Photoplethysmography Blood Pressure Measurement

by Cheah Kim Wei (U016319H)

Department of Mechanical Engineering, National University of Singapore.

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Table of Contents

1. Introduction ............................................................................ 1

1.1 Objective ............................................................................................................. 2

1.2 Blood Pressure Measurement ............................................................................. 3

1.3 Photoplethysmograph ......................................................................................... 5

2. Design....................................................................................... 6

2.1 General................................................................................................................ 6

2.2 Design Elements ................................................................................................. 8

2.3 Method of Measurement ................................................................................... 10

3. Design Improvement ............................................................ 13

3.1 Cuffless Design................................................................................................. 13

3.2 Device Positioning ............................................................................................ 14

3.3 Energy Consumption ........................................................................................ 15

4. Conclusion ............................................................................. 16

4.1 Discussion......................................................................................................... 16

4.2 Recommendation / Future Concern .................................................................. 17

References.................................................................................... 18

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1. Introduction

Blood is being carried from our heart to all parts in our body by blood vessels

called arteries. Blood pressure is the force of blood pushing against the wall of the

arteries. Each time our heart beats it pumps out blood to the arteries. Systolic pressure

which is the highest blood pressure occurs when our heart is pumping. Diastolic pressure

is lowest blood pressure when our heart is resting.

Since blood pressure is an indirect measurement of heart beats, the blood pressure

will changes according to time and emotion. For instance, blood pressure will rise when

we are awaken and excited. The unit for blood pressure is in mmHg and the notation will

be systolic followed by diastolic pressure.

Normal blood pressure is less than 120/80mmHg. Blood pressure of

140/90mmHg is considered high blood pressure or hypertension. Hypertension does not

cause much symptoms but severe case causes headache, sleepiness, confusion and coma.

Therefore it is very important to continuous monitoring the blood pressure of potential

victims such as hypertension patients, elderly etc.

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1.1 Objective

In this design framework, a wearable blood pressure sensor needs to be designed

to suit into the existing MEMSWEAR platform. The blood pressure measurement has to

be semi-continuous which will be elaborated in details in later stage of the report. The

information gathered will be sent by Bluetooth technology to the MEMSWEAR system

to be monitored.

The size for the sensor system must be small and easy to incorporate into the

MEMSWEAR suit. Besides that, the energy consumption of the sensor should not be

large and the sensor can be easily attached and detached.

First the framework will discuss more on methods to measure human blood

pressure and listing out of disadvantages and problems. Next, there will be more

explanations on the solutions to these problems and the design of the sensor. The

following topic of discussion will be on the design elements of the sensor and how these

elements will benefit the wearer. Subsequently, the suggested experimental testing will

be explained. The later part of the report will be on problems faced and future concerns.

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1.2 Blood Pressure Measurement

Non-invasive pressure measurement sensor is preferred as it is easily attached and

detached from the wearer’s body. Non-invasive pressure measurement is an oscillometric

method which applies theory of sphygmomanometer. It is an instrument for measuring

blood pressure in the arteries, especially one consisting of a pressure gauge and a rubber

cuff that wraps around the upper arm and inflates to constrict the arteries.

To take a blood pressure reading, you need to be relaxed and comfortably seated,

with your arm well supported. Alternatively, you can lie on an examination couch.

1. A cuff that inflates is wrapped around your upper arm and kept in place with

Velcro. A tube leads out of the cuff to a rubber bulb.

2. Another tube leads from the cuff to a reservoir of mercury at the bottom of a

vertical glass column. Whatever pressure is in the cuff is shown on the mercury

column. The mercury is held within a sealed system – only air travels in the

rubber tubing and the cuff.

3. Air is then blown into the cuff and increasing pressure and tightening is felt on the

upper arm.

4. The doctor puts a stethoscope to your arm and listens to the pulse while the air is

slowly let out again.

5. The systolic pressure is measured when the doctor first hears the pulse.

6. This sound will slowly become more distant and finally disappear.

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The diastolic pressure is measured from the moment the doctor is unable to hear

the sound of the pulse. The blood pressure is measured in terms of millimetres of mercury

(mmHg).

Figure 1: Sphygmomanometer

The method above needs a trained personnel and significant size equipment which

is not a good option for MEMSWEAR. Besides it is sometimes hurt if the cuff is applied

on our arm. Hence, we need good solutions for these so-called traditional ways.

The sensor has to be smaller in size, can operate without any personnel and does

not hurt too much. Energy consumption of the sensor must be as low as possible.

Therefore, a photoplethysmograph (PPG) sensor is proposed to be used on our finger. In

the next section, the PPG method will be further elaborated.

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1.3 Photoplethysmograph

The photoplethysmograph (PPG) is a non-invasive electro-optical signal to

measure pulsations associated in changes of blood volume. The method was first

introduced by Penaz. The principle of indirect continuous monitoring of blood pressure

waveform is unloading technique proposed first by Marey (1876), later specified by

Shirer (1962). This method is based on idea that if an externally applied pressure in the

cuff is equal to the arterial pressure instantaneously, the arterial walls will be unloaded

(zero transmural pressure) and the arteries will not change in size. In this condition, the

blood volume will not change. This method was attempt to realise for the first time by

Penaz (1973) using photoelectric technique of detecting blood flow, equipped with a

transparent inflatable cuff controlled by a servocontrol system in the human finger [1].

Figure 2: Penaz's method of photoplethysmography

Digital photoplethysmographic sensor has an infrared emitter and photodiode

detector. The intensity of light from infrared emitter which reaches the photodiode

detector in either reflection or transmission will be measured to determine the blood

volume changes.

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2. Design

2.1 General

The photoplethysmographic sensor will be placed below the tip of the finger and

pressure will be applied on the proximal phalanx. Since the cuff is wrapped on the

proximal phalanx of the finger rather than arm, it makes less discomfort for prolonged

used. The blood volume changes on the finger will be notified by the sensor and

transmitted to the system by Bluetooth transmitter.

Figure 3: Location of the sensor and cuff

There is a timer attached on the device to time the cuff pressure applied time and

transmitter so that it is semi-continuous measurement. This is because to decrease the

energy consumption of Bluetooth transmission and to avoid discomfort of the wearer’s

finger due to continuous pressure applied.

The idea of the blood pressure sensor is much like the Photoplethysmograph

Fingernail Sensor for measurement of finger force which is mentioned in a journal by

Stephen A. Mascaro and H. Harry Asada [2]. The finger force is measuring colouration

of fingernail by using reflectance photoplethysmography. The sensor is made into a hand

glove as a more compact device. Below is the picture of the fingernail sensor.

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Figure 4: Fingernail sensor handglove

The proposed blood pressure sensor will be incorporated into a hand glove as well.

The hand glove must be easily attachable and detachable by the wearer to fulfill wearable

requirement.

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2.2 Design Elements

Size/Profile

In order to fulfill the requirement reduce discomfort of the wearer, the sensor

must be thin and unobtrusive. If possible, the sensor can be made in decorative manner

such that it is not obvious to the others. Besides that, the sensor should not be obtrusive to

the wearer’s daily activities.

Sensor Position

Separation between noise and signal is the most crucial part of the sensor position.

It is proposed to be attached rigidly to the fingernail. This will reduce the background

noise of sensor. The interface between the finger and the sensor must be optically

transparent to increase the sensitivity of the sensor.

Shielding of Ambient Light

Due to the sensitivity of the sensor to light, the shielding of the sensor is difficult.

Photodiodes has to be optically shielded from the ambient light to reduce noise caused by

the ambient light. Photodiode detector has to be shielded from the side to prevent

unwanted light from the infrared emitter. This will avoids unwanted voltage from

forming.

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Signal Processing and Transmission

An amplifier such as below is located near to the photodiode detector to amplify

signal from the detector. This is to prevent loss of signal due to long transmission. The

amplified signal will be processed and transmitted to the system by using Bluetooth

transmitter. The figures below show the amplifier circuit of the PPG sensor and the

transmission flowchart of the PPG sensor.

Figure 5: Photoplethysmograph sensor amplifier circuit

PPG Sensor

Microcontroller on Sensor

Bluetooth Transmitter

Bluetooth Receiver

Monitoring System

Mobile Phone

Internet Alarm

Sensing Device

Figure 6: Transmission flowchart

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2.3 Method of Measurement

There are two methods suggested by Ante Fiantii and Miroslav Gaban to measure

the diastolic and systolic blood pressure on the finger [3]. One of them is based on

measurement of pulse time-delay between proximal cuff and distal photoplethysmograph

sensor. The other method is based on pulse amplitude differences which occur during the

cuff deflation.

The systolic pressure can be easily determined with the sensor on the tip of the

finger. The proximal cuff deflates and when the systolic pressure in the proximal cuff is

reached, the first bolus of blood succeeds to reach sensor, what indicate systolic pressure.

However, it is harder for determination of diastolic pressure measurement.

Pulse Time Delay Method

In the previous work of Fiantii and Miroslav Gaban mentions about the method of

measuring the time delay of the transition between proximal cuff and sensor until it

becomes constant [3]. Refer to the figure below, the time delay can be measured in ∆t.

Figure 7: Pulse pressure signal in the proximal cuff 1 and photoplethysmograph sensor 2 near systolic pressure

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However, if we notice different time pulse delay has different peak value. In

previous work by A.Santic, M.R. Neuman, the less changed peak amplitude is taken.

Refer to the figure below; the peak amplitude corresponding to the first minimum of time

pulse changes is taken.

Figure 8: Pulse time delay as a function of pressure in the proximal cuff

Pulse Amplitude Difference Method

The amplitude differences (between cuff pressure and sensor) obtained after the

systolic pressure is the highest systolic pressure. The amplitude differences obtained after

the diastolic pressure is the lowest pressure. Refer to the figure below to see the graph of

pulse signal.

Figure 9: Pulse pressure signals in the proximal cuff 1 and photoplethysmograph

sensor 2 near diastolic pressure with shown amplitude differences

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Figure 10: Pulse amplitude differences a function of pressure in proximal cuff

The differences of amplitude are plotted out in a graph. The diastolic pressure is 45% of

the highest and lowest amplitude differences which the equation below.

∆Adia = 0.45(∆Amax-∆Amin) + ∆Amin

The mentioned methods require sampling of 10 to 20 data before the diastolic

pressure is determined. Obtaining one data sampling requires about one second, hence,

time to take an exact diastolic pressure is 20 seconds. This delay of measurement will be

fatal to the user during the critical period.

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3. Design Improvement

3.1 Cuffless Design

The traditional blood pressure measurement used pressure cuff which could cause

painful or discomfort to the user. Pulse Transit Time (PTT) is the time interval for the

arterial pulse pressure wave to travel from the aortic valve to a peripheral site. PTT is

studied and used to estimate blood pressure for cuffless design. Peripheral pulses are

recorded at the fingertip using photoplethysmography and then compare the result with

Electrocardiograms (ECG). In this case, the pressure cuff can be excluded from the

design. The advantages from not having a pressure cuff are the device is easily

manageable, elimination of bulkiness of the device and energy saving.

The implementation of PPT needs to perform ECG scanning. There are three

ways to implement cuffless blood pressure measurement. The first way is to use PPT

which requires ECG. The second way is to implement Continuous Wavelet Transform

(CWT) [4]. The third way is combining a PPG-based signal with a hydrostatic pressure

reference for absolute sensor calibration [5].

With cuffless design, the monitoring can be done more often than usual pressure

cuff design on the finger. However, cuffless design is hard to determine the diastolic

pressure. Therefore more testing is required to verify the result of the blood pressure from

cuffless design.

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3.2 Device Positioning

Since the design has been improved to cuffless design, the position of the device

can be changed to other parts of the body. The requirement to position the device is

limited blood pressure change which means limited movement. For instance when the

user’s finger is lowered the pressure could increase.

The sensor is encapsulated in a casing which very similar with the hearing assist

tool shown in the figure below. The skin behind the ear is very thin hence suitable for the

PPG sensor to work. The transmission device will be at the same casing with the sensor

to avoid undesired noise during the transmission. The benefit from this position is the

possible integration with the other sensors such as temperature and SpO2 sensor. With the

integration with other sensors, all signals from the sensors can be sent at one time to

reduce energy consumption.

Figure 11: Positioning of the PPG sensor

When the PPG sensor is attached onto the ear, the first calibration of the blood

pressure is taken. In this calibration process, the systolic and diastolic pressure is taken. If

the pressure is above the normal person’s rate, the system will show a warning message.

After it is being calibrated, any abnormal systolic and diastolic pressure will caused the

system to send warning message to the recipients.

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3.3 Energy Consumption

A normal Bluetooth headset is built in with lithium polymer battery with

rechargeable ability. The device will be powered by the similar battery since it is proven

workable. The idea of having semi-continuous monitoring will still applicable to reduce

energy consumption. Since Bluetooth consumed a lot of energy therefore by limiting

numbers of Bluetooth transmission we can reduce energy consumption.

Table 1: Bluetooth energy consumption of handphone headset

Active Idle Bluetooth Activated Disabled

Energy Consumption High. Battery life to 4 hours Low. Stand-by time up to 200 hours.

A small microcontroller can be integrated into the device to perform abnormal

respond switch. The monitoring of blood pressure will be continuous by using PPG

sensor. However when abnormal blood pressure occurs the microcontroller will

immediately executes Bluetooth transmission to the central monitoring system. Refer to

the table above; the energy consumption will be down to a low level when the Bluetooth

is idle.

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4. Conclusion

4.1 Discussion

Photoplethysmographic is a workable solution for blood pressure measurement.

The method uses infrared laser to sense and respond to the system. The advantage of

infrared sensor is fast response and ambient pressure independent. However the sensor

needs accurate and fixed positioning for consistent result. If the sensor is moved

accidentally the measured blood pressure may be inaccurate. The sensor is also

depending on surrounding lighting therefore it requires shielding of lighting to be done.

The systolic pressure can be easily obtained by measuring the first bolus that

reaches the sensor after the proximal cuff deflated. Getting the diastolic pressure is more

complicated, two methods are suggested namely time pulse delay method and amplitude

differences method. The weakness of these methods is the method requires sampling of

10 to 20 wavelengths from the sensor before the diastolic pressure can be obtained. In

this case, the warning message of any critical situation will be delayed.

The original design of sensor can be improved into cuffless design. There are

three different approaches to design a cuffless blood pressure sensor. They are Pulse

Transit Time (PTT), Continuous Wavelet Transform (CWT) and hydrostatic pressure

reference. By implementing the cuffless design, it is possible to place the sensor on the

human ear.

With semi continuous monitoring, the Bluetooth transmission can be reduced. In

order to do that, a microcontroller is used as abnormal detector to send only the abnormal

signal to the monitoring system. This will decrease the sensor energy consumption.

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4.2 Recommendation / Future Concern

The future direction of the design will be emphasized on reducing the energy

consumption and improving battery life. A long battery life is very essential for remote

monitoring system. The failure of the system due to low battery life could be fatal to the

user especially during critical period.

Few sensors that are similar to each other can be integrated together in one system.

These sensors are blood pressure, SpO2 and temperature sensors. Integration of sensors

will also reduce energy consumption since all the signals are sent at one time. On the

other hand, integration of sensors requires more complicated circuitry that may leads to

interference and failure of the system.

The two ways of measuring diastolic pressure require some sampling of

waveforms from the sensor. Therefore the warning message maybe delayed and the result

could be fatal to the user. In the future research on this area, better ways to determine the

diastolic pressure are needed for faster response time of the system.

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References

[1] Pavla Holejšovská, Zdeněk Peroutka, Jiří Čengery, “Non-Invasive Monitoring of

the Human Blood Pressure”, University of West Bohemia, Pilsen, Czech Republic.

[2] Stephen A. Mascaro, Student Member, IEEE, and H. Harry Asada, Member,

IEEE, “Photoplethysmograph Fingernail Sensors for Measuring Finger Forces Without

Haptic Obstruction”, IEEE Transactions on Robotics and Automation, Vol. 17, No. 5,

Oct 2001.

[3] Ante Fiantii, Senior Member, IEEE, Miroslav Gaban, “Two Methods for

Determination of Diastolic and Systolic Pressures in Fingers”, IEEE-EMBC and CMBEC,

1995.

[4] X. F. Teng and Y. T. Zhang, “Continuous and Noninvasive Estimation of Arterial

Blood Pressure Using a Photoplethysmographic Approach”, Proceedings of the 25th

Annual International Conference of the IEEE EMBS, Cancun, Mexico, September 17-21,

2003.

[5] P. Shaltis, A. Reisner, H. Asada, “A Hydrostatic Pressure Approach to Cuffless

Blood Pressure Monitoring”, Department of Mechanical Engineering, Massachusetts

Institute of Technology, Cambridge, MA, USA Massachusetts General Hospital, Boston,

MA, USA, Proceedings of the 26th Annual International Conference of the IEEE EMBS,

San Francisco, CA, USA, September 1-5, 2004.

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