Bio Monitoring 01

Overview of system

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. 5. 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 1 wirelessly

Electrocardiogram (ECG)

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Wireless ECG sensor v What is ECG signal? As the heart undergoes depolarization and repolarization, electrical currents spread throughout the body because the body acts as a volume conductor. The electrical

volume

Entire process of depolarization and repolarization

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Wireless ECG sensor v 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

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Wireless ECG sensor Conventional ECG sensor

1. Standard ECG: 12 leads, specially for hospital use. Advantage: Standard and comprehensive ECG signals Disadvantage: Short sampling and can not detect irregular or intermittent arrhythmia 2. ECG holter: 3 or 5 leads for long-term monitoring Disadvantage: 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 14.Can be unobtrusively worn over a period of several days 3. Continually log heart rate data, provide detection of life threatening events, such as arrhythmia 5

Wireless ECG sensor General processing

Two square pads used as electrodes for testing

Fabricated ECG sensors based on TI MSP430FG439 6

Detectable ECG abnormalities Rate (bpm) and R-R Interval (sec) Ø Normal ~60-90 bpm corresponds to ~ 0.66 – 1sec Ø Tachycardia > 90 bpm corresponds to < 0.65 sec Ø Bradycardia < 60 bpm corresponds to > 1 sec

0.91 s

Normal Regular

0.91 s

Rhythm 0.59 s Ø 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

0.89 s

1.08 s

Ø Normal = 0.06 – 0.10 sec Ø 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 ) Ø > 0.12 sec indicates § Complete LBBB or RBBB § Ventricular tachycardia

Tachycardia Irregular

0.65 s

Bradycardia Irregular

1.08 s

R

Q Wave Ø Width > 0.04 sec or/and height > 25% of R’s heightQ indicates Myocardial Infarction (MI) R R

QRS Complex Typical QRS Q

S S

Q

0.16 s RBBB

Q is 37.5% of R

Abnormal QRS Ø Appear in MI and Hyperkalemia 7 SpO 2

ECG

abnormalities AV (Atrio-Ventricular) block

Atrial fibrillation

8 Exit

ECG

abnormalities Wolff-Parkinson-White syndrome (WPW)

Complete left bundle branch block (LBBB)

Complete right bundle branch block (RBBB) 9 Exit

Saturation of Arterial Oxygen - SpO2

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Blood and Hemoglobin (Hb)

Circulation

v1 red blood cell: ~ 265 million molecules of hemoglobin Common carotid artery Superior vena cava Pulmonary vein Inferior vena cava

Pulmonary artery

v1 hemoglobin molecule: 4 heme and 4 globin units. Each heme and globin unit can carry 1 molecule of oxygen vHemoglobin changes color: Oxygenated (HbO2): bright red Deoxygenated (Hb): dark red This color change is used to measure hemoglobin oxygen saturation.

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Blood and Hemoglobin

Diffusion of oxygen Feeder arteriole

Precapillary sphincter

Tissue cells

Drainage venule

v 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. v Oxygen diffuses into the interstitial fluid and into the True capillary

Shunt

True capillary

Arteriole end

Φ6-8μm

Venule end

Lymph capillary 12

Definition of SO2 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 13 oxygen delivered.

Principle of SpO2 Beer’s law (Beer-Lambert’s law or Bouguer’s law) I 0

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

l

C I

C: concentration l: optical path length

I0 C1 C2

l I 14

Principle of SpO2

Beer’s law for measurement of oxygen εHb: absorption coefficient of Hb saturation I0

εHbO2: absorption coefficient of HbO2

Arterial blood

l

Tissue and capillaries

m

Venous blood

n I

Arterial

Tissue

In arterial blood: CaHb: concentration of Hb CaHbO2: concentration of HbO2 In venous blood: CvHb: concentration of Hb CvHbO2: concentration of HbO2

Venous

( l is variable in pulsed arterial blood) 15

Principle of SpO2 Pulsation of the blood I0 AC

I0 Δl

LED

Pulsating arterial blood Non-pulsating arterial blood Venous blood

DC

Other tissue

Photodiode

time One cardiac cycle

Normalization :

ID

I

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Principle of SpO2 Ratio of normalized signals

LED1 LED2

Wavelength λ1, Wavelength λ2,

Photodiode

Ratio

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

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Principle of SpO2

Criteria for the choice of wavelengths 16

Absorptivity

εHbO2 εHb

12

8

v The red skin pigmentation absorbs a great amount of light at wavelengths shorter than 600 nm. wavelength > 600 nm v Large differences in the absorption coefficients of Hb and HbO2 To get high sensitivity v Flatness of absorption spectra

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660nm

940nm

0 500

600

700

800

900

1000

Wavelength (nm)

Hemoglobin absorbance spectra 18

Principle of SpO2

SpO2 and R

660 nm/ 940 nm 1.2 1

SpO2

0.8 0.6 0.4 0.2 0 0

0.5

1 R

1.5

2

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Measurement of R

Logarithmic method

IRmax IRmin

Infrared transmittance Light intensity

Light intensity

Red transmittance

IIRmax IIRmin

Photo-plethysmogram (PPG) waveforms

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Measurementoneof R cardiac cycle

For λ1, Let

We have

Light intensity

Derivative method

Imax

M

(Imax+Imin)/2 Imin

Δt

Its derivative is

At point M,

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Measurement of R

Comparison

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History of SpO2

v 1982, pulse oximetry was developed by Nellcor (William NEw, Jack LLoyd and Jim CORenman). v 1983, pulse oximetry was introduced into the US operating room market. v By 1987, pulse oximetry was included in the standard of care for the administration of a general anesthetic in the US. v Now, pulse oximetry is widely used in hospitals. v The current researches are focused on: v Noise reduction and signal processing v reduction of motion artifact noise v Venous pulsation v Low perfusion

v Portable and long-time wearable SpO2 v Sensor fusion 23

SpO2 Products

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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 25

Limitation of SpO2

Motion artifact

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Limitation of SpO2

Venous pulsation • •

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. No venous pulsation:

With venous pulsation:

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

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Limitation of SpO2 Venous pulsation The venous pulsation cause the measured SpO2 value lower than

1.1 1 0.9 SpO2

•

0.8 0.7 0.6 0.5 0

Venous blood is accumulated in fingertip

20

40

60 Time (sec)

80

SpO2 measured from fingertip

100

120

Venous blood is minimized 28

Limitation of SpO2 Low perfusion •

Received light intensity is determined by position of LED LED position 1 LED position 2 (correct position) (inappropriate position) Artery

Bone

Vein

Cross section of fingertip •

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

Red

Infrared

Infrared

Position 1: normal signal

Position 2: weak and noisy signal (low perfusion)

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Measurement Position

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Prototype of SpO2

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Prototype of SpO2

Hardware

Mi cr oc on tro lle r

Fingertip/ earlobe

Bl ue to ot h

LED(660nm) LED(940nm)

PDA/Phone

Red/IR bicolor LED: 3.2mm

Photodiode

Light-to-Voltage (analog output)

Voltage

Photodiode (light converter): Light-to-Frequency (digital output) light intensity: low high

Light intensity

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Signal processing

General processing

FIR filter

v v

v

Motion artifact Venous pulsatio n Low perfusio

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

Smooth data without highfrequency noises

One cardiac cycle

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Blood Pressure

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Blood Pressure

Definition of blood pressure

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

Blood pressure

v Blood pressure usually refers to systemic arterial blood pressure, i.e., the pressure in the large arteries delivering blood to body parts. v Systolic pressure is defined as the peak pressure in the arteries during the cardiac cycle. v Diastolic pressure is the lowest pressure at the 35

Blood Pressure

Korotkoff 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.

Add pressure to cuff Flow

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

2) Sound is listened when blood just starts to flow in the artery. The first sound: cuff pressure = systolic BP Release pressure slowly

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

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Blood Pressure

Oscillometric method (1970)

• 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. 1) Artery is completely occluded: cuff pressure > systolic BP 2) 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. Cuff Blood Pressure

Time

cuff pressure: constant

oscillate

constant

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Blood Pressure

Cuff-less method using PTT (pulse transit time) g: 9.8 m/s2 v

a: cross section of artery ΔBP: pressure difference d h m

F Examples:

ρ: density of blood, 1035 kg/m3 PTT: duration while m moves distance d

Systolic (1) Heart Fingertip (2) Heart Fingertip

(3) Heart Earlobe 38

Blood Pressure

PTT extracted from PPG(SpO2) and ECG PPG graph: Point 1: Greatest light intensity -> minimum blood volume in artery Slope 2: Fresh arterial blood is filling artery due to heart 4 contraction. x 10 4

ECG

3.5

3

Light intensity

2.

x 10

PTT (1)

4

PPG

2 .65 2.

(2)

2.5

2 .55 2. 5 2

2

2.

(3)

2

2. 1.5

0

50 0

1 000

1 500

2 000

2 500

Time (ms) 39

1

Demonstration

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

Block diagram of the implemented MEMSWear-Biomonitoring System

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