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Academia Industry Meet, Mumbai, India, June 27, 2003 A THORACIC IMPEDANCE SIMULATOR FOR IMPEDANCE CARDIOGRAPHY Vinod K Pandey (PhD) Manigandan NS (MTech) Guide: Prof PC Pandey BME Group & EE Dept, IIT Bombay Abstract
A THORACIC IMPEDANCE SIMULATOR FOR
Vinod K Pandey (PhD)
Manigandan NS (MTech)
Guide: Prof PC Pandey
BME Group & EE Dept, IIT Bombay
Impedance cardiography is a non-invasive technique for measuring cardiac output and for diagnosing cardiac disorders. During systol, blood is pumped into the thoracic region which changes its basal impedance. High frequency (20 – 100 kHz) current (3 – 10 mA) is injected into the thoracic region through electrodes and the impedance change is sensed, by measuring the voltage through another set of electrodes. The impedance variation is known as impedance cardiogram (ICG) and can be used for estimating stroke volume by using appropriate models of blood flow and can also be used for diagnostic information.
Impedance cardiography R&D involves instrumentation development, study of electrode configurations, model development and validation through clinical trials. We have developed a thoracic impedance simulator for comprehensive testing and calibration of an impedance cardiograph. It has the facility for varying the beat rate, magnitude of common and differential mode ECG signals and a fixed percentage variation in the thoracic impedance as a square wave. Option for feeding external pickups is also incorporated.
• A non-invasive technique for measuring cardiac output and for diagnosing cardiac disorders.
• Senses variation in thoracic impedance caused by blood pumped during systol.
• High frequency (20 – 100 kHz) current (3 – 10 mA) injected, resulting voltage sensed using 4-electrode configuration, and the detected impedance variation (< 2% of basal impedance), known as impedance cardiogram (ICG), used for estimating stroke volume & diagnostic information.
•Study of electrode configurations
•Model development for signal processing and validation through clinical trials
Needed for testing and calibration of impedance cardiograph hardware
• Variable beat rate
• Variable ECG
• Thoracic impedance variation as a square wave (a fixed % of base impedance)
• External pickup simulation
I1, I2 : Current exc. electrodes
E1, E2 : Voltage sens. electrodes
Re’s : Contact resistance
Rs1, Rs2 : Exc. path fixed
Ed : Diff. mode ECG
Ec : Com. mode ECG
Ep : Ext. pickup
Rp : Pickup resistance
Ro, Rs : Parallel column model
R1 = Rs1 + Re1 R2 = Re2
R3 = Re3 R4 = Re4 + Rs2
Ry1´ = Ry1 || ( Rz + ( Ry2 || Rx2 ) )
Ry2´ = Ry2 || ( Rz + ( Ry1 || Rx1 ) )
V1 = Ex1 [ Ry1´ / ( Rx1 + Ry1´ ) ] = Ec + Ed / 2
V2 = Ex2 [ Ry2´ / ( Rx2 + Ry2´ ) ] = Ec - Ed / 2
Ra = Rb = Rs / 2
Rz = Ro
• Operation modes : ECG or ICG
• Waveform : square wave
• Repetition rate :
0.4 – 5 Hz ( 24 – 300 beats/min. )
• ECG Modes (CM/DM) :
Common mode: 0 – 60 mV with < 1 mV diff.
Diff. Mode: 0 – 20 mV with < 2 mV common
• Base impedance : 19.8
• Impedance variation : 0.17
• Power source : 9 V, 10 mA
Thoracic impedance simulator developed for testing and calibration of ICG detection and ECG extraction circuits in impedance cardiograph.