMI EC11, EC13, and EC38 standards, and their respective rationales reveals history and current recommendations for low-frequency response of electrocardiographs. Originally, a 0.05-Hz frequency for single-pole high-pass filter was recommended. The latest adjustment of high-pass filter to 0.67 Hz was based upon heart rate data from Framingham Heart Study and the studies of Simonson. A number of high-pass filters with different cutoff frequency values were evaluated, and their effect on various characteristics of original ECG signal waveform was observed. In particular, we evaluated high-pass filters with linear phase characteristics because
141
nonlinearity alters diagnostic information contained within the original waveform. Traditionally, high-pass filter characteristics are derived from a low-pass prototype using a simple 1/s transform. Unfortunately, this transformation does not preserve phase linearity of the original low-pass filter. We have explored several other techniques to obtain an analog high-pass filter with linear phase characteristics. As a result, a high-pass filter characteristic is recommended to provide a good approximation between attenuation of the direct current components and overall system performance. doi:10.1016/j.jelectrocard.2005.06.077
A new method for measuring high-frequency noise Richard Gregg, James Lindauer, Eric Helfenbein, Min Shao, Sophia Zhou (Advanced Algorithm Research Center, Philips Medical Systems, USA)
Background: In computerized electrocardiography, high levels of muscle noise degrade the performance of analysis programs. Accurate estimates of the high-frequency noise level can be used to reduce false-positive diagnostic calls due to noise. Methods: The new method calculates the mean absolute deviation of the first differences of the average beat after each addition of a new beat. A 1/N curve is fit to the successive mean absolute deviation values to estimate the reduction in noise and the asymptote representing the signal. A second method estimates noise by short-term RMS signal levels. The 2 methods are compared on a set of electrocardiograms (ECGs) including artificial- and low-noise biologic ECGs with known filtered Gaussian noise added. The experiment was repeated for 3 noise bandwidths (BWs). Table 1 shows that the correlation is higher for the curve fit method. For the first difference-based curve fit method, the slope of the line fit decreases as the noise BW decreases because of higher correlation between adjacent noise samples. Because the noise BW is shaped by the input filter of the acquisition device, the BW is known. Conclusion: High-frequency noise in the ECG can be effectively estimated by the curve fit method described previously. The advantage of the curve fit method is simplicity. It is calculated only on the average beat as it is formed. doi:10.1016/j.jelectrocard.2005.06.078
Why does not QT dispersion reflect the heterogeneity in ventricular repolarization? Yang Zhong-fu, An Yong, Li Chuan-yong (Department of Biophysics, Nankai University, Tianjin, 300071, PRC) Introduction: Many discrepancies exist in the feasibility of QT dispersion to represent the heterogeneity in ventricular repolarization referring to experimental results. The computer simulation is established in this article to analyze physical meaning of QT and its dispersion, and then understand the relationship between QT dispersion and the heterogeneity in ventricular repolarization. Methods: A piece of 1-dimensional myocardium of 10 mm was established in the study. When the excitation traveled from one end (endocardium) to the other (epicardium) at 0.5 mm/ms, myocardial extracellular potential was calculated by the myocardial bidomain model. The heterogeneity in ventricular repolarization was modeled by a regional myocardial ischemia, in which the transmembrane potential of ischemic
Table 1 Noise estimation method Curve fit method Minimum RMS method
BW = 150 Hz R
Slope
BW = 100 Hz R
Slope
BW = 50 Hz R
Slope
0.955 0.881
0.728 0.616
0.941 0.836
0.546 0.530
0.899 0.800
0.288 0.456
Living systems function in conditions far from equilibrium. They use energy to evolve to more hierarchically ordered structural configurations and less entropic states in comparison with the surrounding environment. In extreme cases, presaging death, a state approaching maximum equilibrium, is reached. Time irreversibility, defined as lack of invariance of the statistical properties under the operation of temporal inversion, is a fundamental property of nonlinear nonequilibrium systems. Surprisingly, relatively little work has been published on practical implementation of time reversibility to biologic time series. We introduce a time irreversibility measure and show the results of applying it to the analysis of approximately 24-hour human cardiac interbeat interval time series for groups of healthy young (n = 26) and healthy elderly (n = 46) subjects, those with congestive heart failure in sinus rhythm (n = 43), and those with atrial fibrillation (n = 9) from an open-access database (www.physionet.org). The time asymmetry index is significantly higher for young healthy subjects than for both healthy elderly subjects and subjects with pathology ( P b .005). Furthermore, the time asymmetry index is significantly higher for the elderly healthy group than for the pathological groups ( P b .005). These findings provide, for the first time, evidence that supports the hypotheses that (1) time irreversibility is greatest for healthy physiological systems, which exhibit the most complex dynamics; and 2) time irreversibility decreases with aging or pathology, offering a new marker of loss of nonlinear functionality and adaptability. Our results also suggest that a readily implemented quantitative measure of time irreversibility complement traditional indexes of heart rate variability. doi:10.1016/j.jelectrocard.2005.06.076
Electrocardiograph high-pass frequency response revisited Budimir S. Drakulic, PhD, Zoran Konstantinovic, PhD, Marko Kostic, Branislav Vlajinic, Garett Nakatsuka (Recom Managed Systems, Inc, Studio City, California, USA)
The main purpose of the high-pass filter in electrocardiograph equipment is to suppress baseline wander. Review of high-pass filter settings on various modern electrocardiographs shows that manufacturers provide a number of different options. Furthermore, the analysis of American National Standards Institute/AAMI EC11, EC13, and EC38 standards, and their respective rationales reveals history and current recommendations for low-frequency response of electrocardiographs. Originally, a 0.05-Hz frequency for single-pole high-pass filter was recommended. The latest adjustment of high-pass filter to 0.67 Hz was based upon heart rate data from Framingham Heart Study and the studies of Simonson. A number of high-pass filters with different cutoff frequency values were evaluated, and their effect on various characteristics of original ECG signal waveform was observed. In particular, we evaluated high-pass filters with linear phase characteristics because
141
nonlinearity alters diagnostic information contained within the original waveform. Traditionally, high-pass filter characteristics are derived from a low-pass prototype using a simple 1/s transform. Unfortunately, this transformation does not preserve phase linearity of the original low-pass filter. We have explored several other techniques to obtain an analog high-pass filter with linear phase characteristics. As a result, a high-pass filter characteristic is recommended to provide a good approximation between attenuation of the direct current components and overall system performance. doi:10.1016/j.jelectrocard.2005.06.077
A new method for measuring high-frequency noise Richard Gregg, James Lindauer, Eric Helfenbein, Min Shao, Sophia Zhou (Advanced Algorithm Research Center, Philips Medical Systems, USA)
Background: In computerized electrocardiography, high levels of muscle noise degrade the performance of analysis programs. Accurate estimates of the high-frequency noise level can be used to reduce false-positive diagnostic calls due to noise. Methods: The new method calculates the mean absolute deviation of the first differences of the average beat after each addition of a new beat. A 1/N curve is fit to the successive mean absolute deviation values to estimate the reduction in noise and the asymptote representing the signal. A second method estimates noise by short-term RMS signal levels. The 2 methods are compared on a set of electrocardiograms (ECGs) including artificial- and low-noise biologic ECGs with known filtered Gaussian noise added. The experiment was repeated for 3 noise bandwidths (BWs). Table 1 shows that the correlation is higher for the curve fit method. For the first difference-based curve fit method, the slope of the line fit decreases as the noise BW decreases because of higher correlation between adjacent noise samples. Because the noise BW is shaped by the input filter of the acquisition device, the BW is known. Conclusion: High-frequency noise in the ECG can be effectively estimated by the curve fit method described previously. The advantage of the curve fit method is simplicity. It is calculated only on the average beat as it is formed. doi:10.1016/j.jelectrocard.2005.06.078
Why does not QT dispersion reflect the heterogeneity in ventricular repolarization? Yang Zhong-fu, An Yong, Li Chuan-yong (Department of Biophysics, Nankai University, Tianjin, 300071, PRC) Introduction: Many discrepancies exist in the feasibility of QT dispersion to represent the heterogeneity in ventricular repolarization referring to experimental results. The computer simulation is established in this article to analyze physical meaning of QT and its dispersion, and then understand the relationship between QT dispersion and the heterogeneity in ventricular repolarization. Methods: A piece of 1-dimensional myocardium of 10 mm was established in the study. When the excitation traveled from one end (endocardium) to the other (epicardium) at 0.5 mm/ms, myocardial extracellular potential was calculated by the myocardial bidomain model. The heterogeneity in ventricular repolarization was modeled by a regional myocardial ischemia, in which the transmembrane potential of ischemic
Table 1 Noise estimation method Curve fit method Minimum RMS method
BW = 150 Hz R
Slope
BW = 100 Hz R
Slope
BW = 50 Hz R
Slope
0.955 0.881
0.728 0.616
0.941 0.836
0.546 0.530
0.899 0.800
0.288 0.456