Heart beat monitoring with IR using Blood Oxygen

level (SPO2) By measuring Blood Oxygen level and patterns of reflection

Shahab Ali Nasir, Farrukh Junaid & Muhammad Zeeshan

Department of Mechatronics Engineering

College of E&ME,NUST

Rawalpindi,Pakistan

Abstract— It is noted that technology has a lots of usage in the

health industry. Especially its usage increase when the case is

about the heart diseases. Because we can’t really relay upon

human judgment in this kind of disease, main and very common

type of diseases are heart pumping rate and blood pressure. In

this project we are very keen to monitor the blood oxygen level

and blood pumping rate and making use of it we take a look

upon the condition of the body. Furthermore we also know that

oxygenated blood has different light absorption characteristics

than the deoxygenated blood in terms of IR and red LED

wavelength. Our purpose is to examine the heart rate and oxygen

absorption from the finger print. Ti enables an efficient way of

health care. This paper is only to be used by the engineers,

medical staff, and people who are concerned to it.

I. INTRODUCTION

We know very well that to measure the condition of human

cardio vascular properly so we need to measure the condition

of heart i.e. heart rate and blood oxygen level. It is a very old

method to measure the heart beat by counting the pulse rate in

10 sec or more or less and multiply it with suitable no to make

it 60, it is to be noted that heart may not pump same at all that

1 min time. It is also stated that physician may not be giving

proper attention to the pulse, so for that we need some

instrumented thing that is free of all kinds of errors. Also ECG

is one of the most frequently used methods to measure the

heart rate but problem with it is it is not so much economical.

To provide people with economic gadget we need to replace

ECG. The heart rate of a normal adult at rest is 72(±15), the

number is greater for the females. Similarly babies have much

higher heart pumping rate. Now it is to be clarified that heart

rate mostly depends upon the condition, age, health of the

human body.

The project consists of two parts, hardware and

software. Hardware is circuit elements that are used to read

the value of heart beat using IR sensor and the software is the

interface of the result with LABVIEW. It shows us graphs of

heart beat with increasing time interval and also the blood

oxygen level in a graph format.

Fig(i) IR Sensor Module

II. LITERATURE REVIEW

IR sensor is used to control the transmission of infrared

LED through finger tip because there are some parts of human

body where it’s easy to use IR for measurement, which are ear

lobes, fingertips and forehead. Oxyhemoglobin (HbO2)

absorbs visible and infrared (IR) light differently than

deoxyhemoglobin (Hb), and appears bright red as opposed to

the darker brown Hb. Heartbeat can either be measured by

absorption of IR or by Reflection of IR in arteries of the

fingertip. For that purpose AC signal is superimposed on the

DC signal. We are using reflectioin

Equations: R = (ACrms of Red / DC of Red) / (ACrms of IR / DC of IR)

The ratios between different parameters of sensor and their

outcome can be calculated as shown above and this leads us

towards the measurement of SPO2 percentage in the blood % SpO2 = 110 – 25 × R

Accuracy of the calculation will depend on the device and

sensors we use for the analyzing. One method is to send

Different pulses at different timed intervals and average of the

heart rate is calculated using simple codes for average

calculation on Labview or Orduino. Other method is to

determine the time between two pulses and determine the rate

per minute, using the formula beat/min = 60. Third method is

to combine the above two methods by calculating the beat/min

and then taking average of the results. This method is more

affective because it provides greater accuracy in our results.

For calculation of Heart rate the no. of pulses for a given

time are multiplied by 60/T. This gives us Pulse oximetry

relies on measurement of physiological

signal called photoplethismography, which is an optical

measurement of the change in blood volume in the arteries.

Pulse oximetry acquires PPG signals by irradiating two

different wavelengths of light through the tissue, and

compares the light absorption characteristics of blood under

these wavelengths. These absorptions obey Beer Lambertís

law. According to Beer Lambertís law transmittance of light

through the tissue can be calculated using:

Iout=IineA

Where Iout is the light intensity transmitted through fingertip

tissue, Iin is the intensity of the light going into the fingertip

tissue and A is the absorption factor. Oxygenated hemoglobin

absorbs more infrared light and allows more red light to pass

through whereas deoxygenated hemoglobin absorbs more red

light and allows more infrared light to pass through. Red light

is in the 600-750 nm wavelength light band whereas infrared

light is in the 850-1000 nm wavelength light band. In addition

to SpO2 values, the pulse/heart rate variable was able to be

compared with that from two previous studies [14,20]. They

reported that when subjects were fed, pulse rate increased from

baseline. This corroborated the heart rate data in this study that

showed that eating was a variable that increased heart rate for

all 4 groups.

More studies showed ways to achieve great accuracies in

the result of blood oxygen level. These studies [5] took

different factors in account like disabilities and blood oxygen

differences after eating or running. This method of heart rate

measurement can be used as alternative for the blood sampling

techniques specially for those people who have issues while

giving blood samples.

III. Methodology

Fig:(ii) Flow Chart Heart beat rate Calculation through

Fingertip sensor

Above flow chart shows the concept for measuring and

monitoring the SPO2 along with showing data on the labview.

Using reflective photointerupters LTH 1550-01 or Finger Pulse

Sensor HRM-2511E we will measure analog signal. In Finger

Pulse sensor an IR LED and a photodetector are placed on

opposite sides. When a fingertip is plugged into sensor it is

illuminated by the IR light coming from LED. The

photodetector diode receives the transmitted light through

tissue on other side. The light transmitted to diode depends on

the blood volume inside. The transmitted light intensity varies

with pulsing of blood with heart. Then a plot of this variation is

drawn against time to get a photoplethysmographic or PPG

Signal.[7]

The input/ PPG signal is of very low amplitude with

approximately 2% of the signal is of interest. Signal

processing is required to separate desire signal from steady

state signal.

Fig: (iii) Light absorption by different type of tissue [8]

For proper signal conditioning circuit, the amplitude of

analog signal can be converted into a pulse by removing noise

and amplifying to make suitable for microcontroller. For that

purpose two active low pass filters are used to remove noise

from the signal. Using LM 324 op-amp for low pass filter and

the cut off frequency will be about 4 Hz . For cut off

frequency( fc ) the equation is as follow:

Fig: (iv). Design of filters and amplifiers using LM324

PPG Signal

Using Sensor

Signal

Conditioning

Filtering

Using LM 324

Amplifying

using LM324

Comparator

using LM324

Micro

Controller

LABVIEW

SPO2 &

GRAPH

If the cut off frequency is about 4Hz then maximum

measurable heart rate will be 4x60=240bpm. Both low pass

filters have cut off frequency about 4 Hz.

As Amplitude of the signal is very small so weak pulsating

signal is amplified about 100 times to get a good signal to use

it as reference voltage in the comparator.

The output voltage will be fluctuating between 0 to 0.35 V.

this fluctuating voltage is converted in to a 0 to 5V swing using

a comparator through LM 324. The reference voltage of

comparator is set to 0.35V. Whenever the voltage goes above

0.35 comparator goes to zero and when it stay below 0.35V

output of the comparator goes positive [9]. A neat and efficient

amplitude fluctuating pulse is fed to micro-controller and lab-

view. In micro controller we counts the number of pulses in a

specific time . While in lab-view we study the graph of the

fluctuating pulse of heart rate with time and determine the

value of SpO2 and we compare the data of healthy person with

the patient/subject to check any disease in the person.

Microcontroller:

Micro controller will first convert analog signal into

digital signal through built in ADC otherwise external ADC is

be needed. All the calculations are done in micro controller by

using different algorithm such as lookup tables, ratio

calculation and also to calculate the value of SpO2 through

counter, the numbers of pulses are counted. The micro

controller is programed such that first it will take input value

then it will calculate and then will be display on LCD.

Fig(v) LCD interface with 8051 Microcontroller

LABVIEW:

An analog signal will be observed in labview will be

compared with healthy person heart rate with time.

Fig (vi) Healthy Heart rate with time

This will tell us which disease is present in the person when

compare with healthy person. This graph can also be recorded

in labview or may be printed and later may be shown to the

doctor for deep observation. More data sampling of heart rates

of different persons can give us record of discrepencies in the

results and they may vary according to the age and different

respiratary conditions. We used NI ELVIS II for data

acquisition and we got the patteren of absorption of IR and

compared it with standard absorption spectrum of HBO2 in

labview. The comparison of these results with different

conditions and increased no . of subjects with age variation

gives us accurate measurement. We made a graphical user

interface(GUI) on labview for interaction of human with the

computer. We acquired the signals from our hardware module

and made them as input to the data acquisition module (DAQ).

NI ELVIS kit can also be used to reduce the interferences

caused by different parameters.The signal we acquired is of

4Hz and it can be amplified according to requirements ,other

way is to take the values directly from the hardware which will

give us filtered and amplified signals. These signals will be

used to calculate heartbeat in block diagram panel of labview

and the results will be displayed on GUI.

Graphical user interface on the labview is next step in

getting the output signal to work and then analyze it further

using NI ELVIS II for the study of data patterns. The methods

for the measurement of the average heart rate have been

discussed earlier. This is very useful for doctors and its

interface is much simpler and once a person has to be made

familiar with the GUI and the other time he can do it all by

himself and can also help others.

For the calculation of %SPO2 we assume the value of

K=98.5 was selection from the previous work on this field[10].

Normally a blood oxignation percentage from 95% to 100%

can be considered safe and its value drops with age. We will

have to adopt a way of repeated values and then take average

and zero error can be calculated and can be tackled. We are

adding an alarm for the indication of elevated heart rates and

critical blood oxignation level. We use ECG signals acquired

from our simple harware and used them for analyzing heartrate.

Using SPO2 percentage in blood after certain timed intervals

we can differentiate between oxygnated and deoxygnated

blood. An algorithm will be best for the calculation and

handling of different aspects. The absorbed or reflected IR

signal has some AC and DC component. BPM variations with

age are attached below.

Fig (vii) BPM dependence on age

Ways to output heart rate:

We can show the heart rate either on SSDs or on LCD

but for getting LCD to work,first you will have to interface it

with 8051 Microcontroller. For convenience it’s more

convenient to use SSDs due to simpler circuitory and less

requirement of skills. The other way is good and more

desireable to be used that is showing the output on Labview

and calculation the average by the calculation by the heart rate

indicated by the individual pulses.

Normally the output signal by hardware is displayed on the

ECG monitors as shown above and also BPM is showed with

it. Heart rate of a person at an instant is calculated using R-R

interval in second or millimeters. One formula that can be used

for the measurement of heart rate is given below

HR = 60/(RR interval in seconds)

Other formulas can be used if we wish to do it by calculating

the distance between the peak of the above signal and the

interval can either be taken in milliseconds or millimeters.

Fig (viii) output on labview after placing finger on sensor

After using the sensor with the hardware that gives us required

Filtering and amplification we got the above output using NI

ELVIS II and the result was shown on the Oscilloscope after

processing in labview.

Fig (ix) front panel of our labview interface

Fig (x) block diagram of our labview interface

IV. CONCLUSION

We have made this design using the components with least

cost as it is more preffered in engineering design. Moreover

our GUI on labview is easy and simple to such extent that

everybody can use it with great ease. Our circuit and design

can be modified according to need and led and alarms can

be added depending on our requirement. Alarm can be

added to indicate a drop in SPO2 which can be a symptom

for some disease and can also be used for alert in case of

high blood pressure. We showed a way of measurement of

heart rate using NI ELVIS II and Labview integration.

REFERENCES

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[3] Dogan Ibrahim, Kadri Buruncuk, Heart Rate Measurement

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