Ivaylo Christov

Real time electrocardiogram QRS detection using combined adaptive threshold

BackgroundQRS and ventricular beat detection is a basic procedure for electrocardiogram (ECG) processing and analysis. Large variety of methods have been proposed and used, featuring high percentages of correct detection. Nevertheless, the problem remains open especially with respect to higher detection accuracy in noisy ECGsMethodsA real-time detection method is proposed, based on comparison between absolute values of summed differentiated electrocardiograms of one of more ECG leads and adaptive threshold. The threshold combines three parameters: an adaptive slew-rate value, a second value which rises when high-frequency noise occurs, and a third one intended to avoid missing of low amplitude beats.Two algorithms were developed: Algorithm 1 detects at the current beat and Algorithm 2 has an RR interval analysis component in addition.The algorithms are self-adjusting to the thresholds and weighting constants, regardless of resolution and sampling frequency used. They operate with any number L of ECG leads, self-synchronize to QRS or beat slopes and adapt to beat-to-beat intervals.ResultsThe algorithms were tested by an independent expert, thus excluding possible authors influence, using all 48 full-length ECG records of the MIT-BIH arrhythmia database. The results were: sensitivity Se = 99.69 % and specificity Sp = 99.65 % for Algorithm 1 and Se = 99.74 % and Sp = 99.65 % for Algorithm 2.ConclusionThe statistical indices are higher than, or comparable to those, cited in the scientific literature.

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.

Premature ventricular contraction classification by the Kth nearest-neighbours rule

An analysis of electrocardiographic pattern recognition parameters for premature ventricular contraction (PVC) and normal (N) beat classification is presented. Twenty-six parameters were defined: 11 x 2 for the two electrocardiogram (ECG) leads, width of the complex and three parameters derived from a single-plane vectorcardiogram (VCG). Some of the parameters include amplitudes of maximal positive and maximal negative peaks, area of absolute values, area of positive values, area of negative values, number of samples with 70% higher amplitude than that of the highest peak, amplitude and angle of the QRS vector in a VCG plane. They were measured for all heartbeats annotated as N or PVC in all 48 ECG recordings of the MIT-BIH arrhythmia database. Two reference sets for the Kth nearest-neighbours rule were used-global and local. The classification indices obtained with the global reference set were 75.4% specificity and 80.9% sensitivity. Using the local reference set we increased the specificity to 96.7% and the sensitivity to 96.9%. The achieved specificity and sensitivity are comparable with, and greater than, the results reported in the literature.

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.

Comparison of four methods for premature ventricular contraction and normal beat clustering

The learning capacity and the classification ability for normal beats and premature ventricular contractions clustering by four classification methods were compared: neural networks (NN), K-th nearest neighbour rule (Knn), discriminant analysis (DA) and fuzzy logic (FL). Twenty-six morphology feature parameters, which include information of amplitude, area, specific interval durations and measurement of the QRS vector in a VCG plane, were defined. One global and two local learning sets were used. The local classifiers achieved better accuracies because of their good adaptability to the patients, while the capacity of the global classifier to process new records without additional learning was expectedly balanced by lower accuracies. NN assure the best results (high and balanced indices for specificity and sensitivity) using one of the local learning set, while the Knn provides the best results with the other local learning set. Using the global learning set DA and the FL methods perform better than the NN and Knn

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.

Myocardial infarction and ischemia characterization from T-loop morphology in VCG

Some particular aspects of T-wave morphology are considered for the characterization and quantification of heterogeneous repolarization, using the vectorcardiogram (VCG). The orthogonal Frank leads were synthesised from the standard 12-lead ECG. For this purpose, three parameters were obtained from the VCG with two different methods of considering the zero point. The population-based ECG-ILSA (Italian Longitudinal Study on Aging) database was used, and those patients classified as healthy [328], with myocardial ischemia [123], myocardial infarction [172] or both (59) were included in this study. The modified parameters yielded higher discriminative power. From a statistical analysis, it was found that the mean values of all three parameters of the healthy subjects group were statistically different from those of the ischemia or myocardial infarction groups.

Pattern recognition and optimal parameter selection in premature ventricular contraction classification

Analyses of electrocardiographic pattern recognition parameters for premature ventricular contraction (PVC) and Normal (N) beat classification are presented. Twenty-six parameters are defined: 11/spl times/2 for the two ECG leads, 3 for vectorcardiogram (VCG) and width of the complex. Some of them include: amplitudes of maximal positive and negative peaks, area of the absolute values, area of positive and negative values, number of samples with 70% higher amplitude then that of the highest peak, amplitude and angle of the QRS vector in a VCG plane. They are measured for all N and PVC heart beats in the MIT-BIH arrhythmia database. The classification ability of each parameter is tested using discriminant analysis. Considering both leads 7 parameters with highest discriminant power for N and PVC are extracted and a specificity of 96.6% and a sensitivity of 90.5% are obtained. Taking into account relatively all parameters a specificity of 97.3% and a sensitivity of 93.3% are achieved.

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.