Abstract’Electrocardiogram (ECG) contains several noises such as Baseline noise, power line interference & others. Due to presence of Baseline drift Isoelectric line of ECG gets shifted causes distortion in ST segment of the original ECG signal. Thus to filter Baseline drift from ECG signal is important. The function of filter is to remove unwanted parts of the signal to extract useful parts of the signal. This paper presents the comparisons of digital IIR & FIR filter complexity & their performances to remove Baseline noises from ECG signal.
Index Terms’Baseline Drift, Power line Inference, IIR filters, FIR filters.
Introduction
Electrocardiography (ECG) is the acquisition of electrical activity of the heart captured over time by an external electrode attached to the skin. Each of the cell membrane that form the outer covering of the heart cell have an associated charge which is depolarized during every heart beat. These appear as tiny electrical signals on the skin which can be detected and amplified by the ECG. It has considerable diagnostic significance,[1] and application of ECG monitoring are diverse and in wide use. For diagnostic- quality ECG recordings, signal acquisition must be noise free. The signal acquisition is susceptible to the interference from other biological and environmental sources.
A medical topic that attracted the early interest of engineers is that of the electrocardiography. It is one of the most useful non-invasive medical diagnostic tests and is in very wide use (an estimated 200 million ECGs taken each year). Its technological challenge is to improve performance by increasing number of simultaneously recorded signals, raising the signal to noise ratio, and accomplishing these goals, possibly, with the subject undergoing physical activity. Therefore the electrocardiogram (ECG) signal is one of the most important vital signs monitored from cardiac patients. Cardiologist readily interprets the ECG waveforms and classifies them into normal and abnormal patterns.
Prominent parts of the ECG are the P wave, a deflection caused by the current originating in the atrium. [3]The QRS complex, showing the passage of the electrical activity into the ventricles and the T wave, as the ventricles reset themselves. In a normal cardiac cycle, the P wave occurs first, followed by the QRS complex and the T wave.
The sections of the ECG between the waves and complexes are called segments and intervals: the PR segment, the ST segment, the TP segment, the PR interval, the QT interval, and the R-R interval. Intervals include waves and complexes, while segments do not. When electrical activity of the heart is not being detected, the ECG is a straight, flat line, which is called isoelectric line or baseline.
FIG: 1 NORMAL ECG TRACE.
Often ECG signal is corrupted with noise such as Baseline noise, power line Interference, Electrode contact noise, motion artifacts, instrumentation noise, Electrosurgical Noise etc. Due to the time varying nature of ECG signals, ECG noise removal is complicated. Different digital filter structures are available to eliminate these forms of noise sources. Every type of filter has its strong and weak points.
II. BASELINE NOISES
Baseline noise is of two types:
1) Baseline drift and
2) Baseline wander
FIG 2: ECG SIGNAL WITH BASELINE DRIFT(114.DAT)
FIG 3: ECG SIGNAL WITH BASELINE WANDER (113.DAT)
Fig 3: ECG signal with Baseline wander (113.dat)
BRIEF LITERATURE SURVEY
In research paper [1] sources of noise in ECG signal are defined.
In paper [2] out of these different methods for removal of noise like baseline drift and power line noise have been proposed.
While acquisition of the ECG it gets corrupted due to different types of artifacts and interferences such as Power line interference, Electrode contact noise, noise due to Motion artifacts, Muscle contraction, Base line drift, Instrumentation noise generated by electronic devices and Electrosurgical noise. For the meaningful and accurate detection, steps have to be taken to filter out or discard all these noise sources. Analog filters help in dealing with these problems; however, they may introduce nonlinear phase shifts, skewing the signal. Also, the instrumentation depends on resistance, temperature, and design, which also may introduce more error. With more recant technology, Digital filters are now capable of being implemented offering more advantages over the analog one. Digital filters are more precise due to a lack of instrumentation. The work on design and implementation of Digital filter on the ECG signal is in progress in the different part of the world. Different researchers have worked on the reduction of noise in the ECG signal. Power-line interference (either 50 Hz or 60 Hz) is a significant source of noise in biomedical recording. Elimination of power-line interference in the Electrocardiogram (ECG) signal by various methods has been proposed in the past. Mitov IP has investigated the method for removing the power line interference. Figure 1.a shows the typical power spectrum of the ECG signal. Many researchers are working on noise reduction in the ECG signal specifically powerline interference and baseline wander. While removing line interference, Mitov IP has used the sampling rate integer multiple of the nominal power line frequency [1], his method is suitable for off line use than that of the real time because of the computational complexity. Power-line interference (either 50 Hz or 60 Hz) is a significant source of noise in biomedical recording. Elimination of power-line interference in the Electrocardiogram (ECG) signal by various methods has been proposed in the past. Despite the high common mode rejection ratio of the input amplifiers of the Electrocardiographs, recordings are often contaminated by power-line (mains) frequency interference [3]. The efficiency of notch filters and a subtraction procedure for power-line interference cancellation in electrocardiogram (ECG) signals is assessed by the Dotsinsky I, Stoyanov T [4]. Kumaravel N et.al demonstrated a novel method of integrating rule-based system approach with linear FIR filter and also with Wave digital filter has investigated [4]. Hamilton PS in his article compared adaptive and nonadaptive filters for reduction of power line interference in the ECG [7]. Ider YZ, Saki MC, Gcer HA have developed a method for line interference reduction to be used in signalaveraged electrocardiography[8]. Cramer E, McManus CD, Neubert D has introduced a global filtering approach. In this method two types of the digital filters are used. One is using lest square method and other is using special summation method [9]. Different scientists have tried for removing the power line interference and base line wonder specifically from the ECG signal [8-10]. Zschorlich VR and Zschorlich VR, have also designed digital filters to cope with EMGsignals[12-14]. Webster has explained the instrumentation requirements for the ECG[16].
In the proposed task conventional IIR filters will be used. The filters selected are Butterworth and elliptic and there combination has been used.