CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 406

Figure 8.25 Circuit for the dual opposing capacitor discharge intracardiac impedance sensor core. C a 1 (C39) is charged to 1.2 V through
IC9D and C a 2 (C47) is charged to 1.2 V through IC9A with ground established through IC9C. IC9D, IC9C, and IC9A then open while
IC10C connects the reference terminal C39 and C47 to one of the intracardiac electrodes. C a 1 discharges into the heart through IC10D and
C48 for 10 µs. C a 2 then discharges for 10 µs by way of IC10A, C48, and IC10C. IC10C is then opened and IC9C is closed. The differential
compensated impedance measurement is generated by instrumentation amplifier IC11. A sample-and-hold circuit (IC9B, C44, and IC8B)
holds the measurement. The impedance signal is scaled and filtered by IC8C.

Here most of the active signal-processing circuitry (bu ff ers and di ff erential ampli fi er) are

replaced by a switched-capacitor di ff erential to a single-ended converter, which also acts

as a sample-and-hold circuit.
Figure 8.30 shows some impedance signals obtained through an instrument that uses
the sensor circuit of Figure 8.24. These signals were acquired from a human subject using
the electrode con

guration shown in Figure 8.30 b . Theoretical and experimental studies
[Hoekstein and Inbar, 1994] have shown that the largest contributor to the impedance sig-
nal detected through pacing electrodes is the near-

fi

fi eld movement of the cardiac walls in

the largely inhomogeneous fi field around the distal electrode.