Absolute Cardiac Output Values Research Articles

A comparison of hemodynamic measurement methods during orthotopic liver transplantation: evaluating agreement and trending ability of PiCCO versus pulmonary artery catheter techniques

BackgroundSignificant hemodynamic changes occur during liver transplantation, emphasizing the importance of precious and continuous monitoring of cardiac output, cardiac index, and other parameters. Although the monitoring of cardiac output by pulse indicator continuous cardiac output (PiCCO) was statistically homogeneous compared to the clinical gold standard pulmonary artery catheterization (PAC) in previous studies of liver transplantation, there are fewer statistical methods for the assessment of its conclusions, and a lack of comparisons of other hemodynamic parameters (e.g., SVRI, systemic vascular resistance index). Some studies have also concluded that the agreement between PiCCO and PAC is not good enough. Overall, there are no uniform conclusions regarding the agreement between PiCCO and PAC in previous studies. This study evaluates the agreement and trending ability of relevant hemodynamic parameters obtained with PiCCO compared to the clinical gold standard PAC from multiple perspectives, employing various statistical methods.MethodsFifty-two liver transplantation patients were included. Cardiac output (CO), cardiac index (CI), SVRI and stroke volume index (SVI) values were monitored at eight time points using both PiCCO and PAC. The results were analyzed by Bland-Altman analysis, Passing-bablok regression, intra-class correlation coefficient (ICC), 4-quadrant plot, polar plot, and trend interchangeability method (TIM).ResultsThe Bland-Altman analysis revealed high percentage errors for PiCCO: 54.06% for CO, 52.70% for CI, 62.18% for SVRI, and 51.97% for SVI, indicating poor accuracy. While Passing-Bablok plots showed favorable agreement for SVRI overall and during various phases, the agreement for other parameters was less satisfactory. The ICC results confirmed good overall agreement between the two devices across most parameters, except for SVRI during the new liver phase, which showed poor agreement. Additionally, four-quadrant and polar plot analyses indicated that all agreement rate values fell below the clinically acceptable threshold of over 90%, and all angular deviation values exceeded ± 5°, demonstrating that PiCCO is unable to meet the acceptable trends. Using the TIM, the interchangeability rates were found to be quite low: 20% for CO and CI, 16% for SVRI, and 13% for SVI.ConclusionsOur study revealed notable disparities in absolute values of CO, CI, SVRI and SVI between PiCCO and PAC in intraoperative liver transplant settings, notably during the neohepatic phase where errors were particularly pronounced. Consequently, these findings highlight the need for careful consideration of PiCCO’s advantages and disadvantages in liver transplantation scenarios, including its multiple parameters (such as the encompassing extravascular lung water index), against its limited correlation with PAC.

Comparing Doppler Echocardiography and Thermodilution for Cardiac Output Measurements in a Contemporary Cohort of Comatose Cardiac Arrest Patients Undergoing Targeted Temperature Management.

Measuring cardiac output is used to guide treatment during postresuscitation care. The aim of this study was to compare Doppler echocardiography (Doppler-CO) with thermodilution using pulmonary artery catheters (PAC-CO) for cardiac output estimation in a large cohort of comatose out-of-hospital cardiac arrest (OHCA) patients undergoing targeted temperature management (TTM). Single-center substudy of 141 patients included in the TTM trial randomly assigned to 33 or 36°C for 24 hours after OHCA. Per protocol, PAC-CO and Doppler-CO were measured simultaneously shortly after admission and again at 24 and 48 hours. Linear correlation was assessed between methods and positive predictive value (PPV) and negative predictive value (NPV) of Doppler to estimate low cardiac output (<3.5 L/min) was calculated. A total of 301 paired cardiac output measurements were available. Average cardiac output was 5.28 ± 1.94 L/min measured by thermodilution and 4.06 ± 1.49 L/min measured by Doppler with a mean bias of 1.22 L/min (limits of agreements -1.92 to 4.36 L/min). Correlation between methods was moderate (R2 = 0.36). Using PAC-CO as the gold standard, PPV of a low cardiac output measurement (<3.5 L/min) by Doppler was 33%. However, the NPV was 92%. Hypothermia at 33°C did not negatively affect the correlations of CO methods. In the lowest quartile of Doppler, 13% had elevated lactate (>2 mmol/L). In the lowest quartile of thermodilution, 36% had elevated lactate (>2 mmol/L). In ventilated OHCA patients, the two methods for estimating cardiac output correlated moderately and there was a consistent underestimation of Doppler-CO. Absolute cardiac output values from Doppler-CO should be interpreted with caution. However, Doppler can be used to exclude low cardiac output with high accuracy. TTM at 33°C did not negatively affect the correlation or bias of cardiac output measurements. ClinicalTrials.gov ID: NCT01020916.

Non-invasive cardiac output measurement by electrical cardiometry and M-mode echocardiography in the neonate: a prospective observational study of 136 neonatal infants

BackgroundElectrical cardiometry (EC) is a continuous, non-invasive method for measuring cardiac output (CO). This study investigates the correlation and consistency of CO values in newborns obtained by using EC and M-mode echocardiography (Teichholz formula).MethodsIn this prospective observational study, simultaneous measurement of CO was implemented with EC (COec) and M-mode echocardiography (COm) in neonates. The absolute values of CO measured by the two methods were converted to Z-scores. Following that, Pears’s correlation analyses and the Bland–Altman index were employed to analyze the correlation and consistency of COec Z-scores and COm Z-scores.ResultsA total of 136 neonates (93 preterm infants) were enrolled in this study, and EC and M-mode echocardiography comparative studies were conducted 155 times. The mean value of COec and COm demonstrated significant statistical differences (P<0.001). A moderate correlation (r=0.601; P<0.001) was found between the two methods. The Bland-Altman index value was 3.2%, which remained less than 5% in the low birth weight (LBW) (2.1%), non-LBW (3.4%), spontaneous respiration (3.1%), nasal continuous positive airway pressure (nCPAP) (4.0%), mechanical ventilation (2.9%), hemodynamic significance of the patent ductus arteriosus (hsPDA) (4.3%), and non-hsPDA (3.7%) groups, respectively.ConclusionsAlthough the absolute values of CO measured by EC and M-mode echocardiography were not interchangeable, the distribution of CO in EC and M-mode echocardiography was similar.

Can bioimpedance cardiography assess hemodynamic response to passive leg raising in critically ill patients: A STROBE-compliant study.

Passive leg raising (PLR) is a convenient and reliable test to predict fluid responsiveness. The ability of thoracic electrical bioimpedance cardiography (TEB) to monitor changes of cardiac output (CO) during PLR is unknown.In the present study, we measured CO in 61 patients with shock or dyspnea by TEB and transthoracic echocardiography (TTE) during PLR procedure. Positive PLR responsiveness was defined as the velocity-time integral (VTI) ≥10% after PLR. TTE measured VTI in the left ventricular output tract. The predictive value of TEB parameters in PLR responders was tested. Furthermore, the agreement of absolute CO values between TEB and TTE measurements was assessed.Among the 61 patients, there were 28 PLR-responders and 33 non-responders. Twenty-seven patients were diagnosed with shock and 34 patients with dyspnea, with 55.6% (15/27) and 54.6% (18/34) non-responders, respectively. A change in TEB measured CO (ΔCO) ≥9.8% predicted PLR responders with 75.0% sensitivity and 78.8% specificity, the area under the receiver operating characteristic curve (AUROC) was 0.79. The Δd2Z/dt2 (a secondary derivative of the impedance wave) showed the best predictive value with AUROC of 0.90, the optimal cut point was −7.1% with 85.7% sensitivity and 87.9% specificity. Bias between TEB and TTE measured CO was 0.12 L/min, and the percentage error was 65.8%.TEB parameters had promising performance in predicting PLR responders, and the Δd2Z/dt2 had the best predictive value. The CO values measured by TEB were not interchangeable with TTE in critically ill settings.

Evaluation of New Calibrated Pulse-Wave Analysis (VolumeViewTM/EV1000TM) for Cardiac Output Monitoring Undergoing Living Donor Liver Transplantation.

BackgroundIntrapulmonary thermodilution technique using a pulmonary artery catheter is widely used for measuring cardiac output (CO) in patients undergoing liver transplantation. However, its invasiveness and associated complications have led to an interest in less invasive modalities. Thus, we aimed to evaluate whether the new calibrated pulse-wave analysis method monitoring (VolumeViewTM/EV1000TM) is interchangeable with intrapulmonary thermodilution technique.MethodsTwenty-eight patients undergoing living donor liver transplantation were enrolled in this prospective observational study. COs were recorded automatically by the two devices and compared simultaneously at 10-minute intervals. The agreement of absolute CO values and the tracking ability of CO changes trends were compared. A Bland-Altman analysis with percentage errors and concordance rate for trend analysis using both a 4-quadrant plot and a polar plot were performed on the data.ResultsA total of 375 paired datasets from 25 patients were included in analysis. COs measured by intrapulmonary thermodilution ranged from 3.8–13.7 L/min. The mean CO difference between the two techniques was 0.57 L/min, and the 95% limits of agreement were -0.98 L/min to 2.12 L/min with a percentage error of 42.3%. The percentage errors in the dissection, anhepatic, and reperfusion phase were 30.5%, 31.7%, and 27.4%, respectively. The concordance rate between the two techniques was 78.4%.ConclusionThe calibrated pulse-wave analysis and intrapulmonary thermodilution failed to show acceptable interchangeability in terms of both estimating CO and tracking CO changes during living donor liver transplantation.

Do We Need to Monitor Cardiac Output during Major Surgery?

Do We Need to Monitor Cardiac Output during Major Surgery?

051 Can Echocardiography Adequately Measure Cardiac Output Early After Neonatal Surgery for Congenital Heart Disease?

051 Can Echocardiography Adequately Measure Cardiac Output Early After Neonatal Surgery for Congenital Heart Disease?

The effect of anaesthesia and aortic clamping on cardiac output measurement using arterial pulse power analysis during aortic aneurysm repair*

The LiDCO plus monitor (LiDCO Ltd, Cambridge, UK) uses pulse contour analysis of the arterial pressure waveform to indicate changes in stroke volume and cardiac output. Calibration against a lithium indicator dilution method is required to permit display of absolute values in addition to trends. The effect of haemodynamic changes during anaesthesia and surgery on this calibration factor has not previously been studied. Therefore, we investigated whether it remained constant during elective abdominal aortic aneurysm surgery in 15 patients. Comparison between the calibration factor values at different time points was made by repeated recalibration throughout the peri-operative period. Calibration factor increased by a mean of 53% after anaesthesia (epidural plus general) (p = 0.03) and decreased by a mean of 40% after aortic clamping (p = 0.0001). Recalibration should be undertaken after induction of anaesthesia and after aortic clamping if absolute values of cardiac output and stroke volume are required.

NICOM vs LiDCO™ plus during changes in cardiac output in critically ill patients

NICOM (Cheetah) is a new non-invasive cardiac output (CO) monitor based on bioreactance. We tested the agreement of NICOM vs LiDCO™ plus in terms of absolute CO values and of the ability to track changes after therapeutic intervention in critically ill patients.

Pulse contour-derived cardiac output in hemodialysis patients

Reliable methods for cardiac output determination are essential for studying the pathophysiology of intradialytic hypotension. Use of the current gold standard, the Transonic monitor, requires an arteriovenous fistula. We wished to verify the accuracy of a method based on finger pulse contour analysis, namely the Finometer monitor (FNM) for further use on patients dialyzing on a central vascular catheter. Fifty simultaneous cardiac output measurements were obtained during hemodialysis sessions in 25 patients. The internal variability of the FNM measurements was assessed by comparing 24 pairs of immediately successive measurements. The variability of successive FNM measurements was small (bias 0.28%, SD +/- 6.1%; NS). The absolute cardiac output values reported by the FNM were unreliable (bias 20.1%, SD +/- 35.3%; P<0.001) as were the relative intradialytic changes (r(2)=0.01). Excluding participants from the analysis due to old age, high dialysis vintage or a suspicion of atherosclerosis did not improve the results. Our findings do not support the use of pulse contour analysis for measuring cardiac output in hemodialysis patients. Uremic vascular disease may be the cause of the observed inaccuracy.

Evaluation of the Aesculon cardiac output monitor by subxiphoidal Doppler flow measurement in children with congenital heart defects

To evaluate the noninvasive electrical velocimetry (Aesculon) monitor for cardiac output (CO) by subxiphoidal Doppler flow measurement in children. CO was determined at the end of diagnostic or interventional cardiac catheterization for congenital heart defects. Standard ECG surface electrodes were attached in a vertical direction to the patients' left middle and lower neck, and lower thorax at the level of the heart and xiphoid process. Aesculon CO data were compared with a simultaneously measured CO by the subxiphoidal Doppler flow measurement technique. For each patient, measurements were repeated three times within 5 min. Whitney U-test, simple regression and Bland-Altman analysis were performed to compare CO values obtained by the two techniques. Data are given as range (median). A total of 36 children aged 5.7 (0.5-16.0) years were investigated. CO values obtained by Aesculon monitor [0.55-5.58 (2.62) l min] and subxiphoidal Doppler flow measurements [0.62-6.27 (3.05) l min] differed significantly between both methods (P = 0.04). Simple regression analysis revealed moderate correlation between CO values obtained from the two techniques (r = 0.5544, P < 0.001). Bias between the two methods was 0.31 l min with a precision of 1.92 l min. We conclude that electrical velocimetry using the Aesculon monitor does not reliably reflect absolute CO values as compared with subxiphoidal Doppler flow measurement.

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

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. Prospective observational study in a tertiary center for pediatric cardiology of a university children's hospital. Twenty-four pediatric patients with congenital heart disease without shunt undergoing cardiac catheterization under general anesthesia. 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. 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.

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.

Comparison of Cardiac Outputs of Major Burn Patients Undergoing Extensive Early Escharectomy: Esophageal Doppler Monitor Versus Thermodilution Pulmonary Artery Catheter

After major burn injury, severe hemodynamic fluctuations occur in a relatively short time, requiring invasive hemodynamic monitoring. The noninvasive esophageal Doppler monitor offers an alternative to the pulmonary artery catheter. This study compares the cardiac output of the esophageal Doppler monitor with that of a pulmonary artery catheter during the large volume shifts seen in extensive early escharectomy. This study recruited 20 critically ill, major burn patients scheduled for elective early escharectomy. A thermodilution pulmonary artery catheter and esophageal Doppler monitor probes were inserted to measure cardiac output simultaneously at regular intervals. The results were analyzed by simple linear regression and Bland and Altman plots. The analysis included 92 data pairs from 20 patients, most of whom were critically ill with major burns. Escharectomy caused large intraoperative fluid shifts. The cardiac output of the esophageal Doppler monitor correlated moderately well with the cardiac output of the pulmonary artery, although it showed a mean of 15% less than the output of the pulmonary artery catheter. The mean bias was 0.77 L/minute, and the limits of agreement were +/-2.74 L/min. For major burn patients undergoing early escharectomy, the esophageal Doppler monitor allows changes in hemodynamics to be followed, but does not accurately measure the absolute values of cardiac output.

Cardiac Output Estimations by Esophageal Doppler Cannot Replace Estimations by the Thermodilution Method in Off-pump Coronary Artery Bypass Surgery Patients

Background: Esophageal doppler is discribed as a non-invasive alternative to cardiac output (CO) estimation by thermodilution, the current bedside “gold standard”. This study was designed to evaluate the accuracy of CO estimations performed by esophageal doppler (EDCO), compared to those obtained using a continuous CO pulmonary flotation catheter (TDCO). Methods: In 16 patients undergoing off-pump coronary artery bypass surgery, CO was measured simultaneously by the esophageal doppler and the thermodilution method, after induction (A), after sternotomy (B), after coronary revascularization (C), and after sternal closure (D). Agreement between the TDCO and EDCO estimations was assessed by analyzing their mean differences and the distribution of these differences. Relative CO changes (percentages of the previous value) was analyzed by the same method. Results: Both absolute CO values and relative CO changes by esophageal doppler showed a considerable scatter compared to those obtained using the thermodilution method. The bias (EDCO-TDCO) between the two mehtods was -0.8 2.7 L/min for A, -0.9 2.5 L/min for B, -0.9 3.6 L/min for C, and -0.6 2.7 (mean 2 SD) L/min for D. On analyzing changes in CO, no significant method bias was found but 2 SD of the bias were 74% for A to B, 100% for B to C, and 83% for C to D. Conclusions: These results suggest that CO estimations by esophageal doppler cannot replace estimations by the thermodilution method in patients undergoing off-pump coronary artery bypass graft surgery.

Comparison of cardiac output assessed by pulse-contour analysis and thermodilution in patients undergoing minimally invasive direct coronary artery bypass grafting

Objective: To investigate the precision and accuracy of continuous pulse contour cardiac output (PCCO) compared with intermittent transcardiopulmonary (TCPCO) and pulmonary artery thermodilution (TDCO) measurements in patients undergoing minimally invasive coronary bypass surgery (MIDCAB). Design: Prospective, controlled, clinical study. Setting: University hospital. Participants: Twelve patients undergoing MIDCAB. Interventions: Thirty-six measurements of PCCO and thermodilution cardiac output (CO) were simultaneously performed after the start of surgery, during bypass grafting, and at the end of surgery. TCPCO and TDCO were simultaneously assessed by three injections of ice-cold saline randomly spread over the respiratory cycle. The pulse contour device was initially calibrated with an additional set of aortic thermodilution measurements. Measurements and Main Results: Absolute values of CO ranged between 1.6 and 9.2 L/min. A close agreement among the three techniques was observed at all measurements. Mean bias between PCCO and TDCO and TCPCO was 0.003 L/min (2 SD of differences between methods = 1.26 L/min) and 0.27 L/min (2 SD of differences between methods = 1.16 L/min), respectively. The correlation coefficients were r 2 = 0.90 for TCPCO versus PCCO and r 2 = 0.88 for TDCO versus PCCO. Conclusion: The results of the present study show that compared with thermodilution CO, pulse contour analysis enables accurate measurement of continuous CO in patients undergoing MIDCAB.

Comparison of transesophageal echocardiographic, fick, and thermodilution cardiac output in critically ill patients

Purpose: Recent observations have highlighted errors in the thermodilution technique of measuring cardiac output. Thus, cardiac output measurements using transesophageal echocardiography and the Fick method were compared with simultaneous thermodilution measurements. Methods: In 13 mechanically ventilated critically ill patients, cardiac output was determined simultaneously using (1) transesophageal echocardiography (CO TEE), (2) the Fick method (CO FICK), and (3) thermodilution (CO TD) immediately before and after a rapid infusion of 500 mL of saline. Left ventricular end-diastolic and end-systolic areas were measured using the transesophageal echocardiographic transgastric short axis view, and CO TEE was calculated from the corresponding volumes. Absolute cardiac output values and the changes from before to after saline infusion (ΔCO) were compared using analysis of variance, linear regression, and the Bland and Altman method. Results: There were no significant differences between CO TEE (8.0 ± 3.4), CO FICK (8.4 ± 3.3), and CO TD (8.3 ± 3.0) or between ΔCO TEE, ΔCO FCK, and ΔCO TD using analysis of variance. However, correlations between CO TEE and CO TD ( r 2 = 0.46; P < .00001), CO FICK and CO TD ( r 2 = 0.46; P < .00001), and CO TEE and CO FICK ( r 2 = 0.42; P < .00001) were only moderately good. Using the method of Bland and Altman, the mean difference (±2 standard deviations) between CO TEE and CO TD was 0.3 ± 4.3 L/min, between CO FICK and CO TD was −1.0 ± 3.8 L/min, and between CO TEE and CO FICK was 0.6 ± 5.6 L/min, whereas the difference between ΔCO TEE and ΔCO TD was 0% ± 26%, between ΔCO FICK and ΔCO TD was 9% ± 46%, and between ΔCO TEE and ΔCO FICK was 8% ± 39%. Conclusions: There are substantial differences in cardiac output as measured by these three methods, best demonstrated using the method of Bland and Altman. The variability of cardiac output and its derivatives (eg, oxygen delivery) should be borne in mind when making clinical decisions on individual patients.

Time Courses of Cardiac Output and Oxygen Uptake Following Stepwise Increases in Exercise Intensity

The adjustment of pulmonary oxygen uptake (VO2) following a step increase in work rate has been characterized as consisting of an early "cardiodynamic" component with unchanged mixed-venous O2-content ("phase 1") and a subsequent "metabolic" component ("phase 2") starting when venous blood from the muscle arrives at the lungs. The aim of the present study was to investigate whether the onset of phase 2 actually indicates the arrival of blood influenced by the altered muscle metabolism. Parallel measurements of cardiac output (Doppler technique) and VO2 (breath-by-breath measurements at the mouth) were performed in eight subjects during step increases in exercise intensity (from a 20 W baseline to either 80 W, 120 W, 160 W or 200 W). To vary the absolute cardiac output values for given muscle VO2 the subjects exercised both in upright and supine position. Individual time-courses of the arterio-venous O2 difference (a-v delta O2) were computed from cardiac output and VO2 data. Independent of body position two clear-cut phases of similar duration were seen both in VO2 and in the computed a-v delta O2. The duration of the first component with unchanged a-v delta O2 was about 20 s at the lowest step amplitudes (20-80 W and 20-120 W). It decreased to about 15 s for the 20-160 W and 20-200 W steps. At the lower exercise intensities the duration of phase 1 appears too long to be entirely due to the transit time of venous blood from the exercising muscles to the lungs.(ABSTRACT TRUNCATED AT 250 WORDS)

Reliability of impedance cardiography in measuring central haemodynamics

The purpose of the study described here was to investigate the reliability of impedance cardiography (IC) in measuring cardiac output (CO) and central blood volume. Absolute values and changes in these variables obtained by impedance cardiography and by isotope- or thermodilution techniques were compared. The reproducibility of IC within the same day was compared with that of isotope dilution and the reproducibility in IC from day to day was derived. Finally, the effects of the readings of impedance tracings by different observers were quantified. The results are based on 270 measurements in 37 healthy subjects and in 25 unmedicated patients with ischaemic heart disease. We obtained significant correlations between absolute values (y = 0.68x + 1.48) and changes (y = 1.00x + 0.0003) in CO measured by IC and isotope- or thermodilution. IC significantly overestimated absolute values of CO (P less than 0.001). We found a qualitative but no quantitative correlation between thoracic fluid volume measured by IC and central blood volume measured by isotope dilution. IC was highly reproducible both when studies were repeated within the same day (SD on differences in CO = 0.36 1 min-1 for IC; SD on differences in CO = 0.30 1 min-1 for isotope dilution) and on different days (SD on differences in CO = 0.45 1 min-1). A low intra-observer variability was found (SD on differences in CO = 0.12 1 min-1). We conclude that impedance cardiography is reliable in measuring changes in cardiac output and thus suitable for repeated measurements in studies on the haemodynamic effects of physiological or pharmacological intervention. Impedance cardiography is sufficiently reliable for comparison of absolute values of CO between different groups of patients. We cannot recommend impedance cardiography for quantitative studies of central blood volume.

Noninvasive Doppler determination of cardiac output during submaximal and peak exercise.

We compared pulsed Doppler (PD) measurements of stroke volume (SV) and cardiac output (CO) as a function of work load with previously reported values that were obtained by standard invasive methods. Suprasternal notch measurements of Doppler-shifted frequency (delta f) were obtained from the ascending aorta and SV calculated with the Doppler equation and an independent measurement of aortic diameter. Motion artifacts were minimized with the aid of a restraining table cycle ergometer. Signal aliasing was accommodated with manual summation of delta f waveforms. A total of 207 determinations were made in 10 sitting subjects exercising to exhaustion. Linear regression analysis of CO vs. work load was significant (P less than 0.001). The correlation coefficient (r = 0.95) and standard error of estimate value (1.21 1/min) were similar to values from the literature. Absolute values of CO and SV underestimated the literature values across all work loads. Technical reproducibility was assessed by comparing with paired t tests the differences between 65 duplicate serial measurements of CO and SV at rest and exercise. No significant differences (P less than 0.001) were found. We concluded that PD-determined SV and CO are reproducible and correlate linearly with work load in a manner consistent with reported invasive techniques. Thus the PD method appears suitable for use during submaximal and peak exercise.