Comparison of the ability of two continuous cardiac output monitors to detect stroke volume index: Estimated continuous cardiac output estimated by modified pulse wave transit time and measured by an arterial pulse contour-based cardiac output device

Abstract

BACKGROUND:

Estimated continuous cardiac output (esCCO), a non-invasive technique for continuously measuring cardiac output (CO), is based on modified pulse wave transit time, which is determined by pulse oximetry and electrocardiography.

OBJECTIVE:

We examined the ability of esCCO to detect stroke volume index (SVI) and changes in SVI compared with currently available arterial waveform analysis methods.

METHODS:

We retrospectively reanalysed 15 of the cases from our previous study on esCCO measurement. SVI was calculated using an esCCO system, measured using the arterial pressure-based CO (APCO) method, and compared with a corresponding intermittent bolus thermodilution CO (ICO) method. Percentage error measurement and statistical methods, including concordance analysis and polar plot analysis, were performed.

RESULTS:

The difference in the SVI values between esCCO and ICO was $-$ 3.0 $\pm$ 8.8 ml (percentage error, 33.5%). The mean angular bias was 0.8 and the radial limits of agreement were $\pm$ 27.3. The difference in the SVI values between APCO and ICO was 0.9 $\pm$ 11.2 ml (percentage error, 42.6%). The mean angular bias was $-$ 6.8 and the radial limits of agreement were $\pm$ 44.1.

CONCLUSION:

This study demonstrated that the accuracy, precision, and dynamic trend of esCCO are better than those of APCO.

Keywords

Estimated continuous cardiac output haemodynamic monitoring stroke volume index arterial pressure-based CO

1. Introduction

Fluid replacement using goal-directed fluid therapy is strongly recommended in high-risk patients and patients undergoing surgery with large intravascular fluid loss (blood loss and protein/fluid shift) [1]. Some patients require volume therapy where goal-directed boluses of intravenous solutions (usually a colloid) are aimed at maintaining central normovolaemia by utilising changes in stroke volume [2, 3]. Arterial hypotension should be treated with vasopressors, in cases where administering intravenous fluid boluses fails to improve the stroke volume significantly [4, 5]. In high-risk patients undergoing a major abdominal surgery, implementation of an intraoperative goal-directed hemodynamic optimisation protocol using the FloTrac/Vigileo device was associated with a reduced length of hospital stay and a lower incidence of complications compared to a standard management protocol [6]. The FloTrac/Vigileo system consists of a specialised blood pressure sensor and monitor that collects and analyses arterial pressure data in real time, using an algorithm to derive the arterial pressure-based cardiac output (APCO) from the arterial pressure wave. Additionally, fluid optimisation guided by stroke volume variation using the FloTrac/Vigileo device during major abdominal surgery is associated with better intraoperative hemodynamic stability, decreased serum lactate levels at the end of surgery, and a lower incidence of postoperative organ complications [7]. Stroke volume variation monitors changes in arterial pressure waveform amplitudes with regard to breathing patterns, which is an effective method to monitor fluid responsiveness. The FloTrac/Vigileo is a valid device for intraoperative hemodynamic monitoring and is used widely in many countries. However, the FloTrac/Vigileo is costly and requires invasive arterial pressure measurements, while the estimated continuous cardiac output (esCCO) method is non-invasive and is not associated with operational costs. If the accuracy, precision, and dynamic trend of the stroke volume index (SVI) using esCCO are better than those of APCO, esCCO will be a valid device for intraoperative hemodynamic monitoring. In the present study, we retrospectively reanalysed 15 cases from our previous study [8]. SVI was measured using esCCO and APCO and compared with a corresponding intermittent bolus thermodilution cardiac output (ICO) method. Percentage error measurement and statistical methods, including concordance analysis and polar plot analysis, were performed.

2. Methods

After our previous study [8] on esCCO, we retrospectively identified 15 cases. Our previous study was approved by the ethics committee of the Toho University Omori Medical Centre, Tokyo, Japan. All procedures performed were in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. Written informed consent was obtained from all patients who underwent kidney transplant surgery at our hospital between April 2008 and June 2010. After the patient’s condition stabilised, following the start of surgery, the esCCO system was connected to an electrocardiogram (ECG) monitor, an arterial pressure monitor, and a pulse oximetry system; ICO, APCO were then measured, and the esCCO was then calculated; esCCO calibration was conducted using patient information. ECG, pulse oximetry waves, arterial blood pressures, and pulse wave transit times were obtained using a BSM-9101 bedside monitor (Nihon Kohden, Tokyo, Japan). APCO was determined using a FloTrac/VigileoTM system Version 3 (Edwards Lifesciences, CA, USA). A-line catheters were inserted into the radial artery of the patients. Three sets of readings were taken (a) before renal artery clamping, (b) before declamping, and (c) after surgery. ICO measurements were obtained thrice using 10 ml volumes of ice-cold saline solution [9], and the average of these measurements was used. Pulse wave transit time (PWTT) was calculated from the dynamic average data from 64 consecutive heartbeats, and esCCO was then calculated as previously reported [10]. The APCO records hemodynamic variables at 20 s intervals, performing its calculations on the most recent 20 s of data. We performed correlation analyses comparing SVI calculated from ICO versus SVI calculated from esCCO, and SVI calculated from ICO versus from SVI calculated from APCO. We then constructed Bland-Altman plots of SVI calculated from ICO versus from esCCO and SVI calculated from ICO versus from APCO to determine the bias (the mean of the differences) and precision (one standard deviation of differences). The percentage error was calculated using the ratio of two standard deviations of the bias to the mean SVI from ICO. We also performed concordance analyses using four-quadrant plot and polar plots analyses comparing SVI calculated from ICO versus from esCCO and SVI calculated from ICO versus from APCO to determine the ability of the two continuous cardiac output monitors to measure trends [11, 12]. These data were used to set the criteria for good trending ability. After excluding central zone data, good trending ability was associated with concordance rates above 95%, marginal trending ability was associated with concordance rates between 90% and 95%, and poor trending ability was associated with concordance rates below 90%. For polar analysis, ICO was the reference method; good trending required a mean angular bias of $\leqq\pm$ 5 ${}^{\circ}$ and radial limits agreement of $\leqq\pm$ 30 ${}^{\circ}$ .

Figure 1.

Correlation analyses comparing SVI calculated from ICO versus that from esCCO, and SVI calculated from ICO versus that from APCO. A. The correlation coefficient between SVI from ICO and that from esCCO was 0.68. B. The correlation coefficient between SVI from ICO and that from APCO was 0.51.

Figure 2.

Bland-Altman plots analyses comparing SVI calculated from ICO versus that from esCCO, and SVI calculated from ICO versus that from APCO. A. The difference in the SVI values between esCCO and ICO was $-$ 3.0 $\pm$ 0.88 ml (percentage error, 33.5%). B. The difference in the SVI values between APCO and ICO was 0.9 $\pm$ 11.2 ml (percentage error, 42.6%).

3. Results

The patients’ average age was 42.4 $\pm$ 14.9 (mean $\pm$ SD) years, average patient height was 162.7 $\pm$ 9.9 (mean $\pm$ SD) cm, and average patient weight was 57.8 $\pm$ 15.4 kg (mean $\pm$ SD). The correlation coefficient between SVI from ICO and SVI from esCCO was 0.68 (Fig. 1, Left). The correlation coefficient between SVI from ICO and SVI from APCO was 0.51 (Fig. 1, Right). The difference in the SVI values between esCCO and ICO was $-$ 3.0 $\pm$ 0.88 ml (percentage error, 33.5%) (Fig. 2, Left). The difference in the SVI values between APCO and ICO was 0.9 $\pm$ 11.2 ml (percentage error, 42.6%) (Fig. 2, Right). The mean angular bias between SVI from ICO and esCCO was 0.8, and the radial limits of agreement were $\pm$ 27.3 (Fig. 3, Left). For the concordance analysis, we used a four-quadrant plot for SVI from esCCO versus SVI from ICO, which showed a concordance rate of 95.5% (exclusion zone: 8.5 ml/m ${}^{2}$ , concordance: 21, disconcordance: 1) (Fig. 3, Right). The mean angular bias between SVI from ICO and SVI from APCO was $-$ 6.8, and the radial limits of agreement were $\pm$ 44.1 (Fig. 4, Left). The concordance analysis using the four-quadrant plot for SVI from APCO versus SVI from ICO showed a concordance rate of 78.2% (exclusion zone: 8.5 ml/m ${}^{2}$ , concordance: 18, disconcordance: 5) (Fig. 4, Right).

Figure 3.

Concordance analyses and polar plots analyses comparing SVI calculated from ICO versus SVI calculated from esCCO (exclusion zone 8.5 cc/m ${}^{2}$ ; 15% of average SVI). A. The mean angular bias between SVI calculated from ICO and esCCO was 0.8, and the radial limits of agreement were $\pm$ 27.3. B. The concordance analysis for SVI from esCCO versus SVI from ICO showed a concordance rate of 95.5% (exclusion zone 8.5 ml/m ${}^{2}$ ; 15% of average SVI; concordance: 21; disconcordance: 1).

Figure 4.

Concordance analyses and polar plots analyses comparing SVI calculated from ICO versus SVI calculated from APCO (exclusion zone 8.5 cc/m ${}^{2}$ ; 15% of average SVI). The mean angular bias between SVI calculated from ICO and that from APCO was $-$ 6.8 and the radial limits of agreement were $\pm$ 44.1. B. The concordance analysis for SVI from APCO versus SVI from ICO showed a concordance rate of 78.2% (exclusion zone 8.5 ml/m ${}^{2}$ ; concordance: 18; disconcordance: 5).

4. Discussion and conclusion

In high-risk patients undergoing major abdominal surgery, implementation of an intraoperative goal-directed hemodynamic optimisation protocol using the FloTrac/Vigileo device was associated with a reduced length of hospital stay and a lower incidence of complications compared to a standard management protocol [6]. If we obtain a non-invasive continuous stroke volume measurement, it will be possible to reduce the incidence of perioperative complications without the complications associated with radial artery catheterisation. This study examined the ability of esCCO to detect SVI and the changes in SVI, compared with currently available arterial waveform analysis methods.

This study demonstrated that the accuracy and precision of SVI from esCCO are better than those from APCO. We also performed concordance analyses using four-quadrant plot and polar plots analyses comparing SVI calculated from ICO versus from esCCO and SVI calculated from ICO versus from APCO to determine the ability of the two continuous cardiac output monitors to measure trends. This study demonstrated that the dynamic trend of the SVI from esCCO is good trending, and that of the SVI from APCO is poor trending. Hence, this study demonstrated that the accuracy, precision, and dynamic trend of esCCO are better than those of APCO. APCO, with the most recent software, allows for sufficiently accurate and precise cardiac output measurements and trending in normo- and hypodynamic conditions, in the absence of large changes in vascular tone [13]. The reliability of the APCO system to measure cardiac output and track changes in cardiac output with normal (1200–2500 dyn $\cdot$ sec $\cdot$ cm ${}^{-5}$ $\cdot$ m ${}^{2}$ ) systemic vascular resistance index states has been verified [14]. In addition, it has been reported that esCCO can also be used in both children and adult patients [15, 16]. esCCO could be used in a wide range of age groups, and the change of PWTT may be independent of vascular elasticity. Moreover, the accuracy of esCCO may be independent of the systemic vascular resistance index. We verified that the trending ability of the esCCO device was deemed good with APCO with normal systemic vascular resistance index states [17]. esCCO could be used in those with large changes in vascular tone. A limitation of this study is the small number of cases; therefore, further studies are required to validate our findings. In conclusion, this study suggests that esCCO is more valid for use in goal-directed fluid therapy than APCO.

Footnotes

Conflict of interest

The authors declare that they have no conflict of interest.

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