CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 397

R20

C11
0.1uF

5.6M
1%

+5Viso

I

R21
11K
1%

R22
1 1%

+5Viso

Q2
2N3906

Current Injection
Electrode 1

+ C12
10uF

C13
.01uF

R23
22K
1% IC3

TL081

J1
1

I I

7 1

JP1
JUMPER

Inj1 3

2

+
-

6

C1 4

4 5

VTOI_1

R24
22K
1%

.05uF

+ C15

C16
.01uF

-5Viso

Q3
2N3904

10uF

J2
1

I I

R27

Inj2

R25
11K
1%

Current Injection
Electrode 2

R26
1K
1%

1 1%

R28
10M
1%

I

I

-5Viso

IC1A
TL082

8

R29
100K

3
2

+
-

50KHZ

1

JP2

JUMPER

C1 7

4

R30
10K

.047uF

I

Figure 8.17 Voltage-controlled current source of the general-purpose impedance plethysmograph. The output of the oscillator is buffered
by IC1A, scaled via potentiometer R30 and ac-coupled to modulate the current through Q3. IC3, Q2, and Q3 is a class B amplifier that is
made to operate as a constant-current source.

Impedance measurements obtained through the conductance catheter cannot be used
directly, since important intrinsic errors are generated by the nonhomogeneous distribution
of the current lines within the measured volume as well as because of leakage currents
through the heart muscle and extracardiac tissues. The

ects the linear rela-
tionship between conductance and volume, while the latter adds a “phantom” parallel vol-
ume. Special calibration and signal processing techniques are usually applied to scale and
linearize the impedance measurements against true ventricular volume.
Conductance catheters are often used by researchers and physicians to generate a graph
of the left-ventricular pressure-volume relationship. This relationship, known as a PV
loop , gives important information on the type of cardiomyopathy and possible treatments.
Pressure is measured using a miniature sensor that is part of the conductance catheter. PV
diagrams clearly distinguish the four phases of the cardiac cycle (isovolumetric contraction,

fi rst error a ff