Technique For Measurement Of Cardiac Output Research Articles

Evaluation of four cardiac output measurement techniques during a modified passive leg raise maneuver in anesthetized dogs

Introduction: Modified passive leg raise maneuver (PLRM) is a successful volumetric predictor of fluid responsiveness in dogs during experimental induction and correction of hypovolemia using invasive pulmonary artery thermodilution (PACTD) for measuring cardiac output (CO). In this study, other CO methods, minimally-invasive esophageal Doppler (ESD), transesophageal echocardiography (TEE), arterial pressure waveform analysis (APW), noninvasive electrical velocimetry (EV), were investigated during PLRM in anesthetized dogs.

Is There a Correlation Between Left Ventricular Outflow Tract Velocity Time Integral and Stroke Volume Index in Patients Undergoing Cardiac Surgery?

IntroductionLeft ventricular outflow tract velocity time integral (LVOT VTI) is a promising surrogate for stroke volume (SV). However, there is controversy in the literature regarding its correlation with thermodilution or newer cardiac output measurement techniques. This study was conducted to determine the correlation between LVOT VTI determined by transesophageal echocardiography (TEE) with stroke volume index (SVI) calculated by thermodilution.MethodsConsecutive patients older than 17 years undergoing elective cardiac surgery with pulmonary artery catheter (PAC) and TEE monitoring between September 2021 and February 2022 were included in this prospective, descriptive, single-center study. LVOT VTI was measured using TEE after induction of anesthesia but before skin incision and at least four hours after initial LVOT VTI measurement. SVI was simultaneously measured using the continuous thermodilution technique with a PAC. The correlation between LVOT VTI and SVI was determined with Pearson’s correlation index.ResultsTwelve patients were included and 21 paired measurements were compared. Mean SVI was 31.62 ± 10.71 mL/m2 and mean LVOT VTI was 14.74 ± 4.79 cm. The Pearson's correlation index for the two measurements was r = 0.257, p = 0.262.ConclusionThis prospective study demonstrated a weak correlation between LVOT VTI and SVI in patients undergoing cardiac surgery.

Non-invasive methods of cardiac output measurement and their importance in everyday clinical practice: the current state of knowledge

Cardiac output is an important parameter in the evaluation of the patients’ haemodynamic status which can be measured using a number of methods with varying degrees of invasiveness. These include invasive (such as pulmonary artery catheterization, transpulmonary thermodilution, lithium dilution), minimally invasive (catheter-based arterial pressure curve contour analysis, transoesophageal Doppler) and non-invasive (electrical bioimpedance, transthoracic echocardiography, measurement of exhaled CO2, continuous non-invasive blood pressure measurement) techniques. With advances in medicine, the role of non-invasive methods as an alternative to pulmonary artery catheterization continues to grow. The undoubted advantage of non-invasive methods is their safety for the patients and their wide availability. In contrast, invasive methods are characterized by greater measurement precision. It is up to the physician to choose the most appropriate haemodynamic monitoring method depending on the clinical situation. The aim of this article is to discuss non-invasive cardiac output measurement techniques and their application in everyday clinical practice.

Evaluation of concordance among three cardiac output measurement techniques in adult patients during cardiovascular surgery postoperative care

IntroductionThe standard method for cardiac output measuring is thermodilution although it is an invasive technique. Transesophageal Echocardiography (TEE) offers a dynamic and functional alternative to thermodilution. ObjectiveAnalyze concordance between two TEE methods and thermodilution for cardiac output assessment. MethodsObservational concordance study in cardiovascular surgery patients that required pulmonary artery catheter. TEE cardiac output measurement at both mitral annulus (MA) and left ventricle outflow tract (LVOT) were performed. Results were compared with thermodilution. Correlation was evaluated by Lin's concordance correlation coefficient and Bland–Altman analysis. Statistical analysis was undertaken in STATA 13.0. ResultsTwenty-five patients were enrolled. Fifty two percent of patients were male, median age and ejection fraction was 63 years and 35% respectively. Median thermodilution, LVOT and MA -measured cardiac output was 3.25 L/min, 3.46 L/min and 8.4 L/min respectively. Different values between thermodilution and MA measurements were found (Lin concordance=0.071; Confidence Interval 95%=−0.009 to 0.151; Spearman's correlation=0.22) as values between thermodilution and LVOT (Lin concordance=0.232; Confidence Interval 95%=−0.12 a 0.537; Spearman's correlation 0.28). Bland–Altman analysis showed greater difference between MA measurements and thermodilution (DM=−0.408; Bland–Altman Limits=−0.809 to −0.007), than the other echocardiographic findings (DM=0.007; Bland–Altman Limits=−0.441 to 0.428). ConclusionResults from cardiac output measurement by doppler and 2D-TEE on both MA and LVOT do not correlate with those obtained by thermodilution.

Evaluation of concordance among three cardiac output measurement techniques in adult patients during cardiovascular surgery postoperative care

IntroductionThe standard method for cardiac output measuring is thermodilution although it is an invasive technique. Transesophageal Echocardiography (TEE) offers a dynamic and functional alternative to thermodilution. ObjectiveAnalyze concordance between two TEE methods and thermodilution for cardiac output assessment. MethodsObservational concordance study in cardiovascular surgery patients that required pulmonary artery catheter. TEE cardiac output measurement at both mitral annulus (MA) and left ventricle outflow tract (LVOT) were performed. Results were compared with thermodilution. Correlation was evaluated by Lin's concordance correlation coefficient and Bland–Altman analysis. Statistical analysis was undertaken in STATA 13.0. ResultsTwenty-five patients were enrolled. Fifty two percent of patients were male, median age and ejection fraction was 63 years and 35% respectively. Median thermodilution, LVOT and MA -measured cardiac output was 3.25 L/min, 3.46 L/min and 8.4 L/min respectively. Different values between thermodilution and MA measurements were found (Lin concordance=0.071; Confidence Interval 95%=−0.009 to 0.151; Spearman's correlation=0.22) as values between thermodilution and LVOT (Lin concordance=0.232; Confidence Interval 95%=−0.12 a 0.537; Spearman's correlation 0.28). Bland–Altman analysis showed greater difference between MA measurements and thermodilution (DM=−0.408; Bland–Altman Limits=−0.809 to −0.007), than the other echocardiographic findings (DM=0.007; Bland–Altman Limits=−0.441 to 0.428). ConclusionResults from cardiac output measurement by doppler and 2D-TEE on both MA and LVOT do not correlate with those obtained by thermodilution.

Addressing Assumptions for the Use of Non-invasive Cardiac Output Measurement Techniques During Exercise in COPD

The multifactorial functional limitation of COPD increasingly demonstrates the need for an integrated circulatory assessment. In this study cardiac output (Qc) derived from non-inert (CO2-RB), inert (N2O-RB) gas rebreathing approaches and bioimpedance were compared to examine the limitations of currently available non-invasive techniques for exercise Qc determination in patients with chronic lung disease. Thirteen COPD patients (GOLD II-III) completed three constant cycling bouts at 20, 35, and 50% of peak work on two occasions to assess Qc with bioimpedance as well as using CO2-RB and N2O-RB for all exercise tests. Results showed significantly lower Qc using the N2O-RB or end-tidal CO2-derived Qc compared to the PaCO2-derived CO2-RB or the bioimpedance at rest and for all exercise intensities. End-tidal CO2-derived values are however not statistically different from those obtained using inert-gas rebreathing. This study show that in COPD patients, CO2-rebreathing Qc values obtained using PaCO2 contents which account for any gas exchange impairment or inadequate gas mixing are similar to those obtained using thoracic bioimpedance. Alternately, the lower values for N2O rebreathing derived Qc indicates the inability of this technique to account for gas exchange impairment in the computation of Qc. These findings indicate that the choice of a gas rebreathing technique to measure Qc in patients must be dictated by the ability to include in the derived computations a correction for either gas exchange inadequacies and/or a vascular shunt.

Cardiac Output Assessed by Invasive and Minimally Invasive Techniques

Cardiac output (CO) measurement has long been considered essential to the assessment and guidance of therapeutic decisions in critically ill patients and for patients undergoing certain high-risk surgeries. Despite controversies, complications and inherent errors in measurement, pulmonary artery catheter (PAC) continuous and intermittent bolus techniques of CO measurement continue to be the gold standard. Newer techniques provide less invasive alternatives; however, currently available monitors are unable to provide central circulation pressures or true mixed venous saturations. Esophageal Doppler and pulse contour monitors can predict fluid responsiveness and have been shown to decrease postoperative morbidity. Many minimally invasive techniques continue to suffer from decreased accuracy and reliability under periods of hemodynamic instability, and so few have reached the level of interchangeability with the PAC.

Techniques of cardiac output measurement during liver transplantation: Arterial pulse wave versus thermodilution

In this study, we compared continuous cardiac output (CO) obtained from the femoral arterial pressure by simulation of an aortic input impedance model [model-simulated cardiac output (MCO)] to thermodilution cardiac output (TDCO) determined by bolus injection during liver transplantation. Both variables were measured in 39 adult patients (13 females) every 10th minute during liver transplant surgery. Paired measurements were compared during the 4 phases of surgery-dissection, anhepatic phase, early reperfusion (the first 15 minutes after reperfusion), and late reperfusion (15-60 minutes after reperfusion)-without the detection of any significant difference between the 2 estimates of CO. TDCO ranged from 2.3 to 17.2 L/minute, and the bias (the mean difference between MCO and TDCO) prior to calibration was -0.4 +/- 1.6 L/minute (mean +/- standard deviation; 1309 paired measurements; 95% limits of agreement: -3.4 to 2.6 L/minute). After calibration of the first determined MCO by the simultaneously determined TDCO, the bias was 0.1 +/- 1.5 L/minute, with 57% (n = 744) of the comparisons being less than 1 L/minute and 35% (n = 453) being less than 0.5 L/minute; this was independent of the level of CO, and the mutual correlation coefficient was 0.812 (P < 0.001). This study indicates that during liver transplantation surgery, MCO reflects TDCO throughout the operation. Thus, for CO, this less invasive method appears to provide a reliable uninterrupted measurement during orthotopic liver transplantation.

Clinical value of an arterial pressure-based cardiac output measurement device

However, the Bland Altman plot of APCO versus ICO shows a wide spread of data points with a percentage error of 42%. These large variations could lead to a completely different clinical management. Also, we disagree that a percentage error less than 28% is a conservative requirement. By using an error-gram, limits of precision of ±20% for both test and reference method give predicted limits of agreement of 28.3% [2]. These limits should be respected when an alternative cardiac output measurement technique is evaluated because limits of precision in excess of 20% for a single technique are not clinically acceptable. Furthermore, the authors state that they only consider a change in cardiac output of 30% or more clinically relevant. This is in contrast to daily clinical practice in which cardiac output changes of 10% to 15% are frequently used for making decisions regarding therapy. Also, they have calculated the change in cardiac output by dividing the delta cardiac output by the mean value before and after the change. In this way they have artificially decreased the relative change in cardiac output. Subsequently, in the plot showing the change in ICO versus the change in APCO, it can be observed that when changes in ICO of more than 15% are analyzed, in only 35% of the cases did the APCO also change 15% or more in the same direction. Moreover, in 45% of the cases the APCO changed in the opposite direction! Based on the results of this study, we think that APCO is not accurate in measuring absolute values of cardiac output, nor in tracking changes in cardiac output in a general intensive care population.

Cardiac output measurements during off-pump coronary surgery

Reply EDITOR: We thank Dr Renner and colleagues for their interest in our article and their comments. We are grateful for all input into the discussion about our article on cardiac output monitoring during off-pump coronary artery bypass (OPCAB) surgery. We agree with Dr Renner and colleagues that aortic transit-time ultrasound (ATTUS) cardiac output measurement is the true clinical gold standard for instantaneous cardiac output measurements. However, to our knowledge, in the case of OPCAB surgery, ATTUS has not yet been compared with other cardiac output measurement techniques. Indeed, continuous cardiac output measurements with either pulmonary artery catheter or partial CO2 rebreathing (PRCO) techniques do not provide continuous beat-to-beat assessment of cardiac output. Ideally, the best monitoring equipment should be non-invasive and beat-to-beat, and neither PRCO nor ATTUS achieve both of these ideals. Concerning ventilation during OPCAB surgery, our anaesthetists only exceptionally have to decrease tidal volume below 10 mL kg−1 in order to adjust to the needs of the surgeons. End-tidal CO2 is maintained by adjusting respiratory rate. As we mentioned in our article, no changes in cardiac output readings were observed, suggesting that acute haemodynamic events recorded by either method were related to changes in ventilatory settings. The differences in attitudes of surgeons and between institutions towards tidal volume during OPCAB surgery might be the reason why we were not concerned about the theoretical link of low tidal volume and inadequate rebreathing under our operating conditions. In addition, as mentioned by Tachibana and colleagues [1], the accuracy of the non-invasive cardiac output (NICO) technique can be maintained even at small tidal volumes if normocapnia is achieved by adjusting the respiratory rate. Conversely, the same authors also stated that the NICO underreports cardiac output at small tidal volumes. On the other hand, it is the variation of cardiac output which gives more information about acute haemodynamic changes than the absolute number itself. A disadvantage of underestimation of cardiac output could rather serve as an incentive to react quicker to haemodynamic changes. The assumption of a stable pulmonary capillary blood flow during NICO measurement cycles is indeed a drawback, and recalibration is needed in case of changes of SPO2 and FiO2. We stated that the NICO showed a faster response than the pulmonary artery catheter during OPCAB surgery. In fact, this statement was based on observation and it is correct to add that it has not been substantiated by any of our data. Bein and colleagues [2] correctly concluded that NICO as compared to ATTUS is not suitable for cardiac output determination during quickly changing haemodynamic conditions applicable to their setting of aortic reconstruction. However, to our knowledge ATTUS has neither been examined nor brought into practice as a monitoring tool for OPCAB surgery. Therefore, in this context, even with a delayed response time of 165 s, NICO still remains an interesting monitoring option for OPCAB surgery [3].

Comparison of simultaneous estimation of cardiac output by four techniques in patients undergoing off-pump coronary artery bypass surgery- a prospective observational study

We prospectively compared four techniques of cardiac output measurement: bolus thermodilution cardiac output (TDCO), continuous cardiac output (CCO), pulse contour cardiac output (PiCCO), and Flowtrac (FCCO), simultaneously in fifteen patients undergoing off-pump coronary artery bypass grafting (OPCAB). All the patients received pulmonary artery catheter (capable of measuring both bolus thermodilution cardiac output and CCO), PiCCO arterial cannula in the left and FCCO in the right femoral artery. Cardiac indices (CI) were obtained every fifteen minutes by using all the four techniques. TDCO was treated as 'control' and the rest were treated as 'test' values. Interchangeability of techniques with TDCO was assessed by Bland and Altman plotting and mountain plot. Four hundred and thirty eight sets of data were obtained from fifteen patients. The values of cardiac output varied between 1 to 6.9 L/min. We found that the values of all the techniques were interchangeable. At certain times, the values of CI measured by both PiCCO and FCCO appeared erratic. The values of CI measured simultaneously appeared in the following descending order of accuracy; TDCO>CCO>FCCO>PiCCO (the % times TDCO correlated with CCO, FCCO, PiCCO was 93, 86 and 80 respectively). The bias and precision (in L/min) for CCO were 0.03, 0.06, PiCCO 0.13, 0.1 and flowtrac 0.15, 0.04 respectively suggesting interchangeability. We conclude that the cardiac output measured by CCO technique and the pulse contour as measured by PiCCO and FCCO were interchangeable with TDCO more than 80% of the times.

Comparison of invasive and less-invasive techniques of cardiac output measurement under different haemodynamic conditions in a pig model

Despite the introduction of various less-invasive concepts of cardiac output measurement, pulmonary arterial thermodilution is still the most common measurement technique. This prospective controlled study was designed to compare different methods of cardiac output measurement simultaneously. Pulmonary arterial thermodilution, transpulmonary thermodilution (PiCCO), trans-oesophageal echo-Doppler probe (HemoSonic) and partial carbon dioxide rebreathing technique (NICO monitor) were evaluated against a peri-aortic transit-time flow-probe as reference method in a clinically relevant animal model. After approval from the Local Ethics Committee on Animal Research, the investigations were conducted in nine anesthetized domestic pigs. Systemic haemodynamics were modulated systematically by the application of catecholamines, caval occlusion and exsanguination. Statistical analysis was performed with Bland-Altman and linear regression. A total of 366 paired cardiac output measurements were carried out at a reference cardiac output between 0.5 and 7 L min(-1). The correlation coefficients for pulmonary arterial and transpulmonary thermodilution against reference were 0.93 and 0.95, for trans-oesophageal Doppler and partial rebreathing technique 0.84 and 0.77. Pulmonary arterial thermodilution and transpulmonary thermodilution showed comparable bias and limits of agreement. Where HemoSonic showed an overestimation of cardiac output at a higher precision, NICO overestimated low and underestimated higher cardiac output values. Our data suggest that pulmonary arterial thermodilution and PiCCO may be interchangeably used for cardiac output measurement even under acute haemodynamic changes. The method described by Bland and Altman demonstrated an overestimation of cardiac output for both thermodilution methods. HemoSonic and NICO offer non-invasive alternatives and complementary monitoring tools in numerous clinical situations. Trend monitoring and haemodynamic optimizing can be applied sufficiently, when absolute measures are judged critically in a clinical context. The use of the NICO system seems to be limited during acute circulatory changes.

Measuring cardiac output in one-lung ventilation: a comparison of pulmonary artery and transpulmonary aortic measurements in pigs

Objective: The agreement between cardiac output measurements via pulmonary artery thermodilution (CO[PA]) and transpulmonary aortic thermodilution (CO[AT]) during one-lung ventilation was studied. Design: Animal study with repeated simultaneous measurements comparing 2 cardiac output measurement techniques. Setting: Experimental animal facility of a university department. Participants: Forty-eight female pigs (26–42 kg). Interventions: The pigs were anesthetized, tracheally intubated, and mechanically ventilated. After placement of an aortic thermistor catheter via the femoral artery and a pulmonary artery catheter, a double-lumen tube was placed via tracheotomy. During one-lung ventilation in each animal, 3 measurements with pulmonary artery thermodilution and transpulmonary aortic thermodilution were performed in different hemodynamic states. Both thermistors were connected to 1 computer system, and 144 simultaneous cardiac output measurements were analyzed. Measurements and Main Results: Linear regression analyses revealed a close relationship between the 2 methods: CO(AT) = 0.81 CO(PA) + 1.04 (L/min) ( r = 0.96, p < 0.0001). Bland-Altman analysis showed that CO(AT) was slightly higher than the CO(PA) with a bias of 0.2 ± 0.5 L/min. However, in higher CO states, an inversion of this relationship was found, possibly because of indicator loss and recirculation. Conclusions: The pulmonary artery thermodilution and the transpulmonary aortic thermodilution techniques both accurately measure cardiac output during one-lung ventilation.

Comparison of Non-Invasive, Semi-Invasive and Invasive Techniques of Cardiac Output Measurement under Different Haemodynamic Conditions in a Pig Model

Comparison of Non-Invasive, Semi-Invasive and Invasive Techniques of Cardiac Output Measurement under Different Haemodynamic Conditions in a Pig Model

Advances in noninvasive cardiac output monitoring: An update

Routine monitoring of circulatory function during the perioperative period has generally consisted of heart rate, blood pressure, and electrocardiography. Clinical signs, such as urine output, jugular venous distention, skin turgor, and skin perfusion have been used to estimate cardiac function. These signs may be unreliable, particularly in critically ill patients.2-4 Although Harvey discovered the circulation of the blood more than 300 years ago, routine measurements of cardiac output (CO) have been available to clinicians only since 1970.5 Since its introduction in 1970, pulmonary artery catheterization has been considered the gold standard for the measurement of CO in humans. Although its risk-to-benefit ratio has not been determined, this procedure has become one of the most common procedures performed in critically ill patients around the world.6-8 During anesthesia, CO determination is traditionally reserved for patients with severe circulatory instability or patients undergoing major surgery because the standard technique for CO measurement requires right heart catheterization with its attendant costs and risks.6 Ideally, the technology that provides CO estimation should be noninvasive, accurate, reliable, and continuous. At present, no single monitoring method meets all of these criteria.3 New methods of noninvasive CO monitoring have become available for use in the operating room. In addition to being able to use them at the bedside, a major advantage of many noninvasive techniques is that they avoid the risks of morbidity as well as discomfort associated with more invasive techniques. Because many of these techniques have developed so quickly, anesthesiologists may be unfamiliar with all their clinical applications. Even more challenging may be to decide if the new method can replace the old gold standard based on clinical evidence and the appropriate statistical analysis. Previous studies comparing CO measurement techniques suffered from deficiencies in design or suboptimal statistical methodology,9 leading to the erroneous belief that these techniques are interchangeable when significant differences exist. The purpose of this review is twofold: (1) to outline the utility and limitations of new methods of noninvasive CO monitoring (without the use of an intravascular catheter) currently available to anesthesiologists and (2) to assist practitioners to more critically review new reports on these techniques. MAGNITUDE OF THE PROBLEM

Intra-operative evaluation of a continuous versus intermittent bolus thermodilution technique of cardiac output measurement in cardiac surgical patients

The purpose of this study was to analyse the clinical agreement between cardiac output measurements, obtained using a newly available continuous thermodilution technique, and the conventional intermittent bolus technique. Twenty-four cardiac surgical patients were intra-operatively monitored using both techniques. Additionally, two different averaging modes for the continuous thermodilution technique, either the previous 6 min (group 1) or 3 min (group 2) were compared. The mean difference between the continuous thermodilution technique and the intermittent bolus technique (bias), were calculated at eight selected time points. These ranged from -0.09 to 0.42 litres min-1 in group 1 and from -0.02 to 0.18 litres min-1 in group 2. There were significant differences at any time point between the groups. The relative error between continuous thermodilution and intermittent bolus techniques was < 15% for the majority of measurements and was significantly higher in group 1 compared with group 2 just prior to cardiopulmonary bypass. Thus, the continuous thermodilution technique produced a clinically acceptable level of accuracy compared with the intermittent bolus technique measurements, especially when using an averaging mode for the previous 3 min.

Intra-operative evaluation of a continuous versus intermittent bolus thermodilution technique of cardiac output measurement in cardiac surgical patients

The purpose of this study was to analyse the clinical agreement between cardiac output measurements, obtained using a newly available continuous thermodilution technique, and the conventional intermittent bolus technique. Twenty-four cardiac surgical patients were intra-operatively monitored using both techniques. Additionally, two different averaging modes for the continuous thermodilution technique, either the previous 6 min (group 1) or 3 min (group 2) were compared. The mean difference between the continuous thermodilution technique and the intermittent bolus technique (bias), were calculated at eight selected time points. These ranged from −0.09 to 0.42 litres min−1 in group 1 and from −0.02 to 0.18 litres min−1 in group 2. There were significant differences at any time point between the groups. The relative error between continuous thermodilution and intermittent bolus techniques was <15% for the majority of measurements and was significantly higher in group 1 compared with group 2 just prior to cardiopulmonary bypass. Thus, the continuous thermodilution technique produced a clinically acceptable level of accuracy compared with the intermittent bolus technique measurements, especially when using an averaging mode for the previous 3 min.

Intra-operative evaluation of a continuous versus intermittent bolus thermodilution technique of cardiac output measurement in cardiac surgical patients

The purpose of this study was to analyse the clinical agreement between cardiac output measurements, obtained using a newly available continuous thermodilution technique, and the conventional intermittent bolus technique. Twenty-four cardiac surgical patients were intra-operatively monitored using both techniques. Additionally, two different averaging modes for the continuous thermodilution technique, either the previous 6 min (group 1) or 3 min (group 2) were compared. The mean difference between the continuous thermodilution technique and the intermittent bolus technique (bias), were calculated at eight selected time points. These ranged from −0.09 to 0.42 litres min−1 in group 1 and from −0.02 to 0.18 litres min−1 in group 2. There were significant differences at any time point between the groups. The relative error between continuous thermodilution and intermittent bolus techniques was &lt;15% for the majority of measurements and was significantly higher in group 1 compared with group 2 just prior to cardiopulmonary bypass. Thus, the continuous thermo-dilution technique produced a clinically acceptable level of accuracy compared with the intermittent bolus technique measurements, especially when using an averaging mode for the previous 3 min.

A.42 Comparison of cardiac output measurement techniques: Doppler versus Fick method

A.42 Comparison of cardiac output measurement techniques: Doppler versus Fick method

Comparison of cardiac output measurement techniques: thermodilution, Doppler, GO2‐rebreathing and the direct Fick method

Simultaneously measured cardiac output obtained by thermodilution (TD), transcutaneous suprasternal ultrasonic Doppler (DOP), CO2-rebreathing (CR) and the direct Fick method (FI) were compared in eleven healthy subjects in a supine position (SU), a sitting position (SI), and during sitting exercise at a workload of 50 W (EX). The agreements between the techniques, two by two, were expressed as the bias calculated as the averaged differences between the techniques. Precision was expressed as the standard deviation of the bias. The overall agreement (bias +/- precision) between TD, DOP and CR respectively and FI were 2.3 +/- 1.6, -0.1 +/- 1.4, and -0.2 +/- 1.1 l/min. TD overestimated cardiac output consistently in SU, SI and EX. DOP was in-accurate during EX and agreed well with FI in SU and SI. CR agreed closely with FI in SI and EX, but values were underestimated in SU. The overall agreement between DOP and CR, respectively, and TD were 2.5 +/- 2.2 and 2.6 +/- 1.6 l/min. The overall agreement between DOP and CR was 0.1 +/- 1.6 l/min. In conclusion, TD overestimated cardiac output compared to the other techniques and the poor agreement has to be taken into consideration especially in measures of low values. The precision of DOP and CR against FI seems to be within clinically acceptable limits, and these methods may provide interchangeable alternatives to the invasive Fick method.