CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 394

F

(a)

Constant
Current
Source

I

V

Electrode

AC Voltmeter

Tissue

(b)

I

Voltage
Sensing
Electrode

Current-
Injection
Electrode

V

Tissue

Figure 8.14 The most common method for measuring the impedance of tissues involves injecting
an ac current of constant amplitude into the tissue and measuring the potential di erence developed

across two points between the current-injection electrodes. ( a ) Two-terminal measurement in which
the impedance of the system at frequency f is given by Ohm's law R

ff

V/i . ( b ) A four-terminal tech-
nique reduces errors of the two-terminal measurement caused by nonlinear potential di ff erences gen-

erated at the current-injection electrodes by currents fl flowing through the electrode-tissue interface

polarization impedance.

in an impedance cardiograph (Figure 8.15), one set of surface electrodes (usually, two
pairs of gelled disk electrodes or two band electrodes) placed on the upper abdomen and
upper neck are used to inject an ac current, providing more or less homogeneous coverage
of the thorax with a high-frequency field is detected

through a second set of electrodes located at the level of the root of the neck and the
diaphragm.
The circuits of Figures 8.16 to 8.20 implement a general-purpose impedance plethys-
mograph. The circuit of Figure 8.16 is a 50-kHz sinusoidal oscillator that feeds the volt-
age-controlled current source of Figure 8.17 [AAMI, 1994]. The output of the RC
oscillator formed around IC2A is feedback-stabilized by JFET transistor Q1. This transis-
tor is controlled by the error signal between the amplitude setpoint given by R16 and the
average oscillator output signal envelope calculated by the detector formed by D1 and C4.
The output of the oscillator is bu

fi field. The voltage developed by the fi

ered by IC1A, scaled via potentiometer R30 and ac-
coupled to modulate the current through Q3. IC3, Q2, and Q3 is a class B ampli

ff

er that is
made to operate as a constant-current source. The 50-kHz input signal is adjusted via

fi