A comparison of volume clamp method-based continuous noninvasive cardiac output (CNCO) measurement versus intermittent pulmonary artery thermodilution in postoperative cardiothoracic surgery patients

The CNAP technology (CNSystems Medizintechnik AG, Graz, Austria) allows continuous noninvasive arterial pressure waveform recording based on the volume clamp method and estimation of cardiac output (CO) by pulse contour analysis. We compared CNAP-derived CO measurements (CNCO) with intermittent invasive CO measurements (pulmonary artery catheter; PAC-CO) in postoperative cardiothoracic surgery patients. In 51 intensive care unit patients after cardiothoracic surgery, we measured PAC-CO (criterion standard) and CNCO at three different time points. We conducted two separate comparative analyses: (1) CNCO auto-calibrated to biometric patient data (CNCO_bio) versus PAC-CO and (2) CNCO calibrated to the first simultaneously measured PAC-CO value (CNCO_cal) versus PAC-CO. The agreement between the two methods was statistically assessed by Bland–Altman analysis and the percentage error. In a subgroup of patients, a passive leg raising maneuver was performed for clinical indications and we present the changes in PAC-CO and CNCO in four-quadrant plots (exclusion zone 0.5 L/min) in order to evaluate the trending ability of CNCO. The mean difference between CNCO_bio and PAC-CO was +0.5 L/min (standard deviation?±?1.3 L/min; 95% limits of agreement ?1.9 to +3.0 L/min). The percentage error was 49%. The concordance rate was 100%. For CNCOcal, the mean difference was ?0.3 L/min (±0.5 L/min; ?1.2 to +0.7 L/min) with a percentage error of 19%. In this clinical study in cardiothoracic surgery patients, CNCO_cal showed good agreement when compared with PAC-CO. For CNCO_bio, we observed a higher percentage error and good trending ability (concordance rate 100%).     

Comparison of pulmonary artery and arterial thermodilution cardiac output in critically ill patients

Objective: We studied the agreement between cardiac output measurements via pulmonary artery thermodilution [CO(PA)], regarded as the current clinical gold standard, and aortic transpulmonary thermodilution [CO(AORTA)]. Design: Prospective clinical study. Setting: Surgical intensive care unit of a university hospital. Patients: 37 patients with sepsis or septic shock (n = 34) and subarachnoid haemorrhage (n = 3). Measurements and results: We analysed 449 simultaneous cardiac output measurements. All patients were deeply sedated and mechanically ventilated in a pressure controlled mode. Each patient received a 7.5-F five-lumen pulmonary artery catheter and a 4-F aortic catheter with an integrated thermistor. The thermistors of the two different catheters were connected to one computer system (COLD-Z021, Pulsion Medical Systems, Munich, Germany). Linear regression analysis revealed: CO(AORTA) = 0.96 · CO(PA) + 1.02 (l/min) (r = 0.97, p < 0.0001). CO(AORTA) was consistently higher than CO(PA) with a bias of 0.68 (l/min) and a standard deviation of 0.62 (l/min). Conclusion: Cardiac output derived from aortic transpulmonary thermodilution is suitable for measurement in the intensive care unit. Measurements of CO(AORTA) are consistent with, but slightly higher than, those obtained from pulmonary artery thermodilution. Received: 8 February 1999 Final revision received: 26 May 1999 Accepted: 28 May 1999     

Continuous cardiac output by femoral arterial thermodilution calibrated pulse contour analysis: comparison with pulmonary arterial thermodilution

OBJECTIVE: To compare two thermodilution methods for the determination of cardiac output (CO)-thermodilution in the pulmonary artery (COpa) and thermodilution in the femoral artery (COa)-with each other and with CO determined by continuous pulse contour analysis (COpc) in terms of reproducibility, bias, and correlation among the different methods. Good agreement between the methods would indicate the potential of pulse contour analysis to monitor CO continuously and at reduced invasiveness. DESIGN: Prospective criterion standard study. SETTING: Cardiac surgical intensive care unit in a university hospital. PATIENTS: Twenty-four postoperative cardiac surgery patients. INTERVENTIONS: Without interfering with standard hospital cardiac recovery procedures, changes in CO as a result of the postsurgical course, administration of vasoactive substances, and/or fluid administration were recorded. CO was first recorded after a 1-hr stabilization period in the intensive care unit and hourly thereafter for 6 hrs, and by subsequent determinations at 9, 12, and 24 hrs. MEASUREMENTS AND MAIN RESULTS: There were 216 simultaneous determinations of COpa, COa, and COpc. COpc was initially calibrated using COa, and no further recalibration of COpc was performed. COpa ranged from 3.0 to 11.8 L/min, and systemic vascular resistance ranged from 252 to 2434 dyne x sec/cm5. The mean difference (bias) +/-2 SD of differences (limits of agreement) was -0.29+/-1.31 L/min for COpa vs. COa, 0.07+/-1.4 L/min for COpc vs. COpa, and -0.22+/-1.58 L/min for COpc vs. COa. In all but four patients COpc correlated with COa after the initial calibration. Correlation and precision of COpc vs. COa was stable for 24 hrs. CONCLUSIONS: Femoral artery pulse contour CO correlates well with both COpa and COa even during substantial variations in vascular tone and hemodynamics. Additionally, CO determined by arterial thermodilution correlates well with COpa. Thus, COa can be used to calibrate COpc.     

Comparison of uncalibrated arterial waveform analysis in cardiac surgery patients with thermodilution cardiac output measurements

Introduction  

Cardiac output (CO) monitoring is indicated only in selected patients. In cardiac surgical patients, perioperative haemodynamic management is often guided by CO measurement by pulmonary artery catheterisation (CO_PAC). Alternative strategies of CO determination have become increasingly accepted in clinical practice because the benefit of guiding therapy by data derived from the PAC remains to be proven and less invasive alternatives are available. Recently, a device offering uncalibrated CO measurement by arterial waveform analysis (CO_Wave) was introduced. As far as this approach is concerned, however, the validity of the CO measurements obtained is utterly unclear. Therefore, the aim of this study was to compare the bias and the limits of agreement (LOAs) (two standard deviations) of CO_Wave at four specified time points prior, during, and after coronary artery bypass graft (CABG) surgery with a simultaneous measurement of the gold standard CO_PAC and aortic transpulmonary thermodilution CO (CO_Transpulm).     

Uncalibrated pulse power analysis fails to reliably measure cardiac output in patients undergoing coronary artery bypass surgery

Introduction  

Uncalibrated arterial pulse power analysis has been recently introduced for continuous monitoring of cardiac index (CI). The aim of the present study was to compare the accuracy of arterial pulse power analysis with intermittent transpulmonary thermodilution (TPTD) before and after cardiopulmonary bypass (CPB).     

Accuracy of continuous thermodilution cardiac output monitoring by pulmonary artery catheter during therapeutic hypothermia in post-cardiac arrest patients

Purpose

Thermodilution continuous cardiac output measurements (TDCCO) by pulmonary artery catheter (PAC) have not been validated during therapeutic hypothermia in post-cardiac arrest patients. The calculated cardiac output based on the indirect Fick principle (FCO) using pulmonary artery blood gas mixed venous oxygen saturation (FCO-BG-SvO2) is considered as the gold standard. Continuous SvO2 by PAC (PAC-SvO2) has also not been validated previously during hypothermia. The aims of this study were (1) to compare FCO-BG-SvO2 with TDCCO, (2) to compare PAC-SvO2 with BG-SvO2 and finally (3) to compare FCO with SvO2 obtained via PAC or blood gas.

Methods

We analyzed 102 paired TDCCO/FCO-BG-SvO2 and 88 paired BG-SvO2/PAC-SvO2 measurements in 32 post-cardiac arrest patients during therapeutic hypothermia.

Results

TDCCO was significantly although poorly correlated with FCO-BG-SvO2 (R² 0.21, p < 0.01) without systematic bias (−0.15 ± 1.76 l/min). Analysis according to Bland and Altman however showed broad limits of agreement ([−3.61; 3.45] l/min) and an unacceptable high percentage error (105%). None of the criteria for clinical interchangeability were met. Concordance analysis showed that TDCCO had limited trending ability (R² 0.03). FCO based on PAC-SvO2 was highly correlated with FCO-BG-SvO2 (R² 0.72) with a small bias (−0.08 ± 0.72 l/min) and slightly too high percentage error (44%).

Conclusion

Our results show an extreme inaccuracy of TDCCO by PAC in post-cardiac arrest patients during therapeutic hypothermia. We found a reasonable correlation between BG-SvO2 and PAC-SvO2 and subsequently between FCO calculated with SvO2 obtained either via blood gas or PAC. The decision to start or titrate inotropics should therefore not be guided by TDCCO in this setting.     

Effects of intermittent positive-pressure ventilation on cardiac output measurements by thermodilution

Sequential thermodilution measurements of cardiac output in mechanically ventilated patients undergoing cardiac surgery demonstrated a cyclic modulation which correlated with changes in airway pressure, and was not affected by opening the pericardium. There was no satisfactory point for single measurements, which suggests that random thermodilution measurements of cardiac output during intermittent positive-pressure ventilation should be avoided, even when triplicate measurements are performed. To estimate the mean cardiac output, at least two measurements should be made at predetermined points of the ventilatory cycle. We recommend paired measurements at midinspiration and end-expiration.     

Two-beam pulsed Doppler cardiac output measurement: reproducibility and agreement with thermodilution

Two observers used two-beam pulsed Doppler ultrasound, equipped with a suprasternal probe, to measure cardiac output (QtDopp) in 38 ICU patients who had pulmonary artery catheters and in 20 adult volunteers. The two-beam pulsed Doppler method enables one device to measure simultaneously both aortic blood velocity and aortic diameter. Each observer was blind to the other's measurement and to the thermodilution cardiac output measurement (Qttd). Linear regression of the mean of both observer's QtDopp on Qttd showed QtDopp = 0.90.Qttd + 0.01 (see = 1.54 L/min, r = .90). Bias (+/- SD), defined as mean (QtDopp - Qttd) difference, was -0.69 +/- 1.55 L/min. Interobserver agreement was more variable in patients than volunteers; mean (observer 1 - observer 2) difference was 0.14 +/- 1.30 L/min in ICU patients and -0.09 +/- 0.92 L/min in volunteers. Two-beam pulsed Doppler ultrasound is a simpler method of measuring QtDopp than previous pulsed Doppler methods which measure separately the aortic diameter by echocardiography. Although its agreement with Qttd is close to other Doppler methods and has acceptable interobserver reproducibility, its accuracy remains operator-dependent.     

Comparison of pulsed Doppler and thermodilution methods for measuring cardiac output in critically ill patients

We obtained 145 consecutive cardiac output measurements in 38 critically ill patients, using the invasive thermodilution and the noninvasive pulsed Doppler methods. The mean thermodilution cardiac output (TDCO) was 5.7 +/- 1.87 L/min and the mean pulsed Doppler cardiac output (PDCO) was 5.16 +/- 1.66 L/min. The mean difference between the two measurements was 0.51 L/min with an SD greater than 1.6 L/min, reflecting the scattering of results. The overall correlation coefficient was .58. The intercepts were large and the regression equation some way from the line of equal values (TDCO = 2.28 + 0.66 PDCO). When the results were analyzed according to diagnosis or by group experience, there were some differences in the bias of the estimate; however, the SD of the difference between methods was greater than one liter/min in all groups. Thus, the pulsed Doppler method failed to estimate accurately TDCO in critically ill patients.     

Continuous minimally invasive peri-operative monitoring of cardiac output by pulmonary capnotracking: comparison with thermodilution and transesophageal echocardiography

A number of technologies are available for minimally-invasive cardiac output measurement in patients during surgery but remain little used. A system has been developed based on CO₂ elimination (VCO₂) by the lungs for use in ventilated patients, which can be fully integrated into a modern anesthesia/monitoring platform, and provides semi-automated, continuous breath-by-breath cardiac output monitoring. A prototype measurement system was constructed to measure VCO₂ and end-tidal CO₂ concentration with each breath. A baseline measurement of non-shunt cardiac output was made during a brief change in ventilator rate, according to the differential CO₂ Fick approach. Continuous breath-by-breath monitoring of cardiac output was then performed from measurement of VCO₂, using a derivation of the Fick equation applied to pulmonary CO₂ elimination. Automated recalibration was done periodically and data was processed and cardiac output displayed in real time. Measurements were compared with simultaneous measurements by bolus thermodilution in 77 patients undergoing cardiac surgery or liver transplantation. Overall mean bias [sd] for agreement in cardiac output measurement was −0.1 [1.2] L/min, percentage error +44.2%, r = 0.92. Concordance in measurement of changes of at least 15% in cardiac output was 80%. The method followed sudden changes in cardiac output due to arrythmias and run onto cardiopulmonary bypass in real time. The accuracy and precision were comparable to other clinical techniques. The method is relatively seamless and largely automated and has potential for continuous, cardiac output monitoring in ventilated patients during anesthesia and critical care.     

Noninvasive whole-body electrical bioimpedance cardiac output and invasive thermodilution cardiac output in high-risk surgical patients

OBJECTIVE: To evaluate the reliability of whole-body impedance cardiography with two electrodes on either both wrists or one wrist and one ankle for the measurement of cardiac output compared with the thermodilution method. DESIGN: Prospective, clinical investigation SETTING: Surgical intensive care unit of a university-affiliated community hospital. PATIENTS: Simultaneous cardiac output measurements by noninvasive whole-body impedance cardiography (nCO) and invasive thermodilution (thCO) in 22 high-risk surgical patients scheduled for extended surgery requiring perioperative pulmonary artery catheter monitoring. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: A total of 109 sets of measurements consisting of 455 single comparison measurements between nCO and thCO were included in the analysis. The mean cardiac output difference between the two methods was 1.62 L/min with limits of agreement (2 SD) of +/- 4.64 L/min. The inter-measurement variance was slightly higher for nCO. The correlation coefficient between nCO and thCO was r2 = 0.061 (p < .001) for single measurements and r2 = 0.083 (p < .002) for sets of three to six measurements. The two most predictive factors for between-method differences were the absolute thCO value (r2 = 0.13; p < .001) and whether or not a continuous nitroglycerin infusion was used (p < .05, Student's t-test). CONCLUSIONS: Agreement between whole-body impedance cardiography and thermodilution in the measurement of cardiac output was unsatisfactory. Factors that can explain these differences are differences between the populations used for calibration of nCO and the study population, the influence of changing peripheral perfusion, and the effect of a supranormal hemodynamic state on the bioimpedance signal. Whole-body impedance cardiography cannot be recommended for assessing the hemodynamic state of high-risk surgical patients as studied in this investigation.     

Tissue pH monitoring tracks changes in cardiac output following endotoxin administration

We studied the reliability of tissue pH monitoring in tracking changes in cardiac index (CI) following endotoxin administration in ten anesthetized dogs. CI fell significantly during the four hour study period. The changes observed in CI were closely reflected by changes in tissue pH, measured continuously by a probe that was inserted into the dogs' skin. In contrast, other more traditional hemodynamic monitors such as heart rate, mean arterial blood pressure, and arterial and mixed venous blood pH failed to closely reflect the changes in CI. We conclude that tissue pH provides a valuable trend monitor at peripheral tissues where inadequate perfusion results in acid production and a low pH. The reliability and sensitivity of tissue pH monitoring in detecting changes in CI suggests that its use in clinical monitoring during unstable hemodynamic states such as endotoxemia should be considered.     

Emergency physicians can reliably assess emergency department patient cardiac output using the USCOM continuous wave Doppler cardiac output monitor

Objectives: 1 To develop a training package for ultrasonic cardiac output monitor (USCOM) cardiac output assessments and determine the number of proctored studies necessary for skill acquisition. 2 To develop criteria for acceptance of cardiac output results obtained with the USCOM. 3 To evaluate the reliability of USCOM cardiac output assessments in the ED. Methods: The authors developed an audiovisual training package. Four emergency physicians and one geriatrician subsequently underwent hands‐on training, and skill acquisition was assessed at the fifth, 10th, 15th and 20th examinations. Six image‐scoring criteria were developed to assess acoustic image quality. Upon completion of training a protocol was developed to optimize interassessor reliability. Two trained emergency physicians then performed blinded examinations on ED patients using the protocol and interassessor reliability was evaluated. Results: During training average image score improved between the fifth and 20th assessed patient from 4.6 (95% CI 4.0–5.3) to 5.5 (95% CI 5.0–6.0, P_t‐test = 0.02) out of 6 and average intra‐assessor cardiac output difference improved from 17% (95% CI 4–25) to 5% (95% CI 0–11, P_t‐test = 0.02). Analysis of 52 cardiac output assessments in 21 ED patients demonstrated excellent interassessor correlation (r = 0.96, 95% CI 0.90–0.98, P < 0.001). The average interassessor difference in cardiac output and index was 0.2 L/min (4%, 95% CI 3–6) and 0.1 L/min/m² (4%, 95% CI 2–6), respectively. Conclusion: Emergency physicians with no prior ultrasonographic experience can be trained to obtain reliable cardiac output estimations upon conscious ED patients with the USCOM over the course of 20 patient assessments.     

Cardiac output measurement in children: comparison of the Ultrasound Cardiac Output Monitor with thermodilution cardiac output measurement

OBJECTIVE: To compare the assessment of cardiac output (CO) in children using the noninvasive Ultrasound Cardiac Output Monitor (USCOM) with the invasive pulmonary artery catheter (PAC) thermodilution cardiac output measurement. DESIGN AND SETTING: Prospective observational study in a tertiary center for pediatric cardiology of a university children's hospital. PATIENTS: Twenty-four pediatric patients with congenital heart disease without shunt undergoing cardiac catheterization under general anesthesia. MEASUREMENTS AND RESULTS: CO was measured by USCOM using a suprasternal CO Doppler probe in children undergoing cardiac catheterization. USCOM data were compared to CO simultaneously measured by PAC thermodilution technique. Measurements were repeated three times within 5 min in each patient. A mean percentage error not exceeding 30% was defined as indicating clinical useful reliability of the USCOM. CO values measured by PAC ranged from 1.3 to 5.3 l/min (median 3.6 l/min). Bias and precision were -0.13 and 1.34 l/min, respectively. The mean percentage error of CO measurement by the USCOM compared to PAC thermodilution technique was 36.4% for USCOM. CONCLUSIONS: Our preliminary data demonstrate that cardiac output measurement in children using the USCOM does not reliably represent absolute CO values as compared to PAC thermodilution. Further studies must evaluate the impact of incorporating effective aortic valve diameters on CO measurement using the USCOM.     

Comparison of two semicontinuous cardiac output pulmonary artery catheters after valvular surgery

OBJECTIVE: To compare semicontinuous cardiac output (CCO) with bolus cardiac output (BCO), in the immediate postoperative period after valvular surgery, under hypothermic cardiopulmonary bypass with two CCO pulmonary artery catheters, based on the pulsed warm thermodilution technique, i.e., Opti-Q from Abbott or IntelliCath from Baxter-Edwards (Abbott and Baxter groups, respectively). DESIGN: Prospective study. SETTING: University hospital. PATIENTS: Forty-four adult patients scheduled for mitral and/or aortic valve surgery were randomized into two groups. Tricuspid or pulmonary valvulopathy diagnosed by echocardiography was excluded. INTERVENTIONS: Cardiac output was measured every 20 mins during the 3 postoperative hrs. BCO was the mean of three boluses (10 mL) of an ice-cold saline solution injected within 3 secs. CCO was the mean of two CCO values obtained in normal mode immediately before and after BCO measurements. MEASUREMENTS AND MAIN RESULTS: Two groups of 22 patients underwent 198 pairs of cardiac output measurements. The mean difference or bias was calculated as the difference between BCO and CCO, and precision was the SD of the mean bias. The limits of agreement were defined as bias +/- 2 SD. A two-sample Wilcoxon's test was used for comparison of bias and precision in sinus and non-sinus rhythm, and stable and unstable mean arterial pressure in each group and between the two pulmonary artery catheters. The coefficient of correlation was also calculated. Bias +/- precision was 0.066+/-0.526 L/min, r2 = .83, for the Abbott group, and 0.015+/-0.490 L/min, r2 = .85 (not significant), for the Baxter group. There was no significant difference within and between groups for bias and precision in sinus and non-sinus rhythm, nor in stable and unstable mean arterial pressure. CONCLUSIONS: This study, during the immediate postoperative period in valvular surgery under hypothermic cardiopulmonary bypass, showed a satisfactory correlation between CCO and BCO with the two systems.     

Simultaneous comparison of thoracic bioimpedance and arterial pulse waveform-derived cardiac output with thermodilution measurement

OBJECTIVE: To compare the accuracy and reliability of thoracic electrical bioimpedance (TEB) and the arterial pulse waveform analysis with simultaneous measurement of thermodilution cardiac output (TD-CO) in critically ill patients. DESIGN: Prospective data collection. SETTING: Emergency department and critical care unit in a 2,000-bed inner-city hospital. PATIENTS: A total of 29 critically ill patients requiring invasive hemodynamic monitoring for clinical management were prospectively studied. INTERVENTIONS: Noninvasive cardiac output was simultaneously measured by a TEB device and by analysis of the arterial pulse waveform derived from the finger artery. Invasive cardiac output was determined by the thermodilution technique. MEASUREMENTS AND MAIN RESULTS: A total of 175 corresponding TD-CO and noninvasive hemodynamic measurements were collected in 30-min intervals. They revealed an overall bias of 0.34 L/min/m2 (95% confidence interval, 0.24-0.44 L/min/m2; p < .001) for the arterial pulse waveform analysis and of 0.61 L/min/m2 (95% confidence interval, 0.50-0.72 L/min/m2; p < .001) for the TEB. In 39.4% (n = 69) of all measurements, the discrepancy between arterial pulse waveform analysis and TD-CO was >0.50 L/min/m2. The discrepancies of the arterial pulse waveform analysis correlated positively with the magnitude of the cardiac index (r2 = 0.29; p < .001). In 56.6% (n = 99) of all measurements, the discrepancy between TEB and TD-CO was >0.50 L/min/m2. The magnitude of the discrepancies of the TEB was significantly correlated with age (r2 = 0.17; p = .02). Measurements were in phase in 93.2% of all arterial pulse waveform analysis and in 84.9% of all TEB readings (p < .001). CONCLUSIONS: The arterial pulse waveform analysis exhibits a greater accuracy and reliability as compared with the TEB with regard to overall bias, number of inaccurate readings, and phase lags. The arterial pulse waveform analysis may be useful for the monitoring of hemodynamic changes. However, both methods fail to be a substitute for the TD-CO because of a substantial percentage of inaccurate readings.     

Estimated continuous cardiac output based on pulse wave transit time in off-pump coronary artery bypass grafting: a comparison with transpulmonary thermodilution

To evaluate the accuracy of estimated continuous cardiac output (esCCO) based on pulse wave transit time in comparison with cardiac output (CO) assessed by transpulmonary thermodilution (TPTD) in off-pump coronary artery bypass grafting (OPCAB). We calibrated the esCCO system with non-invasive (Part 1) and invasive (Part 2) blood pressure and compared with TPTD measurements. We performed parallel measurements of CO with both techniques and assessed the accuracy and precision of individual CO values and agreement of trends of changes perioperatively (Part 1) and postoperatively (Part 2). A Bland–Altman analysis revealed a bias between non-invasive esCCO and TPTD of 0.9 L/min and limits of agreement of ±2.8 L/min. Intraoperative bias was 1.2 L/min with limits of agreement of ±2.9 L/min and percentage error (PE) of 64 %. Postoperatively, bias was 0.4 L/min, limits of agreement of ±2.3 L/min and PE of 41 %. A Bland–Altman analysis of invasive esCCO and TPTD after OPCAB found bias of 0.3 L/min with limits of agreement of ±2.1 L/min and PE of 40 %. A 4-quadrant plot analysis of non-invasive esCCO versus TPTD revealed overall, intraoperative and postoperative concordance rate of 76, 65, and 89 %, respectively. The analysis of trending ability of invasive esCCO after OPCAB revealed concordance rate of 73 %. During OPCAB, esCCO demonstrated poor accuracy, precision and trending ability compared to TPTD. Postoperatively, non-invasive esCCO showed better agreement with TPTD. However, invasive calibration of esCCO did not improve the accuracy and precision and the trending ability of method.     

CO assessment by suprasternal Doppler in critically ill patients: comparison with thermodilution

Objective: Comparison of suprasternal Doppler (SST) and thermodilution (TD) for the measurement of cardiac output (CO) in critically ill patients.¶Design: Prospective study.¶Setting: Intensive care unit of a university hospital.¶Patients and participants: 65 consecutive critically ill patients requiring a pulmonary artery catheter.¶Interventions: Paired CO measurements were made simultaneously using SST and TD by two independent operators. The time to obtain a CO value by SST was measured. Correlation coefficients and the linear regression equation were determined. A Bland and Altman diagram was plotted. A Bland and Altman diagram was also plotted for the level of cardiac index (CI) values (low: CI < 2.5 l min^–1 m^–2; normal: 2.5 ≤ CI ≤ 4.5 l min^–1 m^–2; high: CI > 4.5 l min^–1 m^–2).¶Measurements and results: In seven patients SST failed to measure CO. In the remaining 58 patients 314 paired CO measurements were performed. The mean time to measure CO by SST was 73 ± 45 s. The equation of linear regression was: SST_CO = 0.84 TD_CO + 1.39. The correlation coefficient was 0.84. The bias between SST and TD was –0.2 ± 1.4 l min^–1. Biases were –0.23 ± 0.50, –0.20 ± 0.68, and 0.25 ± 0.92 l min^–1 m^–2 for low, normal, and high levels of CI, respectively.¶Conclusion: SST does not accurately measure CO but allows a rapid assessment of CI level in critically ill patients.