Beat-to-beat noise removal in non-invasive His-bundle electrocardiogram.

His-bundle electrocardiogram micropotentials are usually obtained by serial-signal averaging, because no acceptable, satisfactory solution for beat-by-beat removal of power-line interference and electromyographic noise has been found. A method has been developed for surface beat-to-beat His-bundle potential recovery, with the hypothesis that no distortion in the signal shape is admissible. It is based on consecutive power-line interference subtraction and wavelet-domain electromyographic noise suppression, modified to match the strict criteria for detecting low-amplitude His potentials. The beat-to-beat wavelet-domain Wiener filtering uses a pilot signal estimate obtained from a limited number (around 20) of heart beat averages. The method resulted in an improvement of more than 4.5 dB in the signal-to-noise ratio and more than 20% reduction in mean absolute error, both measured along the P-Q segment. It is applicable for ECG signals contaminated by moderate electromyogram noise, with an initial signal-to-noise ratio of 15 dB or higher.

Transthoracic defibrillation with chopping-modulated biphasic waveforms.

The superiority of different biphasic pulses for transthoracic defibrillation was proven by several studies. These efficient waveforms were implemented in some commercially available defibrillators. Recently we have devised and evaluated a biphasic waveform with a specially balanced ratio of the first-to-second phase voltages and with 5 kHz frequency 1:1 on-off chopping. It used less than half the energy for successful defibrillation in comparison with the 'classic' monophasic damped sinusoidal wave and showed considerably less post-shock negative effects. This experience led us to try several laws of chopping modulation. A pulse-width modulation, combining low energy with gradual upslope of the modelled transmembrane potential, proved to have better performance than the standard damped sinusoid wave and the non-chopped biphasic truncated exponential pulse. This waveform was tested in a series of animal experiments in comparison with other modulated pulses, with the non-modulated waveform and the standard damped sinusoid wave. The experiments demonstrated the superiority of the modulated waveform, assessed by combining the parameters of threshold defibrillation energy and of post-shock disturbances reduction.

Monitoring of fluctuating airway obstruction and episodes of coughing by thoracic electrical impedance.

The most prominent feature of bronchial asthma is the fluctuating airway obstruction of the affected subjects. Cough is also one of the major symptoms of asthma, but of other conditions as well. The continuous registration of airway obstruction and coughing in the working or open-air environment or at the homes of the potential sufferers may help establish a sometimes elusive diagnosis. To this purpose we developed a method using a measurement of the changes in the chest basal electrical impedance (Z0). A portable device for long-term recording of the Z0 signal was built using a microprocessor system. In order to assess both gradual Z0 changes, suggestive of altering airway patency and coughing episodes (characterized by abrupt changes), a continuous analogue-to-digital conversion (ADC) loop with a sampling frequency of 16 Hz was programmed. After completion of the recording, the memory of the portable instrument can be downloaded to a PC system for assessment and analysis. Appropriate averaging and filtering procedures have been worked out. This device may be further upgraded to give out a signal when Z0 starts to increase, so as to prevent a full-blown asthmatic crisis.

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 crossing. 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 mm s-1 equivalent paper speed and 0.5 mV cm-1 sensitivity; and (ii) with 160 mm s-1 paper speed and vertical scaling ranging from 0.07 to 0.02 mV cm-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 10 ms long adjacent segments along the search interval. The algorithm avoids the use of baseline crossing and 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.

Electrocardiogram signal preprocessing for automatic detection of QRS boundaries.

Automatic detection of QRS onset and offset points with reasonable accuracy has been a difficult task, approached since the first attempts at computerised electrocardiogram interpretation. The problem is additionally complicated by the usual presence of power-line interference, electromyogram artefacts and baseline fluctuation in the original signal, especially in multiphase complexes with small q, r, r', or s' waves. We propose a preprocessing method guaranteeing accurate preservation of the QRS boundaries, even in the existence of strong power-line or electromyogram noise. Examples of detection of QRS onset and offset points and a comparison with observer markings are presented for the assessment of preprocessing efficiency and detection consistency.

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.

Comparison of heart rate variability spectra using generic relationships of their input signals.

Spectral analysis of heart rate variability (HRV) is an accepted method for assessment of cardiac autonomic function and its relationship to numerous disorders and diseases. Various non-parametric methods for HRV estimation have been developed. The spectrum of counts, the instantaneous heart rate spectrum and the interval spectrum are mostly practised. Although extensive literature on their respective properties is available, there seems to be a need for a more complete comparison, eventually resulting in recommendations for applicability. The methods for HRV spectral analysis use their specific transforms of the primary R-R interval series into input signals for spectral computation. This is, in fact, the reason for obtaining different spectra. A basis for comparison is established, revealing the generic relationships of these HRV input signals. It allows for a more systematic evaluation and for further development of the considered methods. The results with simulated and real HRV data show better performance by the spectrum of counts and by a proposed instantaneous heart rate spectrum, obtained using a cubic spline interpolated input signal. The modulation depth of the primary signal can influence the accuracy of the HRV analysis methods.

Power spectra accuracy improvement by optimal signal epoch selection: an heuristic approach.

The problem of epoch selection in frequency analysis of biomedical signals often involves tradeoffs between the physiological system studied stationarity, the spectral resolution required, the need for dynamic investigations implying shorter epochs, etc. The accuracy of the spectra strongly depends on the epoch length. We propose a method for optimal epoch selection introducing the notion of 'quality of power spectra'. Two criteria for quantitative estimation of spectral quality are defined, considering a quality to be high when more power is concentrated in narrow frequency bands. A synthesized signal is used for describing the procedure developed and evaluating the accuracy. Noise sensitivity is assessed by adding intensive noise to the signal. Hundreds of synthesized signals are used for validation of the method and some initial experiments with heart rate variation data are presented.