Effects of Filtering on Right Ventricular Electrograms Recorded from Endocardial Catheters in Humans

Criteria for identifying electrically abnormal myocardium based on analysis of electrogram, amplitude, duration, and morphology have recently been proposed as an aid to catheter mapping in humans. Electrogram characteristics are likely to be influenced by analog filtering and the distance between recording electrodes. These parameters can be varied and differ among electrophysiology laboratories. To determine the magnitude of the effect of low pass and high pass filtering and interelectrode distance on human bipolar right ventricular electrograms eight patients were studied. The signal from a single bipolar electrode pair was recorded simultaneously on five channels with variable filtering. As the high pass filter frequency increased above 10 Hz, the electrogram duration and amplitude decreased. The decrease in amplitude was described by a monoexponential function with a 30% decrease in amplitude at a filter setting of 30 Hz and a 69% decrease at a filter setting of 100 Hz. As the low pass filter frequency decreased from 2,500 to 250 Hz, electrogram characteristics were not significantly altered. Low pas filtering at 100 Hz decreased electrogram amplitude slightly. Neither high nor low pass filter settings or interelectrode distance affected electrogram timing. However, filtering could markedly alter electrogram morphology, introducing the possibility for errors in visual assessment of electrograms. Electrogram duration increased linearly with increasing distance between recording electrodes. The effects of filtering on electrograms were similar regardless of interelectrode spacing. Thus, varying the high pass filter setting over a range that is used clinically (from 1 to 100 Hz) can substantially alter electrogram duration, amplitude, and morphology. Low pass filtering at 250 Hz and above had no effect on these electrograms although it is possible that alterations would be observed in signals from areas of slow conduction that we did not study.(ABSTRACT TRUNCATED AT 250 WORDS)     

The recording of monophasic action potentials with fractal-coated iridium electrodes in humans

The present study was designed to evaluate the feasibility of the recording of monophasic action potentials (MAP) with fractal-coated iridium electrodes in a clinical setting. In 18 patients who underwent an electrophysiological study for various arrhythmias, we performed MAP recordings with both 1.3-mm2 and 6-mm2 tip surface area fractal-coated iridium and standard silver--silver chloride (Ag/AgCl) electrodes in the high right atrium and two ventricular positions. Amplitude and MAP duration at 90%, 50%, and 25% of repolarization were calculated during steady-state pacing at 600, 500, and 400 ms cycle lengths with extrastimuli application. Morphology comparisons of MAP signals recorded with both types of electrodes were performed by regression analysis using 5% of the repolarization segments of the MAP trajectory. Differences between MAP duration at 90%, 50%, and 25% of repolarization recorded with fractal-coated and Ag/AgCl electrodes were statistically insignificant. Amplitude values recorded with 6-mm2 tip electrodes were significantly smaller than those recorded with Ag/AgCl electrodes for all comparisons. During steady-state pacing, the correlation coefficients between Ag/AgCl and fractal-coated 1.3-mm2 and 6-mm2 tip electrodes were within the range of 0.93-0.999 and 0.87-0.999, respectively. The correlation of MAP amplitude and duration at 90%, 50%, and 25% of repolarization following the extrastimulus S2, recorded with both types of electrodes, was significantly weaker for right atrial recordings (r value range 0.78-0.92) as compared to ventricular recordings (r value range 0.92-0.99). The MAP sensing features of fractal-coated iridium and Ag/AgCl electrodes are comparable. The best results for recording of MAPs with fractal-coated electrodes can be achieved with small surface area tip electrodes.     

Frequency filtering improves ultrasonic embolic signal detection.

Problems in detection of Doppler cerebral embolic signals primarily occur for embolic signals of low relative intensity. A characteristic feature of embolic signals is that the intensity increase is maximal over a narrow frequency band. Therefore, frequency filtering of the data might improve embolic signal relative intensity and detectability. We implemented an off-line finite impulse response filter in software running on a commercially available transcranial Doppler system, using the time-domain audio data as input. The range of the filter was chosen by placing a box around the embolic signal on the spectral display. One hundred consecutive embolic signals from patients with carotid stenosis were analyzed; all had been recorded by a bigate system and the signal was analyzed in both proximal and distal channels. There was a highly significant increase in embolic signal relative intensity following frequency filtering; mean (SD) proximal channel prefiltering 12.75 (4.83) dB, postfiltering 16.36 (4.93) dB; distal channel prefiltering 13.42 (4.98) dB, postfiltering 16.60 (5.11) dB, for both p < 0.001. Despite all embolic signals being audible and visible in at least one channel on the frequency spectral display, in 17 cases, the amplitude increase associated with the embolic signal could not be clearly seen in time-domain data of one or both channels prior to filtering. Following frequency filtering, this was reduced to 5. Incorporation of such a frequency-filtering approach to an online system is likely to improve the sensitivity of online detection for embolic signals of low relative intensity.     

Customized spectral band analysis compared with conventional Fourier analysis of heart rate variability in neonates

A customized filtering technique is introduced and compared with fast Fourier transformation (FFT) for analyzing heart rate variability (HRV) in neonates from short-term recordings. FFT is classically the most commonly used spectral technique to investigate cardiovascular fluctuations. FFT requires stability of the physiological signal within a 300 s time window that is usually analyzed in adults. Preterm infants, however, show characteristics of rapidly fluctuating heart rate and blood pressure due to an immature autonomic regulation, resulting in non-stationarity of these signals. Therefore neonatal studies use (half-overlapping or moving) windows of 64 s length within a recording time of 2-5 min. The proposed filtering technique performs a filtering operation in the frequency range of interest before calculating the spectrum, which allows it to perform an analysis of shorter periods of only 42 s. The frequency bands of interest are 0.04-0.15 Hz (low frequency, LF) and 0.4-1.5 Hz (high frequency, HF). Although conventional FFT analysis as well as the proposed alternative technique result in errors in the estimation of LF power, due to spectral leakage from the very low frequencies, FFT analysis is more sensitive to this effect. The response times show comparable behavior for both the techniques. Applying both the methods to heart rate data obtained from a neonate before and after atropine administration (inducing a wide range of HRV), shows a very significant correlation between the two methods in estimating LF and HF power. We conclude that a customized filtering technique might be beneficial for analyzing HRV in neonates because it reduces the necessary time window for signal stability.     

A method for suppressing cardiogenic oscillations in impedance pneumography

The transthoracic electrical impedance signal originates from the cardiac and respiratory functions. In impedance pneumography (IP) the lung function is assessed and the cardiac impedance signal, cardiogenic oscillations (CGOs), is considered an additive noise in the measured signal. In order to accurately determine pulmonary flow parameters from the signal, the CGO needs to be attenuated without distorting the respiratory part of the signal. We assessed the suitability of a filtering technique, originally described by Schuessler et al (1998 Ann. Biomed. Eng. 26 260-7) for an esophageal pressure signal, for CGO attenuation in the IP signal. The technique is based on ensemble averaging the CGO events using the electrocardiogram (ECG) R-wave as the trigger signal. Lung volume is known to affect the CGO waveforms. Therefore we modified the filtering method to produce a lung volume-dependent parametric model of the CGO waveform. A simultaneous recording of ECG, IP and pneumotachograph (PNT) was conducted on 41 healthy, sitting adults. The performance of the proposed method was compared to a low-pass filter and a Savitzky-Golay filter in terms of CGO attenuation and respiratory signal distortion. The method was found to be excellent, exhibiting CGO attenuation of 35.0±12.5 dB (mean±SD) and minimal distortion of the respiratory part of the impedance signal.     

Real time ECG artifact removal for myoelectric prosthesis control

The electrocardiogram (ECG) artifact is a major noise source contaminating the electromyogram (EMG) of torso muscles. This study investigates removal of ECG artifacts in real time for myoelectric prosthesis control, a clinical application that demands speed and efficiency. Three methods with simple and fast implementation were investigated. Removal of ECG artifacts by digital high-pass filtering was implemented. The effects of the cutoff frequency and filter order of high-pass filtering on the resulting EMG signal were quantified. An alternative adaptive spike-clipping approach was also developed to dynamically detect and suppress the ECG artifacts in the signal. Finally, the two methods were combined. Experimental surface EMG recordings with different ECG/EMG ratios were used as testing signals to evaluate the proposed methods. As a key parameter for clinical myoelectric prosthesis control, the average rectified amplitude of the signal was used as the performance indicator to quantitatively analyze the EMG content distortion and the ECG artifact suppression imposed by the two methods. Aiming at clinical application, the optimal parameter assignment for each method was determined on the basis of the performance using the suite of testing signals with various ECG/EMG ratios.     

The wall signal removal in Doppler ultrasound systems based on recursive PCA

In Doppler ultrasound (US) systems, a high-pass filter is usually employed to remove the wall component from the blood flow signal. However, this will lead to the loss of information from the low velocity flow. In this paper, an algorithm based on the principal components analysis (PCA) is proposed, in which singular value decomposition (SVD) is used to extract the main component from the mixed signals. Furthermore, the recursive process is incorporated into the PCA method to improve the performance of wall signal removal. This approach and the traditional high-pass filtering one are, respectively, applied to analyze the computer-simulated in vitro and in vivo Doppler US signals. With the proposed method, the wall signal can be removed while a large portion of low-velocity blood signal remains. Comparison experiments show that this novel approach can satisfy the requirements of Doppler US system and is practicable under a broad range of measurement conditions. Because this algorithm is based on real data, it is currently applied to unidirectional signals.     

Ictal localization by source analysis of infraslow activity in DC-coupled scalp EEG recordings

New bedside long-term DC-coupled EEG techniques have demonstrated that infraslow (<0.5 Hz) activity lateralizes temporal lobe seizures (Vanhatalo, S., Holmes, M.D., Tallgren, P., Voipio, J., Kaila, K., Miller, J.W., 2003a. Very slow EEG responses indicate the laterality of temporal lobe seizures: a DC-EEG study. Neurology 60, 1098-1104). However, even high amplitude infraslow activity is difficult to localize by simple visual inspection if there is overlying faster EEG activity or slow artifact. In this study, we address this with improved DC-coupled EEG recording and analysis techniques and also extend observation to both temporal and extratemporal seizures. Recordings were performed during presurgical evaluation of medically intractable epilepsy, with 20 seizures in 11 patients analyzed. A commercial DC-coupled recording device was used, with sintered Ag/AgCl electrodes in a standard 10-10 system array, with additional anterior temporal and subtemporal electrodes. Seizures were localized with a software package by means of source montage analysis. Infraslow signals occurred with all seizures, often with amplitude orders of magnitude higher than conventional frequencies (0.5 to 70 Hz). The most reliable method to localize these signals and distinguish them from artifacts used a source montage after low-pass filtering below 0.5 Hz. Five of the eight patients who received epilepsy surgery had follow-up documenting significant seizure reduction, and infraslow signal analysis correctly localized the region of seizure onset in all five, while conventional noninvasive EEG recording and analysis localized only three of the five. Several seizures were also analyzed using principle component analysis source localization methods, with the results less consistently localizing than source montage analysis. DC-coupled EEG recordings give clinically useful information to noninvasively localize the seizure focus. The value of this method is increased by source analysis tools that reveal localized changes more clearly than direct visual inspection.     

Adaptive clutter filtering via blind source separation for two-dimensional ultrasonic blood velocity measurement

A method for adaptive clutter rejection via blind source separation (BSS) using principal and independent component analyses is presented in application to blood velocity measurement in the carotid artery. In particular, the filtering method's efficacy for eliminating clutter and preserving lateral blood flow signal components is presented. The performance of IIR filters is compromised by shorth data ensembles (10 to 20 temporal samples) as implemented for color-flow and high frame-rate imaging due to initialization requirements. Further, the ultrasonic imaging system's transfer function maps axial wall and lateral blood motion to overlapping spectra. As such, frequency domain-based approaches to wall filtering are ineffective for distinguishing wall from blood motion signals. Rather than operating in the frequency domain. BSS performs clutter rejection by decomposing the input data ensemble into N constitutive source signals in time, where N is the ensemble length. Source signal energy coupled with respective signal depth and time course profiles reveal which source signals correspond to blood, noise and clutter components. Clutter components may then be removed without disruption of lateral blood flow information needed for two-dimensional blood velocity measurement. A simplistic data simulation is employed to offer an intuitive understanding of BSS methods for signal separation. The adaptive BSS filter is further demonstrated using a Field II simulation of blood flow through the carotid artery including tissue motion. BSS clutter filter performance is compared to the performance of FIR, IIR and polynomial regression clutter filters. Finally, the filter is employed for clinical application using a Siemens Elegra scanner, carotid artery data with lateral blood flow collected from healthy volunteers, and Speckle Tracking; velocity magnitude and angle profiles are shown. Once again, the BSS clutter filter is contrasted to FIR, IIR and polynomial regression clutter filters using clinical examples. Velocities computed with Speckle Tracking after BSS wall filtering are highest in the center of the artery and diminish to low velocities near the vessel walls, with velocity magnitudes consistent with physiological expectations. These results demonstrate that the BSS adaptive filter sufficiently suppresses wall motion signal for clinical lateral blood velocity measurement using data ensembles suitable for color-flow and high frame-rate imaging.     

Application of Beat-to-Beat Techniques

FLOWERS, N.C., ET AL.: Application of Beat-to-Beat Techniques. The focus of this report is to describe a system for recording surface His-Purkinje and ventricular late potentials on a beat-by-beat basis outside of a shielded environment. An AC magnetic field monitoring device was designed for improved site selection, orientation, and quality control of the acquisition. His-Purkinje signals are detected utilizing spatial averaging and specific channel selection algorithms applied to discriminate random noise from signal. Beat-by-beat vectormagnitude complexes were generated from pairs of X, Y, and Z leads. Both infinite impulse response (IIR) filters, modified for beat-by-beat approaches, and finite impulse response (FIR) filters were utilized. Using the IIR filter, beat-by-beat recordings from test subjects were compared to the signal averaged electrocardiogram (SAECG). Measurement parameters from normal test subjects fell within the previously specified normal range for the SAECG. The IIR filter applied to beat-by-beat recordings exhibited sharp frequency response and a precisely defined cutoff frequency allowing maximal attenuation of the low frequency components in the ST segment. While filter ringing was eliminated, discontinuity and distortion of the filtered waveform resulted. The FIR filter with linear phase response retained the integrity and morphology of the complex but because of its flat frequency response, the ST segment was not as well attenuated and it was more difficult to isolate late potentials. A high order FIR filter should be used if the desire is to match the frequency response of the four-pole IIR filter, since the frequency response of the FIR filter is primarily determined by the order of the filter. With the FIR filter the waveform will be widened on both sides in time and therefore a trade-off results between the order of the filter and the cutoff frequency. A low order and a high cutoff frequency were necessary to attenuate the low frequency components of the ST segment without significantly widening the QRS. To record high resolution ECGs without noticeable 60-Hz noise, the magnetic field of 60 Hz should be smaller than 6.6 × 10^-8 Tesla. This study indicates that real-time analysis of both His-Purkinje potentials and late potentials in an unshielded environment is possible. (PACE, Vol. 13, December, Part II 1990)     

Effect of changes in lung volume on acoustic transmission through the human respiratory system

The variation of acoustic attenuation with lung density was determined in experimental studies on seven healthy human volunteers, using a change of lung volume as a means of varying lung density. White noise between 50 and 680 Hz was introduced into the mouth and the transmitted signals were recorded with four microphones on the posterior chest wall (left/right, top/base) at 24, 40, 60 and 80% of total lung capacity. The change in lung volume had a frequency-dependent effect on acoustic attenuation in all subjects. A frequency between 177 and 243 Hz was identified, where altering the lung volume between 24 and 80% of total lung capacity induced a change in attenuation of only 1.0 (+/-0.5) to 2.7 (+/-1.8) dB, while at a frequency of 364-436 Hz marked variations in attenuation 8.9 (+/-2.0) to 21.5 (+/-4.8) dB occurred with similar lung volume changes.     

Reducing CPR artefacts in ventricular fibrillation in vitro

CPR creates artefacts on the ECG, and a pause in CPR is therefore mandatory during rhythm analysis. This hands-off interval is harmful to the already marginally circulated tissues during CPR, and if the artefacts could be removed by filtering, the rhythm could be analyzed during ongoing CPR. Fixed coefficient filters used in animals cannot solve this problem in humans, due to overlapping frequency spectra for artefacts and VF signals.In the present study, we established a method for mixing CPR-artefacts (noise) from a pig with human VF (signal) at various signal-to-noise ratios (SNR) from -10 dB to +10 dB. We then developed a new methodology for removing CPR artefacts by applying a digital adaptive filter, and compared the results with this filter to that of a fixed coefficient filter. The results with the adaptive filter clearly outperformed the fixed coefficient filter for all SNR levels. At an original SNR of 0 dB, the restored SNRs were 9.0+/-0.7 dB versus 0.9+/-0.7 dB respectively (P<0.0001).     

A new and simple Doppler method for measurement of fetal cardiac isovolumetric contraction time.

OBJECTIVES: The fetal cardiac isovolumetric contraction time is defined as the interval between mitral valve closure and aortic valve opening. The objective of this study was to develop a simple and reliable Doppler method for measuring fetal isovolumetric contraction time using a digital filtering and processing system. METHODS: Cardiac Doppler signals were recorded from 40 fetuses at 18-40 weeks' gestation using a continuous-wave ultrasound transducer. The raw signal was digitized, filtered and divided into five different frequency ranges: 250-375, 375-500, 500-750, 750-1000 and 1000-1500 Hz. To determine the most suitable filter setting for detecting mitral valve closure and aortic valve opening signals, we examined whether they were detected clearly in each filter range. RESULTS: Both mitral valve closure and aortic valve opening signals were detected clearly in the 500-1000 Hz range. The atrioventricular flow and outflow noises in the 250-500 and 1000-1500 Hz ranges helped us to identify the signals. It was found that dividing the raw signals into three ranges of 250-500, 500-1000 and 1000-1500 Hz was the most suitable digital-filter setting for measuring isovolumetric contraction time. CONCLUSIONS: We have developed a simple Doppler method for measuring fetal isovolumetric contraction time. The advent of digital processing has simplified the equipment and the simultaneous multidisplay of three different filtered signals enables easy and accurate measurement.     

The Basis for Activity Controlled Rate Variable Cardiac Pacemakers: An Analysis of Mechanical Forces on the Human Body Induced by Exercise and Environment

We conducted tests on six healthy volunteers and six pacemaker patients. With the aid of three straight line frequency acceleration pickups attached to the body, the mechanical signals were recorded on the three axes during different activities. Along with standardized exercise on bicycle and treadmill ergometers, we tested the influence of household activities and interference influences. The results were analyzed in terms of the amplitude and frequency content of the signals. For walking activities, we found a signal amplitude increasing in a largely linear fashion with the walking speed, the signal amplitudes being approximately twice as high on the vertical axis as on the other two axes. Exercise on the bicycle ergometer produced mechanical signals of clearly lower amplitude than comparable walking activities. The Fast-Fourier analysis showed amplitude peaks in the low frequency range of 1 to 4 Hz for all forms of physiological exercise, while interference influences showed amplitude peaks mainly in the range above 8 Hz. The use of a straight line-frequency acceleration pickup and a corresponding low pass filter might be a way of reducing the effect of unphysiological interference influences on an activity controlled pacemaker system. A sensor measuring on the horizontal axis appears to be the most favorable compromise for the various types of exercise. However, due to the considerable difference in signal amplitude for different types of exercise of the same intensity, an activity controlled pacemaker system cannot entirely meet metabolic conditions and requirements.     

The practical significance of two-dimensional deconvolution in echography

This paper evaluates deconvolution (inverse filtering) as applied to ultrasonic imaging systems, and discusses the obstacles which are encountered employing the technique in practice. A minicomputer is used to generate artificial echo signals, simulating rf signals resulting from a set of point reflectors in a homogeneous medium, as recorded by an electronically focused group-steered linear array scanner. Two-dimensional deconvolution in combination with a Wiener noise reduction filter (i.e., a Wiener-Inverse filter) is applied to these simulated rf signals, which were contaminated with white noise. The efficacy of the Wiener-Inverse filter is defined in terms of its ability to resolve two point reflectors with a lateral spacing equal to the local -6 dB width of the ultrasonic beam. In favorable circumstances, the targets are resolved at signal-to-noise ratios (SNR) better than 20 dB, where SNR is defined as the maximum signal power divided by the average noise power level. Nonlinear effects due to quantization or signal clipping are investigated. In order to improve the resolution of an rf signal with a dynamic range of 40 dB, the input signal should be digitized at a minimum of 12 bits. The problem of signal clipping can be circumvented by oversampling. The two-dimensional Wiener-Inverse filter is defined in terms of both temporal and spatial properties of the insonification. Effects of wave diffraction give rise to a depth-dependent ultrasonic beam. As a result of a misfit of the Wiener-Inverse filter and the local properties of the ultrasonic beam, erroneous noisy texture arises in the image. Adaptation of the Wiener-Inverse filter with respect to the beam properties gives acceptable results, at the expense of a rather large computational effort.     

基于盲解卷问题的脉搏波的信号分析

背景人体脉搏信号可以看作是心脏源的激励信号和脉搏系统冲激响应的卷积,目前对脉搏冲激响应有了较详细的认识,但是对激励源信号的研究还需深入.目的提取心脏激励源信号的特征波形.设计随机对照实验.单位中国医科大学附属第二医院,山东大学生物医学工程所.对象2004-3-11在中国医科大学附属第二医院体检的身体健康的成年人.RM6240多道生理信号采集系统.方法将采集到的正常人的脉搏信号,输入到同态解卷的脉搏系统分析模型上,利用脉搏信号盲解卷的原理,基于实际可行的算法,提取心脏激励源信号.主要观察指标①正常的脉搏信号波形.②心脏激励源信号.结果在复时谱的后端(n＞n0),周期性的脉冲和脉搏信号的基本周期一致.证实了脉搏信号中存在一个周期性激励源.如果采用高通滤波器(实验过程中,用到的复时谱滤波器n0选择30左右,效果最佳)进行滤波,得到的复时谱分量再经过傅氏变换,指数运算和傅氏逆变换,就能得到激励源信号波形.结论实际可行的算法为脉搏信号的进一步分析提供了依据.     

Adaptive filtering of ECG interference on surface EEnGs based on signal averaging

An external electroenterogram (EEnG) is the recording of the small bowel myoelectrical signal using contact electrodes placed on the abdominal surface. It is a weak signal affected by possible movements and by the interferences of respiration and, principally, of the cardiac signal. In this paper an adaptive filtering technique was proposed to identify and subsequently cancel ECG interference on canine surface EEnGs by means of a signal averaging process time-locked with the R-wave. Twelve recording sessions were carried out on six conscious dogs in the fasting state. The adaptive filtering technique used increases the signal-to-interference ratio of the raw surface EEnG from 16.7 +/- 6.5 dB up to 31.9 +/- 4.0 dB. In addition to removing ECG interference, this technique has been proven to respect intestinal SB activity, i.e. the EEnG component associated with bowel contractions, despite the fact that they overlap in the frequency domain. In this way, more robust non-invasive intestinal motility indicators can be obtained with correlation coefficients of 0.68 +/- 0.09 with internal intestinal activity. The method proposed here may also be applied to other biological recordings affected by cardiac interference and could be a very helpful tool for future applications of non-invasive recordings of gastrointestinal signals.     

Sampling rate effects on surface EMG timing and amplitude measures

OBJECTIVE: To determine if oversampling the surface electromyographic signal provides any benefit in analyzing common electromyographic timing and amplitude measures used in kinesiological studies. DESIGN: A within subjects (n=8) repeated measures design was used to examine surface electromyographic signals captured under four contraction modes and acquired with five different analog-to-digital sampling rates. BACKGROUND: There is a growing trend to sample surface electromyography at rates higher than the Nyquist rate. Though there is limited evidence to support oversampling, the necessity or benefit of doing so remains unclear. METHODS: Surface electromyography was recorded from the triceps brachii during maximal, submaximal, and fatiguing isometric contractions, as well as dynamic contractions. The analog signals were bandpassed between 20 Hz and 2 kHz, and oversampled at 6 kHz. The signals were then digitally resampled at 3 kHz, 1 kHz, 500 Hz, and 250 Hz without benefit of an anti-aliasing filter. Amplitude and timing variables measured from both the rectified and smoothed signal were compared across sampling rates. RESULTS: Oversampling produced no significant changes in timing and amplitude measures of the rectified or smoothed electromyographic signal. For the smoothed signal, minor undersampling at half the Nyquist rate was sufficient to accurately capture most timing and signal strength measures. CONCLUSIONS: Oversampling is unnecessary to gather typical amplitude and timing measures from the surface electromyographic signal. Electromyography sampled below half the Nyquist rate is likely to result in a poor temporal and amplitude representation of the signal. RELEVANCE: Computer memory and processing resources for analyzing amplitude and timing information need not be expended in oversampling surface electromyography, and results of previous studies need not be outright dismissed because of minor undersampling violations or the lack of an anti-aliasing filter.     

Electrical impedance tomography in extremely prematurely born infants and during high frequency oscillatory ventilation analyzed in the frequency domain

Functional electrical impedance tomography (EIT) measures relative impedance change that occurs in the chest during a distinct observation period and an EIT image describing regional relative impedance change is generated. Analysis of such an EIT image may be erroneous because it is based on an impedance signal that has several components. Most of the change in relative impedance in the chest is caused by air movement but other physiological events such as cardiac activity change in end expiratory level or pressure swings originating from a ventilator circuit can influence the impedance signal. We obtained EIT images and signals in spontaneously breathing healthy adults, in extremely prematurely born infants on continuous positive airway pressure and in ventilated sheep on conventional mechanical or high frequency oscillatory ventilation (HFOV). Data were analyzed in the frequency domain and results presented after band pass filtering within the frequency range of the physiological event of interest. Band pass filtering of EIT data is necessary in premature infants and on HFOV to differentiate and eliminate relative impedance changes caused by physiological events other than the one of interest.