Development of a Continuous Non invasive Respiratory Variation Monitor for Use in Pain Management

KEY POINTS

• Question: How is the performance of the vascular unloading technique for continuous noninvasive arterial pressure measurements (CNAP system; CNSystems, Graz, Austria) compared with invasive arterial pressure measurements in severely obese patients during laparoscopic bariatric surgery?
• Findings: The mean of the differences ± standard deviation (concordance rate) between the 2 methods was 7.9 ± 9.6 mm Hg (97.5%) for mean, 4.8 ± 15.8 mm Hg (95.0%) for systolic, and 9.5 ± 10.3 mm Hg (96.7%) for diastolic arterial pressure.
• Meaning: In the setting of laparoscopic bariatric surgery, continuous noninvasive arterial pressure monitoring with the CNAP system shows good trending capabilities compared with continuous invasive arterial pressure measurements obtained with a radial arterial catheter, but absolute values are not interchangeable.

Increasing rates of obesity are creating new challenges for hemodynamic patient monitoring in the perioperative phase. Obesity and associated comorbidities such as the metabolic syndrome, arterial hypertension, coronary artery disease, and heart failure are increasing the risks for perioperative cardiovascular complications. In severely obese patients, close monitoring of the arterial pressure (AP) is, therefore, important to ensure patient safety during the anesthesiological perioperative care. Different technologies for either intermittent or continuous AP monitoring are available for perioperative AP monitoring. In patients with low cardiovascular risk undergoing low-risk surgical interventions, AP is usually measured intermittently with an oscillometric upper arm cuff. In contrast, the continuous measurement of AP is recommended for severely obese patients to detect hemodynamic changes quickly during operations. ^{1 , 2} Therefore, usually, an arterial catheter is placed if these patients need to undergo surgical interventions under general anesthesia. In obese patients, the placement of an arterial catheter might, however, be difficult due to the anatomic conditions. Moreover, the placement of arterial catheters is associated with risks such as bleeding, ischemia, and infections. ^{3 , 4}

Innovative technologies based on different measurement principles—eg, the vascular unloading technique or radial artery applanation tonometry—allow continuous noninvasive AP measurements. ⁵ Based on the vascular unloading technique, the continuous noninvasive arterial pressure (CNAP) system (CNSystems Medizintechnik AG, Graz, Austria) provides beat-to-beat AP measurements and continuous recording of the AP waveform. ^{6 , 7} The CNAP system has already been the subject of former method comparison studies comparing the CNAP AP measurements with invasive continuous AP measurements obtained with an arterial catheter during various surgical interventions and in intensive care unit patients. ^8–11 However, data on the measurement performance of the CNAP system in severely obese patients are scarce.

Therefore, the aim of this study was to compare continuous noninvasive AP measurements using the CNAP system with invasive AP measurements (radial arterial catheter) in severely obese patients during laparoscopic bariatric surgery.

METHODS

Study Design, Inclusion, and Exclusion Criteria

This prospective method comparison study was reviewed and approved by the ethics committee (Ethikkomission der Ärztekammer Hamburg, Hamburg, Germany). We obtained written informed consent from all patients. Adult patients with severe obesity (defined as a body mass index [BMI] of ≥35 kg/m²) scheduled for elective laparoscopic weight loss surgery (laparoscopic vertical sleeve gastrectomy or laparoscopic Roux-en-Y gastric bypass), in whom continuous AP monitoring with an arterial catheter was planned independently of the study, were eligible for study inclusion. Exclusion criteria were the presence of vascular abnormalities or anatomical deformities of the upper extremities or peripheral edema.

AP Measurements

Before starting the study AP measurements, AP was measured oscillometrically on both upper arms to exclude marked AP differences between the right and the left arm. The CNAP system was set up as described in detail before. ¹² The CNAP system's double finger cuff was attached to the hand opposite to the side on which the arterial catheter was placed. Calibration of the CNAP finger sensor–derived values to upper arm oscillometric values was performed every 15 minutes. CNAP-derived continuous noninvasive AP measurements and invasive AP measurements obtained with the arterial catheter were recorded simultaneously over a period of 45 minutes.

Data Recording and Processing

Noninvasive and invasive AP waveforms were simultaneously measured and displayed on the patient monitor (Dräger Infinity Delta, Dräger, Lübeck, Germany) during recordings. The values for mean, systolic, and diastolic AP from both measurement techniques were extracted using dedicated recording software (Dräger Data Grabber Studies, Dräger, Lübeck, Germany). The AP waveforms and values used in the analysis were the same that were available on the displays of the monitors in clinical practice. Using the recording software, we extracted the numeric AP data second-to-second from the beat values as displayed on the patient monitor. The waveforms were used for the identification of artifacts. A serial interface cable connected the patient monitor and a study laptop computer. Obvious artifacts of invasively or noninvasively obtained AP measurements were excluded after visual inspection of AP waveforms. For comparative statistical analyses, we used paired 10-second averages of noninvasive and invasive AP measurements.

Statistical Analysis

Patients' characteristics are described as absolute numbers or mean with absolute minimum and maximum.

We compared paired 10-second AP value averages measured with the CNAP system (test method) and the arterial catheter (reference method). We plotted AP values assessed with the CNAP system against invasively measured AP values in scatter plots for visual assessment of the distribution and relationship of the AP data. In addition, we used Bland-Altman analysis accounting for multiple measurements per patient ¹³ and calculated the mean of the differences, the standard deviation of the mean of the differences, and the 95% limits of agreement (mean of the differences ± 1.96 × standard deviation of the mean of the differences) to evaluate the agreement between the noninvasive and invasive AP measurements. As described by Bland and Altman, ¹³ we used 1-way analysis of variance (ANOVA) to handle unbalanced data. We calculated the difference between the mean squares for subjects and the residual mean square (taken from ANOVA), then divided this by a factor which depends on the numbers of observation on each subject (which, if the number of observations m is the same for all subjects, reduces to this number m), adding this to the residual mean square to estimate the total variance for single differences on different subjects with varying numbers of observations.

The differences of CNAP-derived and invasive AP measurements were calculated by subtraction of the invasively assessed AP value from the CNAP-derived AP value.

For the assessment of the ability of the CNAP system to follow changes in AP, we computed 4-quadrant plots ¹⁴ and calculated the concordance rate using an exclusion zone of 5 mm Hg. Changes in AP were analyzed on the basis of 2-minute intervals. We calculated the number of data points available for trending analysis, the number of AP data points outside the exclusion zone, and the number as well as the percentage of concordant AP data points for each patient.

To determine the source of the error between the test method (CNAP system) and the reference method (arterial catheter) and to quantify the variability of the AP measurement performance within a patient (intraindividual) and between different patients (interindividual), an ANOVA was performed.

In addition, to evaluate the measurement performance of the CNAP device in comparison with oscillometric upper arm cuff measurements, we compared the invasive arterial catheter–derived AP measurements (reference method) (1) with oscillometric upper arm cuff measurements performed every 15 minutes to calibrate the CNAP system and (2) with the CNAP values obtained just before this calibration. For CNAP- and arterial catheter–derived AP measurements, we used the mean of the AP values obtained during the 30-second interval before the start of the oscillometric upper arm cuff calibration measurement for these comparisons.

For statistical analyses, we used Microsoft Office Excel 2007, MATLAB version 7.8.0 (R2009a) (The MathWorks Inc, Natick, MA), and Python version 2.7 (Python Software Foundation, Beaverton, OR).

RESULTS

Patients and Data

We included 30 patients in this study. One patient was excluded because AP recordings could not be analyzed due to technical problems with data recording. In consequence, AP data from 29 patients (female n = 22, male n = 9) were used for final statistical analysis. Seventeen patients received a laparoscopic vertical sleeve gastrectomy and 12 patients a laparoscopic Roux-en-Y gastric bypass. The mean age was 43 (24–67) years, mean body mass index was 48.1 (37.4–67.8) kg/m². Mean height was 172 (155–192) cm, and mean weight was 142.8 (91–178) kg.

We identified and excluded artifacts in 3.5% of all invasive AP recordings (caused by arterial line flushing or movement) and in 0.5% of all noninvasive AP recordings (caused by technical problems such as external movement of the CNAP finger probe). A total of 6864 pairs of averaged 10-second episodes of simultaneous noninvasive and invasive AP recordings were analyzed.

Method Comparison: CNAP Versus Arterial Catheter

The distribution and relationship of the AP data obtained with the CNAP system and the arterial catheter is illustrated in scatter plots (Figure 1A–C).

Figure 1.:

Scatter plot. This plot illustrates the correlation between arterial pressure (AP) data obtained with the continuous noninvasive arterial pressure (CNAP) system and the arterial catheter (IBP) for mean AP (MAP; A), systolic AP (SAP; B), and diastolic AP (DAP; C).

Bland-Altman analysis (Figure 2A–C) revealed a mean of the differences (±standard deviation, 95% limits of agreement) between the AP values obtained by the CNAP system and the invasively accessed AP values of 7.9 mm Hg (±9.6 mm Hg, −11.2 to 27.0 mm Hg) for mean AP, of 4.8 mm Hg (±15.8 mm Hg, −26.5 to 36.0 mm Hg) for systolic AP, and of 9.5 mm Hg (±10.3 mm Hg, −10.9 to 29.9 mm Hg) for diastolic AP, respectively.

Figure 2.:

Bland-Altman analysis. The Bland-Altman plots show the comparison between measurements from the continuous noninvasive arterial pressure (CNAP) system and the arterial catheter (IBP) for mean arterial pressure (MAP; A), systolic arterial pressure (SAP; B), and diastolic arterial pressure (DAP; C). The mean of the differences (intermediate dashed horizontal line) and the 95% limits of agreement (upper and lower dashed horizontal line) are shown. CI indicates confidence interval.

To determine the source of the error between the test method (CNAP system) and the reference method (arterial catheter) and to quantify the variability of the AP measurement performance within a patient (intraindividual) and between different patients (interindividual), an ANOVA was performed. Supplemental Digital Content, Table 1, https://links.lww.com/AA/C139, shows a high interpatient variability of the mean difference between CNAP- and arterial catheter–derived AP.

Figure 3.:

Four-quadrant plot analysis. The capability of the continuous noninvasive arterial pressure (CNAP) system to track changes in arterial pressure (AP) is shown in 4-quadrant plots indicating AP derived from CNAP (deltaCNAP) with AP values derived from the arterial line (deltaIBP) for mean AP (MAP; A), systolic AP (SAP; B), and diastolic AP (DAP; C). AP changes are averaged over 10 s within a 2-min interval. An exclusion zone of 5 mm Hg was applied.

Based on 4-quadrant plot analysis (Figure 3A–C), we calculated a concordance rate between AP changes observed with the CNAP system and AP changes measured with the arterial catheter (2-minute intervals) of 97.5% for mean AP, 95.0% for systolic AP, and 96.7% for diastolic AP. The number of AP data points available for trending analysis, the number of AP data points outside the exclusion zone, and the number as well as the percentage of concordant AP data points for each patient are shown in Supplemental Digital Content, Table 2, https://links.lww.com/AA/C139.

Method Comparison: Arterial Catheter Versus Oscillometry and Versus CNAP

The comparison between invasive arterial catheter–derived AP measurements (1) with oscillometric upper arm cuff measurements performed every 15 minutes to calibrate the CNAP system and (2) with the CNAP values obtained just before this calibration is shown in Figure 4 (scatter plots) and Figure 5 (Bland-Altman plots).

Figure 4.:

Arterial catheter versus oscillometry and versus continuous noninvasive arterial pressure (CNAP) system: scatter plot. The correlation between arterial pressure (AP) data obtained with the arterial catheter (IBP) and the oscillometric upper arm cuff measurements (OscBP) performed every 15 min to calibrate the CNAP system for mean AP (MAP; A), systolic AP (SAP; B), and diastolic AP (DAP; C) are shown. In addition, the correlation between IBP and the CNAP values obtained just before this calibration (mean of the AP values obtained during the 30-s interval before the start of the oscillometric upper arm cuff calibration measurement) is shown for MAP (D), SAP (E), and DAP (F).

Figure 5.:

Arterial catheter versus oscillometry and versus continuous noninvasive arterial pressure (CNAP) system: Bland-Altman analysis. The Bland-Altman plots on the left show the comparison between arterial pressure (AP) data obtained with the arterial catheter (IBP) and the oscillometric upper arm cuff measurements (OscBP) performed every 15 min to calibrate the CNAP system for mean AP (MAP; A), systolic AP (SAP; B), and diastolic AP (DAP; C). The Bland-Altman plots on the right show the comparison between IBP and the CNAP values obtained just before this calibration (mean of the AP values obtained during the 30-s interval before the start of the oscillometric upper arm cuff calibration measurement) for MAP (D), SAP (E), and DAP (F). The mean of the differences (intermediate dashed horizontal line) and the 95% limits of agreement (upper and lower dashed horizontal line) are shown.

Bland-Altman analysis revealed a mean of the differences (±standard deviation, 95% limits of agreement) between invasive AP measurements and oscillometric upper arm cuff measurements of 4.6 mm Hg (±8.6 mm Hg, −12.2 to 21.5 mm Hg) for mean AP, of 5.8 mm Hg (±14.4 mm Hg, −22.5 to 34.1 mm Hg) for systolic AP, and of 9.3 mm Hg (±7.0 mm Hg, −4.5 to 23.0 mm Hg) for diastolic AP, respectively.

For the comparison between invasive AP measurements and CNAP values obtained before the oscillometric calibration measurement, we observed a mean of the differences (±standard deviation, 95% limits of agreement) of 9.3 mm Hg (±10.6 mm Hg, −11.5 to 30.1 mm Hg) for mean AP, of 6.3 mm Hg (±16.4 mm Hg, −25.7 to 38.4 mm Hg) for systolic AP, and of 9.8 mm Hg (±10.0 mm Hg, −9.8 to 29.3 mm Hg) for diastolic AP, respectively.

DISCUSSION

In this clinical method comparison study, we compared continuous noninvasive AP measurements using the CNAP system with invasive AP measurements (radial arterial catheter) in severely obese patients during laparoscopic bariatric surgery. The main results of this study can be summarized as follows. In this patient population of severely obese patients undergoing bariatric operations, continuous noninvasive AP monitoring with the CNAP system (vascular unloading technique) showed good trending capabilities compared with continuous invasive AP measurements obtained with a radial arterial catheter. However, absolute CNAP- and arterial catheter–derived AP values were not interchangeable (reflected by high mean ± standard deviation of the differences between the measurements). In addition, we observed a high interpatient variability of the mean difference between CNAP- and arterial catheter–derived AP.

The perioperative anesthesiological management in severely obese patients is challenging for several reasons. Besides potential problems regarding airway management and mechanical ventilation, hemodynamic monitoring including the continuous assessment of AP can bear difficulties. On the one hand, cardiocirculatory alterations associated with obesity can make the continuous assessment of AP necessary in these patients. On the other hand, the placement of catheters needed for invasive monitoring can be challenging due to the anatomic situation. Therefore, we hypothesized that continuous noninvasive AP monitoring using the vascular unloading technology might be an elegant way to monitor these patients during intermediate risk surgery such as bariatric operations because this technology is easy to apply even in patients with severe obesity. ⁵

The CNAP system has been demonstrated to provide AP measurements with reasonable accuracy and precision in surgical and intensive care unit patients. ^{8 , 9 , 11 , 15} Compared with these studies not specifically focusing on obese patients, our study results indicate that the CNAP system seems to be less accurate and precise in severely obese patients. Nevertheless, the CNAP system was able to follow AP changes within 2-minute intervals with a high concordance rate.

When discussing results of AP method comparison studies the lack of standards to define acceptable agreement remains to be a problem. ^{16 , 17} The American National Standards Institute/Association for the Advancement of Medical Instrumentation (ANSI/AAMI) defined clinically acceptable agreement between a test method for AP measurement with a reference method as a mean of the differences of ±5 mm Hg with a standard deviation of 8 mm Hg. ¹⁸ However, the ANSI/AAMI standard cannot be applied to our study (in which we evaluated continuous AP measurements with a finger cuff technology) because it only covers nonautomated, automated, or electronic sphygmomanometers "that are used with an occluding cuff for the indirect determination of arterial blood pressure" ¹⁸ In addition, the ANSI/AAMI criteria are debatable because they impose the same absolute AP thresholds for systolic and diastolic AP despite the fact that absolute systolic and diastolic AP values are very different.

Despite the growing number of obese patients undergoing surgery under general anesthesia, studies evaluating noninvasive AP measurement techniques in these patients are scarce. Tobias et al ¹⁹ compared continuous noninvasive AP measurements with the CNAP system with AP measurements derived from a radial arterial catheter in 18 obese patients undergoing bariatric surgery and revealed comparable measurement agreement as compared to our results. Greiwe et al ²⁰ evaluated another technology for continuous noninvasive AP monitoring, the radial artery applanation tonometry, ⁵ in bariatric surgery patients and revealed comparable means of the differences and standard deviations but lower concordance rates between invasive and noninvasive AP measurements.

When discussing the results of our study, several factors related to the underlying measurement principles of the CNAP system have to be considered. The CNAP system provides AP after calibration to oscillometric brachial AP obtained with an upper arm cuff. In our study, we measured the upper arm circumference to choose the correct upper arm cuff size and to exclude AP differences between the left and right arm. It has to be stressed, however, that oscillometric upper arm cuff measurements depend on the cuff size and have repeatedly been demonstrated to be not accurate and precise in obese patients due to the anatomical conditions (eg, the relation of upper arm circumference and upper arm length). ^21–26 Therefore, calibration of the raw CNAP AP signal obtained at the level of the finger arteries with the finger cuff to the upper arm cuff AP values might be a questionable approach in the specific group of severely obese patients. The result from ANOVA, together with the comparison between the invasive arterial catheter–derived AP measurements (1) with oscillometric upper arm cuff measurements performed every 15 minutes to calibrate the CNAP system and (2) with the CNAP values obtained just before this calibration allow a better understanding of the overall measurement performance of the CNAP system. The variability in the mean of the differences between CNAP-derived AP and invasively assessed AP is highly patient specific. On the other hand, in most patients, the standard deviation of the mean of the differences is low. This suggests that the "offset" varies only slightly within patients. Thus, the main source of error between the CNAP-derived and invasively assessed AP seems to stem from the calibration toward oscillometric AP values and only to a lesser degree from the continuous CNAP measurement at the finger sensor.

Because AP is not constant along the arterial tree of the upper extremities, ⁵ another factor contributing to the difference between invasive and noninvasive AP measurements might be that we obtained invasive AP measurements from the radial artery while the CNAP system is calibrated to the brachial artery AP.

Our study has limitations. The study was performed in a single institution and results might therefore not be transferable to other clinical settings. In addition, although this—to the best of our knowledge—is one of the largest studies on continuous noninvasive AP monitoring in bariatric surgery patients, we studied only a limited number of patients not allowing for subgroup analysis to identify specific influencing factors on the measurement performance of the CNAP technology.

In summary, severe obesity is increasingly becoming a challenge in anesthesiology. Reliable continuous AP monitoring during general anesthesia is important in obese patients regarding patient safety. In these patients, however, AP monitoring with an oscillometric upper arm cuff might be difficult and unreliable due to the anatomic conditions. Placement of an arterial catheter for continuous invasive AP monitoring can also be challenging. Noninvasive continuous AP monitoring technologies could be an alternative way of monitoring in this group of patients. ²⁷ The vascular unloading technique allows performing continuous AP monitoring in a noninvasive manner. In addition to AP, the continuous noninvasive recording of the AP waveform allows the assessment of advanced variables by pulse contour analysis ²⁸ and therefore might help to improve the hemodynamic management. However, the results of our study show that this technology needs to be further improved before it can be recommended for routine clinical use in the setting of bariatric surgery.

In conclusion, in the setting of bariatric surgery, continuous noninvasive AP monitoring with the CNAP system (vascular unloading technique) showed good trending capabilities compared with continuous invasive AP measurements obtained with a radial arterial catheter. However, absolute CNAP- and arterial catheter–derived AP values were not interchangeable and we observed a high interpatient variability of the mean difference between CNAP- and arterial catheter–derived AP.

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, performed the statistical analyses, and drafted the manuscript.

Conflicts of Interest: None.

Name: Julia Y. Nicklas, MD.

Contribution: This author helped analyze the data and interpret and critically revise the manuscript for important intellectual content.

Conflicts of Interest: J. Y. Nicklas received refunds of travel expenses from CNSystems Medizintechnik AG (Graz, Austria).

Name: Sebastian A. Haas, MD.

Contribution: This author helped analyze the data and interpret and critically revise the manuscript for important intellectual content.

Conflicts of Interest: None.

Name: Daniel A. Reuter, MD.

Contribution: This author helped analyze the data and interpret and critically revise the manuscript for important intellectual content.

Conflicts of Interest: None.

Name: Bernd Saugel, MD.

Contribution: This author helped conceive and design the study, analyze and interpret the data, draft the manuscript, and supervise the study.

Conflicts of Interest: B. Saugel received refunds of travel expenses from CNSystems Medizintechnik AG (Graz, Austria). Bernd Saugel received honoraria for giving lectures for CNSystems Medizintechnik AG (Graz, Austria).

This manuscript was handled by: Maxime Cannesson, MD, PhD.

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