1. An Embedded Design Example: A Thorax Simulator for Testing and Calibration of Impedance Cardiographs by P. C. Pandey EE Dept., IIT Bombay December 2008

2. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 2/19 Thorax simulator Presentation Overview ▪ Introduction ▪ Thorax simulator model ▪ Simulator circuit : embedded design ▪ Results ▪ Conclusion

3. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 3/19 Thorax simulator 1. Introduction To embed To 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

4. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 4/19 Thorax simulator 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 Design A 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

5. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 5/19 Thorax simulator Bioimpedance Sensing Sensing 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

6. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 6/19 Thorax simulator 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

7. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 7/19 Thorax simulator Impedance Cardiograph Impedance detector Voltage sense amplifier Demodulator Drift cancellation circuit Introduction 5/6

8. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 8/19 Thorax simulator 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

9. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 9/19 Thorax simulator I1 and I2 : Current injection terminals E1 and E2: Voltage sensing terminals R e ’s : Tissue-electrode impedances R s 1 and R s 2 : Tissue impedances (fixed) R s || R o : Thoracic impedance V d and V c : DM and CM voltages V p : Common mode interference 2. Thorax simulator model Thorax simulator 1/2

10. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 10/19 Thorax simulator Schematic of thorax simulator Thorax simulator 2/2 where Model relations with the schematic

11. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 11/19 Thorax simulator 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

12. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 12/19 Thorax simulator Impedance variation Basal resistance: 20 – 200 Ω % Variation: < 2 % If R = 200 Ω and ∆R = 4 Ω then α = 50 Simulator circuit 2/6

13. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 13/19 Thorax simulator Thorax simulator circuit Simulator circuit 3/6

14. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 14/19 Thorax simulator The controller and power supply circuit Simulator circuit 4/6

15. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 15/19 Thorax simulator The controller circuit Microcontroller AT89S52 8 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 Simulator circuit 5/6

16. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 16/19 Thorax simulator Parameter selection Sr.ParametersOptions 1Mode (ECG only)CM / DM 2TypeSine / Square / ECG 3Magnitude (ECG only)0  100 mV (16 steps) 4Frequency1, 2, 4,.. 256 Hz (16 steps) 5Resistance Variation 0.1  1.2 % of base resistance (12 steps) Simulator circuit 6/6

17. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 17/19 Thorax simulator 4. Results ▪ Simultaneous simulation of bioimpedance and ECG ▪ ECG CM: 50 - 100 mV, with < 0.2 mV DM voltage DM: 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

18. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 18/19 Thorax simulator 5. Conclusion A thorax simulator developed using embedded design approach for testing and calibration of the instruments for impedance cardigraphy

19. pcpandey@ee.iitb.ac.in IntroIntro. Simulator circuit Results & con. 19/19 Thorax simulator References [ 1] J. Nyboer, Electrical Impedance Plethysmography, 2 nd ed., Charles C. Thomas, Springfield, Massachusetts, 1970. [ 2] L. Cromwell, F. J. Weibell, and E. A. Pfeiffer, Biomedical Instrumentation and Measurements, 2 nd 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. 2 nd 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. 28 th 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. 28 th 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.