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ENTC 4370 FINAL PROJECT Due : April 26, 2004 Introduction In the area of biomedical signal processing a common problem is the requirement for signals which are free of contamination due to a number of sources. These signals are commonly used, example, for fetal monitoring to assess the effects of the measurement system on the electrical activity of the fetal heart (Westgate, 1994). An ECG is a small electrical signal, which is produced due to the activity of the heart. Its source can be considered as a dipole located in the partially conducting medium of the thorax. This dipole induces a body-surface potential, which can be measured and used for diagnostic purposes. The signal ECG is characterized by five peaks and valleys labeled with successive letters of the alphabet P, Q, R, S, and T (Greene 1987). The ECG is said to consist of the P wave, QRS complex and T wave (Greene 1987). The reciprocal of the heart period is the time interval between the R-to-R peaks (in milliseconds), multiplied by 60,000 gives the instantaneous heart rate. A typical heartbeat has the normal Sinus Rhythm. Impulses originate in the SA node regularly at a rate of 60-100 per minute in adults and at faster rates in older children (90-110), small children (100-120), and infants (120-160). The P waves are upright in L2 and negative in AVR and of uniform size and contour from beat to beat. The PR interval is 0.12-0.20 sec and constant when A-V conduction is normal; PR is pro longed and/or variable when A-V block is present. Each P is followed by a QRS with the resulting P:QRS ratio 1:1. The QRS may be less than 0.11 sec or QRS may be wide and bizarre when bundle branch block is present. The RR intervals may be slightly irregular, especially in the young and elderly. The basic heart rate can be calculated from: Heart rate (beats min 1 ) 1 60,000 heart period (ms) There are many factors that must be considered in the design of a system that is capable of measuring these signals without introducing contamination into the signal. Patients who are having their ECGs taken on either a clinical electrocardiograph, or continuously on a cardiac monitor, are often connected to the pieces of electric apparatus. Each electrical device has its own ground connection either through the power line or, in some cases, through a heavy ground wire attached t0 some point in the room. A ground loop can exist when two or more electrical monitoring devices are connected to the patient. Another problem caused by the ground currents is related to the fact that, because the ground lead of the electrocardiograph usually runs alongside the signal leads, magnetic fields caused by the current in the grounding circuit can induce small voltages in the signal lead wires. This can produce interference on the recorded data. A major source of interference is the electric power system. Besides providing power to the ECG system itself, power lines are connected to other pieces of equipment in a typical hospital or physician’s office. Such interferences can appear on the recorded data as a result of two mechanisms, each operating singly or, in some cases, both together. The first is electric field coupling between the power lines and the electrocardiograph and/or patient and is the result of the electric field surrounding the main power lines. The other source of interference from power lines is magnetic induction. Current in the power lines establishes a magnetic filed in the vicinity of the line. If basic precautions are taken a great deal of this type of contamination can be minimized (Webster 1995). To reduce the above contamination it is possible to use simple signal processing filter techniques. The example shows how LabVIEW can be used to achieve these goals. We begin by modeling the ECG signal, then applying a digital filter to remove a selected component of the signal. The system is also capable of filtering real ECG data. This is demonstrated at the end of the example. The basic aims and objectives of this example are detailed below: 1. To create a VI that will simulate a heartbeat signal that is contaminated by a signal at the mains frequency (50Hz) 2. To develop an HR filter to reduce the contamination 3. To test the Vi’s using real ECG data obtained using a data acquisition card with a medical safe isolation amplifier. 4. To generate a Lab VIEW program to simulate an ECG signal plus 50Hz contamination. 5. To use a standard LabVIEW function to create an HR filter that will notch out the 50HZ contamination and create an appropriate front panel display to show the raw and filtered EGG data. 6. To devise a means of counting the beats per minute. PROCEDURE This example is designed in two parts, the first assumes that the EGG signal is modeled and the second will use real EGG data obtained from a patient. The first requirement is to simulate the heart beat. To achieve this, a simple method can be used which combines the sum of the sinusoidal signals that represent the basic components of an ECG signal. Careful selection of the frequencies can be made to accurately simulate the contamination due to a ground loop. These are 60, 40 and 20Hz respectively (Note the assumption at this system is design for use where the mains frequency is 50Hz). The contamination can vary from this center frequency of 50Hz. To construct a simulation of this a white noise source was chosen. This noise source was fed into a 5th order Butterworth bandpass filter with a bandwidth of 49 to51Hz. The amplitude of the white noise source is set to 150 and the gain of the Sine Pattern generators are unity. The sampling rate was set at 600 samples/second. The details of the VI are illustrated in the following figure.