An Embedded Design Example: A Thorax Simulator for Testing and Calibration of Impedance Cardiographs...

An Embedded Design Example: A Thorax Simulator for Testing and Calibration

of Impedance Cardiographs by

P. C. Pandey

EE Dept., IIT BombayDecember 2008

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Intro. Simulator circuit Results & con. 2/19Thorax simulator

Presentation Overview

▪ Introduction ▪ Thorax simulator model ▪ Simulator circuit : embedded design▪ Results▪ Conclusion

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1. Introduction To embedTo fix firmly in the surrounding mass or environment.

System A set of connected things or parts that form a whole or work together. A set of rules, practices, or principles forming a philosophy or government A method of classification, notation, or measurement.

Embedded Electronic System System with a strong coupling with its surroundings Very tight coupling between I/O, data processing, memory Generally real-time processing and control

Introduction 1/6

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Embedded System H/W Chip count minimization High reliability Compact size Power management

Embedded System S/W Modest ROM and RAM, often no secondary storage Single or limited number of application programs

DesignA plan of detailed steps or drawings for making something Problem specification Conceptualization of parts and interconnections (h/w and s/w partitioning) Simulation Construction or Assembly Testing and Result Analysis Preparation of Design Document

Introduction 2/6

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Bioimpedance SensingSensing the variation in the impedance across a body segment for non-invasive monitoring of the changes in the fluid volume or underlying physiological events.

Impedance plethysmography: Impedance changes in a body segment

Impedance glottography ( electroglottography): Impedance across the larynx for estimation of the variation in the degree of contact between the vibrating vocal folds during speech production.

Impedance cardiography: Thoracic impedance during the cardiac cycle for estimating the stroke volume and the cardiac output.

Introduction 3/6

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Measurement Method

A current (20 kHz - 1 MHz, < 5 mA) passed through a pair of surface electrodes and the resulting amplitude modulated voltage sensed using the same or another pair of electrodes.

Electrode configurations- Two-electrode configuration - Three-electrode configuration (one guard electrode)- Four-electrode configuration (a more uniform current density, reduced effect of the skin-electrode impedance)

Instrumentation Challenges

- Detection of extremely low modulation index (0.2-2 %)- Rejection of interference from other sources (external sources, bioelectric sources, artifacts)

- Testing and calibration (sensitivity, frequency response)

Introduction 4/6

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Impedance detector

ECG extraction

circuit

Current source

Differentiator

Zo(t)

dZ/dt

ECG

Impedance Cardiograph

Impedance detector• Voltage sense amplifier• Demodulator• Drift cancellation circuit

Introduction 5/6

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Methods for testing and calibration

A cardiograph instrument with an internal resistance connected across the electrode terminals for calibration of the current source.

Instrument with testing mode: amplitude modulated current to simulate the modulation of the sensed voltage due to time varying impedance.

Thorax simulator - Time varying impedance for testing of sensitivity and frequency response of the impedance detector- Internal voltage source for measuring the DM gain and CMRR of the ECG amplifier in the impedance cardiograph - External source for measuring the rejection of external interference.

Introduction 6/6

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I1 and I2 : Current injection terminals

E1 and E2: Voltage sensing terminals

Re’s : Tissue-electrode

impedancesRs1 and Rs2 : Tissue

impedances (fixed) Rs || Ro : Thoracic impedance

Vd and Vc : DM and CM

voltages Vp : Common mode

interference

2. Thorax simulator model

Thorax simulator 1/2

Rp

Int. Com.

I1

E1

E2

I2

+ _

Ext.Gnd

Cardiac Cycle

+

+

Ro

2

Ro

2

Vd

2

VcRs

Rs2

Rs1

Vp

Vd

2

Re1

Re2

Re3

Re4

_

_

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Schematic of thorax simulatorThorax simulator 2/2

Rp

Int. Com.

I1

E1

E2

I2

Ext.Gnd

Ry1

Ry2

Vp

Ra

Rb

Rc

Rd

Rx1

Rx2

Vx1

Vx2

Cardiac Cycle

Rv

+

_

+

_

1 1R R Rs e a 1 1R R Rs e a

2R Re b 3R Re c 2 4R R Rs e d

0 1 2 1 2R || R R R || R || R Rs y y v x x

2 1 1 1 1' 'V V / R / R R Vc d y x y x

2 2 2 2 2' 'V V / R / R R Vc d y x y x

where

1 1 2 2'R R || R R || Ry y v x y

2 2 1 1'R R || R R || Ry y v x y

Model relations with the schematic

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3. Simulator Circuit : Embedded Design

Desired Features

● Parameter selection without wiring related pick-ups● Operation with single supply voltage

Realization of Thorax Simulator ● Impedance variation using digital potentiometer

● ECG (DM and CM) using two digital potentiometers ● Parameter selection and waveform generation by microcontroller ● Split power supply for analog circuit

Simulator circuit 1/6

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Impedance variation

R αR Req

0

50

100

150

200

250

0 15 30 45 60 75 90 105

120

135

150

165

180

195

210

225

240

255

Descrete steps of R3

Req

11R Req

RR

Basal resistance: 20 – 200 Ω % Variation: < 2 % If R = 200 Ω and ∆R = 4 Ω then α = 50


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Thorax simulator circuit


R3

R2

R4 R6

R8

R9

R7

R5

IC4B

IC4C

I1

I2

E2

R24 R21

R23 R20

R14 R15

IC8 CD4066

1

2

13

7 JP5 JP6

R13

P3.3

Rpot

CS

SDI

SCKL

CS

SDI

SCKL

A

C

E

C

A

D

IC2 MCP4105

IC3 MCP4105

1

3

2

2

3

1

A

W

B

5

6

7

5

6

7

A

W

B

VCC+

VCC-

EpR16

Ex1

Ex2

9

10

8

6

5

VCCGND84

C10

0.1µ

VSS

VCC

VSS

14

7

A

B

EN

VDD

+

–

–

+

E1

33K

15K15K

33K

2.2K

2.2K

2.2K

100

100

5K

220

220

220

220

15033

3.3K

CS

SDI

SCKL

B

A

C

IC5 MCP4105

1

3

26

7

5

A

W

B

R103.3K

C11

VSS

0.1µVDD

C12

VSS

0.1µVDD

4 8GND VCC

VCCGND84

C13

VSS

0.1µVDD

Ex

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CC

4.7 k

4.7 k

5

R1

R19

R22

1

2345

ABCDE

VCC+

In Out

Com

1

2

3

C60.1µ

R114.7M

R12

4.7MC70.1µ

C80.1µ

AGnd

2

3

1

IC77805

VCC-

In Out

ComS1

C9

0.1µ

1

C5

0.1µ2

3IC6

7805

C41µ

9V

C16

0.1µ

1

2Sync. Test Outputs

Digi. Pot. Control signals

IC4

+

–

IC4ALM324

VDD

VDD

VDD

VDD

4

11

10µ

0.1µ

LCD1

The controller and power supply circuit


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The controller circuit

Microcontroller AT89S528 Kbytes ROM, 256 bytes RAM, 3 timers, In-system programmable

Digital potentiometers MCP 4105 (SPI controlled)

Controls: P1.0 : CLK, P1.2 : Data, P1.1, P1.3, P1.4 : Individual IC select

Lookup tables: various wave shapes (sinusoidal, square, ECG)

Synchronization test outputs: Port pins P3.6 and P3.7

User Interface: 2 line x 16 char. LCD display, two soft keys

LCD : 4-bit parallel (P2.7-P2.4), CS, RS, R/W - GND (unidirectional data flow)

Soft keys: P0.0 and P0.1 with s/w debouncing


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Parameter selection

Sr. Parameters Options

1 Mode (ECG only) CM / DM

2 Type Sine / Square / ECG

3 Magnitude (ECG only) 0 100 mV (16 steps)

4 Frequency 1, 2, 4,.. 256 Hz (16 steps)

5 ResistanceVariation

0.1 1.2 % of baseresistance (12 steps)


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4. Results ▪ Simultaneous simulation of bioimpedance and ECG ▪ ECG

CM: 50 - 100 mV, with < 0.2 mV DM voltageDM: 0 - 50 mV, with < 0.9 mV CM voltage

▪ Base resistance: 23.77 Ω, 28.24 Ω, 84.98 Ω,and 196.07 Ω

▪ Resistance variation: 0.1 - 1.2 % of selected base resistance

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5. Conclusion

A thorax simulator developed

using embedded design approach

for testing and calibration of

the instruments for impedance cardigraphy

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References

[1] J. Nyboer, Electrical Impedance Plethysmography, 2nd ed., Charles C. Thomas, Springfield, Massachusetts, 1970.[2] L. Cromwell, F. J. Weibell, and E. A. Pfeiffer, Biomedical Instrumentation and Measurements, 2nd ed., Prentice Hall, New Delhi, 1990.[3] A. K. Krishnamurthy and D. C. Childers, "Two-channel speech analysis", IEEE Trans. Acoustics, Speech Signal Proc., vol. ASSP - 34, pp. 730 - 743, 1986.[4] L. E. Baker, "Principles of impedance technique", IEEE Eng. Med. Biol. Mag., vol. 8, pp. 11 - 15, 1989.[5] R. P. Patterson, "Fundamentals of impedance cardiography", IEEE Eng. Med. Biol. Mag., vol. 8, pp. 35 - 38, 1989.[6] J. Wtorek and A. Polinski, "Multifrequency impedance plethysmograph", in Proc. IEEE Inst. Meas. Tech. Conf., Brussels, Belgium, 1996, pp. 1452 - 1455.[7] J. Fortin, W. Habenbacher, and A. Heller, "Non-invasive beat-to-beat cardiac output monitoring by an improved method of transthoracic bioimpedance measurement", Comp. Bio. Med., vol. 36, pp. 1185 - 1203, 2006.[8] F. Mora, G. Villegas, A. Hernandez, W. Coronado, R. Justiniano, and G. Passariello, "Design of an instrumentation system for a neurocardiology laboratory", in Proc. 2nd Int. Conf. IEEE Dev., Cir. and Sys., Carcas, Venezuela, 1998, pp. 272 - 277.[9] V. Vondra, J. Halamek, I. Viscor, and P. Jurak, "Two channel bioimpedance monitor for impedance cardiography", in Proc. 28th Annu. Int. Conf. IEEE Eng. Med. Biol. Soc., 2006, pp. 6061 - 6063.[10] G. Panfili, L. Piccini, L. Maggi, S. Parini, and G. Andreoni, "A wearable device for continuous monitoring of heart mechanical function based on Impedance Cardiography", in Proc. 28th Annu. Int. Conf. IEEE Eng. Med. Biol. Soc., 2006, pp. 5968 - 5971. [11] G. D. Jindal and J. P. Babu, "Calibration of dz/dt in impedance plethysmography" , Med. Biol. Eng. Comp., vol. 23, pp. 279 - 280, 1985.[12] V. K. Pandey, P. C. Pandey, and J. N. Sarvaiya, "Impedance simulator for testing of instruments for bioimpedance sensing", IETE Journal of Research, vol. 54, no. 3, pp. 203 - 207, 2008.[13] N. S. Manigandan, V. K. Pandey, and P. C. Pandey, "Thoracic simulator for impedance cardio‑ graphy", in Proc. National Symposium on Instrumentation (NSI-28), Pantnagar, India, 2003.[14] L. Venkatchalam / P. C. Pandey (Supervisor), “Development of hardware for impedance cardio‑graphy”, M.Tech. Dissertation, Biomedical Engineering Group, School of Biosciences and Bioengineering, Indian Institute of Technology Bombay, 2006.[15] B. M. Oliver, Square-wave and pulse testing of linear systems, in B. M. Oliver and J. M. Cage (Eds.), Electronic Measurements and Instrumentation, McGraw Hill, Singapore 1975.

An Embedded Design Example: A Thorax Simulator for Testing and Calibration of Impedance Cardiographs...

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