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Correspondence: Gianfranco Parati, Department of Cardiology, S. Luca Hospital, Istituto Auxologico Italiano, Piazzale Brescia 20, 20149 Milano, Italy.
Tel: 39 02 619112949. Fax: 39 02 619112956. E-mail: gianfranco.parati@unimib.it
(Received 26 March 2013 ; accepted 28 May 2013 )
ORIGINAL ARTICLE
Cardiac index assessment: Validation of a new non-invasive very low
current thoracic bioimpedance device by thermodilution
ANDREA FAINI
1
, STEFANO OMBONI
2
, MARIUS TIFREA
3
, SERBAN BUBENEK
3
,
OVIDIU LAZAR
4
& GIANFRANCO PARATI
1
1
Department of Cardiology, IRCCS Ospedale San Luca, Istituto Auxologico Italiano and Department of Health
Sciences, University of Milano Bicocca, Milano, Italy,
2
Italian Institute of Telemedicine, Varese, Italy,
3
Institute of
Emergency for Cardiovascular Diseases “ Prof. Dr. C.C.Iliescu ” , Bucharest, Romania;
4
“ Agrippa Ionescu ” Military
Emergency Hospital, Balotesti Center of Cardiovascular Disease, Bucharest, Romania
Abstract
Introduction. The accuracy of impedance cardiography for cardiac index assessment is matter of debate, with available stud-
ies reporting inconsistent results. Our study aimed at evaluating the agreement between measurements of cardiac index
provided by a new-generation thoracic electrical bioimpedance device (Hotman System) and an invasive approach based
on thermodilution in humans. Methods. Cardiac index was assessed simultaneously with thoracic electrical bioimpedance
and conventional thermodilution through comparison of fi ve consecutive measurements in 51 cardiac patients, hospitalized
in an intensive care unit (mean SD age: 60 11 years; 68% males). The agreement between cardiac index values measured
by both methods was assessed by the Bland – Altman approach, adjusted for repeated measures. The repeatability coeffi cient
and the intraclass correlation coeffi cient were used to assess reproducibility of replicates. Results. Average ( SD) cardiac
index was 3.05 0.91 l/min/m
2
with Hotman System and 3.14 1.12 l/min/m
2
with thermodilution. The bias of precision
was 0.09 0.41. The coeffi cients of repeatability and intraclass correlation coeffi cients were high and similar for the two
techniques (0.95 l/min/m
2
and 0.91 for Hotman System vs 0.78 l/min/m
2
and 0.90 for thermodilution). Conclusions. Cardiac
index values yielded by Hotman system compares favorably with that obtained with thermodilution in cardiac patients.
Key Words: Cardiac output , hemodynamic monitoring , impedance cardiography , thermodilution
Introduction
Critically ill patients often require continuous cardiac
function assessment, and cardiac output monitoring
represents an important diagnostic and prognostic
tool in these conditions (1). The thermodilution
technique is the clinical standard for cardiac output
estimation, but it requires an invasive and costly
procedure, not free from complications (2 – 5). An
alternative to such an approach might be the use of
impedance cardiography or thoracic electrical bio-
impedance. This technique is based on recording of
changes in the electrical resistance of the chest dur-
ing heartbeat: these variations are then converted to
changes in volume over time and used to derive
stroke volume (6 – 9). Compared with thermodilu-
tion, impedance cardiography allows hemodynamic
measurements to be carried out more easily and
quickly, and without the risk of infection or other
complications associated with pulmonary artery
catheterization (10). In addition, unlike thermodilu-
tion, the bioimpedance technique enables monitor-
ing of cardiac output in a continuous mode at the
patient ’ s bedside, with a low cost of testing (10).
Since its introduction several years ago, impedance
cardiography has demonstrated its usefulness in
various populations and conditions (10). However, a
number of validation studies suggest a poor correla-
tion between data provided by most impedance
devices and invasive measurements of cardiac outputs
and a wide range of variability in the agreement
between the two methods for the different devices
available on the market (6,9,11 – 14). This evidence
has questioned the reliability of bioimpedance for
hemodynamic monitoring of critically ill patients,
Blood Pressure, 2013; Early Online: 1–7
ISSN 0803-7051 print/ISSN 1651-1999 online © 2013 Scandinavian Foundation for Cardiovascular Research
DOI: 10.3109 / 08037051.2013.817121
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2 A. Faini et al.
and has hindered the broad adoption of this technique
as a tool for managing cardiac patients, limiting its
diffusion mainly to physiological studies or peri-
operative applications (1).
Recently, attempts have been made to provide
more reliable non-invasive measurements of hemo-
dynamic parameters through a very low current
thoracic electrical bioimpedance device (Hotman
System, Hemo Sapiens Medical Inc., Sedona, AZ,
USA), making use of a more accurate algorithm for
the calculation of cardiac output (15). Preliminary
evidence exists on the ability of such device to yield
a reproducible non-invasive estimation of cardiac
output in healthy volunteers, but no data on its
clinical accuracy is yet available (16).
The purpose of this study was thus to investigate
the agreement between measurements of cardiac
output simultaneously provided at rest by a reference
invasive measuring methodology (thermodilution)
and by the Hotman non-invasive electrical bioimped-
ance device in patients hospitalized in an intensive
care unit.
Materials and methods
Study design and population
This was a non-randomized, multicenter, national
study, involving two centers located in Romania. The
study was designed to compare the accuracy of the
cardiac output determination by a thoracic electrical
bioimpedance system (Hotman System, Hemo Sapi-
ens Medical Inc., Sedona, Arizona, USA) (15) versus
that obtained with pulmonary artery catheter stan-
dard bolus thermodilution, used as reference gold
standard. The study was performed in 51 cardiac
patients of either gender, aged 18 – 72 years, hospital-
ized in the intensive care units of Institutul de Urgenta
pentru Boli Cardiovasculare “ Prof. Dr. C.C. Iliescu ”
( n 44) and Spitalul Militar de Urgenta “ Prof. Dr.
Agrippa Ionescu ” ( n 7), Bucharest, Romania.
Patients were included in the study if there was a
clinical indication for pulmonary artery catheteriza-
tion (patients undergoing cardiac surgery or a cath
lab test), and following initial treatment and stabili-
zation in the critical care unit.
Patients could not be enrolled in the study if
meeting at least one of the following exclusion
criteria: (i) ventricular or dual chamber pacemaker
wearers; (ii) presence of a severe aortic insuffi ciency;
(iii) patients with left to right shunt; (iv) patients with
terminal illness; (v) extremely obese patients (body
mass index 35 kg/m
2
); (vi) patients displaying a
high level of anxiety; (vii) patients undergoing hemo-
dialysis, ultrafi ltration, mechanical ventilation with
continuous positive airway pressure, or life-saving
treatments other than mechanical ventilation, or who
were using a left ventricular assist device (including
intra-aortic balloon pump); (viii) previously enrolled
subjects.
The study was approved by the local ethics
committee of each center. All participants provided
a written informed consent prior to any study
procedure.
Patients were studied in the immediate post-
operative period (all patients had been subjected to
open heart surgery, 33 patients with coronary artery
bypass graft and 18 with valvular problems, aortic
and/or mitral), and cardiac output was determined
at the arrival in the cardiac intensive care unit. Assess-
ment was done in the supine position, simultane-
ously by thermodilution and non-invasive thoracic
electrical bioimpedance: fi ve consecutive determina-
tions at 3-min intervals were scheduled in each sub-
ject. The investigators had to start the recording of
non-invasive hemodynamics each time a bolus was
injected for the thermodilution measurement.
Transpulmonary thermodilution
Thermodilution data were obtained by the fl uid
bolus technique (2,4). A 7.5 French pulmonary
artery catheter (Swan – Ganz) was introduced into the
right internal jugular vein and advanced to the pul-
monary artery through the right heart. A 10-ml bolus
of ice-cold 5% dextrose was injected in less than 4 s
in the right atrium. According to the thermodilution
principle, the change in temperature detected in the
blood of the pulmonary artery by a thermistor
positioned at the end of the catheter was used to
calculate cardiac output, using the modifi ed Stewart –
Hamilton indicator dilution equation. In order to
improve the accuracy of the determination, each
measurement was based on the average of three
repeated passes, all within 10% of each other, by
excluding the fi rst injection. In total, fi ve consecutive
measurements within a 3-min interval were per-
formed. Measurements with more than 10% variation
compared with the average of all fi ve measurements
had to be excluded, being considered errors given by
the injection time.
Non-invasive thoracic electrical bioimpedance
Cardiac output was determined also non-invasively
by a novel device based on the thoracic electrical
bioimpedance method (Hotman System) (15). The
system allows a complete non-invasive assessment of
patient ’ s hemodynamic with two important improve-
ments compared with other previous similar devices:
(i) use of a very low current (7 μ A, 300 – 400-fold
lower than that used by other products, making it
safer for the patient) with a current frequency of
65 kHz, and (ii) use of a new data signal processing
and of an improved mathematical algorithm. The
Hotman system consists of a patient interface for
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Hemodynamic assessment by thoracic bioimpedance 3
automatic acquisition of blood fl ow and left ventric-
ular parameters through electrodes positioned on
the patient ’ s chest and of a computer equipped with
analysis software. Measured hemodynamics can be
integrated by entering in the software additional
information such as the upper arm blood pressure
and the oxygen saturation level, which are processed
by the software with calculation of additional para-
meters. Measurement of hemodynamic parameters
is pursued through generation of a very low current
circulating between two pairs of solid gel electrodes,
one pair located in the upper part of the laterocervi-
cal region (root of the neck) and the other at the level
of the upper abdomen (midaxillary line at the level
of the xiphoid process): these are the “ current elec-
trodes ” (Figure 1). As the alveoli are fi lled with air
(non-conducting medium), the electrical current is
conducted mainly through the thoracic aorta and
the venae cavae. Other four electrodes ( “ measuring
electrodes ” ) are placed in close proximity to the cur-
rent electrodes as shown in Figure 1: they record the
electrocardiogram as well as the voltage of the elec-
trical current that crosses the thorax. This current is
proportional to the thoracic impedance, namely to
the tissue resistance when crossed by the electrical
current. As this current is directed by the blood fl ow
through the venae cavae and the thoracic aorta, and
since the blood is the best electrical conductor of the
human body, blood fl ow variations may be translated
in variations of the thoracic impedance.
For the purpose of the validation study, because
of the presence of the pulmonary artery catheter over
the right internal jugular vein, the upper right-sided
sensor of the neck was positioned behind the right
ear lobe and the upper left-sided sensor, just before
the left ear lobe. Before applying the electrodes, the
skin was carefully cleaned with alcohol, to ensure
good adhesion of electrodes and low skin-to-sensor
impedance. The electrodes were then connected
through a patient cable to the bioimpedance system.
Prior to starting hemodynamic measurements, height
and weight values were entered in the system and
body surface area automatically computed, in order
to be used for determination of cardiac indexes
for both non-invasive and invasive methods. The
non-invasive blood fl ow measurement was assessed
concomitantly with the thermodilution measurement
and cardiac index automatically calculated.
Data analysis
The parameter evaluated during the study and
compared between methods was the cardiac index,
defi ned as the ratio between cardiac output and body
surface area and expressed in l/min/m
2
. Body surface
area was calculated by the Du Bois & Du Bois
formula (17).
The study specifi cally aimed at assessing the
accuracy of the non-invasive method and the repeat-
ability of the measurements.
Figure 1. Illustration of the electrode arrays for the thoracic bioimpedance system used in the validation study. Position of current injecting
electrodes and voltage measuring electrodes is reported.
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4 A. Faini et al.
The agreement between cardiac index values
measured by thermodilution and thoracic bioimped-
ance was assessed by calculating the mean between-
method difference (bias or accuracy), and the
standard deviation (SD) of the difference and the
95% confi dence limits of agreement (precision) as
described by Bland & Altman (18,19). For each
patient and for each single measurement, the differ-
ence between the cardiac index detected by the ther-
modilution method (reference) and by the test device
(thoracic electrical bioimpedance), as well as the
average of the cardiac index value obtained with the
two methods, were computed. Agreement between
the reference and test device was checked by Bland –
Altman graphs of reference-test device differences
plotted vs average of the two methods, by consider-
ing the average of fi ve measurements obtained in
each individual (18,19). Percentage error, defi ned as
the ratio between the limits of agreement of the bias
and mean reference cardiac index for the two meth-
ods together, multiplied by 100, was calculated for
interchangeability of the two methods according to
the criterion described by Critchley & Critchley (13).
Acceptance of a new technique should rely on a per-
centage error of up to 30% (13). Kendall ’ s rank
correlation tau ( τ ) test was used to seek a possible
relation between average values and between-method
differences.
Repeatability of replicated measurements for
each method was also assessed by calculating the
within-subject SD of the fi ve replicates and then
the repeatability coeffi cient, defi ned by 1.96 √ 2
multiplied by the within subject SD (18,20). The
repeatability coeffi cient represents the value below
which the absolute difference between two single test
results may be expected to lie with a 95% probability
(20). In addition to the repeatability coeffi cient, the
intraclass correlation coeffi cient was calculated in
order to assess agreement between methods over
repeated measurements (21). The intraclass correla-
tion coeffi cient is measured on a scale of 0 to 1,
where 1 represents perfect reliability with no mea-
surement error and 0 indicates no reliability.
Wherever it was deemed necessary, a correction for
repeated measured was applied, as required by the
local calculation.
Results
A total of 51 cardiac patients were studied, 68% of
which were males. The patients ’ mean age ( SD)
was 60 11 years (range 28 – 72). Mean cardiac index
( SD) was 3.05 0.91 l/min/m
2
by the thermodilu-
tion method and 3.14 1.12 l/min/m
2
by bioimped-
ance. The correlation between the subject mean
values showed a strong relationship ( τ 0.71;
p 0.0001) between the two methods (Figure 2).
The correlation coeffi cient within subjects was 0.97;
this value showed that an increase in thermodilu-
tion indices within individual was associated with
increase in bioimpedance indices.
The mean thermodilution – bioimpedance differ-
ence was 0.09 l/min/m
2
. The relative error of the
bioimpedance method was 26%, namely less than
the 30% maximum acceptable threshold (13). Figure 3,
panel A, shows the Bland – Altman graph for
12345678
1
2
3
4
5
6
7
8
CI thermodilution method
[
l/min/m
2
]
CI biompedance method [l/min/m
2
]
Figure 2. Plot of the 255 pairs of cardiac index (CI) estimation derived by bioimpedance ( y -axis) and thermodilution technique ( x -axis).
The continuous line refers to the line of complete agreement (identity).
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Hemodynamic assessment by thoracic bioimpedance 5
comparison of cardiac index obtained by the two
methods. The limits of agreement using repeated
measurements ( 1.21 and 1.03) were small, sug-
gesting that the test method might be reasonably
accurate. The 95% confi dence interval for the limits
of agreement was also small: 1.41 to 1.01 for the
lower limit of agreement and 0.83 to 1.23 for the
upper limit of agreement. In addition, only one point
(2%), laid outside the 95% limits of agreement, con-
fi rming that accuracy of the test method was accept-
able for the majority of subjects. The between-method
difference, using the average values for each subjects,
did not systematically vary over the range of mea-
surements as demonstrated by a small and not sta-
tistically signifi cant Kendall ’ s rank correlation tau
( τ 0.17; p 0.089), in spite of the tendency for an
overestimation bias by bioimpedance at higher values
of cardiac index (Figures 2 and 3).
The repeatability coeffi cient for cardiac index
determined by thermodilution was 0.78 l/min/m
2
,
while it was 0.95 l/min/m
2
for the thoracic electrical
bioimpedance method, indicating only a slightly
better internal agreement for the reference method.
The intraclass correlation coeffi cients for fi xed raters
and the assessment for interreader agreement for
random raters were similarly high for both the
thermodilution [0.90 for both (95% confi dence
interval: 0.86, 0.94); p 0.0001] and for the bio-
impedance method [0.91 for both (0.87, 0.94);
p 0.0001], indicating high reliability.
Discussion
In the literature, a fl urry of reports on validation of
different bioimpedance devices vs thermodilution,
the clinical standard for cardiac output determina-
tion (3,4,22), is available. However, so far no
consensus has been reached on the accuracy of
impedance cardiography in the measurement of
cardiac output since in some validation studies the
method was evaluated as highly accurate, whereas in
others more dispersion between the two methods
was found (12,13).
In our paper, we report on a validation study of
a new impedance device against thermodilution by
presenting bias, limits of agreement, percentage error
and repeatability coeffi cients, as currently universally
recommended (13,18,19,23). The Bland – Altman
method adjusted for repeated measures was used for
analysis of the validation data because it measures
the extent of deviations from the line of complete
agreement (no bias 0) between the methods
(18,19). This is a much more reliable estimation of
agreement between methods than the correlation
coeffi cient, often used in the past, which measures
how close to a straight line the pairs of measurements
lie, which does not automatically imply that the line
is the one of complete agreement.
We found a good agreement between the cardiac
index measurements determined by non-invasive
and invasive device: the bias between the two meth-
ods was 0.09 l/min/m
2
. The percentage error was
1234567
−2
−1
0
1
2
Average CI thermodilution and biompedance method [L/min/m
2
]
CI difference thermodilution and biompedance method [L/min/m
2
]
−0.09
−1.21 [−1.41;−1.01]
1.03 [0.83;1.23]
Figure 3. Plots of difference in cardiac index (CI) estimation between the reference (thermodilution) and test method (thoracic electrical
bioimpedance system) ( y -axis) vs the average cardiac index value of test and reference method ( x -axis). Data are shown for the 51 cardiac
patients. Dashed lines refer to bias, and upper and lower limits of agreement.
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6 A. Faini et al.
26%, namely within the 30% limit of agreement:
this indicates that the test method is no less accurate
than the reference method (13), in spite of the ten-
dency towards a small overestimation error by the
bioimpedance device at higher values of cardiac
index. The use of the percentage error strengthens
the study results, since this measure incorporates
not only the error of the method to be tested but
also adjustments for errors in the reference method
itself (13).
In our study, we were also able to obtain repeated
measurements in the same subject. This further
increases the power of our results, but also allowed
us to assess the degree of (short-term) repeatability
of the methodology under test. The coeffi cients of
repeatability were similar for the two methods (0.78
for thermodilution and 0.95 for bioimpedance) and
the intraclass correlation coeffi cients were high (0.90
thermodilution and 0.91 biompedance), both indi-
cating a performance of the non-invasive method
comparable with that of thermodilution.
Although we acknowledge that we were able to
assess only short-term repeatability of cardiac index
by bioimpedance, nevertheless we must emphasize
that limited evidence exists in literature for this
important aspect. Previous evidence suggests that
bioimpedance may have an extremely variable ability
to adequately refl ect cardiac output changes over
time in different situations or during postural changes
(24 – 27). In addition, it has been shown that the
reproducibility of cardiac index under standardized
conditions is better in the short than in the long term.
Verhoeve and coworkers (28) showed a high degree
of reproducibility of measurements obtained by
means of bioimpedance cardiography during the
same day (at few minutes interval) and after 24 h, in
96 cardiac patients enrolled in a rehabilitation clinic
(28). Intra-day correlation coeffi cient showed a 95%
range of error of 0.92 – 0.96, while the percentage of
variability in cardiac index from day 1 to day 2 was
6% with a correlation coeffi cient of 0.79 (28). In
another study, performed in 31 consecutive patients
admitted in an intensive care unit, the coeffi cient of
reproducibility for cardiac output obtained with
bioimpedance was very close to that obtained through
thermodilution (0.6 l/min vs 1.0 l/min), but there
was no agreement between the absolute values of
cardiac output (29). Finally, Jewkes and coworkers
(30) evaluated the reproducibility of cardiac output
estimation in resting supine volunteers by a thoracic
electrical bioimpedance cardiograph over a short
time period (30 min) and over several days. The
reproducibility was better on the short term and
worsened on the long term, with an average variation
coeffi cient increasing from 5% to 11%. Unfortu-
nately, in most of the previously referred studies,
an inappropriate or not completely appropriate
analytical approach was employed for assessing
reproducibility.
Limitations of the study
When interpreting the data presented in our study,
some methodological aspects and limitations must
be considered. First, it may be argued that the obser-
vations made are based on a relatively small number
of patients and thus may be of a limited value.
However, the overall number of cardiac index deter-
minations collected in our 51 subjects ( n 255) was
suffi ciently high to empower the study results. Sec-
ond, we assessed reproducibility over a very short
time interval and in standard supine position only.
We cannot exclude, as suggested by the literature for
most devices, that reproducibility of cardiac index
estimation by impedance cardiography with the test
device may worsen when assessed over a longer
observation period or in different postures. Third, we
did not test accuracy in situations or conditions
known to negatively affect reliability of impedance
measurement. We also acknowledge that further stud-
ies are needed to establish the validity of impedance
cardiography by the Hotman device in unconven-
tional situations.
Conclusions
At present, there is much controversy regarding
the utility and safety of the pulmonary artery cath-
eterization for the evaluation of cardiac output.
Non-invasive techniques may represent an attractive
alternative to increase the chance of assessing cardiac
output also in subjects with contraindications to
invasive procedures, but these techniques have not
always met accuracy standards. In our validation
study, we demonstrated that a totally non-invasive
method of cardiac output monitoring by a new-
generation thoracic bioimpedance device may per-
form well as a moderately invasive tool in high risk
cardiac patients. Our results suggest that progress
of hardware and software, including digital signal
processing and new algorithms, may improve the
quality of the results of non-invasive cardiac output
estimation by new devices. However, since accuracy
of measurements by impedance cardiography may be
negatively affected by different conditions and pos-
tures, further studies are required to demonstrate the
reliability of the Hotman device in such situations.
Additional studies are also needed to investigate
more in depth the tendency for an overestimation
bias by the bioimpedance approach at higher values
of cardiac index, observed in our study.
Competing interests
This study was supported by an unrestricted uncon-
ditional grant from the manufacturer of the Hotman
bioimpedance device (Hemo Sapiens Inc. Sedona,
Arizona, USA). The authors have no fi nancial rela-
tionship with any product or manufacturer mentioned
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Hemodynamic assessment by thoracic bioimpedance 7
in the article. The authors declare that they have no
competing interests.
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