Most of the solid state electronic engineering of the system now generally known as the Minnesota Impedance Cardiograph was performed with the support of a five-year contract, NAS9-4500, with the NASA Lyndon B. Johnson Space Center, Houston, Texas. This contract ran from 1965 to 1970. In addition to the engineering design and development of the hardware, the contract called for testing on both animals and human subjects. This project also provided funds to construct twenty impedance cardiographs and place them in selected research and clinical facilities for further evaluation. This, then, led to the First Symposium on Impedance Cardiography, held at the NASA Lyndon B. Johnson Space Center, Houston, Texas, 2-4 June 1969. Twenty-four excellent papers were presented

Kubicek, W. G.

Tracy, R. A.

English

NASA Technical Reports Server

N94- 28367
Cardiac Output and Cardiac Contractility by Impedance Cardiography
during Exercise of Runners
W. G. Kubicek, BSEE, Ph.D. and R. A. Tracy, Ph.D.
Department of Physical Medicine and Rehabilitation
University of Minnesota Medical School
Minneapolis, Minnesota
-
Historical Perspective
Most of the solid state electronic engineering of the system now generally known as the Minnesota
Impedance Cardiograph was performed with the support of a 5-year contract, NAS9-4500, with
the NASA Lyndon B. Johnson Space Center, Houston, Texas. This contract ran from 1965 to
1970, W. G. Kubicek, Principal Investigator. In addition to the engineering design and
development of the hardware (Figure 1), the contract called for testing on both animals and human
subjects. This project also provided funds to construct twenty impedance cardiographs and place
them in selected research and clinical faci!itie_ for further evaluation. This, then, led to the First
Symposium on Impedance Cardiography, held at the NASA Lyndon B. Johnson Space Center,
Houston, Texas, June 2 to June 4, 1969. Twenty-four excellent papers were presented.
Constant
Current
Oscillator
100 KHZ
Xiphlsletnol Join/
°m
' t / ToE.',.,..,
Eel Inlet
Figure . A schematic diagram of the main elements of the Minnesota Impedance
Cardiograph. From left to right, the constant current oscillator at 100 kJ-Iz, 4 ma
(RMS). L--distance between electrodes 2 and 3 in cm, Zo--longitudinal thoracic
impedance between electrodes 2 and 3 in ohms, AZ=magnitude of impedance
change during cardiac cycle in ohms, dZ/dt=prst time derivative of AZ in ohms per
second, ECG=electrocardiogram and finally, a phonocardiogram output.
CARDIACotrn,trr ANDCARDIACCON'I_.AC'mXrY 141
https://ntrs.nasa.gov/search.jsp?R=19940023864 2020-06-16T15:01:04+00:00Z
A separatecontract was negotiated by the NASA Center with Space Labs, Inc., Van Nuys,
California, to construct six miniaturized impedance units as shown in Figures 2 and 3. Testing of
one of the systems showed fairly good function (Figure 4). However, a satisfactory calibration
system was not incorporated into the design, making use of the device very difficult. One of these
units was used during space flight by Dr. William Thornton (NASA).
Figure u
Figure 3.
A photograph of one of the six miniaturized impedance cardiographs, patterned
after the circuitry shown in Figure 1. A satisfactory calibration circuit was not
incorporated into these units. These units were constructed by Space Labs, Inc.,
Van Nuys, California, about 1968.
=
A photograph of the unit in Figure 2 worn in a "cartridge belt" suspension.
142 CARDIAC otrn_rrA_ CARI)IACcogrRh_
Figure 4. A photograph of the laboratory arrangement for bicycle ergometer exercise of six
middle distance runners (4,000 meters) . From left to right, Dr. R. A. Tracy, a
runner in place on the bicycle ergometer with a ,face mask attachment for measuring
oxygen consumption rate, and the strip chart recorder with the impedance
cardiograph mounted on top with the technician ready to start the recorder, when
needed.
An excellent comprehensive review by Miles and Gotshall [1] on the subject, "Impedance
Cardiography: Noninvasive Assessment of Human Central Hemodynamics at Rest and During
Exercise," is most appropriate for this conference.
Scientific Observations of Importance during Exposure to Microgravlty
Two major scientific achievements were made during our NASA contract. First, the derivation of
a stroke volume formula and second, the use of the first time derivative (dZ/dt) of thoracic
impedance change (AZ) during cardiac cycle to refine the stroke volume formula as widely used
today [2].
Later experiments on anesthetized dogs revealed that peak (dZ/dt) occurs simultaneously with peak
ascending aortic blood flow and that peak (dZ/dt) is directly proportional to peak aortic flow [3].
An example of a strip chart recording of (dZ/dt), phonocardiogram and ECG on a normal
individual is shown in Figure 5. In addition to use in computing stroke volume and cardiac output,
this recording to determine several systolic time intervals and contractility indices. It would appear
that these parameters would be of great importance if there is deterioration in cardiac mechanical
function during prolonged space flight.
CARDIACOtrlrtrr AND CARDIACCO1¢I__ 143
, i ¸
r i -]
Contractility and Systolic Time
Figure 5. An example of a strip chart recording on a normal individual at rest. From left to
right, (Q-Z) interval in seconds=time from Q wave to peak dZ/dt. PEP preejection
period in seconds, VET=ventricular ejection time in seconds, QS2=total systolic
time interval in seconds, (dZ/dt)min in ohms per second, "0" wave=rapid
ventricular filling wave, (RR)=ime interval between two heart beats in seconds.
This recording illustrates the type of recording used to compute or measure the
various parameters shown in Figures 6 to 10.
It is generally accepted that when a normal individual changes from a standing position to supine
(at one g) cardiac output increases due to an augmented venous return from the lower parts of the
body. Table 1 illustrates the change in several parameters obtained by impedance cardiography
when normal individuals changed from standing to supine. Of special importance here is the
decrease in 7-.0 in the supine position. This indicates an increase in fluid in the pulmonary vascular
bed.
144 CARDIAC OU'I?UTAND CARDIAC CONTRA_
Table 1. Standing and supine data from 19 (8 female, 11 male) normal individuals, aged 21
to 54 years (Mean _:SD)
Standing Supine (1 to 2 rain)
SV 67 ±17 123 ±30
HR 79 -,-13 64 ±12
CO 5.3 ±1.4 7.9 ±2.0
CI 2.9 ±0.8 4.3 ±1.2
dZdt 2.4 ±0.7 2.6 + 0.8
ET 230 -,- 21 306 --25
PEP 117 ±14 88 ±10
Q-Z 157 ±15 142 ±12
HI 15 ±5.5 19 ± 6.5
7-o 29 -*-3.7 26 ± 3.5
Zo/L 1.0 ±0.17 0.9 ±0.15
SV=ventricular stroke volume (CC), HR--heart rate in beats per minute, CO=cardiac output in liters per minute,
CI=cardiac index, CO/body surface area (M2)=CO/M2, dZ/dt=first time derivative of thoracic impedance change (AZ)
during cardiac cycle in ohms/second, (dZ/dt)min (Figure 5), ET=ventricular ejection time in milliseconds, (VET
Figure 5), PEP=preejection period in milliseconds (Figure 5), Q-Z=time interval between ECG Q wave and peak
dZ/dt in milliseconds (Figure 5), HI=Heather index (d7_,/dt)/(Q-Z) in ohms/sec/sec, 7-,o thoracic impedance (ohms)
between electrodes 2 and 3 (Figure 1), Zo/L=ohms/cm.
SV=pL2/Z2o T(dZ/dt)min
where p=150 ohm era, T=ejection time (see references 2 and 3), CO=SV.heart rate/10OO=liters per minute
It would be reasonable to predict that immediately after reaching microgravity, the cardiovascular
response would be similar to a ground-based individual changing from standing to supine. Zo
Table 1 over an extended time, any adaptation to microgravity should be apparent.
If there is any tendency toward congestive heart failure during prolonged space flight, these
relatively simple measurements could be of great importance.
Response to Exercise of Well Conditioned Runners
An exercise study was performed on six well-conditioned middle distance (4000 meters) runners.
The primary objective was to determine if a special 6-weeks training program would improve their
overall cardiac function. A perusal of the results presented graphically in Figures 6 to 10 indicates
that the special training program had no effect on cardiac performance.
CARDIACotrrptrr AND_tAC CON'D__ 145
25
20
15
1-0
,y"
1 I I 1 I
REST 100 200 300 400
WORK LOAD : WATTS
Figure 6. Relationship between mean cardiac output and bicycle ergometer work load for six
middle distance runners before and after 6 weeks of special training.
zN
53
0 PRIOR TO TRAINING
X AFTER TRAINING
X
00
X
0
0
X
I I I I i ! I [ I
z'est lO0 200 300 4DO
WDRK LOAD : WATTS
Figure e A graph showing the relationship between mean dz/dt values and increasing work
loads on the bicycle ergometer for six middle distance runners before and after 6
weeks of special training.
146 CARDIACOUTPUTANDCARDIACCONTRACI'ILnT
Figure
/
<t
>
t_
LtJ
I--
Z
8,
200 -
15C --
I0C --
50-
(
It
m
Q-_-Z lNTE RVAL
l i I l l i ! i i
REST 100 200 300 400
WORK LOAD : WATTS
u
m
200
- 150
_ i00
-, 50
//
.r_
C,
e_
e¢
I
The relationship between the mean heart rate values and mean Q-Z interval values
and increasing work loads on the bicycle ergometer for six middle distance runners
before and after 6 weeks of special training.
6O
5O
60
3O
2O
10
O PRIOR TO TRAINING
X AFTER TRAINING
0 I I I I I I [ I I
REST 100 200 300 _.00
WORK LOAD : WATTS
Figure u A graph illustrating the relationship between the mean Heather Index values from
rest to maximal bicycle ergometer exercise for six middle distance runners before
and after 6 weeks o[ special training.
CARDL_C OUTPUT AND CARDIAC CONTgACrnaTY 147
d
_)
E
.°
t---
300 -
250 -
200
150 -
I /
p,ATE
0
l 0
X
X
o
_x
0
i
X 0
I 1 I I I I I I i
REST 100 200 300 400
_dORK LOAD z WATTS
200
0
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- 150
- I00
- 50
-¢
d
.r.,i
E
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w.
iii
I,-,...
I'-'-
,.,,-
Lt.I
"t"
Figure 10. A graph showing the relationship between mean T (ejection time in milliseconds)
values and mean heart rate values and increasing work loads on the bicycle
ergometer for six middle distance runners before and after 6 weeks of special
training.
These data are presented here as an illustration of how impedance cardiography could be used in
relation to space flight and microgravity. Careful and repeated ground-based testing of the
response to appropriate exercise could be carried out to establish a pattern of response prior to
space flight. These baseline data could then be used for comparison, either for pre- and postflight
testing for short-duration flights or for onboard testing during prolonged space flight.
Summary
Impedance cardiography is here to stay. About 500 published articles, utilizing impedance
cardiography, are available. It is safe, convenient, noninvasive, and cost effective. It is the only
proven method available to NASA to monitor the various parameters, described here, related to the
intimate physiology of cardiac dynamics. In addition, changes in thoracic fluid volume can be
followed. If desired, peripheral circulation can be studied by impedance.
Cardiac output is the historic standard for measuring cardiac mechanical function. However,
changes in parameters such as the Heather contractility index fill) may provide an early warning of
impending serious myocardial dysfunction before any changes in cardiac output occur.
148 CARDIAC OUTPUT AND CARDIAC CONTRAcnurY
References
[1]
[2]
MILES, D. S. AND R. W. GOTSHALL.
Impedance Cardiography: Noninvasive
Assessment of Human Central
Hemodynamics at Rest and During
Exercise. In: Exercise and Sport Sciences[3]
Reviews, edited by K. B. Pandolf.
Baltimore, MD: Williams & Wilkens,
1989, p. 231-263.
KUBICEK, W. G., J. M. KARNEGIS,
R. P. PATTERSON, D. A. WlTSOE, AND
Evaluation of an Impedance Cardiac Output
System. Aerospace Medicine 37:1208-
1212, 1966.
KUBICEK, W. G., F. J. KOTTKE, M.
U. RAMOS, R. P. PATTERSON, D. A.
WlTSOE, J. W. LABREE, W. REMOLE,
T. E. LAYMAN, H. SCHOENING, AND
J. T. GARAMELLA. The Minnesota
impedance cardiograph-theory and
applications. Biomedical Engineering
9:410-416, 1974.
Acknowledgment
The material shown in Figures 4, 6, 7, 8, 9, 10, is part of an unpublished Ph.D. thesis study
carried out in our laboratory by Robert Tracy (1971).
CARDIACotrrptrr ANDCARDIACCONTRACrILr_ 149



Cardiac output and cardiac contractility by impedance cardiography during exercise of runners

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