Download reliability of cardiac output calculation by the fick principle and

Journal of Clinical Monitoring and Computing
DOI: 10.1007/s10877-008-9143-y
Springer 2008
RELIABILITY OF CARDIAC OUTPUT
CALCULATION BY THE FICK PRINCIPLE
AND CENTRAL VENOUS OXYGEN SATURATION
IN EMERGENCY CONDITIONS
Weinbroum AA, Biderman P, Soffer D, Klausner JM, Szold O. Reliability of cardiac output calculation by the Fick principle and central
venous oxygen saturation in emergency conditions.
J Clin Monit Comput 2008
Avi A. Weinbroum, MD1,2, Philippe Biderman1,
Dror Soffer3, Joseph M. Klausner3 and Oded Szold3
been the gold standard for determining cardiac output in the
critically ill patients. Less invasive methods have recently been
introduced. This study aimed at evaluating the agreement
between cardiac output (CO) measured by a new Fick method,
using central venous saturation (Scvo2), and that measured by
the classic thermodilution technique, in patients requiring
emergent CO evaluation. Settings. Prospective clinical study
in a university-affiliated, tertiary hospital, at surgical and general
intensive care units. Patients and methods. Fifteen
mechanically ventilated patients arriving in the emergency
department in hemodynamic shock, had immediately a
pulmonary artery catheter introduced under fluoroscopy upon
arrival into the ICU. Cardiac output (CO) was obtained in each
patient via both thermodilution and the Fick method, using
oxygen consumption, SpO2 and Scvo2. Results. COs ranged
between 2 and 2.3 (in the Fick and thermodilution methods,
respectively) and 19 or 19.5 l/min (respectively). Mean
thermodilution-derived CO was 6.2 ± 4.2 l/min whereas the
Fick’s was 7.0 ± 4.3 l/min. There was statistical significant
correlation between the two modalities of measurements, with
an r2 = 0.9 (P < 0.001). Conclusions. The new method of
Fick assessed emergent CO as reliably as the thermodilution,
regardless of whether it was low or high. The use of Scvo2
allows for prompt bedside calculation for most emergency
patients.
ABSTRACT. Background. For many years thermodilution has
KEY WORDS. cardiac output, Fick, thermodilution, Scvo2, Svo2.
INTRODUCTION
Avi A. Weinbroum and Philippe Biderman concurred equally to
the present investigation.
From the 1Department of Anesthesiology and Critical Care,
Tel-Aviv Sourasky Medical Center and Sackler Faculty of
Medicine, Tel-Aviv University, Tel-Aviv, Israel; 2Post Anesthesia
Care Unit and Animal Research Laboratory, Tel Aviv Sourasky
Medical Center and Sackler Faculty of Medicine, Tel-Aviv
University, 6 Weizman Street, Tel Aviv 64239, Israel; 3Division of
Surgery and Surgical Intensive Care Unit, Tel-Aviv Sourasky
Medical Center and Sackler Faculty of Medicine, Tel-Aviv
University, Tel-Aviv, Israel.
Received 22 May 2008. Accepted for publication 22 September
2008.
Address correspondence to A. A. Weinbroum, Post Anesthesia
Care Unit and Animal Research Laboratory, Tel Aviv Sourasky
Medical Center and Sackler Faculty of Medicine, Tel-Aviv University, 6 Weizman Street, Tel Aviv 64239, Israel.
E-mail: [email protected]
Quick but reliable determination of cardiac output (CO)
and stroke volume is of essential importance for the
management of patients arriving in the emergency
department (ED) or intensive care unit (ICU), especially
among patient who are of high risk. Optimization of
hemodynamic conditions is also essential during the early
phase of resuscitation of septic patients [1]. Since its
introduction in the 1970s, the thermodilution technique,
through a pulmonary artery catheter (PAC, e.g., Swan
Ganz catheter), has been routinely and satisfactorily used
[2]. Nevertheless, the difficulties and risks associated with
its insertion and maintenance, may account for the
emergence of new, less invasive, modalities to monitor
the hemodynamic status in the critically ill patient [3, 4].
Several previous studies have reported contradictory
results when assessing CO using Fick’s principle in the
critically ill patient. In a study by Bremer et al. [5], the
Journal of Clinical Monitoring and Computing
and ‡10 min of hemodynamic stability prior to measurements.
average CO obtained by this method was higher than that
obtained using thermodilution; however, the difference
was not statistically significant. In another study, involving
fifteen critically ill patients, the bias between the two
methods was 1.7 ± 3.8 l/min, the values obtained by
Fick’s method yielding the higher results [6]. In these and
other studies, mixed venous O2 saturation (Svo2) measurements required the placement of a PAC, with risks
that may surplus its benefits [4, 7, 8].
The central venous catheter is routinely introduced in
ED patients and used in most – if not all – critically ill
patients, both for the administration of medications and
for right heart monitoring. In recently published
guidelines, central venous oxygen saturation (Scvo2) and
Svo2 were equally suggested for the management of the
septic patient [9]; some authors, however, reported
conflicting data [10–12].
The primary aim of this study was to assess whether CO
diagnostic data obtained by Fick’s method upon the arrival of patients in the ED or ICU were as reliable as the
thermodilution-originated values. The secondary goal was
to determine the usefulness of Scvo2 as a clinically reliable
tool during such fast measurements in critically ill patients.
Thermodilution cardiac output (TCO)
Correct placement of a PAC (Arrow, Arrow International, Reading, PA, USA) was verified by pressure
waveform during the insertion and by later chest radiography. Thermodilution-derived CO (TCO) was obtained
by injecting 10 ml of 5% dextrose in water at roomtemperature into the proximal injection port of the PAC
within a 5 s period, at random times throughout the
respiratory cycle. The average of three satisfactory curves
yielded mean cardiac output measurement value.
A commercial CO monitor (S/5 Critical Care Monitor,
Datex-Ohmeda, Madison, WI, USA) was used to
calculate the end results.
Fick’s method of cardiac output measurement (FCO)
These measurements were accomplished simultaneously
with the TCO ones. Specifically, oxygen consumption
was determined from the measurements of carbon dioxide
and oxygen contents in the inspired and expired gases,
using a standard metabolic monitor (S/5 M-COVX
Datex-Ohmeda, Madison, WI, USA) calibrated prior to
the beginning of each series of measurements in each
patient. Ten min of metabolic steady state was required
( £ 5% changes in the respiratory quotient [R], oxygen
consumption [Vo2] and carbon dioxide production
[Vco2]). Blood gas analysis and hemoglobin concentrations were simultaneously retrieved from arterial, central
venous and mixed venous blood, using the RapidLab 865
Analyzer (Bayer Healthcare Diagnostics, Munich,
PATIENTS AND METHODS
Fifteen mechanically ventilated patients were studied
(Table 1). Criteria for inclusion were their recent (within
2 h) arrival in the ED or in ICU; the placement of PAC by
the physician in charge; controlled mechanical ventilation
without patient’s attempting spontaneous respiration, the
requirement for fractionated inspired oxygen partial pressure (FiO2) £ 0.6, stable level of inspired oxygen fraction
Table 1. Individual data of the reported patients
Patient
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Age
Gender
1ry Diagnosisa
VO2
Svo2
Scvo2
TCO
FCO
66
M
1
207
63
62
4.9
5.3
73
M
1
300
75
78
7.4
10.2
53
F
1
289
69
70
8.1
8.7
28
F
2
150
70
71
3.1
3.7
72
M
2
350
66
68
5.7
7.6
71
M
1
150
70
72
3.1
3.7
71
F
1
305
75
74
6.3
9.6
53
M
1
140
74
74
3.1
3.7
57
F
1
167
63
76
4.1
3.4
75
F
2
318
64
61
6.1
6.8
53
M
1
363
72
70
8.5
10.4
24
M
3
170
82
80
2.3
2
72
F
1
265
62
70
5.3
5.9
56
M
1
330
80
84
19.5
19
64
F
1
230
63
61
4.2
4.6
a
Hemodynamic shock due to 1 = sepsis; 2 = cardiogenic; 3 = hypothermia.
Abbreviations: BMI: body mass index, TCO: thermodilution-derived cardiac output, CI: cardiac index, FCO: Fick-derived cardiac output,
VO2: oxygen consumption, Svo2: mixed venous O2 saturation, Scvo2: central venous oxygen saturation, SpO2: arterial saturation.
Weinbroum et al.: Emergency Fick Cardiac Output Measurement
Germany). The mixed venous sample was obtained via
the distal port of the PAC, the arterial blood was drown
via radial or femoral arterial catheter and the central venous sample was taken through a subclavian or jugular
line, which site of placement was verified previously by a
chest X-ray. Thus, CO was determined using Fick’
metabolic equation (FCO), as expressed by the ratio of
Vo2 to the difference between the arterial oxygen content
and the central venous oxygen content (Cao2 - Cvo2) 9
10, where the variables were defined as oxygen consumption [Vo2], arterial and central oxygen contents
[Cao2 - Cvo2].
Statistical analysis
The statistical analyses were performed at the Statistical
Laboratory of the School of Mathematics, Tel-Aviv
University, using the SPSS Release for Windows, Version
11.01 (Chicago, IL, 2001). A linear regression test was
done to correlate CO data of the two modalities, using the
equation of Y(=FCO) = A + B*X(X = TCO). In
addition, looking at the same variable at the same time
under identical conditions, and since previous data have
shown that despite the good correlation, poor agreement
could be demonstrated [13], we also analyzed the data by
the Bland and Altman method. We also determined CO
data obtained in the same patient by the two methods,
which the difference between them reached a maximum
of 20%. This was done based on previous reports indicating this difference as clinically acceptable. Values are
expressed as mean ± standard deviation (SD), with significance defined as P £ 0.05.
RESULTS
Patients’ average age was of 62 (range 24–73) years. Eleven of the patients suffered from septic shock, three from
cardiogenic shock, and one patient from low cardiac
output due to severe hypothermia (Tables 1, 2). CO
measurements ranged between 2 and 19 l/min (by Fick
method) and 2.3–19.5 l/min (thermodilution method).
The mean cardiac output measurements by the two
modalities were similar as well (Table 2).
Data correlation
Correlation between the CO data obtained by the thermodilution and the Fick modalities are reported in
Figure 1A. There was a statistical significant correlation
between the two series of measurements, with an
r2 = 0.925 (P = 0.0001). The correlation between the
Table 2. Demographic, physical and measurements values
(mean ± SD, or absolute values)
Parameters
Values
Age (years)
Gender (M/F)
BSA
1ry Diagnosis (1/2/3)a
FiO2
TCO (l/min)
CI (l/min/kg)
FCO (l/min)
VO2 (ml/min)
SvO2 (%)
ScvO2 (%)
SpO2 (%)
Hemoglobin (g %)
DAV (mmHg)
62 ± 14
8/7
2.0 ± 0.3
11/3/1
0.5 ± 0.1
6.2 ± 4.2
3.5 ± 0.6
7.0 ± 4.3
248 ± 79
70 ± 6.4
70.8 ± 6.8
97.0 ± 1.6
10.3 ± 1.4
27.7 ± 4.9
a
Hemodynamic shock due to 1 = sepsis; 2 = cardiogenic;
3 = hypothermia.
Abbreviations: BSA: body surface area, TCO: thermodilutionderived cardiac output, CI: cardiac index, FiO2: fractionated
inspired oxygen partial pressure, FCO: Fick-derived cardiac
output, VO2: oxygen consumption, Svo2: mixed venous O2
saturation, Scvo2: central venous oxygen saturation, SpO2:
arterial saturation, DAV: arterio-venous oxygen content.
Svo2 and the Scvo2 saturation values also reached a statistical significance (Figure 1B).
Following the Bland and Altman analysis of data, the
thermodilution and the Fick determinations of CO were
found interchangeable, with (Figure 2) an excellent
degree of agreement between the two series of data, both
of the CO and of the venous saturations.
In this study, 11 out of 15 (73%) CO measurements
agreed within 20% of each other, while the other four
agreed within 35% of their values. When comparing the
saturation data, agreement within 20% of each other
included all sets of values in either method.
LIMITATIONS
This report indicates the usefulness of Fick’s method upon
the arrival of patients in the ED or ICU. Further assessment of the reliability of the method as compared to the
thermodilution during patients’ stay is currently under
investigation. Also, the method could possibly result in
inaccuracies in some critical patients such as septic patients
and those with acute respiratory failure: Scvo2 values may
vary significantly from Svo2, and there may exist a significant level of pulmonary shunting, respectively.
Journal of Clinical Monitoring and Computing
A 20.00
Cofick
15.00
10.00
5.00
R Sq Linear= 0.925
0.00
0.00
10.00
5.00
15.00
20.00
CO
B 85.00
80.00
Scvo2
75.00
70.00
65.00
R Sq Linear = 0.84
60.00
60.00
65.00
70.00
75.00
80.00
85.00
Svo2
Fig. 1. (A) Correlation between the Fick metabolic (COfick) and the
thermodilution-derived (CO) cardiac output. (B) Comparison between the
central venous oxygen saturation (Scvo2) and the mixed venous oxygen
saturation (Svo2). Both regression equations are shown along with their
relative confidence limits.
Fig. 2. The Bland and the Altman method’s correlation for both the two
CO measurements (thermodilution-derived CO [CO] and Fick-derived
CO [COfick]) and blood saturations (central venous oxygen saturation
[Scvo2] and the mixed venous oxygen saturation [Svo2], A, B, respectively).
DISCUSSION
The present study demonstrates, apparently for the first
time, that the thermodilutionally-measured CO and the
reported Fick-based CO value – that uses Scvo2 –
correlated closely. Furthermore, this extreme similarity
existed both in septic and in non-septic patients. It appears
that the newly-introduced non-invasive methodology of
FCO is as reliable as TCO and therefore clinically
promising even in ED patients, even in patients with both
extremely low (2 l/min) and very high (19 l/min)
Weinbroum et al.: Emergency Fick Cardiac Output Measurement
CO. Furthermore, most (73%) of the cardiac output
values showed good agreement within 20% difference
from one modality value to another, while in the rest the
agreement laid within a 35% difference. Given these side
by side statistical agreements, we believe that for an initial,
quick but reliable diagnostic assessment of the hypotensive
patient in the ED or ICU, the FCO method does not
differ from the TCO methods and is of clinical reliability
as is the latter.
Our results are in agreement with the report by Capderou et al. who assessed hemodynamic patterns by the
Fick method during cardiac catheterization in children
[14]. Similarly to us, they found that the Fick method
correlated linearly with TCO values. In addition, and as
demonstrated previously by Bremer et al. [5], and by
Engoren and Barbee [6], we found that the two modalities
are equivalent in all measurements, even though average
Fick’s CO measurements were slightly higher than the
results obtained by the thermodilution method. Nevertheless, in view of the correlation data obtained by the
two statistical methodologies, these differences are of no
clinical relevance for the clinician during the initial
emergent diagnostic bed-side measurements.
Importance of the Fick’s method
The use of TCO has been considered a cornerstone in
cardiovascular and hemodynamic evaluation. In an early
publication, Nishikawa and Dohi [15], described 13 factors that can result in false results when using TCO,
leading to the assertion that this may not be the ‘‘Gold
Standard’’ for the measurement of cardiac output. In another report [16], variance of up to 13% between the
results of three measurements of cardiac output was
considered acceptable variability. Based on the prior reports by Sageman and Amundson [17] and by Baylor [13],
we opted for 20% acceptable clinical variability between
the series of data originating from the two techniques (i.e.,
thermodilution vs. Fick). Our results are, indeed, superior
to those of Baylor [13] that found only 57% of the cardiac
output values in agreement within 20% of each other, as
compared to 73% in the present study. This difference in
the cohorts is probably due to the fact that instead of using
an average Fick’s technique-related Vo2 values obtained
over 20 min before the TCO measurement, we used the
values obtained simultaneously with the TCO measurement. Furthermore, the present close correlation coefficients between the two groups of CO data existed both
when measuring both very high and extremely low values. Also, all Svo2 and Scvo2 data herein presented were
within the mentioned 20% of agreement in all patients.
An additional support to the accuracy of the Fick method
herein proven originates from an animal study involving
mechanically ventilated septic pigs [18]. In that study,
Marx et al. found good correlation between cardiac
output measurements by arterial trans-cardiopulmonary
and arterial thermodilution with the Fick’s method even
during hemodynamic instability.
The usefulness of simultaneously Svo2 and Scvo2
measurements
Unlike previous reports that looked into the Fick’s method
that used an average of Vo2 over 30 min, we measured
simultaneously Svo2 and Scvo2 as well; these also correlated significantly and closely between themselves. As reported by Reinhart [19, 20] and others [21, 22] earlier, the
habitual difference of 2–3% between Svo2 and Scvo2 may
change under conditions of shock. In septic shock, for
example, oxygen consumption of the gastrointestinal tract
may increase despite the increased regional blood flow,
while cerebral blood flow is constant over time. In those
cases, Scvo2 may exceed by as much as 8% the Svo2 [19].
These data, however, must be considered cautiously, since
several recent publications [13] suggested that Scvo2 can
not replace Svo2 value in managing the septic patient.
In retrospect, finally, importance must also be given to
the changes data may undergo during the first stages of
treatment. Such assessments could be of interest and
warrant additional investigations.
CONCLUSIONS
In summary, to the best of our knowledge, we are the first
to demonstrate that the Fick method determination of
CO, using Scvo2 measured via central venous catheter is
as reliable and accurate as the thermodilutionally-obtained
CO, both in low and high COs. Since central vein
catheterization is placed in most critically ill patients upon
their arrival in the ED, at least for the initial and emergent
determination of the nature of the shock, determination
of CO in such patients can be obtained rapidly and not
invasively, which facilitates treatment and circumvents
complications. Further studies are needed to determine
whether this method is as useful beyond the initial diagnostic CO measurement, as for continuous management
of the hemodynamically unstable patient in the ICU.
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