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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.