Four-electrode impedance plethysmograph measures ventricular stroke volume of cardiac output of humans. The instrument is automatic, operates with only one recording channel, and minimizes patient discomfort

Kubicek, W. B.

Patterson, R. P.

Witsoe, D. A.

English

NASA Technical Reports Server

Sample and 
Hold with 
Automatic 
Reset 
Null Balance 
Readout 
for Z0
Z Amplifier 
Differentiator 
and Filter 
Differentiator 
and Filter 
•	 Placement of Band Electrodes
Z Output 
dZ/dt Output 
d2Z/dt2 Output 
June 1968	 Brief 68-10220 
1 NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
New Electrical Impedance Plethysmograph Monitors Cardiac Output 
Constant	 High Input 
Current	 Impedance	 Linear 
100 kHz	 Differential	 Detector 
Oscillator	 Amplifier
ZJ Output 
A four-electrode impedance plethysmograph has 
been developed to satisfy a need in medical clinics 
and research laboratories for a simple, bloodless 
method of measuring ventricular stroke volume or 
cardiac output of humans. In using the instrument, 
two band electrodes are placed around the subject's 
neck, a third band is placed around the thorax at the 
level of the xiphisternal joint, and a fourth band 
around the lower abdomen. The electrodes at the 
upper neck and the abdomen are excited by a 100 kHz 
sinusoidal current and the resultant voltage (imped-
ance) changes occurring with the cardiac cycle are 
monitored from the other two electrodes. Stroke 
volume is calculated from the first time derivative 
(dz/dt) of the impedance change data, using a formula
relating impedance change to volume change in a con-
ducting medium. The new instrument is automatic, 
operates with only one recording channel, and mini-
mizes patient discomfort. 
As shown in the block diagram, current from the 
100-kHz oscillator is fed to the thorax (bands I and 4) 
by means of coaxial cables. These have electrically 
driven shields to reduce the effective cable capacity 
and thereby maintain high output impedance at the 
electrodes. The voltage from the thorax (bands 2, 3) 
is recorded by a differential amplifier having coaxial 
leads. The use of this amplifier allows a ground to be 
placed anywhere on the body. The differential input 
impedance of the amplifier measured at the cable ends 
using 100 kHz current is 10 ohms. Positive feedback 
(continued overleafl 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19680000220 2020-03-12T05:34:45+00:00Z
is used on the input to reduce the effects of cable 
capacity. The common mode rejection is 50 dB at 100 
kHz. The output of the differential . amplifier is fed to a 
linear detector, whose output in turn is fed to a stable 
sample and hold circuit and to the AZ (impedance 
change) amplifier. This amplifier also receives an 
input from the sample and hold circuit and subtracts 
the tvo inputs to give the AZ output. 
The sample portion of the sample and hold circuit 
can be operated either manually by a switch or auto-
matically. The automatic operation is achieved by 
sensing the output of the AZ amplifier and then 
sampling when the output exceeds an adjustable preset 
limit. This allows the system to record the small AZ 
changes due to the cardiac signal; but if respiration, 
movement, or other artifact causes a larger change in 
Z, the system will automatically rebalance in ap-
proximately 40 milliseconds. The output of the detec-
tor is also fed to two analog differentiators to give the 
first and second time derivatives (dz/dt and d2z/dt2 
respectively). The null balance circuit connected to the 
sample and hold output, nulls an accurate dc voltage 
against the output of the sample and hold circuit to 
accurately measure the value of the mean impedance 
zo. 
Significant electrical features of this impedance 
plethysmograph are as follows: 
1. High output impedance (105 ohms) at the elec-
trodes for the current source.
2. High input impedance (10 5
 ohms) at the electrodes 
for the voltage pickup system. 
3. Use of a floating electrode system, not requiring 
grounding. 
4. One percent absolute accuracy and linearity 
5. Fully automatic operation, with a balancing time 
of 40 milliseconds. 
6. Manual balancing that requires only the movement 
of one switch. 
7. A low-noise first derivative output (80 m y
 peak to 
peak). 
Note: 
Complete technical details may be Obtained from: 
Technology Utilization Officer 
Manned Spacecraft Center 
• Houston, Texas 77058 
• Reference: B68-10220 
Patent status: 
No patent action is contemplated by NASA. 
Source: W. B. Kubicek, R. P. Patterson, 
and D. A. Witsoe 
•	 of the University of Minnesota 
.under contract to 
Manned Spacecraft Center 
(MSC-1 1447) 
Brief 68-10220	 Category 01


New electrical plethysmograph monitors cardiac output

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