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International Journal of Latest Research in Science and Technology ISSN (Online):2278-5299
Volume 3, Issue 5: Page No 148-152. September-October 2014
https://www.mnkpublication.com/journal/ijlrst/index.php
148
Publication History
Manuscript Received
: 20 October 2014
Manuscript Accepted
: 26 October 2014
Revision Received
: 28 October 2014
Manuscript Published
:
31 October 2014
AN OVERVIEW ON HEART RATE MONITORING
AND PULSE OXIMETER SYSTEM
1Esrat Jahan, 1Tilottoma Barua, 1Umme Salma
1Department of EEE, Chittagong University of Engineering and Technology (CUET), Chittagong-4349, Bangladesh
Abstract- Use of technology in healthcare is growing importance as a result of the tendency to acquire chronic disease like heart attack and high
blood pressure. Heart rate and blood oxygen saturation is a couple of such biometrics that is monitored in this project to provide information
regarding the health of the body. By measuring the intensity change of light transmitted through tissue due to arterial blood, heart rate is
measured. Furthermore, oxygenated blood has different light absorption characteristics than deoxygenated blood under red and infrared
wavelengths. Comparing the absorptions produce an estimate of the oxygen saturation of blood. The purpose is to examine how heart rate and
the oxygen saturation of subject is measured from finger and then processed and displayed. The design, is small in size, easy to use, allows a non-
invasive, real time method to provide information regarding health. This enables an efficient and economical means for managing the health
care. This document is intended to be used by engineers, medical equipment developers, anyone related to medical practice and interested in
understanding the operation of pulse oximeter and heart rate monitoring system.
Keywords - Beat per minute(BPM), Pulse oxymetry, Oxygenated Hemoglobin (HBO2), SPO2, Photoplethismography
I. INTRODUCTION
Measurement of heart rate and pulse oximetry are very
important factors to access the condition of human
cardiovascular system. Heart rate is formerly measured by
placing the thumb over the arterial pulsation, and counting the
pulses usually in a 30 second period. Heart rate is then found
by multiplying the obtained number by 2. This method
although simple, is not accurate and can give errors when the
rate is high [1]. In clinical environment, heart rate is measured
under controlled conditions like blood measurement, heart
voice measurement, and Electrocardiogram (ECG). ECG is
one of frequently used and accurate methods for measuring the
heart rate. But ECG is not economical [2]. The heart rate of a
healthy adult at rest is around 75(±15) (or greater for females)
beats per minute (bpm). Athletes normally have lower heart
rates than less active people. Babies have a much higher heart
rate at around 120 bpm, while older children have heart rates
at around 90 bpm. Heart rate varies significantly between
individuals based on fitness, age and genetics [3].
On the other hand the percentage of arterial blood saturated
with oxygen helps to determine the effectiveness of a patient’s
respiratory system. The technique by which blood oxygen
saturation is determined is called Pulse Oximetry [4]. In
earlier days, the common method used to measure blood
oxygen saturation was arterial blood gas measurement. An
Arterial Blood Gas is a blood test that involves puncturing an
artery with a thin needle and syringe and drawing a small
volume of blood [5]. This method was invasive, expensive,
difficult, painful and potentially risky. So, Pulse Oximeter is
introduced, operation of which is non-invasive and based on
measuring the absorption of red and infrared light that passes
through a patient's finger or ear lobe by using light sensors.
Acceptable normal ranges for patients are from 95 to 100
percent, those with a hypoxic drive problem, would expect
values to be between 88 to 92 percent. Due to its non-invasive
nature, high precision, and reasonable cost, optical pulse
oximetry and heart rate measurement system is widely
adopted as a standard patient monitoring technique [6].
Diagnosis of heart disease can be achieved by correlating the
pattern of measured value with a typical healthy signal,
characterizing the measured value with basic logic decisions.
II. LITERATURE REVIEW
According to Handbook of Biomedical Instrumentation by
R.S. Khandpur [7], techniques of measuring heart rate are:
Average Calculation:
An average rate is calculated by counting the number of
pulses in given time. This method does not show changes in
time between beats and thus does not represent the true picture
of heart’s response to exercise, stress and environment
Beat To Beat Calculation:
This is done by measuring the time (T) in seconds, between
two consecutive pulses, and converting the time into
beats/min, using the formula beat/min = 60/T.
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International Journal of Latest Research in Science and Technology.
ISSN:2278-5299 149
Combination Of Beat To Beat Calculation With
Averaging:
This is based on four or six beats average. The advantage of
this technique over the averaging techniques is its similarity
with beat to beat monitoring system.
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 Lambert’s
law. According to Beer Lambert’s law transmittance of light
through the tissue can be calculated using:
Iout = Iin eA (1)
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[8].
According to Yousuf Jawahar, Pulse oximetry can be done
by two methods [9]:
Transmittance Method: In this method, light is
transmitted through tissue using the LED and is detected on
the other end using a photo-detector. It is more suited to the
areas of body that lend themselves better to light
transmittance through them, e.g. fingers or ear lobe. This
configuration cannot be used in other areas of body when
there are obstacles such as bones or muscles.
Fig. 1 Transmittance Method
Reflectance Method: In reflectance pulse oximetry it uses
a photo detector on the same side as the LED to detect the
light reflected by the tissue. This method is more useful
where the vasculature is available close to the surface of
skin e.g. forehead, wrist, forearm.
Fig. 2 Reflectance Method
Based on all these review, there are two methods are chosen
to calculate heart rate and blood oxygen saturation level.
Heart rate calculation: In this project is based on the beat
to beat heart rate calculation process. In this process, number
of pulses for a given period T is calculated and converted to
bpm by multiplying with 60/T, that gives the instantaneous
heart rate in bpm. So this can be expressed as:
Heart rate bpm (2)
Calculation of blood oxygen saturation level: The
principle of pulse oximetry is based on the red and infrared
light absorption characteristics of oxygenated and
deoxygenated hemoglobin. 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. The absorption
relationship is shown in following figure:
Fig. 3 Absorption Relationship Of Oxygen Levels In The
Blood For The Red And Infrared Wavelengths
Because the flow of blood is pulsatile in nature, the
transmitted light changes with time. A normal finger has light
absorbed from bloodless tissue, venous blood, and arterial
blood. The volume of arterial blood changes with pulse, so the
absorption of light also changes. The light detector will
therefore see a large DC signal representing the residual
arterial blood, venous blood, and bloodless tissue. A small
portion will be an AC signal representing the arterial pulse.
Because this is the only AC signal [10].
Fig. 4 Light Absorption By Tissue Type
Now the basic formula to calculate oxygen saturation level
can be stated as:
SPO2 = (3)
Where, hemoglobin with oxygen molecules is considered as
oxygenated hemoglobin (HbO2). When it is carrying less
oxygen molecules, then it is considered reduced (Hb).
To find oxygen saturation, first calculate the ratio R:
International Journal of Latest Research in Science and Technology.
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R =
= (4)
The maximum and minimum value will be used to calculate
the ratio R. Using this value oxygen saturation level can be
measured [5]. So, if we consider any linear equation then, the
oxygen saturation level can be calculated as,
SPO2 = a – bR (5)
Where a and b can be considered as the calibrated values,
which are constants.
III. SYSTEM OVERVIEW:
Sensor System:
The optical sensor is consisting of light emitter and detector
and control circuit, which are discussed below:
Light Emitter And Detector:
In sensor system, the light source and photo detector are on
the opposite sides of the tissue inside a fingerclip. The photo
detector measures the intensity of light transmitted through the
tissue. For measuring SPO2 level, it requires two LEDs of
different wavelengths to compare the absorption level of
oxygenated and deoxygenated blood. Thus the optical sensor
is comprised of two LEDs emitting visible and infrared light
as emitter and a photo-detector[10].
Control Circuit:
Since both the red and infrared LEDs are detected by the
same photodiode, the photodiode cannot distinguish between
red and infrared light. But to accommodate this, LEDs is
pulsed alternatively[14]. For this purpose, a driver system
alternately turns each LED on and off. The device repeatedly
samples the photodiode output while the red LED is on, while
the infrared LED is on and while both are off. By sampling
with both LED’s off, the pulse oximeter is able to subtract any
ambient light that may be present during measurement[9].For
this reason, the RED and IR LEDs are lit alternately every
400ms. The typical pulsating scheme of the LEDs is shown in
fig. 6:
Fig. 6 Typical Pulsating Scheme Of Red And Infrared LED
Pre-Processing:
The input signal is of very low amplitude and the
approximately 2% of the signal is of interest. The signal
processing is required to separate the desired signal from the
steady state signal to give accurate readings. With a proper
signal conditioning circuit, little changes in the amplitude of
the detected light in sensor unit can be converted into a pulse,
suitable for processing by the microcontroller [11].
Fig. 7 Control Circuit For Red And Infrared LED
In this project, low power dual operational amplifier
LM358 op-amp is chosen. According to its data sheet, it
consists of dual op-amps in a single package and it can operate
at supply voltages as low as 3.0 V or as high as 32 V with low
quiescent currents according to its data sheet. The single stage
of this op-amp can achieve voltage swings of 0VDC to
common-collector voltage Vcc (pin 8) −1.5V [13].
The gain of OP AMP is calculated as:
á = 1 + (6)
= 1 +
= 101
The gain of each stage is set to 101, giving the total
amplification of about 10000.
The purpose of the filter circuit is to keep any frequency
content between 0-2.5Hz and eliminate above and below this
range[11].
The cut off frequency of LPF is calculated as:
fC = 1/ (2ðRf Cf) (7)
=1/(2×3.1416×680k × 0.1e-6)
=2.34 Hz
The cut-off frequency of active low pass filters is about
2.34 Hz. This means the maximum measurable heart rate is
about 2.34×60=140 bpm [12].
There are two stage operational amplifiers configured as
active low pass filters. The cut-off frequencies of both the
filters are set to about 2.34 Hz, and so it can measure the pulse
rate up to 2.34*60 = 140 bpm. A 2.34Hz filter was chosen so
that the fundamental and second harmonic of the cardiac beat
could be captured.Further the gain of each filter is 100, which
gives the total 2-stage amplification of 10000. This is good
International Journal of Latest Research in Science and Technology.
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enough to convert the weak pulsating signal into a TTL pulse
[13].
Microcontroller:
To implement the advanced signal processing algorithms on
microcontroller in real-time for pulse oximeter and heart rate
monitor, the computation involves ratio calculation and look
up table implementation to calculate final SpO2 for display.
The microcontroller (PIC18F452) is programmed to switch
and control the timing and intensity of IR and RED LEDs.
The output signal from the amplifier will be supplied to the
PIC18F452 which will be converted from analog signal into
digital signal through the built-in ADC. The microcontroller
computes the received red and infrared light intensity ratio and
hence to derive SpO2 value. At the same time it calculates the
number of beats per minute [14].
Serial Transfer and Display:
For the device to be user friendly, the output is displayed
via LCD: SPO2 concentration in percentage and pulse rate in
bpm. A 16x2 LCD display is very basic module and is
preferred over seven segments and other multi segment LEDs.
Fig. 8 Serial Transfer
As LCDs are economical; easily programmable; have no
limitation of displaying special & even custom characters,
animations and so on.
In order to transfer data to the doctor, the users need to
connect the system to the PC parallel port.
So the other type of display is a PC serial (USART) port. The
microcontroller PIC18F452 has built in USART on board at
pins 25 an d 26 (RC6/TX and RC7/RX), that can send data to
PC via RS-232 interface and display information of SPO2
concentration and heart rate.
Alarming System :
The output unit consists of an alarming system to indicate
whether the heart rate and percentage of oxy-hemoglobin is
within the reference values. If the heart rate counter and SPO2
level is different from reference then a LED indicator is
lightened and an audio signal is generated. Preset values SPO2
and heart rate are interpreted by the table I and II:
TABLE I PRESET VALUES OF SPO2
SPO2 Reading (%) Interpretation
95-100 Normal
91-94 Mild Hypoxemia*
86-90 Moderate Hypoxemia*
<85 Severe Hypoxemia*
*Hypoxemia is defined as decreased partial pressure in blood
and oxygen available to the body or an individual tissue or
organ.
The output of heart rate is compared with the references
representing bradycardia and tachycardia for adult or children.
These referenced values were taken by statistical computation.
TABLE III PRESET VALUES OF HEART RATE
Age Heart Rate (BPM) Interpretation
15 years – adult < 60 Bradycardia
1–2 days > 159 Tachycardia
3–6 days >166 Tachycardia
1–3 weeks >182 Tachycardia
1–2 months >179 Tachycardia
3–5 months >186 Tachycardia
6–11 months >169 Tachycardia
1–2 years >151 Tachycardia
3–4 years >137 Tachycardia
5–7 years >133 Tachycardia
8–11 years >130 Tachycardia
12–15 years >119 Tachycardia
>15 years – adult >100 Tachycardia
International Journal of Latest Research in Science and Technology.
ISSN:2278-5299 152
Fig. 11 Physical Appearance Of The Designed Device
IV. CONCLUSION
The Heart rate monitoring and pulse oximeter device
available in market are high pricing where the designed device
is the cheapest one. The design proposes small size, light
weight, low power consumption, standardized signal
processing capabilities. This device is able to produce highly
reliable test results for both heart rate and SpO2 level. Our
designed device has the advantage that it can be used by non-
professional people at home to measure the heart rate and
SPO2 level easily and safely. At the same time abnormal
condition can be detected easily and data can be sent to doctor
from PC through email for further diagnosis.
ACKNOWLEDGMENT
We like to express sincere appreciation and deep gratitude
to all participants in this work.
REFERENCES :
[1] Dogan Ibrahim, Kadri Buruncuk, “Heart Rate Measurement From
The Finger Using A Low-Cost Microcontroller”, pp 1-4,
September, 2005.
[2] Ken Li Chong, David Holden, Tim Olin, “Heart Rate Monitor”, vol-
1, pp 2-10, October,2010.
[3] http://en.wikipedia.org/wiki/Heart_rate, October 17, 2011.
[4] http://en.wikipedia.org/wiki/Pulse_oximetry, September 14, 2011.
[5] Dilpreet Kaur, Sukhwinder Kumar, Shashi Sharma, “Online
Graphical Display of Blood Oxygen Saturation and Pulse Rate”,
International Journal of Scientific & Engineering Research Volume
2, ISSN 2229-5518, Issue 6, June-2011
[6] http://www.ocw.uc3m.es/tecnologia-electronica/IE_Project-
3_OCW.pdf, November 30, 2011
[7] R.S. Khandpur, ISBN10:0-07-047355-2 “Handbook of Biomedical
Instrumentation ”, Tata Macgraw-Hill Education, April 1, 2003,
second edition.
[8] Saly J. 2000. Neonatal and Pediatric Pulse Oximetry. Respiratory
Care 286-289.
[9] Yousuf Jawahar, ” Design Of An Infrared Based Blood Oxygen
Saturation And Heart Rate Monitoring Device”, pp 9-25, April 10,
2009.
[10] Y. Iyriboz , J. Morrow , D. Ayers MS and G. Landry MS,
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Oximetry system “, pp 37-40, December 2008.
[12] http://www.embedded-lab.com/, October 28,2011.
[13] Robert F. Coughlin, Frederick F. Driscoll, ISBN-81-203-2096-4,
“Operational Amplifier and Linear Integrated Circuits”, Hall Of
India Private Limited, New Delhi-110001, 2003, Sixth Edition.
[14] Dilpreet Kaur, Sukhwinder Kumar, Shashi Sharma, “Online
Graphical Display of Blood Oxygen Saturation and Pulse Rate”,
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2, ISSN 2229-5518, Issue 6, June-2011
