SpO2 Measurement Using ECG

Abstract:

          ECG is the most basic electric potential of our body. In fact any the disease related to the heart can be easily determined using this “ECG”. But in our body each and every activity is interrelated with each other. From a minute cell to any system such as cardiac system, respiratory system is interrelated. 

         Each and every signal coming out from the brain is related with the various muscular activities, various metabolic activities are related with the pumping activity of our heart and also on various muscle movements.  

         Till now we have technology where we can measure parameter of one system by using one technique i.e. using ECG we can only determine the parameters of cardiac system but not able to determine the parameters of respiratory. In our project we tried to solve this limitation by using ECG we can measure SPO2 in the arterial blood. To establish the relationship between heart rate and SPO2 we have taken help of applied mathematics.

Introduction :

HEART RATE:

No of times our heart beats per minute. The normal heart rate of a normal adult person/human is around 70-72 beats/min and the respiration rate is 12-15 breaths/min.

CARDIAC OUTPUT: 

The cardiac output is the amount of blood ejected from the heart. If the hear rate is 70-72 per minute then cardiac output would be 5litres per minute. This increases during the exercises.

STROKE VOLUME: 

The amount of blood expelled by each contraction of the ventricles is the stroke volume. When increased blood supply is needed to meet increased tissue requirements of the oxygen and nutrients, heart rate or stroke volume can be increased.

SPO2: 

oxygen saturation, which is the ratio of summation of concentration of oxyhemoglobin and concentration of hemoglobin to the concentration of oxyhemoglobin.

Now the intake of oxygen (VO2) during one minute is 5 lit/min.

As we know,

VO2 = Stroke Volume* Heart Rate

& CARDIAC OUTPUT (C.O) = Stroke Volume*Heart Rate

So we can say VO2 is directly related to C.O.

VO2 =Cardiac Output

Now taking 72 breaths as a standard value at that time the value of Cardiac Output (C.O.) is 5lit/min so for X pulses (diseased condition) we can determine the value of Cardiac Output (C.O.)

Cardiac Output = (number of pulses)* 5/72

Considering normal beats i.e. 72 beats/min the saturation of oxygen in the arterial blood (SPO2) is 100% so for x heart beats (diseased condition) we can determine the saturation of oxygen in the arterial blood (SPO2).

SPO2 = (number of pulses)* 100/72

Block Diagram:

Description of Block Diagram: 

Electrodes: 

          Various types of “ECG electrodes” are available in market, but the most simplest and common one is CLAMP ELECTRODES which we have used in our project. The ECG clamp electrodes pack contains three reusable clamp electrodes which can be attached to wrists or ankles. The electrode is 3 x 8 centimeters providing a 24 cm2 area to maximize contact. 

      The ECG clamp electrodes connect to any Bio Amp Cable with snap-connect Lead Wires.The basic function of electrodes is to pick up the signals (bio-potentials) from the body. We are using lead II configuration in which the electrodes are connected to the one hand and two legs. The signals are very small in amplitude, so they are given to the amplifier for getting proper measurable amplitude.
 

Instrumentation amplifier: 

     We have used INA 126p instrumentation amplifier IC. Instrumentation Amplifier is high input impedance and low output impedance amplifier. We have chosen the gain of amplifier 1000, so as to get proper magnitude of signals. Normally the amplitude of “ECG” is 1mV so by keeping the gain 1000, we can get 1V as output.

Notch filter: 

        Notch filter is the band reject filter. It is commonly used to reject a single frequency. In our project as the ECG signals may get distorted due to the frequency of power supply (50Hz), we have designed 4th order band reject filter of 50Hz.

Calculations for the notch filter are given below

Fn=1/(2*pi*R*C)

Here we have FN=50Hz; so the value of C=0.1µF

So R ~=33 KHz
Low pass filter: 

       The frequency range of this filter is from 0.5-150Hz; which is the frequency range of ECG signals. So we have designed low pass filter of 150Hz. By selecting this range we are able to reduce the external noise as much as possible. To get proper and sharp cut-off we have selected forth order filter. We have used UAF42AP universal filter IC.designing and more information of IC shown in datasheet.

SCHMITT TRIGGER:

It is an inverting comparator with positive feedback. This circuit converts an irregular shaped waveform to a square wave or pulse. The input voltage VIN triggers the output VO every time it exceeds certain voltage levels. In our circuitry, the reference voltage is around 0.5V. So the output of the circuit comes out in the form of pulses. These pulses are fed to the micro controller, where they are processed, using mathematical calculations to give output in the form of SPO2.

The circuit diagram of Schmitt trigger is shown below--- 

Micro Controller Flow Chart:

Materials Required:

• Power supply
• Frequency generator.
• Instrumentation Amplifier. 
• Notch filter.
• Low pass filter. 
• Comparator. 
• Micro controller . 
• LCD display. 

 

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