Hemodynamic goal-directed strategies have been shown to decrease perioperative morbidity in patients undergoing high-risk surgery, most notably using cardiac output (CO) monitoring.1,2 Among the numerous available devices, the pulmonary artery catheter (PAC) is still considered to be the reference method for CO determination,3 but the invasive nature of this technique is decreasing its popularity.4 Therefore, a safe, reliable and accurate method for monitoring changes real time of CO (ΔCO) has become increasingly warranted.5 While there is currently a growing market of available options, 2 devices with limited clinical evidence were chosen for evaluation in this study: the esCCO™ (estimated electrocardiogram [ECG]-SpO2 Continuous Cardiac Output, Nihon Kohden, Tokyo, Japan) and ECOM™ (Endotracheal Cardiac Output Monitor, ECOM Medical, Inc., San Juan Capistrano, CA). These devices are both minimally invasive, easy to use, both of which could enhance their potential acceptance. The esCCO and ECOM have been previously compared with the PAC for tracking CO in clinical studies with varying results, biases, and conflicts of interest.6–9 Further validation is thus mandatory before recommending a wider use of either device at the bedside, especially with regard to the ability to detect rapid CO changes during various clinical conditions.
One approach for testing the clinical applicability of such monitoring devices in these conditions is to analyze their accuracy during typical interventions known to cause significant hemodynamic changes. One such intervention is the use of an alveolar recruitment maneuver (ARM), which is widely used for its ability to improve arterial oxygenation.10–12 Through cardiopulmonary interactions intrinsic to the ARM, this maneuver has been shown to reversibly and safely influence CO.13,14 As a result of this known perturbation, examining the accuracy of these 2 minimally invasive CO monitoring devices during such a maneuver allows for a strong and necessary step forward in defining their clinical potential. The primary aim of this study was to evaluate the accuracy and the precision of the esCCO and ECOM monitors when compared to intermittent bolus thermodilution (TD) for measuring absolute values and changes of CO during an ARM in postoperative cardiac surgical patients.
This article has been written following the recommendations of the STrengthen the Reporting of OBservational studies in Epidemiology (STROBE) statement.15
After approval by the local ethics committee (Comité de Protection des Patients Sud-Est 3, Lyon, France) and receiving written, informed consent, 32 patients undergoing cardiac surgery were enrolled between November 2012 and April 2013. Exclusion criteria were patients with arrhythmias, circulatory support systems, pacemakers, left or right ventricular dysfunction, hemodynamic instability (use of norepinephrine >0.2 μg·kg−1 min−1, epinephrine >0.5 μg kg−1 min−1, dobutamine >10 μg kg−1 min−1, or milrinone >0.3 μg kg−1 min−1), respiratory failure, or known significant tricuspid valve insufficiency. Before surgery, patients were tracheally intubated with an ECOM endotracheal tube, 7.5 or 8 mm, depending on the height of the patient. A 4-cm 5-Fr tipped catheter (Arrow International®, Reading, PA) was inserted in the left or right radial artery, and a PAC (7.5-Fr Swan-Ganz catheter, Edwards®, Irvine, CA) was inserted in the right internal jugular vein. After surgery, in the intensive care unit (ICU), pressure transducers (Medex Medical, Rossendale, Lancashire, UK) were placed on the midaxillary line and fixed to the bed to keep the transducer at the right atrial level. All transducers were then zeroed to atmospheric pressure. Measurements began 2 hours after the end of cardiopulmonary bypass in the ICU.
Devices and Measurement Procedures
After arriving in the ICU, anesthesia was maintained with propofol 2 mg·kg−1·h−1 and morphine (bolus of 2–4 mg when needed) to maintain the Ramsay score above 5 (no response to nociceptive stimulus or ventilator settings).16 A heat blanket was set to maintain a core temperature >36°C. All patients were mechanically ventilated using volume-controlled mode with an Inspiratory/Expiratory ratio of 1:2, tidal volume of 8 mL·kg−1 ideal weight, a respiratory frequency between 12 and 16 to maintain an end-tidal CO2 between 30 and 40 mm Hg, an active inspiration with a constant flow of 40 L·min−1, and an oxygen inspiratory fraction of 0.5 (Evita XL, Dräger Medical®, Lübeck, Germany). Positioning of the PAC was verified by pressure tracing and thoracic radiography. Invasive blood pressure measurements were performed in the radial artery on the same side as the pulse oximetry measurements. The pulse oximetry wave, ECG, noninvasive blood pressure, and continuous CO estimation (ESco) were obtained using an esCCO monitor (VISMO PVM-2703, Nihon Kohden, Tokyo, Japan). Acquisition of ESco has been detailed elsewhere.6,17,18 Briefly, ESco is calculated according to the formula ESco = K × (α × PWTT + β) × HR, where HR is the heart rate measured by the ECG, K is based on the pulse pressure measured by the noninvasive arterial pressure, and α and β are experimental constants and defined by the demographic data patient. PWTT is the pulse wave transit time (defined as the time interval between the ECG R wave and the pulse plethysmograph upstroke); it is correlated with the stroke volume. The device measures the mean of 64 successive PWTT. The ECOM monitor (ECOM-MG, software 2.4) was connected to the radial arterial catheter via the arterial pressure cable (ECOM-APC) and to the ECOM endotracheal tube impedance wires via the tube cable (ECOM-TC) to provide beat-to-beat CO.19 The ECOM system provides CO measurements according to a trademark algorithm evaluating: (1) the electrical impedance changes caused by the flow in the ascending aorta, which is measured by probes placed on the cuff of a dedicated endotracheal tube, (2) the pulse pressure measured from an arterial line used as a calibration adjustment, and (3) the systolic timing from R wave measured by ECG. These comparisons are made about 1200 times per second and are triangulated against each other. These 2 devices were calibrated using demographics and noninvasive data: age, gender, height, weight, and a measurement of noninvasive blood pressure for esCCO. Reference CO (TDco) was measured by right heart TD using the PAC, from an average of 3 to 5 successive measurements (exclusion of 2 extremes values in case of a variation >20% of CO after 3 measurements) obtained by injection of 10 mL cooled 5% dextrose solution combined with an Vigilance II Monitor (Edwards). Measurements of TDco and continuous CO by esCCO (ESco) and ECOM (ECco) were performed before, during, and after an ARM3 at 3 defined time points: T1 at a positive end expiratory pressure (PEEP) of 0 cm H2O, T2 at a PEEP of 15 cm H2O, and T3 once PEEP returned to 0 cm H2O. At each step, the following variables were recorded: mean, systolic, and diastolic arterial systemic blood pressure; HR; end-expiratory central venous pressure (CVP); mean, systolic and diastolic arterial pulmonary pressure; pulse oximetry; peak and plateau airway pressure; static compliance of the respiratory system; and perfusion index from the esCCO. Systemic vascular resistance (SVR) was calculated. A noninvasive measurement of blood pressure from the esCCO was collected 1 minute after each PEEP modification. Signal quality of esCCO (signal quality index [SQI] 1–4) and ECOM (1–6 verticals bars) were also recorded. Measurements were made after a 2-minute stabilization phase after each PEEP modification. Measurements consisted of the mean value of 3 recordings of ESco and ECco immediately after the TD curve was accepted, but before the numerical TDco value was displayed on the monitor. If non-ARM-induced hemodynamic instability occurred, or if plateau airway pressures increased above 30 cm H2O, the protocol was immediately stopped. Recordings were started only if no vasoactive drug modifications were made in the 10 minutes before the protocol start point.
Distributions of values were evaluated by a Shapiro-Wilk test. Normal distributed values were compared between the 3 time plots within concomitant percentage changes using an ANOVA for repeated measurements with a P value < 0.05 considered as significant. Nonnormal distributed values were compared using a Friedman test within concomitant percentage changes. If significant differences did occur (P < 0.05), a paired Wilcoxon signed rank test was run to evaluate a difference between each time plot. Significant difference was established after a Bonferroni adjustment, that is, the initial significance level (0.05) divided by the number of time plots (3) (P < 0.017).
Precision of the Technique
The CO precision error (%) of each device at each time point for each patient was calculated using:
where CV is the coefficient of variation of each measurement (
) and 3 is the number of replications kept for each measurement at each time point for each patient.20–22 To ensure that trending CO was not biased by the precision of the technique (i.e., the minimal change between successive measurements that can be considered a real change and not due to random error with a probability of 95%), we calculated the least significant changes (LSCs) of CO proposed by Cecconi20,21 adapted from previous studies23,24 and used recently22,25 where
Mean and SD or median and interquartile range of precision error and LSC were then calculated according to mean values of each time plot for each patient.
Agreement and Responsiveness
The agreement between the measurements obtained with the tested devices (esCCO and ECOM monitors) and those obtained with TD was assessed using the Bland-Altman method.26 The measurements corresponded to the mean of each of the 3 replicates performed at T1, T2, and T3. After checking the normality of Bland-Altman bias by a Shapiro-Wilk test, agreement (1.96 × SD of the bias) between methods of measurement with multiple observations per individual was calculated.27
Agreement of the change in CO (ΔCO: difference of the measurements between T1 and T2 or between T2 and T3) between each new device (ΔESco and ΔECco) and ΔTDco was assessed using the coefficient of correlation concordance (CCC; ρc),28 the 4-quadrant plot, and the polar plot approach.29 Additional relationship between percentage changes in mean arterial pressure (ΔMAP) and ΔTDco was assessed with the CCC. The CCC corresponds to the product of the measurement of precision ρ (Pearson correlation that measures how far each observation deviates from the best-fit line) with the measurement of accuracy Cb (bias correction factor that measures how far the best-fit line deviates from the 45° line through the origin). A CCC of 1 corresponds to perfect agreement, whereas a value <0.9 has been considered to indicate poor strength of agreement.30 The precision and agreement of successive ΔESco, ΔECco, and ΔMAP measurements compared with ΔTDco were analyzed according to CCC with calculated 95% confidence intervals (CIs). Values presenting a mean ΔCO [(ΔTDco + ΔCO tested method)/2] < 0.75 L/min or a mean percentage of ΔMAP and ΔTDco <15% were excluded for the analysis (central exclusion zone).31 The ability to accurately trend changes in the esCCO and ECOM systems was also assessed using the polar plots method proposed by Critchley et al.29,32 Changes in CO values were calculated by subtracting the previous CO value from the current CO value. Circle polar plots are shown with data transformed to positive directional data using a half-moon plot after removing central zone data (<0.5 L/min).33 The distance from the center of the plot represents the mean change in CO. When TDco is the reference method, an angular bias no greater than ±5° and radial limits agreement no greater than ±30° represent a good trending ability.32
Concordance rate was calculated according to the 4-quadrant plot (proportion of data points in which both methods demonstrate change in the same direction) and the polar plot (mean polar angle of the tested method lies between ±30° radial limits of agreement). The 95% CI for the concordance rate was calculated using
, where n is the number of ΔCO pairs and p is the concordance rate.34
We calculated that 30 triplicate sets of ΔCO data would be sufficient to obtain a polar concordance rate of 0.8 with a 95% CI <10%. Using this estimation and knowing that a PEEP of 15 cm H2O would induce a change of TDco >0.5 L/min CO in 60% of the sets,13,35,36 we determined the need for at least 25 patients in our study. All the analysis was carried out using the MedCalc statistical software version 14.8.1 (MedCalc® Software bvba, Ostend, Belgium). Polar coordinate format was performed using an Excel spreadsheet (Office Excel® 2013, Microsoft Corporation, Redmond, VA).
Thirty-two patients were enrolled. Five were excluded. Three were excluded secondary to difficult PAC placement, and 2 additional patients were not monitored due to complications associated with the central venous line puncture (1 pneumothorax and 1 carotid puncture). Intubation with the ECOM endotracheal tube was easily accomplished in all patients. We observed no adverse events or other difficulties related to the use of esCCO or ECOM. ECOM did not display any values twice at T1 for patients 8 and 17 and twice at T2 for patients 4 and 17). Table 1 shows selected clinical characteristics, including information regarding surgery and inotropic support. The random error of each technique is reported in Table 2. The precision error was below 20% and independent of the device and the time plot.
Impact of ARM on Hemodynamic Values
Parameters measured before (T1), during (T2), and after (T3) the ARM are reported in Table 3. CO values were normally distributed. As shown, TDco dropped by 31% (95% CI, 26–35; P < 0.001) between T1 and T2 and increased by 51% (95% CI, 42–63; P < 0.001) between T2 and T3. ESco dropped by 6% (95% CI, 2–11; P = 0.048) between T1 and T2 and increased by 8% (95% CI, 3–14; P = 0.016) between T2 and T3. ECco dropped by 16% (95% CI, 11–22; P < 0.0001) between T1 and T2 and increased by 20% (95% CI, 10–31; P < 0.0001) between T2 and T3. MAP dropped by 13% (95% CI, 6–21; P = 0.006) between T1 and T2 and increased by 21% (95% CI, 10–32; P = 0.002).
We analyzed means of 3 CO measurements at each time, resulting in 81 ESco and 77 ECco values being collected. Bland-Altman plots are displayed in Figure 1. Bias between both tested methods and TDco were normally distributed. Between esCCO and TDco, the mean bias was +0.7 L/min with limits of agreement of −2.1 L/min and +3.5 L/min. Between ECOM and TDco, the mean bias was +0.2 L/min with limits of agreement of −2.0 L/min and +2.4 L/min.
CCC and 4-Quadrant Plot Analysis
Fifty-four pairs of ΔESco/ΔTDco as well as 48 pairs of ΔECco/ΔTDco were collected. After exclusion of any values with mean ΔCO <0.75 L/min, 32 pairs of ΔESco/ΔTDco data and 33 pairs of ΔECco/ΔTDco data were ultimately analyzed. Results are presented in Figure 2. The CCC between ΔMAP and ΔTDco was 0.69 (95% CI, 0.54–0.80) with a Pearson correlation of 0.76 and a bias correction factor of 0.91. The CCC between ΔECco and ΔTDco was significantly superior to the CCC between ΔESco and ΔTDco (P = 0.0065) and was equivalent to the CCC between ΔMAP and ΔTDco (P = 0.16). The 4-quadrant plot analysis showed a concordance rate of 81% (95% CI, 74–88) between ΔESco and ΔTDco and 100% between ΔECco and ΔTDco.
Polar Plot Analysis
Fifty-four pairs of ΔESco/ΔTDco and 48 pairs of ΔECco/ΔTDco were analyzed after exclusion of any values with mean ΔCO <0.5 L/min. Results are presented in Figure 3. The polar plot analysis for ΔESco showed a bias of −29° with radial limits of agreement of −67° to +8° and a polar concordance rate of 41% (95% CI, 34–48). The polar plot analysis for ΔECco showed a bias of −15° with radial limits of agreement of –44° to + 14° and a polar concordance rate of 85% (95% CI, 79–90).
This study is the first to evaluate the accuracy and tracking abilities of esCCO and ECOM when compared to TD using PAC during an ARM in postoperative cardiac surgery patients. Our results demonstrate that both monitors have a consistent negative polar bias greater than the ±5° suggested.29 In other words, they both underestimate the successive decrease and increase during an ARM when compared to TDco. The large polar limits of agreement values observed with esCCO (–67° to +8°) are notably worse than the acceptable polar limits of agreement values of ECOM (within the ±30° of angular bias). This result is further exemplified by: (1) the significantly larger CCC of ΔECco compared to ΔESco and (2) an ECOM concordance rate acceptable for the polar plot (85%) and the 4-quadrant plot (100%). However, in this particular setting with no significant variation of SVR, absence of CCC differences between ECOM and MAP indicates that MAP may be as accurate as ECOM for tracking changes in CO. It is also noteworthy that neither device is clinically interchangeable with TD due to their own large clinical limits of agreement.
The intervention (ARM) had a significant impact on TDco: the lower limit of the 95% CI for all ΔTDco was 26%, a value significantly different from the LSC of TDco (7%).21 Despite its known precision error,37,38 we chose PAC bolus TD due to its ability to track CO.38 Significant changes in CO allowed us to include a high proportion of paired of ΔCO values to calculate the concordance rate. Indeed, a large central zone for the 4-quadrant plot analysis and opposite paired of ΔCO values for the polar plot analysis are excluded for the concordance rate, therefore decreasing the power of the analysis. Interestingly, the 4-quadrant plot view showed a concordance rate of 81% for esCCO despite a low CCC (ρc = 0.42) and a poor polar concordance rate (41%). The large difference in the concordance rate can be explained partly by the wider acceptance of ΔCO values for the 4-quadrant plot analysis. From an angular description, as many plots range between 30° and 45° (circle plots near the horizontal line, Fig. 2) from the identity line that is a diagonal, these plots are accepted in the 4-quadrant plot analysis and rejected in the polar plot analysis (wherein identity line is horizontal with predefined limits of agreement of 30°). Therefore, the threshold ratio used to calculate the concordance rate markedly differs. Therefore, we also chose to use CCC which can explore both the precision and accuracy between the tested devices and the reference method. A good agreement between 2 methods requires a CCC >0.9.30 Hence, the combination of 3 “concordance” statistical analyses highlights the potential limits of one and demonstrates the need for additional 2 analyses. Of note, the CCCs of ΔMAP versus ΔTDco and ΔECco versus ΔTDco were not statistically different, questioning the idea that either of the CO monitors evaluated in this particular setting are more clinically useful that arterial blood pressure monitoring for following changes in CO.
The negative angular bias for both methods is intriguing, but has been previously shown with esCCO8,9 and is due to either underestimation by the 2 devices or overestimating by TDco of the true changes in CO during an ARM. As TDco measures the right-sided CO, it may be possible that right-sided CO is transiently more sensitive to acute preload decreases and acute afterload increases (such as occurs during an ARM) when compared to a measure of left-sided CO.39 The 2-minute period of stabilization after each PEEP modification may have not lasted long enough to allow the equilibration of left- and right-sided CO.36 However, the precision of each technique was below 20%, therefore acceptable (Table 2), and not different regardless of the time plot, demonstrating that variation of CO at each time plot was not significant. Another factor affecting measurements is that PEEP is known to occasionally cause significant tricuspid regurgitation.14 This is due to an increased portion of right outflow returning to the right atrium, in which case right-sided TD underestimates pulmonary valve outflow.40 Interestingly, this physiological condition should have caused a positive angular bias for esCCO and ECOM measurements. A smart approach to elucidate these potential limitations would have been to add a left-sided CO measurement, such as echocardiography, which could have measured the left outflow tract and also evaluated tricuspid regurgitation. When questions of this quantity and diversity arise, validating tested devices with a true gold standard CO measurement like transit time ultrasonic flowprobes29,41 in animal studies or cardiac surgery (where great vessels are accessible) should be considered.
These results agree with the findings of previous comparisons between these devices and TD that found large limits of agreement depicted by percent errors between 44% and 80% for esCCO8,42–44 and between 40% and 55% for ECOM.45–49 The trending ability of these techniques have been studied much less, with one previous study14 showing a concordance rate of 96% for esCCO with an exclusion zone of <0.5 L/min (although it has been suggested to be <0.75 L/min).31,34 Another difference between that study and ours is that they used an invasive calibration method, which may less clinically relevant, as this monitor is designed to be used in a completely noninvasive manner. Additionally, one of the authors had significant conflicts of interest.9 In addition, limited other studies report similar findings. For example, after cardiac surgery Ishihara et al.50 found an acceptable angular bias of −1.6° for esCCO using an invasive calibration method (TDco). They, however, found wide radial limits of agreement (±53°), similar to our study (±37°), indicating the poor trending ability of the esCCO system. Regarding the ECOM system, 5 studies have examined the trending availability.46–49,51 Our results contrast with Fellahi et al.47 and Møller-Sørensen,48 which both demonstrated a poor 4-quadrant trending ability (recalculated using displayed graphs in both studies)47,51 in patients after different functional changes of positioning and a fluid challenge. These different ways of changing CO, with the potential motion of bioimpedance sensors48,51 and a different reference method,47,51 could partly explain these results. Using a polar plot analysis, 2 other studies showed that 83% and 87% of ΔECco measurements were within the clinically acceptable limit of 0.5 L/min, although they reached opposite conclusions on the clinical usefulness of these values.46,49 The absence of an exclusion zone and different limits of good trending result in a difficult comparison with our results. Finally, all these different methods of analyzing trending availability (linear regression, 4-quadrant plot, and polar plot with radial limits of agreement or with a fixed ΔCO threshold) could be a source of confusion when comparing different articles concerning one device, therefore calling for a standardization of CO trending analysis.
Weaknesses of the Study
Several factors of our experiment could explain the poor results of esCCO. First, the small sample size of this study could not represent a representative cardiac population, but our power calculation was based on a predefined polar concordance rate value of 0.8, a threshold that we consider clinically acceptable. Of note, the lower limit of an acceptable polar concordance rate is discussed by the authors who originally proposed this trending analysis.32,34 Second, we feel the need to stress that we only tested 1 approach to modify CO. For example, it was shown here that MAP significantly tracked CO during the various interventions, as SVR was only minimally affected by the ARM. Additional studies are required that address clinical situations where SVR changes, such as testing vasoconstrictors, fluid challenges, or a longer experiment duration.
Additionally, this study was conducted after cardiac surgery where inflammation and vasoactive drugs could affect the quality of the recorded peripheral signal.52 However, a recent multicenter study did not find any influence of cardiopulmonary bypass on esCCO values when compared to other ICU patients.7 Additionally, the esCCO pulse oximeter was placed on the same side as the arterial line, which may have risked decreasing the quality of the peripheral signal.9 Another potential limitation is the effect of ARM-induced sympathetic tone on the plethysmographic SQI. This is not likely a major issue, as SVR was relatively unchanged during the entire experiment (Table 2). During an ARM, the increase in CVP may have lead to “venous pulsation,”53 which can affect the plethysmographic waveform as well as the pulse index, both of which are used in the estimation of PWTT. Indeed, pulse index significantly decreased during the ARM in our study. All of these considerations further demonstrate why the esCCO is inappropriate for tracking the hemodynamic effects of an ARM. However, the SQI of both tested devices were acceptable, and the SQI of esCCO during the ARM (T2) was not significantly different from T1 and T3. Only in 1 time point of 1 patient was the SQI of ECOM too low to display a CO value. One may argue that the breadth of esCCO monitoring should not include perioperative cardiac surgery, but there is strong precedent for testing this device in such conditions42,43,54 to allow for ethical PAC insertion. Notably, the rate of complications with central venous access in this study was surprisingly high, contrasting with previous results from the same team that demonstrated no complications.35,55
When attempting to define the clinical applicability of novel CO monitoring devices with the intention of improving the overall perioperative process, it is important to evaluate the devices in the context of known hemodynamic perturbations. We have demonstrated that in postoperative cardiac surgery patients, absolute CI values of esCCO and ECOM are not interchangeable with TD during the known hemodynamic changes of an ARM. EsCCO failed to adequately track CO changes during this intervention, whereas ECOM is as accurate as invasive arterial pressure, although at a higher cost and lower availability.
Name: Magalie Thonnerieux, MD.
Contribution: This author was the first author and contributed to study design, conduct of the study, data collection, data analysis, and manuscript preparation.
Attestation: Magalie Thonnerieux approved the final manuscript.
Name: Brenton Alexander, BS.
Contribution: This author helped analyze the data and write the manuscript.
Attestation: Brenton Alexander approved the final manuscript.
Name: Catherine Binet, MD.
Contribution: This author helped with patient inclusion and data collection.
Attestation: Catherine Binet approved the final manuscript.
Name: Jean-François Obadia, MD, PhD.
Contribution: This author helped with patient inclusion.
Attestation: Jean-François Obadia approved the final manuscript.
Name: Olivier Bastien, MD, PhD.
Contribution: This author helped study design, conduct of the study, data collection, and manuscript preparation.
Attestation: Olivier Bastien approved the final manuscript.
Name: Olivier Desebbe, MD.
Contribution: This author was the co- first author and helped design the study, conduct of the study, data collection, data analysis, and manuscript preparation. Olivier Desebbe was the archival author.
Attestation: Olivier Desebbe approved the final manuscript. He attests to the integrity of the original data and the analysis reported in this manuscript.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
The authors thank Nihon Kohden Corporation and Sylvain Thuaudet, MD (I.S.T. Cardiology, Saint Contest, France), for kindly providing all the facilities necessary for hemodynamic monitoring with the esCCO™ and ECOM™ devices.
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