DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

402 CARDIAC PACING AND DEFIBRILLATIONVsrcRleadS1CHRHsRHPS3OUTS/H+CaCdc_blockLEADRionicRtissue(Cardiac, etc)-CpS4S5RpassViegmS/HS2HEARTVnoiseS1Switch ClosedSwitch OpenS2Switch ClosedS3S4S5Switch OpenTCCD = 10µsTCCD = 10µsS/HSampleHoldFigure 8.22 The CCD impedance sensor for implantable cardiac stimulators: (a) simplified circuit diagram of the sensor; (b) simplifiedtiming diagram. At the beginning of each measurement cycle, C a is charged to V src while C p is discharged. C p is then connected to the body,allowing it to sample the potential across the lead system for a brief interval t CCD . Immediately thereafter, C a is discharged across the leadsystem for the same amount of time t CCD . The subtraction of V Cp from V Ca is a value proportional to the tissue impedance,t CCDR C a ln{[V Ca (t CCD ) V Cp (t CCD )]/V src }In reality, however, other sources in the circuit (e.g., intrinsic electrical activity of the heart,electrode polarization potentials) have a strong effect on V Ca (t) and make the measurementof R imprecise.By using the voltage sampled in C p , the effects of these sources of error can be canceled.This compensation process is carried out by subtracting V Cp from V Ca before determiningthe resistive component R of the impedance:R tC a ln{[V Ca (t) V Cp (t)]/V src }