The most important finding of our study is that the esCCO system, when using an arterial line source for pulse pressure, reliably trends directional changes in cardiac output. Regression analysis revealed high correlation between ΔCOTD and ΔCOES (r = 0.85). However, the slope of the regression line (0.68) is less than that of the line of identity. This indicates that the esCCO system may underestimate the magnitude of changes in cardiac output. Biancofiore et al.11 recommended the 4-quadrant plot of serial changes in cardiac output and concordance analysis for evaluation of new methods of cardiac output measurement. This was done in this study, as was a Bland-Altman plot comparing changes. The esCCO system correctly assessed the direction of serial changes in cardiac output, with a concordance rate of 96.0% with COTD. However, wide limits of agreement on Bland-Altman analysis indicate that trending of the magnitude of change is less accurate, particularly when large changes in cardiac output occur. Previous reports have suggested the threshold of concordance should be within 90% to 95% for assuming good trending ability when exclusion criteria of 0.5 to 1.0 L·min−1 are applied,7–9 and these data meet that criterion.
Head-up tilt and head-down tilt position changes and fluid administration using hydroxyethyl starch are often used as first-step maneuvers to treat hemodynamic instability.11–15 Most of the hemodynamic changes associated with the various maneuvers in this study have been described.11–13 With fluid administration, systemic vascular resistance decreased, whereas MAP, CVP, mPAP, pulmonary capillary wedge pressure, and COTD all increased. During the Pringle maneuver, COTD and CVP decreased and systemic vascular resistance increased, as previously noted.16,17 Release of the Pringle maneuver was associated with a statistically nonsignificant decrease in MAP. In 9 cases, however, arterial blood pressure after release of the Pringle maneuver was supported with phenylephrine or dopamine.
There was a low concordance rate (83.3%) at change from Pringle maneuver to release, although there were good concordance rates at changes between all other time points. This demonstrates that ΔCOES may not have clinically good trending ability when vasopressors are used to treat hypotension associated with decreased systemic vascular resistance. However, Figure 2 failed to show a correlation between systemic vascular resistance and bias. We believe drug-induced acute changes of vascular conditions might influence the relationship of pulse wave transit time and ΔCOES because pulse wave transit time is influenced both by changes of cardiac contractility and arterial compliance.18 The Pringle maneuver and its release caused hemodynamic instability as a consequence of changes of preload and systemic vascular resistance. Sudden drug-induced acute changes of systemic vascular resistance (e.g., administration of vasoconstrictor or vasodilator) are likely not physiologically identical to changes in systemic vascular resistance, resulting from changes in loading conditions and might lead to a relatively low relationship (0.85 with lower 95% confidence interval = 0.73). More investigation needs to be done in this area for clarification.
There are some real and potential limitations of the esCCO method. First, this system needs calibration with another cardiac output measurement system at the start of continuous measurement. Second, for calibration, it needs at least 3 minutes of hemodynamic stability. Third, this system has not been evaluated in situations that would likely render the system inaccurate: patients with arrhythmias such as atrial fibrillation and ventricular premature contractions, use of either a cardiac pacemaker or intraaortic balloon counterpulsation, and/or suboptimal signal for ECG or pulse oximetry (e.g., hypothermia, cold environment). Continued improvements in the system, as well as studies of its clinical utility, are needed. Perhaps, in the future, a version of this system that does not require external calibration, using patient demographic data and other factors to calculate a calibration constant (similar in concept to the Edwards FloTrac™), may be developed. The potential utility of the system in its current form might be in cases in which a more invasive method of cardiac output measurement was used (e.g., pulmonary artery catheter) and then discontinued. Perhaps, this system could be calibrated to the invasive method before its discontinuation and then used subsequently for hemodynamic management and goal-directed fluid therapy. It is not yet known, however, if or how often this system would require recalibration. Future research should be done to determine the time course and frequency with which the esCCO system would require recalibration.
There are limitations in this study. First, low-risk patients scheduled for only partial hepatectomy were enrolled, and patients with severe hypotension were excluded. Further investigations will be required to assess this technique in higher-risk and hemodynamically unstable patients. Second, the investigator recording COTD and COES was not blinded to the results, potentially leading to bias. Third, only patients under general anesthesia were studied. Fourth, an existing arterial line was used, instead of a noninvasive arterial blood pressure device, for determining pulse pressure. The most recent esCCO software (version 01-04), which was not available at the time of this investigation, allows for use of either an arterial line or noninvasive arterial blood pressure measurement. Further study is required to determine whether using noninvasive arterial blood pressure measurement will result in accuracy comparable with that found in this investigation.
This study showed that during partial hepatectomy the esCCO system provides good trending ability (96% concordance rate) when compared with the thermodilution technique in patients under general anesthesia with preload and systemic vascular resistance changes. The current system may be clinically useful as a substitute for the thermodilution method except perhaps when vasopressors are used to treat hypotension associated with decreased systemic vascular resistance or when the cardiac output is particularly low. With future engineering and technical improvements, use of the arterial pulse wave and pulse transit time offers the possibility of measuring cardiac output accurately, quickly, and noninvasively.
In the esCCO method, the pulse pressure (PP), an index of arterial pulsatility, and pulse wave transit time, an index of vascular resistance, compliance, and cardiac contractility, are used to calculate stroke volume (SV).4,19 PP is proportional to SV, with K, a “resistance/compliance” coefficient, further defining the relationship. The components of pulse wave transit time are illustrated in Figure 3. In the esCCO algorithm, pulse wave transit time is calculated as the time from the electrocardiogram (ECG) R-wave peak to the rise point of the pulse oximeter wave. The rise point of the pulse wave was defined as the point at which the differentiated pulse wave reached 30% of its peak amplitude.18
CO is derived as follows:
K is derived from pulse wave transit time, heart rate (HR), and cardiac output (CO) at the start of measurement in each patient. Accordingly, a CO value derived by another CO measurement system is required at the start of esCCO measurement as a calibration. In this study, COTD was used for this purpose. SV is derived as follows:
Data Processing and Exclusion Criteria
Pulse wave transit time is calculated by averaging 64 consecutive data of heart beats. Average pulse wave transit time and HR are based on data retrieved within every 1-second interval. Data with a large variability in pulse wave transit time (>20 milliseconds) or pulse amplitude deviating from median values (>30%) during calculation are excluded. In addition, calculation process of pulse wave transit time is automatically inhibited when >25% of the 64 heart beats data are excluded in the following conditions:
- Either ECG or pulse-oximetry pulse wave signal is not obtained.
- Either R wave on ECG or the start point of the ascending portion of pulse-oximetry wave is not clearly identified.
COES is then calculated using these calculated pulse wave transit time and HR. Data of inadequate conditions are also excluded automatically. Pulse wave transit time, HR, and PP are averaged for at least 3 minutes to calculate K and β.
Name: Masato Tsutsui, MD, PhD.
Contribution: This author helped design and conduct the study, analyze the data, and write the manuscript.
Attestation: Masato Tsutsui has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Yoshiyuki Araki, MD.
Contribution: This author helped conduct the study.
Attestation: Yoshiyuki Araki approved the final manuscript.
Name: Kenichi Masui, MD, PhD.
Contribution: This author helped analyze the data.
Attestation: Kenichi Masui has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Tomiei Kazama, MD.
Contribution: This author helped design and conduct the study and write the manuscript.
Attestation: Tomiei Kazama approved the final manuscript.
Name: Yoshihiro Sugo.
Contribution: This author helped write the manuscript.
Attestation: Yoshihiro Sugo approved the final manuscript.
Name: Thomas L. Archer, MD, MBA.
Contribution: This author helped write the manuscript.
Attestation: Thomas L. Archer approved the final manuscript.
Name: Gerard R. Manecke Jr, MD.
Contribution: This author helped analyze the data and write the manuscript.
Attestation: Gerard R. Manecke Jr has seen the original study data and approved the final manuscript.
This manuscript was handled by: Dwayne R. Westenskow, PhD.
We specially thank Dr. H. Ishihara, Department of Anesthesiology, Hirosaki University, Japan, for providing theory of the esCCO system.
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© 2013 International Anesthesia Research Society
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