CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 395

<5mA

50kHz

Z(t)

Demodulator

AC
Current
Source

Voltage
Amplifier

Figure 8.15 An impedance cardiograph uses two pairs of gelled disk electrodes placed on the upper abdomen and upper neck to inject an
ac current, providing a more or less homogeneous coverage of the thorax with a high-frequency field. The voltage developed by the field is
detected through a second set of electrodes located at the level of the root of the neck and the diaphragm. The demodulated voltage is
inversely proportional to the content of fluids within the intrathoracic space. On a cardiac beat-by-beat basis, the majority of the impedance
signal ∆ Z ( t ) is caused by changes in aortic blood volume and blood velocity.

potentiometer R30 to cause the output current to have a sinusoidal waveform of su cient

amplitude (e.g., 100 A to 1 mA) to allow detection of impedance changes associated with

the phenomenon under study. Please note that this circuit is not designed to be connected
to electrodes that can cause a signi

µ

cant portion of the current to cross the heart.
The 50-kHz voltage across the potential-measurement electrodes is demodulated using
the lock-in ampli

fi

fi er circuit of Figure 8.18. A lock-in ampli fi er is a standard linear ampli-

fi fier that has its inputs protected

by resistors R33 and R36 and by diodes D3-D6), followed by a synchronous detector
(IC6). A synchronous detector has two inputs, one of which comes from the ampli

fier (in this case, IC5, an INA110 IC instrumentation ampli fi

er, the
other being a signal of the same frequency in phase with the signal of interest. The syn-
chronous detector is similar to an RF mixer in that the output includes the product of the
two input signals. When the frequencies are the same, the output will contain a dc com-
ponent whose value is proportional to the amplitude of the inputs and the sine of the phase
angle between them. The reference signal is bu

fi

ered through IC1B. Demodulation is max-
imized by correcting the reference signal phase through the phase-shifting circuit built
around IC4B. The amplitude of the reference signal is controlled via potentiometer R48.
The lock-in ampli

ff

fi er's output is scaled and low-pass fi filtered via the circuit built around

IC4A.
Since the ac current delivered to the tissue is constant, the demodulated output voltage
is related directly to the impedance of the tissue between the voltage-sensing electrodes.
As shown in Figure 8.19, this signal is galvanically isolated from recording instruments
via isolation ampli

fi er IC7. A notch fi filter built around IC8 is available in case the imped-

ance signals need to be fi filtered from power line interference. Note that a precision

full-wave recti er (Figure 8.20) built around IC14A is available for testing purposes.

It provides the absolute value of the voltage generated by the current source across the
current-injection electrodes.

fi