1

Overview of system

ECG sensor

Integrated SpO2/Temperature sensor

Central processing unit

Wireless Body Area Network (WBAN) Bluetooth

Sentinel events / Request to view real-

time vital signs

GSM

Wearer’s PDA phoneDoctor’s PDA phone

1. Four physiological signs, e.g., Electrocardiogram (ECG), SPO2, body temperature and blood pressure will be continuously acquired or derived from two wireless sensor node - ECG sensor and integrated SpO2/Temperature sensor node. 2. At the wearer’s location, the PDA-based monitor can be used to acquire real-time and continuous waveform.3. Upon detection of sentinel events, the abnormal vital signs would be sent wirelessly through GSM to doctor/caregiver’s PDA phone.4. Doctor/caregiver can promptly view and analyze the received abnormal vital signs forfurther treatment strategy decision

2

Electrocardiogram (ECG)

3

Wireless ECG sensor

As the heart undergoes depolarization and repolarization, electrical currents spread throughout the body because the body acts as a volume conductor. The electrical currents generated by the heart are commonly measured by electrodes placed on the body surface and the resulting tracing is called an electrocardiogram (ECG).

What is ECG signal?

volume conductor

Entire process of depolarization and repolarization

4

Wireless ECG sensor What is ECG signal?

P wave: the sequential activation (depolarization) of the right and left atria

QRS complex: right and left ventricular depolarization

ST-T wave: ventricular repolarization

5

Wireless ECG sensorConventional ECG sensor1. Standard ECG: 12 leads, specially for hospital use.

Advantage: Standard and comprehensive ECG signalsDisadvantage: Short sampling and can not detect irregular or intermittent arrhythmia

2. ECG holter: 3 or 5 leads for long-term monitoringDisadvantage: Receive power from an electrical outlet, outstanding heavy and not

essentially portable or wearable

Objective:1. A medical grade and battery-supplied

Lead I ECG sensor 2. Can be unobtrusively worn over a

period of several days3. Continually log heart rate data, provide

detection of life threatening events, such as arrhythmia

6

Wireless ECG sensorInput noisy ECG signal

Low-Pass FIR FilterRemove high frequency

noise, such as power line interference

High-Pass FIR Filter

Remove low frequency noise, such as EMG

noise (muscle), electrode contact noise

U3 algorithm Remove base line drift

QRS peak detection

Characterization of QRS width, height, R-R interval

Arrhythmia detection

0 500 1000 1500 2000

Samples

ECG Signal

U3 Response

ECG Electrodes

Battery

Bluetooth module

MCU board

General processing

Fabricated ECG sensors based on TI MSP430FG439

Two square pads used as electrodes for testing

7

Detectable ECG abnormalitiesRate (bpm) and R-R Interval (sec)

Normal ~60-90 bpm corresponds to ~ 0.66 – 1sec Tachycardia > 90 bpm corresponds to < 0.65 sec Bradycardia < 60 bpm corresponds to > 1 sec

Rhythm Difference between the Longest and Shortest R-R

interval detected within 3 sec is Irregular if > 0.12 sec,which indicates

AV block Atrial Fibrillation

QRS Width Normal = 0.06 – 0.10 sec 0.1 – 0.12 sec indicates

Wolff-Parkinson-White syndrome (WPW) Non-specific intraventricular conduction delay (IVCD) Incomplete right or left bundle branch block (RBBB or LBBB)

> 0.12 sec indicates Complete LBBB or RBBB Ventricular tachycardia

Q Wave Width > 0.04 sec or/and height > 25% of R’s height

indicates Myocardial Infarction (MI)

QRS Complex

0.16 s

RBBB

Normal Regular

TachycardiaIrregular

0.59 s0.89 s

0.65 s

BradycardiaIrregular

0.91 s0.91 s

1.08 s 1.08 s

Typical QRS

Q

R

S

Abnormal QRS Appear in MI and Hyperkalemia

Q

R

S

Q

R

Q is 37.5% of R

SpO2

8

AV (Atrio-Ventricular) block

ECG abnormalities

Atrial fibrillation

Exit

9

Wolff-Parkinson-White syndrome (WPW)

ECG abnormalities

Complete left bundle branch block (LBBB) Complete right bundle branch block (RBBB)

Exit

10

Saturation of Arterial Oxygen- SpO2

11

Blood and Hemoglobin (Hb)Circulation

Common carotid artery

Superior vena cava

Pulmonary vein

Inferior vena cava

Pulmonary artery

1 red blood cell: ~ 265 millionmolecules of hemoglobin

1 hemoglobin molecule: 4 heme and 4 globin units.

Each heme and globin unit can carry 1 molecule of oxygen

Hemoglobin changes color: Oxygenated (HbO2): bright red Deoxygenated (Hb): dark red

This color change is used to measure hemoglobin oxygen saturation.

12

Blood and HemoglobinDiffusion of oxygen

Feeder arteriole

Precapillary sphincter

Drainage venule

Tissue cells

Shunt True capillary

Once blood is oxygenated, although it may pass through oxygen depleted tissue, oxygen does not diffuse until it reaches the capillaries with one cell thickness in the wall.

Oxygen diffuses into the interstitial fluid and into the cells.

True capillary

Lymph capillary

Arteriole end

Venule end

Φ6-8μm

13

Definition of SO2

2

2

22

2

100% 100%HbO

HbO Hb

CHbOSO

HbO Hb C C

SO2: Saturation of Oxygen

Percent of oxygen present in the hemoglobin present in blood

SaO2: Arterial oxygen saturation (in arterial blood)

Normal range for a health adult: 95 - 100 %

SpO2: Oxygen saturation derived from pulse oximetry.

Non-invasive method

SvO2: Venous oxygen saturation (in venous blood)

The normal SvO2 is 75%, which indicates that under normal conditions, tissues extract 25% of the oxygen delivered.

14

Principle of SpO2

( )0

ClI I e

Beer’s law (Beer-Lambert’s law or Bouguer’s law)

lCε(λ): absorption coefficient of the substance at a specific wavelength λ.

C: concentration

l: optical path length

I0

I

1 1 2 2( ) ( )0

C l C lI I e lC1

I0

I

C2

15

Principle of SpO2

2 2 2 2 20

Hb aHb HbO aHbO t HbO vHb HbO vHbOC l C l A m C n C nI I e e e

Arterial Tissue Venous

Beer’s law for measurement of oxygen saturationεHb: absorption coefficient of HbεHbO2: absorption coefficient of HbO2

In arterial blood:

CaHb: concentration of Hb

CaHbO2: concentration of HbO2

In venous blood:CvHb: concentration of HbCvHbO2: concentration of HbO2

l

n

I0

I

Arterial blood

Tissue and capillaries

Venous blood

m

( l is variable in pulsed arterial blood)

16

2 2( ) ( )( ) ( )0

Hb Hb HbO HbODC DC DC C C lC lI I e e

timeOne cardiac cycle

Other tissue

Venous bloodNon-pulsating arterial blood

Pulsating arterial blood

I0

I

DC

AC

I0

IDC

Δl

( ) ( )0

DC DC DCC lDCI I e

2 2( ) ( )Hb Hb HbO HbOC C l

DC

IN e

I Normalization:

Principle of SpO2

Pulsation of the blood

Photodiode

LED

17

2 2 1

1

( 1) ( 1)Hb Hb HbO HbOC C lN e

Principle of SpO2

Ratio of normalized signals

Wavelength λ1,

2 2 2

2

( 2) ( 2)Hb Hb HbO HbOC C lN e

Wavelength λ2,

1

2

1 2 1 2 1

2 2 2 2 2

ln ( ) ( )

( ) ( )lnHb Hb HbO HbO

Hb Hb HbO HbO

N C C lR

C C lN

Ratio

When the optical path lengths for the two wavelengths (λ1 λ2) are equal, Δlλ1 = Δlλ2

2 1 2

22 1 2 1 2 2 2

( ) ( )100%

( ) ( ) ( ) ( )HbO Hb Hb

Hb HbO Hb HbO Hb HbO

C RSpO

C C R

LED2

Photodiode

LED1

18

Principle of SpO2Criteria for the choice of wavelengths

500 600 700 800 900 1000

16

12

8

4

0

Absorptivity

Wavelength (nm)

εHbO2

εHb

660nm 940nm

The red skin pigmentation absorbs a great amount of light at wavelengths shorter than 600 nm.

wavelength > 600 nm

Large differences in the absorption coefficients of Hb and HbO2

To get high sensitivity

Flatness of absorption spectra

To get high stability

Hemoglobin absorbance spectra

19

Principle of SpO2SpO2 and R

0 0.5 1 1.5 20

0.2

0.4

0.6

0.8

1

1.2

R

SpO

2660 nm / 940 nm

20

Measurement of R

1

2

lnln ln

lnRmin IRmin

Rmax IRmax

N I IR

I IN

Red transmittance Infrared transmittance

Lig

ht

inte

nsi

ty

Lig

ht

inte

nsi

ty

IRmin

IRmax IIRmax

IIRmin

Logarithmic method

Photo-plethysmogram (PPG) waveforms

21

Measurement of R

1 1 ( )0 0

l L tI I e I e

Lig

ht

inte

nsi

ty

Imin

Imax

Derivative method

( )l L t

1

1

( )0 ( )

0 1 1

( ) ( )L t

L td I edI dL t dL t

I e Idt dt dt dt

1

( )dI dt dL t

I dt

1 2 1 2 1

2 2 2 2 2

( ) ( )

( ) ( )Hb Hb HbO HbO Rmax Rmin IRmax IRminR IR

R IR Rmax Rmin IRmax IRminHb Hb HbO HbO

C C I I I IdI dt dI dtR

I I I I I IC C

1 2 1 2( ) ( )0

Hb Hb HbO HbOC C lI I e

1 1 2 1 2( ) ( )Hb Hb HbO HbOC C Let

We have

Its derivative is

Δt

At point M, 2 2min min max min maxI I I I IdI I

dt t t t

one cardiac cycle

(Imax+Imin)/2M

2max minI I I

For λ1,

22

Measurement of R

Logarithmic method Derivative method

Complicated calculation

Requires powerful computation capability

Simple calculation

Suitable for real-time computation in microcontroller applications

ln lnRmin IRmin

Rmax IRmax

I IR

I I

Comparison

Rmax Rmin IRmax IRmin

Rmax Rmin IRmax IRmin

I I I IR

I I I I

23

History of SpO2 1982, pulse oximetry was developed by Nellcor (William NEw,

Jack LLoyd and Jim CORenman).

1983, pulse oximetry was introduced into the US operating room market.

By 1987, pulse oximetry was included in the standard of care for the administration of a general anesthetic in the US.

Now, pulse oximetry is widely used in hospitals.

The current researches are focused on:

Noise reduction and signal processing reduction of motion artifact noise

Venous pulsation

Low perfusion

Portable and long-time wearable SpO2

Sensor fusion

24

SpO2 Products

25

Limitation of SpO2• Motion artifact• Pulsed venous blood• Low perfusion states• Abnormal hemoglobins (primarily carboxyhemoglobin [COHb]

and met-hemoglobin [metHb])• Intravascular dyes• Exposure of measuring probe to ambient light during

measurement• Skin pigmentation• Inability to detect saturations below 83% with the same

degree of accuracy and precision seen at higher saturations• Inability to quantitate the degree of hyperoxemia present

26

Limitation of SpO2Motion artifact

27

Limitation of SpO2• The amplitude of the plethysmographic wave form is directly proportional to the

vascular distensibility, which is significantly greater in the arterial system.• However, the venous signal can have significant impact on the calculation of the

SpO2 if it reaches the threshold for a pulsation.

Venous pulsation

1

1

( 1) lN e

2

2

( 2) lN e

1

2

1

2

ln ( )

( )ln

NR

N

1 1

1

( 1) ( 1)' l nN e

2 2

2

( 2) ( 2)' l nN e

1

2

1 1 1 1

2 2 1 1

ln ' ( ) ( )'

( ) ( )ln '

N n lR

n lN

1 2l l 1 2l l 1 2n n

No venous pulsation: With venous pulsation:

'R R 2 2 'SpO SpO

1 2( ) ( ) Assuming CHbO2= 75%, CHb= 25% in venous blood, for 660nm/940nm LEDs,

28

Limitation of SpO2

• The venous pulsation cause the measured SpO2 value lower than the real value.

Venous pulsation

SpO2 measured from fingertip

0 20 40 60 80 100 1200.5

0.6

0.7

0.8

0.9

1

1.1

Time (sec)

SpO

2

Venous blood is accumulated in fingertip

Venous blood is minimized

29

Limitation of SpO2

• Received light intensity is determined by position of LED emitter.

Low perfusion

Cross section of fingertip

LED position 1(correct position)

Bone

LED position 2 (inappropriate position)

Artery

Vein

Position 1: normal signal Position 2: weak and noisy signal (low perfusion)

Red

Infrared

Red

Infrared

• Signal from photodiode (Photo-plethysmogram waveform,PPG):

30

Measurement Position

Fingertip Earlobe

Severe motion artifact noise

Problem of venous pulsation

Problem of low perfusion (inappropriate sensor position -> weak signals)

Long-time wearable: it may influence human activity

31

Prototype of SpO2

MCU board

Thermopile

SpO2 Probe

Bluetooth

32

Prototype of SpO2Hardware

LED(660nm) LED(940nm)

Photodiode

Mic

roco

ntro

ller

3.2mm

Red/IR bicolor LED:

Photodiode (light converter):Light-to-Frequency

Light-to-Voltage

Light intensity

Vo

lta

ge

(digital output)

(analog output)

light intensity: low high

PDA/Phone

Blu

etoo

th

Fingertip/earlobe

33

Signal processing

0 20-20-10

0

Frequency (Hz)

Magnitude Response (dB)

General processing

IR

red

MAX

MIN

Interval

U3

IR

RED

Band-pass

Segmentation

IR

Band-pass

Max Min extraction

SpO2

0 20-20-10

0

Frequency (Hz)

Magnitude Response (dB)

Signal processing

One cardiac cycle

FIR filter

Raw data with high-frequency noises (ambient light, circuit, etc)

Smooth data without high-frequency noises Motion

artifact

Venous pulsation

Low perfusion

34

Blood Pressure

35

Blood PressureDefinition of blood pressure

Blood pressure is the pressure exerted by the blood at the normal direction to the walls of the blood vessels.

Blood pressure usually refers to systemic arterial blood pressure, i.e., the pressure in the large arteries delivering blood to body parts.

Systolic pressure is defined as the peak pressure in the arteries during the cardiac cycle.

Diastolic pressure is the lowest pressure at the resting phase of the cardiac cycle.

Blood pressure

36

Blood PressureKorotkoff method (1905)

No sound when blood is flowing through smooth vessel (normal condition)

Sound is generated when blood is passing through interrupt changed cross section.

1) The artery is completely occluded. No blood flow, no sound.

2) Sound is listened when blood just starts to flow in the artery.

The first sound: cuff pressure = systolic BP

3) Silent when cuff pressure drops below the diastolic blood pressure.

Cuff pressure = diastolic BP

Add pressure to cuff

Release pressure slowly

Release pressure slowly

Flow

37

Blood Pressure• Oscillometric method is functionally the same as for the auscultatory

method. It also requires a cuff.

• An electronic pressure sensor (transducer) fitted in the cuff detects blood flow.

Oscillometric method (1970)

1) Artery is completely occluded: cuff pressure > systolic BP2) Blood flow is unimpeded: cuff pressure < diastolic BP

cuff pressure is constant.

3) Blood flow is present: diastolic BP < cuff pressure < systolic BP

cuff pressure varies periodically in synchrony with the cyclic expansion and contraction of the artery, i.e., it oscillates.

Blood Pressure

Time cuff pressure: constant oscillate constant

Cuff

38

Blood Pressure21

2Fd mv mgh

Cuff-less method using PTT (pulse transit time)

m

F

h

d

v

F BP a

21

2

m mBP v gh

ad ad

m

ad

a: cross section of artery

ΔBP: pressure difference

ρ: density of blood, 1035 kg/m3

g: 9.8 m/s2

dv

PTT PTT: duration while m moves distance d

21 1

0.7 0.7 2

BP dBP gh

PTT

0.6h m

Examples: 0.4h m 0.4d h m (1) Heart - Fingertip

(2) Heart - Fingertip

(3) Heart - Earlobe

Systolic

39

Blood Pressure

0 500 1000 1500 2000 2500 30001

1.5

2

2.5

3

3.5

4x 10

4

PTT

ECG

0 500 1000 1500 2000 25002.3

2.35

2.4

2.45

2.5

2.55

2.6

2.65

2.7x 10

4

PTT extracted from PPG(SpO2) and ECG

(2)

(3)

(1)

PPG graph:Point 1: Greatest light intensity -> minimum blood volume in arterySlope 2: Fresh arterial blood is filling artery due to heart contraction. Point 3: Lowest light intensity -> maximum blood volume

Ligh

t in

tens

ity

Time (ms)

PPG

40

Demonstration

SpO2 & Temperature data packet

GSM

Bluetooth

Bluetooth

Bluetooth

ECG Electrodes

SpO2 ProbeLight-to-

frequency photodetector

IR/R LED driver and control

TI MSP430F1611

Thermopile

ECG Sensor

Integrated SpO2 & Temperature Sensor

Central Processing

Unit

(Deriving blood pressure and saving all vital

signs)

ECG data packet

Physiological signs acquisition module

Real-time Display

Alarm

Local storage

Patient’s PDA phone

Offline data display

Alarm

Local storage

Doctor’s PDA phone Sentinel vital signs

Command for real-time display / Sentinel events

Lead ITI MSP430FG439 Analog front-end

Block diagram of the implemented MEMSWear-Biomonitoring System

ECG data packets (bytes)

E GEight ECG signals Heart beat type

Peak position

SpO2 data packets (bytes)

SIR RED

Peak/non-Peak

T

Temperature

O

Heat beat rate

Sampling rate: ECG (512Hz), SpO2 (64Hz)

41