Technology, Computing, and Simulation
Section Editor: Maxime Cannesson
Continuous Noninvasive Arterial Pressure Monitoring in
Obese Patients During Bariatric Surgery: An Evaluation
of the Vascular Unloading Technique (Clearsight system)
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Dorothea E. Rogge, MD,* Julia Y. Nicklas, MD,* Gerhard Schön, MSc,† Oliver Grothe, PhD,‡
Sebastian A. Haas, MD,* Daniel A. Reuter, MD,* and Bernd Saugel, MD*
BACKGROUND: Continuous monitoring of arterial pressure is important in severely obese
patients who are at particular risk for cardiovascular complications. Innovative technologies for
continuous noninvasive arterial pressure monitoring are now available. In this study, we compared
noninvasive arterial pressure measurements using the vascular unloading technique (Clearsight
system; Edwards Lifesciences Corp, Irvine, CA) with invasive arterial pressure measurements
(radial arterial catheter) in severely obese patients during laparoscopic bariatric surgery.
METHODS: In 35 severely obese patients (median body mass index, 47 kg/m2), we simultaneously recorded noninvasive and invasive arterial pressure measurements over a period of 45 minutes. We compared noninvasive (test method) and invasive (reference method) arterial pressure
measurements (sampling rate 1 Hz = 1/s) using Bland-Altman analysis (accounting for multiple
measurements per subject), 4-quadrant plot/concordance analysis (2-minute interval, 5 mm Hg
exclusion zone), and error grid analysis (calculating the proportions of measurements in risk
zones A–E with A indicating no risk, B low risk, C moderate risk, D significant risk, and E dangerous
risk for the patient due to the risk of wrong clinical interventions because of measurement errors).
RESULTS: We observed a mean of the differences (±SD, 95% limits of agreement) between the
noninvasively and invasively assessed arterial pressure values of 1.1 mm Hg (±7.4 mm Hg,
−13.5 to 15.6 mm Hg) for mean arterial pressure (MAP), 6.8 mm Hg (±10.3 mm Hg, −14.4 to
27.9 mm Hg) for systolic arterial pressure, and 0.8 mm Hg (±6.9 mm Hg, −12.9 to 14.4 mm
Hg) for diastolic arterial pressure. The 4-quadrant plot concordance rate (ie, the proportion of
arterial pressure measurement pairs showing concordant changes to all changes) was 93% (CI,
89%–96%) for MAP, 93% (CI, 89%–97%) for systolic arterial pressure, and 88% (CI, 84%–92%)
for diastolic arterial pressure. Error grid analysis showed that the proportions of measurements
in risk zones A–E were 89.5%, 10.0%, 0.5%, 0%, and 0% for MAP and 93.7%, 6.0%, 0.3%, 0%,
and 0% for systolic arterial pressure, respectively.
CONCLUSIONS: During laparoscopic bariatric surgery, the accuracy and precision of the vascular unloading technique (Clearsight system) was good for MAP and diastolic arterial pressure,
but only moderate for systolic arterial pressure according to Bland-Altman analysis. The system showed good trending capabilities. In the error grid analysis, >99% of vascular unloading
technique–derived arterial pressure measurements were categorized in no- or low-risk zones.
(Anesth Analg 2019;128:477–83)
KEY POINTS
• Question: How does the vascular unloading technique (Clearsight system; Edwards Lifesciences
Corp, Irvine, CA) for noninvasive continuous arterial pressure monitoring perform during laparoscopic bariatric surgery in comparison with invasive continuous arterial pressure measurements?
• Findings: The mean of the differences ± SD and 4-quadrant plot concordance rate was 1.1 ± 7.4
mm Hg and 93% for mean, 6.8 ± 10.3 mm Hg and 93% for systolic, as well as 0.8 ± 6.9 mm Hg
and 88% for diastolic arterial pressure. In the error grid analysis, >99% of vascular unloading
technique–derived arterial pressure measurements were categorized in no- or low-risk zones.
• Meaning: In the setting of laparoscopic bariatric surgery, arterial pressure monitoring with the
vascular unloading technique (Clearsight system) shows good accuracy, precision, and trending
capabilities for mean arterial pressure and error grid analysis indicates that monitoring of arterial
pressure with the vascular unloading technique does not result in clinical risk for the patient.
From the *Department of Anesthesiology, Center of Anesthesiology and
Intensive Care Medicine, and †Department of Medical Biometry and
Epidemiology, University Medical Center Hamburg-Eppendorf, Hamburg,
Germany; and ‡Institute of Operations Research, Karlsruhe Institute of
Technology, Karlsruhe, Germany.
Funding: Edwards Lifesciences Corp (Irvine, CA) provided the technical
equipment for the study. Edwards Lifesciences was not involved in the collection of the data, drafting of the manuscript, or the decision to submit the
manuscript for publication.
Accepted for publication October 18, 2018.
The authors declare no conflicts of interest.
Sebastian A. Haas, MD, and Daniel A. Reuter, MD, are currently affiliated with the Department of Anesthesiology and Intensive Care Medicine,
University Medical Center Rostock, Rostock, Germany.
Reprints will not be available from the authors.
Copyright © 2019 International Anesthesia Research Society
DOI: 10.1213/ANE.0000000000003943
March 2019 • Volume 128 • Number 3
Address correspondence to Dorothea E. Rogge, MD, Department of Anesthesiology, Center of Anesthesiology and Intensive Care Medicine, University
Medical Center Hamburg-Eppendorf, Martinistrasse 52, 20246 Hamburg,
Germany. Address e-mail to d.rogge@uke.de.
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Vascular Unloading Technique in Bariatric Surgery
T
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he assessment of arterial blood pressure is a mainstay
of hemodynamic monitoring ensuring the patients’
safety in the perioperative setting. Recent studies
revealed that intraoperative hypotension is associated with
myocardial and acute kidney injury1–3 which emphasizes the
importance of continuous arterial pressure monitoring during the perioperative phase. The clinical reference method
for continuous beat-to-beat arterial pressure monitoring
is the invasive measurement with an arterial catheter; the
oscillometric technique remains the standard for automated
intermittent noninvasive arterial pressure measurement.4
During the recent years, new noninvasive arterial pressure
measurement systems became available. One of these systems
is the Clearsight system (Edwards Lifesciences Corp, Irvine,
CA). The Clearsight system uses an enhanced vascular unloading technology, also known as volume clamp method, based
on a modified Penaz principle to measure arterial pressure
noninvasively at the finger with a finger cuff and to reconstruct
brachial arterial pressure.4,5 Continuous noninvasive arterial
pressure measurements with the Clearsight system have been
validated against invasive arterial pressure measurements in
various clinical studies with contradicting results.6 It has been
suggested that in nonobese hemodynamically stable patients
under general anesthesia, the Clearsight system might be
interchangeable with intermittent upper arm cuff oscillometry
regarding the measurement performance compared with invasive arterial pressure measurements, with the advantage of
continuous arterial pressure monitoring.7 However, to the best
of our knowledge, there is no study comparing the vascular
unloading technology using the Clearsight system in severely
obese patients during bariatric surgery.
Patients suffering from obesity have a substantially
higher risk regarding cardiovascular and pulmonary complications in the perioperative phase.8–10 In bariatric surgery,
the combination of obesity-related physiological alterations,
comorbidities, and the surgical procedure (capnoperitoneum) under general anesthesia as well as positioning of
the patient can lead to hemodynamic instability. To this end,
continuous arterial pressure monitoring might be superior to
intermittent automated oscillometric arterial pressure monitoring with an upper arm cuff. However, placing an arterial
catheter can bare technical difficulties in this specific group
of patients and is associated with risks, such as infection,
ischemia, and bleeding (although the rate of severe arterial
catheter-related complications is low11). We hypothesized
that a new noninvasive measurement system that enables
arterial pressure to be monitored continuously might close
the gap between noninvasive intermittent and invasive continuous arterial pressure monitoring techniques.12
Hence, the aim of our study was to evaluate the arterial
pressure measurement performance of the vascular unloading technology with the Clearsight system using invasive
radial arterial catheter–derived arterial pressure measurements as the reference method in severely obese patients
undergoing laparoscopic weight loss surgery.
METHODS
Study Design, Inclusion and Exclusion Criteria
This prospective method comparison study was performed in the Department of Anesthesiology, Center of
Anesthesiology and Intensive Care Medicine, University
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Medical Center Hamburg-Eppendorf between June 2016
and February 2017 after approval by the ethics committee
(Ethikkommission der Ärztekammer Hamburg, Hamburg,
Germany). We obtained written informed consent from
all patients. Adult patients with severe obesity (defined as
a body mass index of >35 kg/m2) scheduled for elective
laparoscopic bariatric surgery (laparoscopic vertical sleeve
gastrectomy or laparoscopic Roux-en-Y bypass), in whom
continuous arterial pressure monitoring with an arterial
catheter was planned independently of the study, were
eligible for inclusion in the study. We measured arterial
pressure oscillometrically on both upper arms to identify
patients with severe arterial pressure differences between
the left and right arm. Patients were not eligible for study
inclusion if they had a systolic arterial pressure difference
of ≥10 mm Hg. In addition, patients were not included if
they presented with peripheral vascular disease (Fontaine
stadium >2), atrial fibrillation, anatomic deformities of the
upper extremities, or peripheral edema.
Anesthesia Management
Anesthesia was induced with remifentanil 0.5 µg/kg/min
(ideal body weight +20%), target-controlled infusion of propofol according to Marsh (4 µg/mL actual body weight),
and rocuronium (1 mg/kg ideal body weight). During surgery, patients were positioned in anti-Trendelenburg position with both arms spread out on arm-positioning devices.
Anesthesia was maintained with target-controlled infusion
of propofol and continuous infusion of remifentanil.
Invasive Arterial Pressure Measurement
An arterial catheter was placed into the radial artery and
arterial pressure was measured continuously after checking the damping properties of the fluid-filled tubing system
with a fast-flush test and zeroing the arterial pressure monitoring system.
Noninvasive Arterial Pressure Measurement
With the Vascular Unloading Technology
(Clearsight system)
After induction of anesthesia and patient positioning in the
operating room, we attached the Clearsight system’s finger
cuff to the fingers of the hand contralateral to the side the
arterial catheter was placed and started the measurement
after zero calibration of the heart reference system.
Data Recording and Data Processing
We recorded arterial pressure measurements of the test
and reference method in parallel for about 45 minutes.
Therefore, we connected the Clearsight monitor with an
interface cable to the patient monitor (Dräger Infinity Delta;
Dräger, Lübeck, Germany). Both arterial pressure waveforms and values assessed noninvasively with the vascular unloading technology and invasively with the arterial
catheter were shown on the patient monitor. The patient
monitor was connected to a personal computer via a serial
interface cable for data recording. To record the arterial
pressure waveforms of the test and reference method, we
used dedicated recording software (Data grabber studies;
Dräger, Lübeck, Germany) and stored the data on the study
ANESTHESIA & ANALGESIA
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computer for statistical analyses. Obvious artifacts of the
invasively or noninvasively obtained arterial pressure measurements were excluded after visual inspection of the arterial pressure waveforms.
Statistical Analysis
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Patients’ characteristics are presented as median with 25th
and 75th percentile or absolute numbers (percentages). We
calculated the mean ± SD of arterial pressure measurements
assessed with the vascular unloading technology and with
the radial arterial catheter. To account for repeated measurements per patient, we applied a random intercept model to
compute the mean ± SD.
We compared values measured using the vascular
unloading technology (test method) and the arterial catheter (reference method) for mean arterial pressure (MAP),
systolic arterial pressure, and diastolic arterial pressure
(sampling rate 1 Hz = 1/s for both the test and reference
method). For visual assessment of the distribution and
relationship of the arterial pressure data, we plotted arterial pressure values obtained with the vascular unloading
technology against the invasively obtained arterial pressure
values by the arterial catheter in scatter plots.
Furthermore, Bland-Altman analysis was performed
accounting for multiple measurements per patient.13,14 We
calculated the mean of the differences between the test and
reference method (by subtracting the noninvasively assessed
arterial pressure values [vascular unloading technology]
from the invasively assessed arterial pressure values [arterial catheter]), the SD of the mean of the differences, and the
95% limits of agreement (with CIs around the limits of agreement) accounting for multiple measurements per patient.13,14
In addition, the ability of the vascular unloading technology
to track changes in arterial pressure values (direction of change
analysis) on the basis of 2-minute intervals was investigated.
Therefore, we computed 4-quadrant plots and performed
a concordance analysis for MAP, systolic arterial pressure,
and diastolic arterial pressure.15 The 4-quadrant plot shows
changes between consecutive arterial pressure measurements
(here 1 measurement every 2 minutes) obtained using the vascular unloading technology (y-axis) and the arterial catheter
(x-axis) in a scatter plot. If both methods agree with respect
to the direction of the arterial pressure change, the resulting
points lie in the first or third quadrant of the plot (so-called
concordant points). If the methods do not agree, the points lie
in the second or fourth quadrant of the plot (discordant points).
Because no clinically applicable arterial pressure measurement
system is perfectly accurate and precise, very small changes in
the measurements are usually attributed to noise and therefore
excluded from the trending analysis. Therefore, we defined
a 5 mm Hg exclusion zone at the center of the plot and thus
excluded very small arterial pressure changes driven by noise
to increase the signal-to-noise ratio (specifically this 5 mm Hg
exclusion zone means that we did not count measured changes
which are <5 mm Hg for the assessment of the trending ability).15 Based on the data points outside the exclusion zone, we
calculated the concordance rate as the proportion (percentage)
of concordant data pairs to all data pairs.15 We computed the
CIs of the concordance rate using a random intercept model
for a binary outcome. The random effect “patient” accounts for
the within-subject correlation.
March 2019 • Volume 128 • Number 3
To assess the clinical relevance of differences between the
noninvasive arterial pressure measurements and reference
arterial pressure measurements, we performed error grid
analysis as proposed by Saugel et al.16 Based on an survey
among experts, the error grid analysis enables a risk level to
be assigned to each pair of measured arterial pressure value
(test method) and “true” arterial pressure value (reference
method) for systolic arterial pressure and MAP; the risk
level ranges from no risk to dangerous risk depending on
whether or not a difference between the measured arterial
pressure value (test method) and the reference arterial pressure value (reference method) would trigger a therapeutic
intervention that can harm the patient given her or his true
reference arterial pressure value.16 In detail, the risk levels
are as follows:
A: No risk (ie, no difference in clinical action between the
reference and test method).
B: Low risk (ie, test method values that deviate from the
reference but would probably lead to benign or no
treatment).
C: Moderate risk (ie, test method values that deviate from
the reference and would lead to unnecessary treatment
with moderate nonlife-threatening consequences for the
patient).
D: Significant risk (ie, test method values that deviate from
the reference and would lead to unnecessary treatment
with severe nonlife-threatening consequences for the
patient).
E: Dangerous risk (ie, test method values that deviate from
the reference and would lead to unnecessary treatment
with life-threatening consequences for the patient).16
To quantify the results of the error grid analysis, we
calculated the proportions of measurements in risk zones
A–E.16
In addition to the discrete risk levels, we present a continuous version of the error grid that aggregates the single
risk assessments of the 25 experts into a continuous risk
level ranging from 0% to 100%.16 The background colors
in the continuous error grid correspond to the continuous
risk level for each pair of measurement; the color scheme
is chosen as proposed by Saugel et al16 and Klonoff et al.17
Note that the data points overlap in the central parts of the
continuous error grid figures due to the large numbers of
measurements. To avoid subjective overassessment of the
few outliers, we used transparent markers for single data
points.16
The primary endpoint of this study was the analysis of the
differences between the test and reference method (BlandAltman analysis). The very narrow CIs around the 95% limits of agreement of the mean of the differences between the
test and reference method show that the sample size of 35
subjects and the number of available arterial pressure data
pairs enabled this primary endpoint to be investigated with
sufficient precision.
For statistical analysis, we used Microsoft Office Excel
2010 (Microsoft Corp, Redmond, WA), the statistical software package R version 3.4.3 (R Foundation for Statistical
Computing, Vienna, Austria), and MATLAB 2017a (The
Mathworks Inc, Natick, MA).
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Vascular Unloading Technique in Bariatric Surgery
RESULTS
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We included 35 patients in the study. The patients’ characteristics are shown in Table 1.
We analyzed 97,623 pairs of simultaneously recorded
noninvasive and invasive arterial pressure values (Table 2).
We excluded 1.7% of arterial pressure data pairs because of
artifacts or technical problems during signal recording with
the vascular unloading technology or the arterial catheter.
The distribution and relationship of arterial pressure data
obtained with the vascular unloading technology and the
arterial catheter are illustrated in scatter plots (Figure 1A–C).
Bland-Altman analysis showed a mean of the differences
(±SD, 95% limits of agreement) between the noninvasively
(vascular unloading technology) and invasively (arterial
catheter) assessed arterial pressure values of 1.1 mm Hg
(±7.4 mm Hg, −13.5 to 15.6 mm Hg) for MAP, of 6.8 mm
Hg (±10.3 mm Hg, −14.4 to 27.9 mm Hg) for systolic arterial
pressure, and of 0.8 mm Hg (±6.9 mm Hg, −12.9 to 14.4 mm
Hg) for diastolic arterial pressure (Table 2; Figure 2A–C).
The concordance rate (ie, the proportion of arterial pressure measurement pairs showing concordant changes to all
changes) between arterial pressure changes observed with
the vascular unloading technology and measured with the
arterial catheter was 93% (CI, 89%–96%) for MAP, 93% (CI,
89%–97%) for systolic arterial pressure, and 88% (CI, 84%–
92%) for diastolic arterial pressure (Figure 3A–C).
The error grid analysis revealed that the proportions of
measurements in risk zones A–E were 89.5%, 10.0%, 0.5%,
0%, and 0% for MAP and 93.7%, 6.0%, 0.3%, 0%, and 0%
for systolic arterial pressure, respectively. The risk zones
describe the risk from no risk to dangerous risk depending
Table 1. Patients’ Characteristics
Demographic and biometric data
Sex, male/female, n (%)
Age, y
Height, cm
Body weight, kg
Body mass index, kg/m2
Surgery
Laparoscopic Roux-en-Y gastric bypass, n (%)
Laparoscopic vertical gastric sleeve, n (%)
Anesthesia
Highest perioperative dose of
norepinephrine, µg/kg/min
Amount of intravenous crystalloids, mL
25 (71), 10 (29)
53 (41–59)
170 (164–176)
140 (122–150)
47 (42–53)
11 (31)
24 (69)
0.06 (0.05–0.08)
1500 (1000–1500)
Data are given as absolute numbers (percentages) or median (25th–75th
percentile).
on whether or not the measurement error (ie, the difference between the vascular unloading technology–derived
arterial pressure value and the reference arterial catheter–
derived arterial pressure value) can trigger a therapeutic
intervention that can harm the patient.
Continuous error grids for MAP and systolic arterial
pressure are shown in Figure 4A, B, respectively. It is clearly
visible that the majority of the measurement pairs lie in
regions with no or only very low risk for the patients.
DISCUSSION
We evaluated the arterial pressure measurement performance of the vascular unloading technology (Clearsight
system) using invasive radial arterial catheter–derived
arterial pressure measurements as the reference method
in severely obese patients undergoing laparoscopic weight
loss surgery. We found that the accuracy and precision of
the vascular unloading technology during laparoscopic
bariatric surgery was good for MAP and diastolic arterial
pressure, but only moderate for systolic arterial pressure.
The vascular unloading technology showed good trending
capabilities. According to error grid analysis, >99% of vascular unloading technology–derived arterial pressure measurements were categorized in no- or low-risk zones.
The vascular unloading technology using the Clearsight
system has been validated against invasive reference techniques for both arterial pressure and cardiac output estimation with reasonable results regarding accuracy, precision,
and trending in nonobese patients in various clinical settings.6 Pouwels et al18 compared the Clearsight system with
intermittent noninvasive arterial pressure measurements
with an upper arm cuff in obese patients. According to their
predefined criteria, the Clearsight system did not pass validation requirements. However, validating the Clearsight
system against intermittent noninvasive arterial pressure
measurements in severely obese patients might be a questionable approach, because oscillometric measurements
depend on the appropriate cuff size to retain reliable arterial
pressure values19–21 and have been shown to be not interchangeable with arterial pressure measurements with a
radial arterial catheter in obese patients.22 Apparently, in the
group of severely obese patients, upper arm cuff–derived
arterial pressure measurements might not be the optimal
clinical reference method, when evaluating the vascular
unloading technology for noninvasive continuous arterial
pressure monitoring, because of alterations in the relation
between the circumference and the length of the upper arm.
Table 2. Arterial Pressure Values Determined With the Vascular Unloading Technique and With the Arterial
Catheter
Mean arterial pressure
Vascular Unloading
Technique (mm Hg)
80.5 ± 11.8
Radial Arterial
Catheter
(mm Hg)
81.5 ± 12.7
Mean of the
Differences
(mm Hg)
1.1
SD of the Mean
of the Differences
(mm Hg)
±7.4
Systolic arterial pressure
109.1 ± 17.0
116.0 ± 19.4
6.8
±10.3
Diastolic arterial pressure
63.8 ± 9.2
64.4 ± 10.5
0.8
±6.9
95% Limits of
Agreement (CI)
−13.5 (−13.5 to −13.4) to
15.6 (15.5–15.7) mm Hg
−14.4 (−14.5 to −14.3) to
27.9 (27.8–28.0) mm Hg
−12.9 (−13.0 to −12.8) to
14.4 (14.3–14.5) mm Hg
Arterial pressure values (mean ± SD) assessed with the vascular unloading technique and with the radial arterial catheter are shown. To compare the 2 methods,
we show the mean of the differences, the SD of the mean of the differences, and the 95% limits of agreement.
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ANESTHESIA & ANALGESIA
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Figure 1. A, Scatter plot for mean arterial pressure (MAP). This plot is illustrating the correlation between MAP data obtained with the
Clearsight system (MAP-vascular unloading technology [VUT]) and the arterial catheter (MAP-art). B, Scatter plot for systolic arterial pressure
(SAP). This plot is illustrating the correlation between SAP data obtained with the Clearsight system (SAP-VUT) and the arterial catheter (SAPart). C, Scatter plot for diastolic arterial pressure (DAP). This plot is illustrating the correlation between DAP data obtained with the Clearsight
system (DAP-VUT) and the arterial catheter (DAP-art). The density of the transparent data point markers illustrates the frequency distribution
of data points.
Figure 2. A, Bland-Altman plot mean arterial pressure (MAP). Bland-Altman plot is showing the comparison between measurements from the
Clearsight technology (MAP-vascular unloading technology [VUT]) and the arterial line (MAP-art). B, Bland-Altman plot for systolic arterial pressure (SAP). Bland-Altman plot is showing the comparison between measurements from the Clearsight technology (SAP-VUT) and the arterial
line (SAP-art). C, Bland-Altman plot for diastolic arterial pressure (DAP). Bland-Altman plot is showing the comparison between measurements
from the Clearsight technology (DAP-VUT) and the arterial line (DAP-art). In each plot, the intermediate dashed horizontal line represents the
mean of the differences and the upper and lower dashed horizontal lines represent the 95% limits of agreement. The density of the transparent data point markers illustrates the frequency distribution of data points.
To the best of our knowledge, this is the first study evaluating the arterial pressure measurement performance of the
Clearsight system in obese patients using invasive arterial
pressure measurements as the reference method.
In general, data on the measurement performance of
innovative technologies for continuous noninvasive monitoring in obese patients are scarce.
Tobias et al23 compared continuous noninvasive arterial
pressure measurements with the CNAP system (CNSystems
Medizintechnik GmbH, Graz, Austria), that also uses the
vascular unloading technology, with arterial pressure
measurements derived from a radial arterial catheter in
18 patients undergoing bariatric surgery and concluded
that the measurement performance of the CNAP system in
severely obese patients is poorer than in nonobese patients.
In a previous study of our group performed in a very similar setting compared to the one of the present study, the
CNAP system showed good capabilities to follow arterial
pressure changes compared with the invasive reference
March 2019 • Volume 128 • Number 3
measurements but failed to provide interchangeable absolute arterial pressure values in patients undergoing laparoscopic bariatric weight loss surgery.24 Of note, the CNAP
system—in contrast to the Clearsight system—uses arterial
pressure values obtained with an upper arm cuff (oscillometry) to calibrate the finger arterial pressure values and
reconstruct brachial arterial pressure. The Clearsight system
uses an internal calibration mechanism, the “Physiocal”
method. This key difference in the measurement principle
might explain differences in the arterial pressure measurement performance in severely obese patients.
In our study, the overall measurement performance of
the Clearsight system was good, but the vascular unloading technology underestimated the systolic arterial pressure value failing the Association for the Advancement of
Medical Instrumentation standards for noninvasive arterial
pressure measurement defining clinically acceptable agreement as a mean of the differences of 5 mm Hg with an SD
of ±8 mm Hg between test and the reference method.25 The
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Vascular Unloading Technique in Bariatric Surgery
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Figure 3. A, Four-quadrant plot for concordance of mean arterial pressure (MAP). The capability of Clearsight system (vascular unloading technology [VUT]) to track changes (delta [Δ]) in MAP is shown in this 4-quadrant plot indicating arterial pressure derived from Clearsight system
(Δ MAP-VUT) with arterial pressure values derived from the arterial line (Δ MAP-art). B, Four-quadrant plot for concordance of systolic arterial
pressure (SAP). The capability of Clearsight system (VUT) to track changes (Δ) in SAP is shown in this 4-quadrant plot indicating arterial pressure derived from Clearsight system (Δ SAP-VUT) with arterial pressure values derived from the arterial line (Δ SAP-art). C, Four-quadrant plot
for concordance of diastolic arterial pressure (DAP). The capability of Clearsight system (VUT) to track changes (Δ) in DAP is shown in this
4-quadrant plot indicating arterial pressure derived from Clearsight system (Δ DAP-VUT) with arterial pressure values derived from the arterial
line ([Δ] DAP-art). In all 4-quadrant plots, arterial pressure changes are averaged over a period of 10 s within a 2-min interval. An exclusion
zone of 5 mm Hg was applied.
Figure 4. A, Error grid analysis for systolic arterial pressure (SAP). This figure shows the error grid for the test method (Clearsight system [SAPvascular unloading technology {VUT}]) in comparison with the reference method (arterial catheter [SAP-art]) for all measurements from the 35
patients regarding SAP. B, Error grid analysis for mean arterial pressure (MAP). This figure shows the error grid for the test method (Clearsight
system [MAP-VUT]) in comparison with the reference method (arterial catheter [MAP-art]) for all measurements from the 35 patients regarding
SAP. The background colors correspond to the continuous risk level for each pair of measurement. The continuous risk level ranges from 0% to
100% as shown below. Note that the points overlap in the central part of the figures due to the large numbers of measurements. This might
lead to subjective overassessment of the few outliers, which we tried to lessen by using transparent markers.
underestimation of systolic arterial pressure may be due to
the reconstruction of the measured finger arterial pressure
to brachial arterial pressure.
It has to be emphasized that neither the Association for
the Advancement of Medical Instrumentation definition
nor Bland-Altman or trending analysis give information
about the clinical importance of arterial pressure differences
between 2 measurement methods. Therefore, we proposed
482 www.anesthesia-analgesia.org
to use error grid analysis in arterial pressure measurement
studies comparing a test and reference method.16 Error grid
analysis gives a visual impression and quantitative analysis
of the clinical relevance of the differences in arterial pressure measurements and the clinical risk related to them.16
Our study has limitations. It was performed in a single
institution; thus, results might not be transferable to different clinical settings. The limited number of patients did not
ANESTHESIA & ANALGESIA
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allow for subgroup analysis to identify factors influencing
the measurement performance of the Clearsight system.
In conclusion, the accuracy and precision of the vascular
unloading technology (Clearsight system) was good for MAP
and diastolic arterial pressure, but only moderate for systolic
arterial pressure during laparoscopic bariatric surgery. The
system showed good trending capabilities. According to
error grid analysis, >99% of Clearsight arterial pressure measurements were categorized in no- or low-risk zones. Besides
that, the Clearsight system is easy to attach also in obese
patients and might, therefore, be an interesting arterial pressure monitoring device in severely obese patients. E
DISCLOSURES
Name: Dorothea E. Rogge, MD.
Contribution: This author helped conceive and design the study, was
responsible for acquisition of data, was responsible for data analysis
and interpretation, helped perform the statistical analyses, helped
draft the manuscript, and read and approved the final version of the
manuscript and agreed to be accountable for all aspects of the study.
Name: Julia Y. Nicklas, MD.
Contribution: This author helped with the data analysis and interpretation, critically revised the manuscript for important intellectual content, and read and approved the final version of the
manuscript and agreed to be accountable for all aspects of the study.
Name: Gerhard Schön, MSc.
Contribution: This author helped analyze the data; perform statistical analyses, statistical testing, and interpretation; draft the manuscript; and read and approved the final version of the manuscript
and agreed to be accountable for all aspects of the study.
Name: Oliver Grothe, PhD.
Contribution: This author helped analyze the data; perform statistical analyses, statistical testing, and interpretation; draft the manuscript; and read and approved the final version of the manuscript
and agreed to be accountable for all aspects of the study.
Name: Sebastian A. Haas, MD.
Contribution: This author helped with the data analysis and interpretation, critically revised the manuscript for important intellectual content, and read and approved the final version of the
manuscript and agreed to be accountable for all aspects of the study.
Name: Daniel A. Reuter, MD.
Contribution: This author helped with the data analysis and interpretation, critically revised the manuscript for important intellectual content, and read and approved the final version of the
manuscript and agreed to be accountable for all aspects of the study.
Name: Bernd Saugel, MD.
Contribution: This author helped conceive and design the study,
was responsible for data analysis and interpretation, drafted the
manuscript, supervised the study, and read and approved the final
version of the manuscript and agreed to be accountable for all
aspects of the study.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
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