I. Daskalov

Removal of power-line interference from the ECG: a review of the subtraction procedure

BackgroundModern biomedical amplifiers have a very high common mode rejection ratio. Nevertheless, recordings are often contaminated by residual power-line interference. Traditional analogue and digital filters are known to suppress ECG components near to the power-line frequency. Different types of digital notch filters are widely used despite their inherent contradiction: tolerable signal distortion needs a narrow frequency band, which leads to ineffective filtering in cases of larger frequency deviation of the interference. Adaptive filtering introduces unacceptable transient response time, especially after steep and large QRS complexes. Other available techniques such as Fourier transform do not work in real time. The subtraction procedure is found to cope better with this problem.MethodThe subtraction procedure was developed some two decades ago, and almost totally eliminates power-line interference from the ECG signal. This procedure does not affect the signal frequency components around the interfering frequency. Digital filtering is applied on linear segments of the signal to remove the interference components. These interference components are stored and further subtracted from the signal wherever non-linear segments are encountered.ResultsModifications of the subtraction procedure have been used in thousands of ECG instruments and computer-aided systems. Other work has extended this procedure to almost all possible cases of sampling rate and interference frequency variation. Improved structure of the on-line procedure has worked successfully regardless of the multiplicity between the sampling rate and the interference frequency. Such flexibility is due to the use of specific filter modules.ConclusionThe subtraction procedure has largely proved advantageous over other methods for power-line interference cancellation in ECG signals.

Filtering of electromyogram artifacts from the electrocardiogram

Electromyogram (EMG) artifacts often contaminate the electrocardiogram (ECG). They are more difficult to suppress or eliminate, compared for example to the power line interference, due to their random character and to the considerable overlapping of the frequency spectra of ECG and EMG signals obtained from the same pair of electrodes. The usually applied low-pass filtering (cutoff frequency of minimum 35 Hz) results in limited suppression of the EMG artifact and considerable reduction of sharp Q, R and S ECG wave amplitudes. A solution to this problem is proposed by applying approximation filtering with dynamically varied number of samples and weighting coefficients, depending on the ECG signal slope. The slope measure used is the absolute value of the product of the tilts of two adjacent 10 ms segments sliding along the signal. The results obtained show a slight widening of some sharper QRS complexes, but a virtual preservation of their amplitudes and a considerable reduction of the EMG artifact.

Improvement of resolution in measurement of electrocardiogram RR intervals by interpolation.

The measurement of successive RR intervals, obtained from the electrocardiogram (ECG), is the basis of any subsequent method for the assessment of variations in heart rate, for example using intervalograms, histograms, trend curves, spectral analysis, etc. The accuracy of measurement directly depends on the sampling rate used for the acquisition of the ECG signal. Depending on specific applications, rates down to 128 Hz are not uncommon. This considerably limits the resolution if RR interval data are to be derived. Interpolation is often employed for solving problems of this type. Linear, cubic and spline interpolations, obtained directly from the MATLAB software product, were applied and compared for the purpose of RR interval measurement. The cubic method was found to combine the improvement of resolution (+/-1 ms) at sampling rates successfully down to 100 Hz, with a rapid operating speed. Comparison examples are given with ECG signals acquired with 1000 Hz, 16-bit sampling.

Peripheral nerve magnetic stimulation: influence of tissue non-homogeneity

BackgroundPeripheral nerves are situated in a highly non-homogeneous environment, including muscles, bones, blood vessels, etc. Time-varying magnetic field stimulation of the median and ulnar nerves in the carpal region is studied, with special consideration of the influence of non-homogeneities.MethodsA detailed three-dimensional finite element model (FEM) of the anatomy of the wrist region was built to assess the induced currents distribution by external magnetic stimulation. The electromagnetic field distribution in the non-homogeneous domain was defined as an internal Dirichlet problem using the finite element method. The boundary conditions were obtained by analysis of the vector potential field excited by external current-driven coils.ResultsThe results include evaluation and graphical representation of the induced current field distribution at various stimulation coil positions. Comparative study for the real non-homogeneous structure with anisotropic conductivities of the tissues and a mock homogeneous media is also presented. The possibility of achieving selective stimulation of either of the two nerves is assessed.ConclusionThe model developed could be useful in theoretical prediction of the current distribution in the nerves during diagnostic stimulation and therapeutic procedures involving electromagnetic excitation. The errors in applying homogeneous domain modeling rather than real non-homogeneous biological structures are demonstrated. The practical implications of the applied approach are valid for any arbitrary weakly conductive medium.

Automatic detection of the electrocardiogram T-wave end

Various methods for automatic electrocardiogram T-wave detection and Q-T interval assessment have been developed. Most of them use threshold level corrsing. Comparisons with observer detection were performed due to the lack of objective measurement methods. This study followed the same approach. Observer assessments were performed on 43 various T-wave shapes recorded: (i) with 100 mms−1 equivalent paper speed and 0.5mVcm−1 sensitivity; and (ii) with 160 mms−1 paper speed and vertical scaling ranging from 0.07 to 0.02 m Vcm−1, depending on the T-wave amplitude. An automatic detection algorithm was developed by adequate selection of the T-end search interval, improved T-wave peak detection and computation of the angle between two 10ms long adjacent segments along the search interval. The algorithm avoids the use of baseline crossin direct signal differentiation. It performs well in cases of biphasic and/or complex T-wave shapes. Mean differences with respect to observer data are 13.5 ms for the higher gain/speed records and 14.7 ms for the lower gain/speed records. The algorithm was tested with 254 various T-wave shapes. Comparisons with two other algorithms are presented. The lack of a ‘gold standard’ for the T-end detection, especially if small waves occur around it, impeded adequate interobserver assessment and evaluation of automatic methods. It is speculated that a simultaneous presentation of normal and high-gain records might turn more attention to this problem. Automatic detection methods are in fact faced with ‘high-gain’ data, as high-resolution analogue-to-digital conversion, is already widely used.

Two-electrode telemetric instrument for infant heart rate and apnea monitoring

Infants may be at risk of life threatening episodes caused by still unknown factors. The Sudden Infant Death Syndrome (SIDS) is believed to be connected with respiration problems due to immature and developing brain breathing control, and many other factors. Currently heart rate and respiration monitoring is considered of major importance to be applied in the hospital and eventually at hoinc. The objective is accurate detection of bradycardia (heart rate below a selected limit, usually between 80 and 100 beats/min) and apnea (cessation of breathing for more than 20 s). A telemetric instrument was developed for monitoring the heart rate and respiration by extraction of the respiration signal from the electrocardiogram QRS complex peak-to-peak amplitude. It makes use of two electrodes integrated in a transmitter module, thus avoiding use of leads and also reducing artifacts to an acceptable minimum. The receiver station can be a simple detector with apnea and bradycardia alarms, a more complicated recorder or a fully developed signal analyser.

Accuracy of 50 Hz interference subtraction from an electrocardiogram

THE FILTERING of power-line interference from the electrocardiogram (ECG) is a very important procedure in any modem electrocardiograph. The requirements for this type of procedure are generally well known and have been very well formulated by Cramer et al. (eRASER et al. , 1987). Despite the high common mode rejection ratio of the input amplifiers of the electrocardiographs, recordings are often contaminated by mains frequency interferences, usually phase-shifted with respect to the mains voltage. This event is due to parasitic currents flowing through the patients body and the connecting cables, and to the difference in the electrode impedances, converting the common mode interference voltages into unwanted differential signals (HUHTA and WEBSTER, 1973; TOWE, 1981). The use of conventional filters affects the signal components surrounding the mains frequency. A variety of adaptive filters for this purpose have been proposed (FtJRNO and TOMPKINS, 1983; AHLSTROM and TOMPK1NS, 1985; FERRARA and WIDROW, 1982), but they require an adaptation period after each interference or abrupt signal change and are not so effective in cases of extra-systoles and arrhythmias. Another problem is the disturbance of the adaptation process by the ECG signal itself, as can be seen in the work of Thakor and Zhu (THAKOR and ZHU, 1991) where, after each QRS complex, a burst of interference reappears.

Electrical field and current distributions in electrochemotherapy.

Electrochemotherapy is a method for cancer treatment consisting in combining intratumor injection of cytotoxic agent with the application of intensive electrical stimuli. Thus transient cell membranes permeabilization is created, allowing the agent to better exercise its destroying effect. Positive results have been published in the treatment of cutaneous malignant formations and other types of cancer are under consideration. The electrode configurations presently used are based mainly on empirical treatment results. In vivo imaging of stimulation currents was attempted in animal models. A preliminary study revealed that having in view the relatively high voltages and currents, there was a virtually resistive load to the electrical source. Assuming a homogeneous medium, potential and current distributions were modeled and studied. The results could help in selection of specific electrode designs, depending on tumor size and location. An optimization of the voltages and/or currents by different electrode arrays can lead to obtaining desired field distribution.

Electrical current distribution under transthoracic defibrillation and pacing electrodes.

The known effect of high current density under the perimeter of defibrillation electrodes, leading to skin damage and even severe burns in some cases, has been considered by many investigators. Two main approaches for improvement were proposed: (i) interfacing with layers of varying and high resistivity and (ii) lengthening and shaping the perimeter line. Using finite element and physical modelling, it is shown that the second approach does not yield significant improvement in the distribution uniformity. Moreover, the application of high resistivity layers is unacceptable in defibrillation. The use of a low resistance layer with a diameter covering and extending over the metal plate by at least 2.5 mm results in better uniformity. A similar effect can be obtained by recessing the metal plate in an isolating support--an approach adopted from implantable neurostimulation electrodes. These two versions can be applied in combination.

Powerline interference suppression in high-resolution ECG

The efficacy of four powerline interference suppression methods was tested for application in high-resolution electrocardiogram (ECG). The goal was minimal distortion of the original signal micro-potential waveform, combined with maximum noise reduction. Simulated low amplitude His-bundle potentials were used for the evaluation. Several objective parameters were measured, such as mean square error and mean absolute error. The methods were applied for His-bundle potentials recovery from real surface ECG signals. Synchronous intracardiac signals with well expressed His potential were recorded and used for reference. Modified time-domain subtraction and regression subtraction methods were found superior to notch filters and spectral interpolation.