The FloTract System—
Measurement of Stroke Volume
and the Assessment of Dynamic
Fluid Loading
Brian Hashim, MD
Adam B. Lerner, MD
Beth Israel Deaconess Medical Center
Boston, Massachusetts
The FloTrac system was introduced in the United States by Edwards
Lifesciences of Irvine, CA in April of 2005. As with other potentially
competing systems which are discussed elsewhere within this publication, namely PiCCO and LiDCO, this system has been marketed as a
device that can provide accurate and reliable measurements of cardiac
output (CO) in a continuous fashion as well as calculations of systemic
vascular resistance (SVR) and the dynamic parameter of stroke volume
variation (SVV) (FloTrac System Brochure, Edwards Lifesciences, 2007).
SVV, important for its ability to predict fluid responsiveness, is also
discussed in detail elsewhere within this publication.
The heart and soul of the FloTrac system is a proprietary software
algorithm that analyzes characteristics of the arterial pressure waveform
and uses this analysis along with patient-specific demographic information to determine blood flow. Obviously, not all elements of this
algorithm have been revealed by Edwards Lifesciences so as to protect
their product from competitors. However, the foundation of this
algorithm has been made available by them and can be summarized
as follows. (Case Study—‘‘Getting ml/beat from mmHg’’ Arterial
REPRINTS: ADAM B. LERNER MD, DEPARTMENT OF ANESTHESIA, CRITICAL CARE, AND PAIN MEDICINE, BETH
ISRAEL DEACONESS MEDICAL CENTER, 1 DEACONESS ROAD CC470, BOSTON, MA 02215; E-MAIL: ALERNER@
BIDMC.HARVARD.EDU
INTERNATIONAL ANESTHESIOLOGY CLINICS
Volume 48, Number 1, 45–56
r 2010, Lippincott Williams & Wilkins
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Hashim and Lerner
Pressure-based Cardiac Output, The Edwards FloTrac Algorithm, Edwards
Lifesciences, 2008.)
Starting with CO = heart rate (HR) stroke volume (SV), the system
first uses the arterial trace to determine a pulse rate (PR) calculated by
counting the number of pulsations in 20 seconds. Though PR represents
only perfused cardiac beats and therefore is not always equivalent to
HR, the algorithm modifies the equation to CO = PR SV. This
modification can become important when comparing SV measurements
derived from other techniques, such as thermodilution, when HR and
PR may diverge. For this reason, and because of their impact on the
arterial waveform itself, persistent dysrhythmias can have significant
impact on the accuracy of FloTrac-derived data. In these settings, the
FloTrac system-derived measurements should probably not be used to
make clinical decisions.
For the determination of SV, the algorithm uses the following
equation: SV = the standard deviation of the arterial pressure (sAP) chi (w). The standard deviation of the arterial pressure (sAP) is
practically related and proportional to pulse pressure and to SV. The
sAP around the mean arterial pressure (MAP) is used in an attempt to
eliminate the effects of changes in vascular tone. The sAP is then used to
determine the standard deviation of the pulse pressure as this is directly
proportional to SV. Changes in sAP also provide information on the
amplitude of the pressure waveform. When this amplitude information
is put into the context of the kurtosis of the waveform, that is, the
concentration of the waveform about the mean pressure or the
distinctness of the peak, it allows for an attenuation of the impact of
pressure wave reflectance and differences in arterial compliance
between different arterial sites. (Fig. 1) This is what allows for the
potential use of any arterial cannulation site in the FloTrac algorithm.
In the current version of the FloTrac algorithm, version 1.10, sAP is
calculated after 20 seconds of waveform retrieval are analyzed at 100
times per second (2,000 generated data points).
One of the most essential components of the algorithm is w. It is a
proprietary polynomial equation that relates the impact of vascular tone
on pulse pressure. The variables used to determine w are PR, sAP, MAP,
and an estimate of arterial compliance based on patient demographics
such as body surface area, age, sex, and on the shape of the arterial
waveform. When referring to shape, the algorithm specifically analyzes
the kurtosis, described above, and skewness, or lack of symmetry,
inherent in the waveform. These characteristics of overall shape are
directly related to the resistance and compliance of the arterial tree.
(Fig. 1) The ‘‘rough’’ FloTrac algorithm estimates of aortic compliance
are based on the work carried out by Langewouters et al.1,2 Using
cadavers, Langewouters et al were able to find a direct correlation
among age, sex, and MAP as they relate to aortic compliance and
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Figure 1. The impact of arterial resistance and compliance on arterial waveform skewness and
kurtosis, respectively. As depicted by the waveforms on the left, as arterial resistance increases, the
skewness or lack of symmetry of the arterial waveform also increases. As depicted on the right, as
arterial compliance increases, the kurtosis or distinctness of the peak of the arterial waveform
decreases. The skewness and kurtosis of the arterial waveform are measured by the FloTrac system
every 60 seconds to assess the impact of arterial compliance and resistance on pressure. This is used to
determine w. (Adapted from: Case Study—‘‘Getting mL/beat from mm Hg’’ Arterial Pressure-based
Cardiac Output, The Edwards FloTrac Algorithm, Edwards Lifesciences, 2008.)
developed a mathematical formula to determine compliance based on
these variables. The algorithm uses the analysis of the skewness and
kurtosis of the arterial waveform to further refine the resistance and
compliance estimates.
Using all of these variables, the algorithm makes a determination of
w. The first version of FloTrac software, version 1.01, performed this
determination or calibration of w every 10 minutes. As changes in
hemodynamics and clinical interventions can occur in a much shorter
time frame, this 10-minute interval was seen as a potential limitation to
the accuracy of FloTrac-derived data. This seemed to be demonstrated
in several of the investigations of the device with this software version.3–7
The software has undergone 3 revisions to deal with this issue. The
newest version of software, version 1.10, and its immediate predecessor,
version 1.07, perform the recalibration of w every 60 seconds. This modification has been demonstrated by several investigators to improve the
accuracy of the system’s results.8–10
Once w and sAP are calculated, SV is determined and output to the
Vigileo display system. Calculations of CO, SVR, and SVV can then
follow from the SV determination.
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The FloTrac has some features that are appealing to the clinician.
First off and as mentioned earlier, the system can theoretically be used
with any arterial line; there is no need to place a specially designed
proprietary catheter. As many cardiac surgical as well as critically ill
patients already have an arterial line in place, there is no additional risk
involved with the placement of other invasive vascular lines. In addition,
there is no apparent limitation as to the location of the arterial
cannulation. This flexibility is enabled by the FloTrac algorithm’s
supposed ability to adjust for differences in waveform that result from
these variations. In support of this, the system has been shown by some
investigators to provide reliable measurements from either a radial or
femoral arterial catheter location.7,11 However, other investigators have
expressed concern about differences in measurements of pressure from
different sites, particularly in the setting of hemodynamic instability
and/or the use of vasopressors. Compton et al12 found significant
introduction of error in FloTrac measurements of CO when differences
in MAP between 2 different sites were more than 5 mm Hg. Although
the site of cannulation may not be important, the quality or ‘‘fidelity’’ of
the tracing is extremely significant. As the system relies on an analysis of
waveform shape, proper dampening of the arterial monitoring system is
required to improve the accuracy of the FloTrac system’s calculations.
(Case Study—‘‘Getting ml/beat from mm Hg’’ Arterial Pressure-based
Cardiac Output, The Edwards FloTrac Algorithm, Edwards Lifesciences, 2008.)
The second major appealing feature to the clinician is the fact that
the FloTrac system does not require external calibration. As described in
detail, the FloTrac algorithm does its own calibration based on patient
demographics and waveform analysis. The PiCCO and LiDCO systems
first released into clinical practice required that a manual calibration be
performed in the individual patient and at relevant intervals to allow for
accurate calculations. The PiCCO system requires a transpulmonary
thermodilution for calibration whereas the LiDCO system requires lithium
dilution.13,14 These calibrations allow for adjustments to individual patient
variables such as vascular tone. Of course this lack of need for external
calibration in the FloTrac system is only a potential benefit; it is realized
only when and if the internal calibration results in valid and accurate
measurements.
There are several principal limitations to the FloTrac system as
outlined by the manufacturer. First off, the device has only been studied
in and is only recommended for use in adult patients. Second, as there is
total dependence on the arterial line waveform for accurate calculations,
the FloTrac system requires a consistent and regular waveform that is of
high fidelity. The presence of dysrhythmias or improper dampening
from kinking or extra tubing and stopcocks can prevent accurate results.
In addition, because of the impact on the arterial waveform, Edwards
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specifically considers the presence of significant aortic regurgitation as a
limitation. This is based on the work of Lorsomradee et al4 who, using
software version 1.07, showed a loss of accuracy in CO measurements in
the presence of ‘‘significant’’ aortic regurgitation. Of note, in this study,
‘‘significant’’ aortic stenosis did not seem to impact accuracy. Third, the
device has not been validated in the setting of intra-aortic balloon pumps
(IABP) or ventricular assist devices. Lorsomradee et al4 showed that the
presence of an IABP prevented the FloTrac system from obtaining
results in several patients and that when results were obtained, the
accuracy was significantly affected.
Lastly, and perhaps most troubling, is the manufacturers consideration of ‘‘shock states or hypothermic episodes’’ as limitations for use as
these conditions ‘‘may influence values with radial arterial locations.’’
(Case Study—‘‘Getting ml/beat from mm Hg’’ Arterial Pressure-based
Cardiac Output, The Edwards FloTrac Algorithm, Edwards Lifesciences,
2008.) The issue with this limitation is that it leads to speculation as to what
degree or type of ‘‘shock’’ or degree of hypothermia is problematic in
individual circumstances. This naturally leads to concern with how the
FloTrac device will perform in patient situations where there are rapid
changes in vascular tone, patient positioning, patient temperature, and
volume status, to name just a few. Similarly, as the FloTrac calculations are
based on algorithm tuned estimates of vascular compliance and resistance,
seemingly any significant change in vascular tone, whether induced by
reflex or by medications, could impact the accuracy of the data provided.
Although these issues may impact radial arterial catheters to a greater
degree, they could potentially affect arterial catheters in any location.
Similarly, any deviations in arterial compliance from FloTrac estimates will
introduce error. This sentiment of concern has been echoed by several
investigative groups.4,5,11,12,15,16 It may in fact be the case that in these
patient situations the actual individual FloTrac-derived calculations may be
of less value than their general trends over time particularly as an
assessment of response to therapy.
An overview of the potential benefits and weaknesses of the FloTrac
system is provided in Table 1. The rest of this chapter will provide a
review of the current literature and its assessment of the system in a
variety of clinical environments.
’
Intravascular Fluid Assessment
and the FloTrac System
As mentioned earlier, in addition to CO, the FloTrac system provides
other data that can be useful in patient care. These data include SV,
SVR, and the dynamic index of SVV. SVV, which will be discussed in
detail elsewhere within this text, has shown great promise in assessing
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Table 1. Strengths and Weaknesses of the FloTrac Monitoring System
Strengths
Ability to use any arterial line
No need for central venous access
No required external calibration
No need for recalibration
Weaknesses
Requirement of consistent arterial
waveform with proper dampening
Decreased accuracy in setting of altered
vascular tone (eg, sepsis, vasopressor
use, etc)
Dependence on a regular rhythm
Limited accuracy during times of
hemodynamic instability
the ‘‘fluid responsiveness’’ of mechanically ventilated patients in a
variety of clinical situations.17–19 Briefly, SVV is caused by the interaction
of the cardiac and respiratory systems; that is, the changes in intrathoracic
pressure during controlled ventilation have an impact on SV. This impact
on SV increases as preload decreases.20 In addition to preload, other
factors affecting SVV include chest wall compliance and ventilation
parameters, including tidal volumes and airway pressures. In situations
wherein chest wall compliance and respiratory parameters are held
relatively constant, SVV can be used as a guide to establishing whether or
not a given patient will respond to fluid loading by increasing CO.
To our knowledge, the ability of FloTrac system assessments of SVV
to predict fluid responsiveness is limited to 6 investigations. Three of
these investigations were performed in cardiac surgical patients, 2 in
patients undergoing esophageal surgery, and 1 in patients undergoing
liver transplantation. de Waal et al21 assessed FloTrac SVV (version 1.01)
in 18 postoperative coronary artery bypass grafting (CABG) patients.
They found that FloTrac SVV could not predict fluid responsiveness
and suggested further investigation with the newer software versions.
Hofer et al16 compared SVV measurements determined by PiCCO and
FloTrac (version 1.07) in 40 postoperative cardiac surgical patients.
They found that both devices performed comparably in terms of
predicting fluid responsiveness but that the FloTrac-derived SVV had a
lower threshold value for determining responsiveness (9.6% compared
with 12.1% for PiCCO). Cannesson et al18 found that FloTrac SVV
(version 1.10) predicted fluid responsiveness with ‘‘acceptable sensitivity
and specificity’’ (82% and 88%, respectively). Kobayashi et al22 found in
2 separate investigations concerning patients requiring major esophageal surgery, 1 postoperative and the other intraoperative, that FloTrac
SVV provided reliable data in determining fluid responsiveness.23 Biais
et al17 investigated whether FloTrac SVV (version 1.07) was able to
predict fluid responsiveness in 35 patients after liver transplantation.
They found that SVV greater than 10% was highly sensitive and specific
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for fluid responsiveness (94% and 94%, respectively). They also found
a significant correlation between the decrease in SVV with volume
expansion and the subsequent increase in CO. Also relevant was the fact
that 5% of patients initially enrolled in the study had to be excluded due
to dysrhythmias.
Although only a small number of patients have been studied, it does
appear that the newer FloTrac software versions allow for meaningful
assessments of SVV. Further studies can serve to reinforce this in other
patient populations and to establish what, if any, adjustment to SVV
threshold for determining fluid responsiveness is necessary and specific
to the FloTrac system.
’
The FloTrac System as a CO Monitor
Before entering into a discussion or review of the FloTrac system’s
accuracy as a monitor of CO, it is important to talk about the assessment of
accuracy. The first issue to consider is the reference point for comparison.
In the case of CO measurement, thermodilution performed with a
pulmonary artery catheter is generally considered as the practical gold
standard. This is not at all unreasonable given the relatively thorough
investigation of thermodilution that has occurred over several decades.
When a new technique for measuring a given parameter is to be assessed
against a gold standard, evaluating correlation between them is not
adequate as correlation says little about how close any pair of
measurements actually is. Correlation only gives information as to how
the series of measurements from the 2 techniques relate to one another.
This shortcoming becomes even more relevant when the standard
technique has significant error in and of itself. Typically, Bland-Altman
analyses, wherein the mean of the result of the 2 techniques is plotted
against the difference in their measurements, are performed.24 These
analyses allow for the determination of bias (average difference), precision
(1 standard deviation of the result comparisons), limits of agreement
(2 standard deviations of the result comparisons), and the percentage
error [( ± 2 standard deviations)/mean result] between the 2 techniques.
It is important to take into account that the inherent error for
thermodilution measurements of CO are in the 10% to 20% range.25
On the basis of this error inherent to the gold standard, Critchley
and Critchley26 recommended that any new method of measurement
for CO be judged acceptable if it also had a similar inherent error. This
led to their generally accepted conclusion that so long as the limits of
agreement between the new method and thermodilution were within
± 30%, the technique should be considered as acceptable. Given this
backdrop, a review of the literature on the FloTrac system as a monitor of
CO now follows.
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To our knowledge, there have been 18 studies performed to assess the
FloTrac system’s accuracy as a CO monitor. Most of these investigations,
that is, 12 of the 16 (75%), have occurred in cardiac surgical patients.3–5,7–
9,11,15,27–31
Some of the studies are limited to the intraoperative period,
some to the postoperative period, and others include both of these
environments. Most have looked at patients that had their surgery
performed with cardiopulmonary bypass3–5,7,9,15,28,30 whereas some have
looked specifically at patients undergoing off-pump CABG.8,29,31 The
likely reason for this focus on cardiac surgical patients is that in most
centers a pulmonary artery catheter is routinely placed in these patients
and, presumably, all centers place an invasive arterial catheter. Therefore,
such study requires little deviation from institutional standards of care for
monitoring in this patient population. Of the remaining 5 studies, one
involved ICU patients with sepsis,6 one investigated ‘‘unstable’’ patients in
a medical ICU,12 one looked at a combination of surgical and medical
ICU patients,10 one looked at neurosurgical ICU patients,32 and one
involved patients undergoing liver transplantation.33 In total, 575 patients
have been involved in these assessments of the FloTrac system’s accuracy.
Using the original version 1.01 of the FloTrac algorithm, Mayer et al
studied 40 patients undergoing CABG or cardiac valve procedures.
Some of the measurements were made intraoperatively and some
postoperatively, all at predetermined times. 33% of patients had to be
excluded from the intraoperative study period and 15% from postoperative period, mostly due to dysrhythmias. They found unacceptable
limits of agreement (>30%) between CO measurements made with
FloTrac and intermittent bolus thermodilution in both the intra and
postoperative periods (51% and 42%, respectively) As a result, they
recommended that FloTrac’s use be limited to the monitoring of trends.
In their initial study, Zimmermann et al, using FloTrac version 1.01
performed a very similar study in cardiac surgical patients and had very
similar results. They concluded that the FloTrac system’s accuracy did
not meet Critchley and Critchley criterion for acceptable accuracy.7
Interestingly, though these two studies led to concern about version
1.01, the rest of the published studies that used this initial software
found that CO measurements were within the acceptable range for
accuracy in certain situations. de Waal et al’s study of intraoperative and
postoperative CABG patients compared FloTrac to bolus thermodilution
and PiCCO. They found that accuracy criteria were successfully met
during the post cardiopulmonary bypass (CPB) and postoperative
periods but not during the pre-CPB period.3 Manecke et al also found
acceptable accuracy compared to both bolus and continuous thermodilution techniques in post-operative cardiac surgical patients deemed as
‘‘reasonably stable.’’11
Most of the studies published in the literature on FloTrac CO
measurements were performed on intermediate versions of the software,
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namely 1.03 and 1.07. Chatti et al, in a multi-center study, compared CO
measurements made using FloTrac versions 1.03 and 1.07 to those from
esophageal Doppler probes in ICU patients.10 They found improvements in accuracy with version 1.07 but that the limits of agreement
were still only 59%. Sakka et al also found unacceptable accuracy in
septic patients receiving norepinephrine with version 1.07.6 However, in
this study, the reference used for comparison was PiCCO derived
transpulmonary thermodilution. They found that that the FloTrac
system generally underestimated the CO as compared to the data
provided by PiCCO, a finding similar to that of Compton et al.12 Prasser
et al’s study in neurosurgical ICU patients using version 1.03 also found
unacceptable accuracy.32 Lorsomradee et al, in their study of intraoperative CABG patients, also found unacceptable accuracy of CO
measurements made with version 1.07 compared to continuous thermodilution.4 Of note, the accuracy was particularly worsened in the
presence of aortic regurgitation or an IABP.
Senn et al found acceptable accuracy of version 1.07 compared to
intermittent thermodilution CO measurements in post-operative cardiac
surgical patients.8 This finding was echoed by Mehta et al who compared
version 1.07 to intermittent thermodilution during the intraoperative
period of off-pump CABG procedures.31 Although Button et al.
concluded that FloTrac version 1.07 was comparable to PiCCO,
continuous thermodilution, and intermittent measurements of CO in
intraoperative and postoperative cardiac surgical patients, they did not
report their limits of agreement.28 This prevents a true assessment of the
degree of accuracy within their study.
The newest version of the FloTrac software seemed to bring some
improvement in accuracy, at least in cardiac surgical patients. Most
encouraging was the fact that when the Mayer and Zimmermann groups
repeated their assessments with the most current software version,
version 1.10, they found improvements in the accuracy of the CO
measurements. Mayer et al30 found acceptable accuracy in CABG patients
during both the intraoperative and postoperative periods, with limits of
agreement of 28.3% and 20.7%, respectively. Zimmermann et al,9
although finding improvements in accuracy compared with their previous
study, still did not find acceptable limits of agreement and reaffirmed
their recommendation that FloTrac ‘‘not be used during and after cardiac
surgery.’’ Prasser et al27 found acceptable accuracy in postoperative CABG
patients compared to bolus thermodilution.
The only other studies that evaluated FloTrac version 1.10 were in the
noncardiac surgical environment. Both of them found that FloTrac’s
accuracy was not acceptable in their study population. Compton et al12
studied 25 ‘‘unstable’’ medical ICU patients. The criteria for being
considered ‘‘unstable’’ were defined as the need ‘‘for fluid resuscitation
and/or vasopressor therapy.’’ It is important to note that in their study the
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standard for measurement was PiCCO-derived pulse contour CO, not
bolus thermodilution through a pulmonary artery catheter. They found a
high percentage of error (58.8%) between the 2 techniques and concluded
that FloTrac was ‘‘not suitable to replace invasive CO monitoring.’’ Biancofiore et al33 compared FloTrac and continuous thermodilution
CO measurements during the intraoperative and postoperative phases of
liver transplantation. They also found unacceptable limits of agreement.
Summarizing the published experience with the FloTrac system is
extremely difficult given the heterogeneity of the available studies.
Differences in patient populations, study environments (intraoperative,
postoperative, nonsurgical), FloTrac software versions, ventilatory settings, medical interventions, and in the reference standard(s) used
(intermittent thermodilution CO, continuous thermodilution CO, esophageal Doppler, PiCCO), combined with the relatively small patient
numbers in each study, are all central to this issue. However, there are
several general summary statements that seem warranted. First off, there
seems to be enough data available to question the accuracy of
the system’s measurements in a variety of situations. This seems to be
particularly true in ‘‘unstable’’ conditions; situations where there are rapid
and significant changes in temperature, vascular tone (from any cause),
and/or intravascular volume. In addition, in patient populations wherein
significant alterations in vascular tone are inherent (sepsis, liver failure)
there also seems to be unacceptable inaccuracies. Obviously, these are the
situations where FloTrac-derived data would likely be most helpful in
developing care plans. To what extent or how the loss of accuracy in these
settings should impact the use of this device has not yet been determined.
Second, although it does seem that the newer FloTrac software versions
have improved the accuracy of the system’s ability to determine CO, this
has not consistently led to sufficient limits of agreement with standard
techniques. Lastly, the FloTrac system certainly does seem to be able to
accurately measure CO in several challenging environments. This, in
addition to its ease of use, makes FloTrac a potentially useful device in the
management of many challenging patient care situations. Future
improvements in system software and the study of more patients and
patient settings will be necessary to better define the role of this unique
monitor in the care of the critically ill patient.
’
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