Cardiac output measurements via echocardiography versus thermodilution: A systematic review and meta-analysis

Authors: Yun Zhang ^aff001; Yan Wang ^aff002; Jing Shi ^aff001; Zhiqiang Hua ^aff001; Jinyu Xu ^aff001
Authors place of work: Department of Emergency Medicine, Wuxi People’s Hospital Affiliated to Nanjing Medical University, Wuxi, Jiangsu, China ^aff001; Education Department, Wuxi People’s Hospital Affiliated to Nanjing Medical University, Wuxi, Jiangsu, China ^aff002
Published in the journal: PLoS ONE 14(10)
Category: Research Article
doi: https://doi.org/10.1371/journal.pone.0222105

Summary

Echocardiography, as a noninvasive hemodynamic evaluation technique, is frequently used in critically ill patients. Different opinions exist regarding whether it can be interchanged with traditional invasive means, such as the pulmonary artery catheter thermodilution (TD) technique. This systematic review aimed to analyze the consistency and interchangeability of cardiac output measurements by ultrasound (US) and TD. Five electronic databases were searched for studies including clinical trials conducted up to June 2019 in which patients’ cardiac output was measured by ultrasound techniques (echocardiography) and TD. The methodological quality of the included studies was evaluated by two independent reviewers who used the Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2), which was tailored according to our systematic review in Review Manager 5.3. A total of 68 studies with 1996 patients were identified as eligible. Meta-analysis and subgroup analysis were used to compare the cardiac output (CO) measured using the different types of echocardiography and different sites of Doppler use with TD. No significant differences were found between US and TD (random effects model: mean difference [MD], -0.14; 95% confidence interval, -0.30 to 0.02; P = 0.08). No significant differences were observed in the subgroup analyses using different types of echocardiography and different sites except for ascending aorta (AA) (random effects model: mean difference [MD], -0.37; 95% confidence interval, -0.74 to -0.01; P = 0.05) of Doppler use. The median of bias and limits of agreement were -0.12 and ±0.94 L/min, respectively; the median of correlation coefficient was 0.827 (range, 0.140–0.998). Although the difference in CO between echocardiography by different types or sites and TD was not entirely consistent, the overall effect of meta-analysis showed that no significant differences were observed between US and TD. The techniques may be interchangeable under certain conditions.

Keywords:

Surgical and invasive medical procedures – echocardiography – Cardiac output – Blood flow – vascular surgery – cardiac surgery – Coronary artery bypass grafting

Introduction

Continuous or dynamic cardiac function monitoring plays a crucial role in the diagnosis, assessment, treatment, and prognosis of critically ill patients. Cardiac output (CO) measurement is one of the most important parameters in cardiac function monitoring. The commonly used CO measurement methods include indirect Fick methods, thermodilution (TD), Doppler ultrasound (US) or echocardiography, partial carbon dioxide (CO₂) rebreathing, thoracic electrical bioimpedance, and magnetic resonance imaging [1, 2]. TD via the pulmonary artery (PA) catheter is still considered to be the gold standard method in the clinical setting. However, this method has disadvantages because it is invasive and can lead to severe complications [3]. Echocardiography, as a noninvasive or semi-invasive method for the assessment of cardiac anatomy and function, is favored in clinical practice. The methods commonly used for echocardiography include transthoracic echocardiography (TTE), transesophageal echocardiography (TEE), ultrasonic CO monitor (USCOM), noninvasive continuous CO system (NICO), and ultrasound dilution (UD). Several sites can be used for CO measuring. The velocity time integral (VTI) and cross-sectional area (CSA) of the ascending aorta (AA) [4], PA [5], aortic valve (AOV) [6], mitral valve [7], or left ventricular outflow tract (LVOT) [8]can be used to calculate the stroke volume (SV) using CSA×VTI and the CO = SV×heart rate. Simpson’s rule [9] was the first method to delineate the innermost endocardial border of the left ventricle using the trackball at end systolic and end diastolic and then to calculate the left ventricular end-diastolic volumes (LVEDV) and left ventricular end-systolic volumes (LVESV): CO = (LVEDV-LVESV)×heart rate. Some studies used the common carotid artery in point-of-care US to estimate the CO [10]. UD [11] technology is also used to measure hemodynamic variables based on the Stewart-Hamilton principle. This method utilizes an extracorporeal arteriovenous tubing loop (AV loop) inserted between existing arterial and venous catheters and isotonic saline as an indicator [12].

Whether echocardiography can replace TD method in CO measurement remains controversial. Some studies [13–15] have revealed that echocardiography is a rapid, accurate, and noninvasive monitoring technology suitable for patients in ICU. Although differences were observed, some studies [10, 16] showed the correlation was good. However, some studies [17, 18] suggested that echocardiography is not interchangeable with TD for measuring CO. Therefore, this study aimed to evaluate the consistency and interchangeability of cardiac output measurements in US and TD and to find the most optimal types or sites used of echocardiography for CO measuring if possible.

Materials and methods

This review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis of diagnostic test accuracy (PRISMA-DTA) statement. Ethics committee approval was not required, as it was a review of published data.

Search strategy

An electronic literature search was performed in PubMed, EMBASE (using OVID), Cochrane Controlled Trials Registry, China National Knowledge infrastructure, and Wanfang Data from their inception up to June 2019. The EndNote X6 software (Thomson Reuters Corporation, New York, NY, USA) was used to eliminate duplicates and manage these citations. The following search strategy was used to identify studies:

transtho*[Title/Abstract] OR transeso*[Title/Abstract] OR echocard*[Title/Abstract] OR cardiac ultrasound [Title/Abstract] OR Doppler [Title/Abstract] OR USCOM [Title/Abstract]
cardiac output [Title/Abstract]) AND thermodilution [Title/Abstract]
#1 AND #2

Study selection

The inclusion criteria were (1) critically ill patients, (2) clinical trials, (3) studies that used echocardiography to measure the CO, (4) studies that used TD technique as the reference technique, and (5) studies in which outcomes of interest included the data of CO or all the differences between the techniques (bias) and standard deviations (SDs) or bias and limits of agreement (LOA).

The exclusion criteria were (1) reviews or case reports, (2) animal studies, (3) studies published in languages other than English, (4) studies only published as an abstract, and (5) studies with no mean and SD of CO and without bias and LOA/SD between two techniques.

Study selection and data extraction were performed by two independent reviewers (YZ and JS). Disagreements were resolved through consensus.

Data extraction

A data collection form was developed prior to data extraction. Two authors (YZ and YW) extracted relevant data from included articles. The extracted data included (1) first author and year of publication; (2) number of patients, sex, and age; (3) the data of CO in both groups; (4) the type of ultrasound and sites; (5) the bias, SD, LOA, and percentage error (PE); (6) the Pearson R coefficient and linear equations; and (7) patient population.

When the results of the trial were reported as median and quartile, the Stela Pudar-Hozo method was used to estimate the mean and standard deviation. Bias was defined as the mean of the two measurement differences, and LOA was defined as bias±1.96SD (some studies defined LOA as bias±2SD). The PE was defined as 1.96SD divided by the mean CO of the two methods. Posteriori probability was also calculated.

Ethics approval was waived for this study as patient consent was obtained within the individual trials and all data were anonymized.

Quality assessment

Studies with critically ill patients who needed CO monitoring were included. The CO measured by thermodilution was the reference standard, regardless of other modes of CO monitoring. Study quality was assessed using QUADAS-2, which was tailored for our systematic review (S1 Table). The quality of each paper was evaluated by two authors (YW and JS) independently, and any disagreements were resolved by consensus.

Statistical methods

The systematic reviews were conducted in compliance with the PRISMA guidelines. The Review Manager Software version 5.3 for Windows (The Cochrane Collaboration, 2014) was used to perform the meta-analysis. The STATA version 12.0 (StataCorp, College Station, TX, USA) was used to analyze the publication bias (Egger’s test). The Cochrane Q-test was used for heterogeneity analysis. A fixed-effect model was used when P>0.1 and I²<50%; otherwise, a random effects model was adopted. If necessary, a sensitivity analysis was performed to reduce the heterogeneity to P>0.1 and I²<25% by omission of some studies as few as possible. All P-values were two-sided, and P<0.05 was considered significant.

Results

Search results

Of the initial 808 records identified, 676 remained after duplicates were removed. Then, 477 records were excluded based on the title, and 83 records excluded based on the abstract; 116 articles were evaluated in full text. Forty-eight full-length articles were excluded, including 33 articles with missing data and 15 articles that used TD and US techniques, but not CO measurement, in the optimization of cardiac preload. Finally, 68 and 43 studies were included in qualitative synthesis and quantitative synthesis, respectively (meta-analysis) (Fig 1). All studies were published between 1971 and 2018 (Table 1).

**Fig. 1. Flowchart of identification of eligible studies.**

**Tab. 1. Main characteristics of the included studies.**

Characteristics and qualities of included studies

Sixty-nine articles involving 1996 subjects were included. Of these studies, the number of patients ranged from 6 to 89. CO measurements were performed using TTE in 19 studies[2, 13, 23, 26, 27, 41, 44–46, 49–51, 58, 64, 68, 70, 73, 75], TEE in 24 [6–8, 18, 20, 30–32, 34, 42, 47, 48, 52–57, 59–63, 77], USCOM in 14 [4, 19, 22, 25, 28, 39, 40, 43, 65–67, 69, 72, 74], UD in 4 [29, 33, 36, 71], and other types of echocardiography in 7 [10, 21, 24, 35, 37, 38, 61]. CO measurements were performed in the AA in 13 studies [2, 4, 19, 24, 27, 28, 32, 35, 37, 41, 45, 47, 70], AOV in 18 [6, 13, 22, 23, 30, 43, 44, 46, 48, 51, 55, 65, 66, 69, 72, 73, 75–77], and LVOT in 12 [8, 18, 21, 31, 34, 49, 50, 54, 57, 64, 68, 77] with VTI. Further, CO measurements were performed in 5 studies in the PA [2, 5, 53, 60, 61], and CO measurement using the Simpson method in 3[20, 38, 56]. Of these studies, Bland-Altman analyses were used in 56 studies, and the LOA and bias were available in 59. Linear regression analyses were used in 54 studies, and 35 regression equations were acquired. Correlation analyses were used in most studies, and the correlation coefficient (R value) was obtained except for the other 15 studies (Table 2). The methodological qualities of the included studies were evaluated according to the tailored QUADAS-2. The results are shown in Fig 2 and S1 Fig.

**Fig. 2. Diagram demonstrating the studies’ percentage compliance, risk of bias, and applicability concerns.**

CO evaluation using different types of Doppler

Of these included studies, there were 41 studies with 49 CO measured results, and 1522 patients were included in the meta-analysis; no significant differences were observed between US and TD (random effects model: MD, -0.14; 95% confidence interval [CI], -0.30 to 0.02; P = 0.08). The subgroup analyses were conducted using different types of echocardiography techniques. In 19 of the TTE studies, 12 with 14 sets of data and 290 patients were included in the meta-analysis. The result showed no significant differences between TTE and TD (random effects model: MD, -0.28; 95% CI, -0.71 to 0.15; P = 0.20). In 24 of the TEE studies, 13 with 19 sets of data and 606 patients were included in the meta-analysis. The result showed that no significant differences were observed between TEE and TD (random effects model: MD, 0.00; 95% CI, -0.12 to 0.11; P = 0.98). In 13 of the USCOM studies, 10 with 10 sets of data and 356 patients were included in the meta-analysis. No significant differences were observed between USCOM and TD (random effects model: MD, -0.16; 95% CI, -0.61 to 0.28; P = 0.47). No significant differences were observed in four studies between UD and TD (random effects model: MD, 0.00; 95% CI, -0.43 to 0.44; P = 0.99), and no significant differences were observed in the other 4 studies between other types of methods and TD (random effects model: MD, -0.56; 95% CI, -1.25 to 0.14; P = 0.12) (Table 2 and S2 Fig). The sensitivity analysis showed that no change occurred in the overall effect and subgroup analysis effect when some studies were omitted up to the acceptable heterogeneity (Table 3 and S3 Fig).

**Tab. 2. Meta-analyses of the cardiac output measurement by echocardiography (US) vs. thermodilution (TD).**

**Tab. 3. Sensitivity analysis of high heterogeneity outcomes in meta-analysis.**

CO evaluation at different sites

In six studies, the AA was used to measure CO by Doppler, and six studies were included in the meta-analysis. Significant differences were observed in the use of US at AA with TD (random effects model: MD, -0.37; 95% CI, -0.71 to -0.11; P = 0.05). Moreover, no significant differences were observed in the use of US at AOA (random effects model: MD, -0.03; 95% CI, -0.31 to 0.25; P = 0.83), LVOT (random effects model: MD, -0.06; 95% CI, -0.32 to 0.21; P = 0.67), PA (random effects model: MD, -0.09; 95% CI, -0.63 to 0.44; P = 0.73), AV loop (random effects model: MD, -0.01; 95% CI, -0.46 to 0.45; P = 0.97), TPF/TAF (random effects model: MD, 0.05; 95% CI, -0.58 to 0.68; P = 0.88), and other sites (random effects model: MD, -0.53; 95% CI, -1.40 to 0.33; P = 0.23) (Table 2 and S4 Fig). The sensitivity analysis showed no changes in the overall effect and the subgroup analyses (Table 3 and S5 Fig).

Bland-Altman analyses and regression analyses

In all studies, the median of bias between US and TD was -0.12 (ranged from -2.50 to 3.04 L/min). The median of LOA was 0.94 L/min (ranged from ±0.05 to ±4.72 L/min). Twenty-eight studies reported that the PE and the median were 24.3% (ranged from 7.2% to 53%). The median of R (correlation coefficient) was 0.827 (ranged from 0.140 to 0.998). The slope ranged from 0.126 to 1.58, and the intercept ranged from -1.91 to 3.55 in the 35 regression equations (Table 1).

Publication bias

The funnel plot was roughly symmetrical (S5 Fig). Egger’s test revealed no publication bias in the literature (P = 0.500) (S6 Fig).

Discussion

In this systematic review, we included 68 studies, of which 43 studies reported data on CO measurement and were included in the meta-analysis. The overall effect showed that no significant difference was observed between echocardiography and TD in measuring CO; the subgroup analysis showed no significant differences in the different types. In all sites, the difference was founded only in AA. Further, the sensitivity analysis showed no change in the results. However, there was a wide range in bias, LOA, PE, and correlation coefficient of the two technologies and was beyond the clinically acceptable range in some studies.

In these different types of echocardiography, the sensitivity analysis showed that the TEE, USCOM, and UD had hairline bias (≤0.1 L/min) with TD; TEE had the lowest standard error and maximum weight followed by TTD. The UD had a small mean difference, but with wide 95% CI. CO could be easily overestimated using TTE instead of using TD, and underestimated using other types of echocardiography (ACM, NICO, point-of-care US, and Doppler PA catheter) despite the differences not having statistical significance. Therefore, we speculate that TEE was the preferred method because it can obtain more accurate results in the measurement of CO. Considering that TEE is semi-invasive and UD can only be used for specific population, USCOM can be the first choice for noninvasive echocardiography for the measurement of CO.

The sites of the ultrasonic probe will also have an effect on the test results. In our meta-analysis, no statistical difference was found except for the measurement of CO in the AA. The lowest mean difference of CO comes from PA, compared with the TD, followed by AV loop. However, the method of measuring PA CO used a PA catheter ultrasound probe and was an invasive procedure [59]. The AV loop used in UD is also based on an invasive procedure[28, 33, 35, 68]. In these studies, more researchers were willing to measure CO in the AOV and LVOT, with mean differences of <0.1 L/min and a narrower LOA. Therefore, the AOV and LVOT as the recommended sampling locations for CO detection are feasible. This finding is also consistent with the recommendation of the American ultrasound guidelines [78].

In all these studies, the largest bias and LOA (bias = 3.04 L/min, LOA = ±9.44 L/min) were found in a study with the ABCOM 1 transtracheal Doppler (TTD) versus TD [34] and with the lowest correlation coefficient (R = 0.14). TTD system requires a special TTD endotracheal tube, in which the tip was embedded with an ultrasonic probe; it can only be used in patients with mechanical ventilation. In this study, seven patients with 36 simultaneous measurements were compared. We speculated that the TTD measurements accounted for most of the between-technique variability. Obtaining and maintaining good Doppler signals were difficult and time-consuming in TTD and were considered possible causes of error. Hausen et al. [37] also compared TTD and TD. They suggested that the TTD system does not provide accurate CO determinations (bias = 1.70 L/min, LOA = ±3.29 L/min, R = 0.248) and that several reasons can affect its accuracy and restrict its wider use, such as cuff deflation for >10 min, which cannot be tolerated by ICU doctors, sensitivity to patient movement, and that an optimal signal cannot often be attained if the probe is not in the appropriate place.

The bias in three studies[2, 21, 25] was > 2.0 L/min. One of these studies[21] used the ACM to monitor the CO in patients with high cardiac output (pregnant and pre-eclamptic women) and found that it was inaccurate compared with TD. Another study[2] found that the mean difference was larger in the spontaneous ventilation state than the intermittent positive pressure ventilation and apnea state. One possible reason was that the patients were not sedated during spontaneous ventilation; thus, the CO was increased. Cariou et al. [25] compared the descending aortic blood flow using a pulse Doppler velocimeter with CO. Although the authors thought that the descending aortic blood flow determination had good correlation and consistency with TD in CO and that descending aortic blood flow provided a reliable noninvasive tool for estimating CO, the mean difference was obviously due to the descending aortic blood flow as a fraction of the CO.

Critchley and Critchley [79] thought that it can be acceptable clinically when the PE is <30%. They suggest that if the PE between the two methods is ≤±30%, then the two methods are interchangeable. In our studies, although the median of PE was 23.8%, the PE of the six studies [2, 8, 18, 24, 43, 51] was >30%. Missant et al. [51] used Doppler echocardiography during off-pump coronary artery bypass grafting and believed that Doppler echocardiography was not always feasible when the heart was displaced from the esophagus and had lower accuracy; The accuracy in CO measurement may have been affected in three studies that included special patients or scenarios (Knirsch et al. [43] had used USCOM in children with congenital heart disease; Botero et al. [24] used NICO during cardiopulmonary bypass; and Castor et al. [2] used NICO in patients with low sedation levels during spontaneous ventilation). Moller-Sorensen et al. [18] thought that the possible explanation was that the SV is calculated from two variables (LVOT, CSA, and the VTi); measurements were made irrespective of the ventilatory cycle, arrhythmias, and the patients with different scenarios. Therefore, we should pay more attention to the evaluation of ultrasound CO results, when the cardiac function or physiological structural change occurs in some patients with heart disease or in special situations.

Moreover, imprecision in echocardiography CO measurements may be induced by technical or operator factors. By improving the operation level, repeated measurement may reduce the measurement error. In our meta-analysis, self-control was used in all studies, and most of the studies used repeated measurements and blinding methods. Therefore, the quality of literature was not evaluated.

Other limitations of this study include the following: (1) no further subgroup analysis was conducted on the research subjects and disease types due to the limitation of data integrity and the diversity of diseases; (2) determination of the best CO test method was difficult, as both have advantages and limitations; (3) the linear equations were not overfitted because finding a general linear equation to express the relationship between US and TD in CO measurement for the inconsistency of the research subjects, ultrasonic type, and sites is difficult.

Conclusions

This systematic review and meta-analysis showed that the overall effect of the CO measurements by echocardiography or TD has no significant difference. TEE can be the preferred method with accurate results and USCOM can be a good choice for its noninvasiveness in CO measurement; the AOV and LVOT can be the recommended sampling location. However, in some special scenarios, such as high CO, low sedation, or with physiological structural changes, the accuracy of CO measurement by echocardiography is questionable.

Supporting information

S1 Table [doc]
Tailored QUADAS-2 to assess risk of bias and applicability judgments.

S1 Checklist [doc]
PRISMA 2009 Checklist.

S1 Fig [tif]
Diagram demonstrating the assessment of bias for each study included in our analysis.

S2 Fig [tif]
Forest plot of comparison in cardiac output measurement and subgroup analysis with different types of echocardiography (US) vs. thermodilution (TD).

S3 Fig [tif]
Forest plot of sensitivity analysis of cardiac output measurement and subgroup analysis with different types of echocardiography (US) vs. thermodilution (TD).

S4 Fig [tif]
Forest plot of comparison of cardiac output measurement and subgroup analysis with different sites for echocardiography (US) vs. thermodilution (TD).

S5 Fig [tif]
Forest plot of sensitivity analysis of cardiac output measurement and subgroup analysis with different sites for echocardiography (US) vs. thermodilution (TD).

S6 Fig [tif]
Funnel plot for publication bias in cardiac output measurement between echocardiography (US) vs. thermodilution (TD) (Egger’s test, = 0.500).

Zdroje

1. Ehlers KC, Mylrea KC, Waterson CK, Calkins JM. Cardiac output measurements. A review of current techniques and research. Annals of biomedical engineering. 1986;14(3):219–39. doi: 10.1007/bf02584272 3532871.

2. Castor G, Klocke RK, Stoll M, Helms J, Niedermark I. Simultaneous measurement of cardiac output by thermodilution, thoracic electrical bioimpedance and Doppler ultrasound. British journal of anaesthesia. 1994;72(1):133–8. doi: 10.1093/bja/72.1.133 8110539.

3. Litton E, Morgan M. The PiCCO monitor: a review. Anaesthesia and intensive care. 2012;40(3):393–409. doi: 10.1177/0310057X1204000304 22577904.

4. Chand R, Mehta Y, Trehan N. Cardiac output estimation with a new Doppler device after off-pump coronary artery bypass surgery. Journal of cardiothoracic and vascular anesthesia. 2006;20(3):315–9. doi: 10.1053/j.jvca.2005.05.024 16750729.

5. Savino JS, Troianos CA, Aukburg S, Weiss R, Reichek N. Measurement of pulmonary blood flow with transesophageal two-dimensional and Doppler echocardiography. Anesthesiology. 1991;75(3):445–51. doi: 10.1097/00000542-199109000-00011 1888051.

6. Chew HC, Devanand A, Phua GC, Loo CM. Oesophageal Doppler ultrasound in the assessment of haemodynamic status of patients admitted to the medical intensive care unit with septic shock. Annals of the Academy of Medicine, Singapore. 2009;38(8):699–703. 19736574.

7. Estagnasie P, Djedaini K, Mier L, Coste F, Dreyfuss D. Measurement of cardiac output by transesophageal echocardiography in mechanically ventilated patients. Comparison with thermodilution. Intensive care medicine. 1997;23(7):753–9. doi: 10.1007/s001340050405 9290989.

8. Hammoudi N, Hekimian G, Laveau F, Achkar M, Isnard R, Combes A. Three-dimensional transoesophageal echocardiography for cardiac output in critically ill patients: A pilot study of ultrasound versus the thermodilution method. Arch Cardiovasc Dis. 2016;110(1):7–13. doi: 10.1016/j.acvd.2016.04.009 28017278.

9. Ong G, Redfors B, Crowley A, Abdel-Qadir H, Harrington A, Liu Y, et al. Evaluation of left ventricular reverse remodeling in patients with severe aortic regurgitation undergoing aortic valve replacement: Comparison between diameters and volumes. Echocardiography. 2018;35(2):142–7. doi: 10.1111/echo.13750 29193376.

10. Gassner M, Killu K, Bauman Z, Coba V, Rosso K, Blyden D. Feasibility of common carotid artery point of care ultrasound in cardiac output measurements compared to invasive methods. J Ultrasound. 2015;18(2):127–33. doi: 10.1007/s40477-014-0139-9 26191100; PubMed Central PMCID: PMC4504859.

11. Lindberg L, Johansson S, Perez-de-Sa V. Validation of an ultrasound dilution technology for cardiac output measurement and shunt detection in infants and children. Pediatric critical care medicine: a journal of the Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies. 2014;15(2):139–47. doi: 10.1097/PCC.0000000000000053 24366506.

12. Krivitski NM, Kislukhin VV, Thuramalla NV. Theory and in vitro validation of a new extracorporeal arteriovenous loop approach for hemodynamic assessment in pediatric and neonatal intensive care unit patients. Pediatric critical care medicine: a journal of the Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies. 2008;9(4):423–8. doi: 10.1097/01.PCC.0b013e31816c71bc 18496416; PubMed Central PMCID: PMC2574659.

13. Tchorz KM, Chandra MS, Markert RJ, Healy M, Anderson H 3rd, Ekeh AP, et al. Comparison of hemodynamic measurements from invasive and noninvasive monitoring during early resuscitation. J Trauma Acute Care Surg. 2012;72(4):852–60. doi: 10.1097/TA.0b013e31824b1764 22491596.

14. Huang H, Liu SF. The clinical evaluation of ultrasonic cardiac output monitor in patients with mechanical ventilation in intensive care unit. Chinese Journal of Clinicians(Electronic Edition). 2013;7(23):11013–4.

15. Yuan J, Chen Y, Ma Y, Yu HD, Yang F, Shao J. Clinical research of the accuracy of cardiac output in ICU patients with cardiac shock by echocardiography. The Journal of Medical Theory and Practice. 2013;26(20):2678–9.

16. Zhang L, Zhu FX, An YZ. Comparison of ultrasonic cardiac output monitor and pulse indicated continuous cardiac output monitor on determination of hemodynamic parameters in critical patients. Chinese Critical Care Medicine. 2016;28(9):796–800. doi: 10.3760/cma.j.issn.2095-4352.2016.09.006

17. Wetterslev M, Moller-Sorensen H, Johansen RR, Perner A. Systematic review of cardiac output measurements by echocardiography vs. thermodilution: the techniques are not interchangeable. Intensive care medicine. 2016;42(8):1223–33. doi: 10.1007/s00134-016-4258-y 26932349.

18. Moller-Sorensen H, Graeser K, Hansen KL, Zemtsovski M, Sander EM, Nilsson JC. Measurements of cardiac output obtained with transesophageal echocardiography and pulmonary artery thermodilution are not interchangeable. Acta Anaesthesiol Scand. 2014;58(1):80–8. doi: 10.1111/aas.12227 24192143.

19. Arora D, Chand R, Mehta Y, Trehan N. Cardiac output estimation after off-pump coronary artery bypass: a comparison of two different techniques. Ann Card Anaesth. 2007;10(2):132–6. 17644886.

20. Axler O, Tousignant C, Thompson CR, Dall'ava-Santucci J, Phang PT, Russell JA, et al. Comparison of transesophageal echocardiographic, fick, and thermodilution cardiac output in critically ill patients. J Crit Care. 1996;11(3):109–16. 8891961.

21. Basdogan F, Visser W, Struijk PC, Jansen JR, Vletter WB, Wladimiroff JW, et al. Automated cardiac output measurements by ultrasound are inaccurate at high cardiac outputs. Ultrasound Obstet Gynecol. 2000;15(6):508–12. doi: 10.1046/j.1469-0705.2000.00128.x 11005119.

22. Beltramo F, Menteer J, Razavi A, Khemani RG, Szmuszkovicz J, Newth CJ, et al. Validation of an Ultrasound Cardiac Output Monitor as a Bedside Tool for Pediatric Patients. Pediatr Cardiol. 2016;37(1):177–83. doi: 10.1007/s00246-015-1261-y 26364291.

23. Bojanowski LM, Timmis AD, Najm YC, Gosling RG. Pulsed Doppler ultrasound compared with thermodilution for monitoring cardiac output responses to changing left ventricular function. Cardiovasc Res. 1987;21(4):260–8. doi: 10.1093/cvr/21.4.260 3652093.

24. Botero M, Kirby D, Lobato EB, Staples ED, Gravenstein N. Measurement of cardiac output before and after cardiopulmonary bypass: Comparison among aortic transit-time ultrasound, thermodilution, and noninvasive partial CO2 rebreathing. Journal of cardiothoracic and vascular anesthesia. 2004;18(5):563–72. doi: 10.1053/j.jvca.2004.07.005 15578466.

25. Cariou A, Monchi M, Joly LM, Bellenfant F, Claessens YE, Thebert D, et al. Noninvasive cardiac output monitoring by aortic blood flow determination: evaluation of the Sometec Dynemo-3000 system. Critical care medicine. 1998;26(12):2066–72. doi: 10.1097/00003246-199812000-00043 9875922.

26. Chandraratna PA, Brar R, Vijayasekaran S, Chen QX, Niguse GT, Shaikh Y, et al. Continuous recording of pulmonary artery diastolic pressure and cardiac output using a novel ultrasound transducer. Journal of the American Society of Echocardiography: official publication of the American Society of Echocardiography. 2002;15(11):1381–6. doi: 10.1067/mje.2002.125921 12415232.

27. Coats AJ, Murphy C, Conway J, Sleight P. Validation of the beat to beat measurement of blood velocity in the human ascending aorta by a new high temporal resolution Doppler ultrasound spectral analyser. Br Heart J. 1992;68(2):223–9. doi: 10.1136/hrt.68.8.223 1389745; PubMed Central PMCID: PMC1025022.

28. Corley A, Barnett AG, Mullany D, Fraser JF. Nurse-determined assessment of cardiac output. Comparing a non-invasive cardiac output device and pulmonary artery catheter: a prospective observational study. Int J Nurs Stud. 2009;46(10):1291–7. doi: 10.1016/j.ijnurstu.2009.03.013 19423107.

29. Crittendon I 3rd, Dreyer WJ, Decker JA, Kim JJ. Ultrasound dilution: an accurate means of determining cardiac output in children. Pediatric critical care medicine: a journal of the Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies. 2012;13(1):42–6. doi: 10.1097/PCC.0b013e3182196804 21499176; PubMed Central PMCID: PMC3176999.

30. Darmon PL, Hillel Z, Mogtader A, Mindich B, Thys D. Cardiac output by transesophageal echocardiography using continuous-wave Doppler across the aortic valve. Anesthesiology. 1994;80(4):796–805; discussion 25A. doi: 10.1097/00000542-199404000-00011 8024133.

31. Descorps-Declere A, Smail N, Vigue B, Duranteau J, Mimoz O, Edouard A, et al. Transgastric, pulsed Doppler echocardiographic determination of cardiac output. Intensive care medicine. 1996;22(1):34–8. doi: 10.1007/bf01728328 8857435.

32. DiCorte CJ, Latham P, Greilich PE, Cooley MV, Grayburn PA, Jessen ME. Esophageal Doppler monitor determinations of cardiac output and preload during cardiac operations. Ann Thorac Surg. 2000;69(6):1782–6. doi: 10.1016/s0003-4975(00)01129-2 10892923.

33. Eremenko AA, Safarov PN. Flow-regulated extracorporeal arteriovenous tubing loop for cardiac output measurements by ultrasound velocity dilution: validation in post-cardiac surgery intensive care unit patients. ASAIO journal. 2010;56(6):522–6. doi: 10.1097/MAT.0b013e3181effdf8 21245798.

34. Feinberg MS, Hopkins WE, Davila-Roman VG, Barzilai B. Multiplane transesophageal echocardiographic doppler imaging accurately determines cardiac output measurements in critically ill patients. Chest. 1995;107(3):769–73. doi: 10.1378/chest.107.3.769 7874951.

35. Froese N, Friesen R. Measurement of cardiac output—transtracheal Doppler versus thermodilution. Canadian journal of anaesthesia = Journal canadien d'anesthesie. 1991;38(7):931–4. doi: 10.1007/BF03036977 1742833.

36. Galstyan G, Bychinin M, Alexanyan M, Gorodetsky V. Comparison of cardiac output and blood volumes in intrathoracic compartments measured by ultrasound dilution and transpulmonary thermodilution methods. Intensive care medicine. 2010;36(12):2140–4. doi: 10.1007/s00134-010-2003-5 20689918.

37. Hausen B, Schafers HJ, Rohde R, Haverich A. Clinical evaluation of transtracheal Doppler for continuous cardiac output estimation. Anesthesia and analgesia. 1992;74(6):800–4. doi: 10.1213/00000539-199206000-00004 1595910.

38. Hoole SP, Boyd J, Ninios V, Parameshwar J, Rusk RA. Measurement of cardiac output by real-time 3D echocardiography in patients undergoing assessment for cardiac transplantation. Eur J Echocardiogr. 2008;9(3):334–7. doi: 10.1016/j.euje.2007.03.033 17618836.

39. Horster S, Stemmler HJ, Strecker N, Brettner F, Hausmann A, Cnossen J, et al. Cardiac Output Measurements in Septic Patients: Comparing the Accuracy of USCOM to PiCCO. Crit Care Res Pract. 2012;2012:270631. doi: 10.1155/2012/270631 22191019; PubMed Central PMCID: PMC3235433.

40. Horster S, Stemmler HJ, Sparrer J, Tischer J, Hausmann A, Geiger S. Mechanical ventilation with positive end-expiratory pressure in critically ill patients: comparison of CW-Doppler ultrasound cardiac output monitoring (USCOM) and thermodilution (PiCCO). Acta Cardiol. 2012;67(2):177–85. doi: 10.2143/AC.67.2.2154208 22641975.

41. Huntsman LL, Stewart DK, Barnes SR, Franklin SB, Colocousis JS, Hessel EA. Noninvasive Doppler determination of cardiac output in man. Clinical validation. Circulation. 1983;67(3):593–602. doi: 10.1161/01.cir.67.3.593 6821902.

42. Izzat MB, Regragui IA, Wilde P, Angelini GD, Bryan AJ. Transesophageal echocardiographic measurements of cardiac output in cardiac surgical patients. Ann Thorac Surg. 1994;58(5):1486–9. doi: 10.1016/0003-4975(94)91941-0 7979680.

43. Knirsch W, Kretschmar O, Tomaske M, Stutz K, Nagdyman N, Balmer C, et al. Cardiac output measurement in children: comparison of the Ultrasound Cardiac Output Monitor with thermodilution cardiac output measurement. Intensive care medicine. 2008;34(6):1060–4. doi: 10.1007/s00134-008-1030-y 18297271.

44. Lee W, Rokey R, Cotton DB. Noninvasive maternal stroke volume and cardiac output determinations by pulsed Doppler echocardiography. Am J Obstet Gynecol. 1988;158(3 Pt 1):505–10. doi: 10.1016/0002-9378(88)90014-2 3348311.

45. Levy BI, Payen DM, Tedgui A, Xhaard M, McIlroy MB. Non-invasive ultrasonic cardiac output measurement in intensive care unit. Ultrasound Med Biol. 1985;11(6):841–9. doi: 10.1016/0301-5629(85)90078-x 3913083.

46. Marcelino P, Germano N, Marum S, Fernandes AP, Ribeiro P, Lopes MG. Haemodynamic parameters obtained by transthoracic echocardiography and Swan-Ganz catheter: a comparative study in liver transplant patients. Acta Med Port. 2006;19(3):197–205. 17234080.

47. Mark JB, Steinbrook RA, Gugino LD, Maddi R, Hartwell B, Shemin R, et al. Continuous noninvasive monitoring of cardiac output with esophageal Doppler ultrasound during cardiac surgery. Anesthesia and analgesia. 1986;65(10):1013–20. 3530048.

48. Maslow A, Comunale ME, Haering JM, Watkins J. Pulsed wave Doppler measurement of cardiac output from the right ventricular outflow tract. Anesthesia and analgesia. 1996;83(3):466–71. doi: 10.1097/00000539-199609000-00004 8780264.

49. Mayer SA, Sherman D, Fink ME, Homma S, Solomon RA, Lennihan L, et al. Noninvasive monitoring of cardiac output by Doppler echocardiography in patients treated with volume expansion after subarachnoid hemorrhage. Critical care medicine. 1995;23(9):1470–4. doi: 10.1097/00003246-199509000-00005 7664547.

50. McLean AS, Needham A, Stewart D, Parkin R. Estimation of cardiac output by noninvasive echocardiographic techniques in the critically ill subject. Anaesthesia and intensive care. 1997;25(3):250–4. doi: 10.1177/0310057X9702500307 9209605.

51. Missant C, Rex S, Wouters PF. Accuracy of cardiac output measurements with pulse contour analysis (PulseCO) and Doppler echocardiography during off-pump coronary artery bypass grafting. Eur J Anaesthesiol. 2008;25(3):243–8. doi: 10.1017/S0265021507002979 17996125.

52. Moxon D, Pinder M, van Heerden PV, Parsons RW. Clinical evaluation of the HemoSonic monitor in cardiac surgical patients in the ICU. Anaesthesia and intensive care. 2003;31(4):408–11. doi: 10.1177/0310057X0303100410 12973965.

53. Muhiudeen IA, Kuecherer HF, Lee E, Cahalan MK, Schiller NB. Intraoperative estimation of cardiac output by transesophageal pulsed Doppler echocardiography. Anesthesiology. 1991;74(1):9–14. doi: 10.1097/00000542-199101000-00003 1986664.

54. Parra V, Fita G, Rovira I, Matute P, Gomar C, Pare C. Transoesophageal echocardiography accurately detects cardiac output variation: a prospective comparison with thermodilution in cardiac surgery. Eur J Anaesthesiol. 2008;25(2):135–43. doi: 10.1017/S0265021507001354 17672920.

55. Perrino AC Jr., Harris SN, Luther MA. Intraoperative determination of cardiac output using multiplane transesophageal echocardiography: a comparison to thermodilution. Anesthesiology. 1998;89(2):350–7. doi: 10.1097/00000542-199808000-00010 9710392.

56. Pinto FJ, Siegel LC, Chenzbraun A, Schnittger I. On-line estimation of cardiac output with a new automated border detection system using transesophageal echocardiography: a preliminary comparison with thermodilution. Journal of cardiothoracic and vascular anesthesia. 1994;8(6):625–30. doi: 10.1016/1053-0770(94)90192-9 7880989.

57. Poelaert J, Schmidt C, Van Aken H, Hinder F, Mollhoff T, Loick HM. A comparison of transoesophageal echocardiographic Doppler across the aortic valve and the thermodilution technique for estimating cardiac output. Anaesthesia. 1999;54(2):128–36. doi: 10.1046/j.1365-2044.1999.00666.x 10215707.

58. Pombo JF, Russell RO Jr., Rackley CE, Foster GL. Comparison of stroke volume and cardiac output determination by ultrasound and dye dilution in acute myocardial infarction. The American journal of cardiology. 1971;27(6):630–5. doi: 10.1016/0002-9149(71)90227-x 4932616.

59. Ryan T, Page R, Bouchier-Hayes D, Cunningham AJ. Transoesophageal pulsed wave Doppler measurement of cardiac output during major vascular surgery: comparison with the thermodilution technique. British journal of anaesthesia. 1992;69(1):101–4. doi: 10.1093/bja/69.1.101 1637593.

60. Sato M, Kunisawa T. Reliability of Cardiac Output Measurements Using LiDCOrapid and Calibration by Transesophageal Echocardiography With the Continuous Pulmonary Artery Thermodilution Method in Patients Undergoing Aortic Valve Replacement for Aortic Stenosis. Journal of cardiothoracic and vascular anesthesia. 2018;32(6):2495–502. doi: 10.1053/j.jvca.2018.04.018 29801725.

61. Segal J, Gaudiani V, Nishimura T. Continuous determination of cardiac output using a flow-directed Doppler pulmonary artery catheter. Journal of cardiothoracic and vascular anesthesia. 1991;5(4):309–15. doi: 10.1016/1053-0770(91)90151-i 1831393.

62. Shimamoto H, Kito H, Kawazoe K, Fujita T, Shimamoto Y. Transoesophageal Doppler echocardiographic measurement of cardiac output by the mitral annulus method. Br Heart J. 1992;68(5):510–5. doi: 10.1136/hrt.68.11.510 1467040; PubMed Central PMCID: PMC1025199.

63. Shimamoto H, Shimamoto Y, Kito H, Kawazoe K, Fujita T, Okamoto M. Doppler echocardiographic measurement of cardiac output in man using mitral annulus method. Journal of clinical ultrasound: JCU. 1992;20(4):263–70. doi: 10.1002/jcu.1870200407 1315800.

64. Souto Moura T, Aguiar Rosa S, Germano N, Cavaco R, Sequeira T, Alves M, et al. The accuracy of PiCCO(R) in measuring cardiac output in patients under therapeutic hypothermia: Comparison with transthoracic echocardiography. Medicina intensiva. 2017;42(2):92–8. doi: 10.1016/j.medin.2017.03.007 28552462.

65. Su BC, Lin CC, Su CW, Hui YL, Tsai YF, Yang MW, et al. Ultrasonic cardiac output monitor provides accurate measurement of cardiac output in recipients after liver transplantation. Acta Anaesthesiol Taiwan. 2008;46(4):171–7. doi: 10.1016/S1875-4597(09)60005-9 19097964.

66. Su BC, Yu HP, Yang MW, Lin CC, Kao MC, Chang CH, et al. Reliability of a new ultrasonic cardiac output monitor in recipients of living donor liver transplantation. Liver Transpl. 2008;14(7):1029–37. doi: 10.1002/lt.21461 18581505.

67. Tan HL, Pinder M, Parsons R, Roberts B, van Heerden PV. Clinical evaluation of USCOM ultrasonic cardiac output monitor in cardiac surgical patients in intensive care unit. British journal of anaesthesia. 2005;94(3):287–91. doi: 10.1093/bja/aei054 15653709.

68. Temporelli PL, Scapellato F, Eleuteri E, Imparato A, Giannuzzi P. Doppler echocardiography in advanced systolic heart failure: a noninvasive alternative to Swan-Ganz catheter. Circulation Heart failure. 2010;3(3):387–94. doi: 10.1161/CIRCHEARTFAILURE.108.809590 20197560.

69. Thom O, Taylor DM, Wolfe RE, Cade J, Myles P, Krum H, et al. Comparison of a supra-sternal cardiac output monitor (USCOM) with the pulmonary artery catheter. British journal of anaesthesia. 2009;103(6):800–4. doi: 10.1093/bja/aep296 19864307.

70. Tibbals J, Osborne A, Hockmann M. A comparative study of cardiac output measurement by dye dilution and pulsed Doppler ultrasound. Anaesthesia and intensive care. 1988;16(3):272–7. doi: 10.1177/0310057X8801600306 3056088.

71. Tsutsui M, Matsuoka N, Ikeda T, Sanjo Y, Kazama T. Comparison of a new cardiac output ultrasound dilution method with thermodilution technique in adult patients under general anesthesia. Journal of cardiothoracic and vascular anesthesia. 2009;23(6):835–40. doi: 10.1053/j.jvca.2009.03.007 19464193.

72. Van den Oever HL, Murphy EJ, Christie-Taylor GA. USCOM (Ultrasonic Cardiac Output Monitors) lacks agreement with thermodilution cardiac output and transoesophageal echocardiography valve measurements. Anaesthesia and intensive care. 2007;35(6):903–10. doi: 10.1177/0310057X0703500608 18084981.

73. Warth DC, Stewart WJ, Block PC, Weyman AE. A new method to calculate aortic valve area without left heart catheterization. Circulation. 1984;70(6):978–83. doi: 10.1161/01.cir.70.6.978 6499155.

74. Wong LS, Yong BH, Young KK, Lau LS, Cheng KL, Man JS, et al. Comparison of the USCOM ultrasound cardiac output monitor with pulmonary artery catheter thermodilution in patients undergoing liver transplantation. Liver Transpl. 2008;14(7):1038–43. doi: 10.1002/lt.21483 18581504.

75. Wong DH, Mahutte CK. Two-beam pulsed Doppler cardiac output measurement: reproducibility and agreement with thermodilution. Critical care medicine. 1990;18(4):433–7. doi: 10.1097/00003246-199004000-00017 2180638.

76. Wong DH, Tremper KK, Stemmer EA, O'Connor D, Wilbur S, Zaccari J, et al. Noninvasive cardiac output: simultaneous comparison of two different methods with thermodilution. Anesthesiology. 1990;72(5):784–92. doi: 10.1097/00000542-199005000-00002 2187376.

77. Zhao X, Mashikian JS, Panzica P, Lerner A, Park KW, Comunale ME. Comparison of thermodilution bolus cardiac output and Doppler cardiac output in the early post-cardiopulmonary bypass period. Journal of cardiothoracic and vascular anesthesia. 2003;17(2):193–8. doi: 10.1053/jcan.2003.46 12698401.

78. Cheitlin MD, Armstrong WF, Aurigemma GP, Beller GA, Bierman FZ, Davis JL, et al. ACC/AHA/ASE 2003 Guideline Update for the Clinical Application of Echocardiography: summary article. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (ACC/AHA/ASE Committee to Update the 1997 Guidelines for the Clinical Application of Echocardiography). Journal of the American Society of Echocardiography: official publication of the American Society of Echocardiography. 2003;16(10):1091–110. doi: 10.1016/S0894-7317(03)00685-0 14566308.

79. Critchley LA, Critchley JA. A meta-analysis of studies using bias and precision statistics to compare cardiac output measurement techniques. Journal of clinical monitoring and computing. 1999;15(2):85–91. 12578081.