Automated non-invasive measurement of cardiac output by the carbon dioxide rebreathing method: comparisons with dye dilution and thermodilution

Cardiac output measurement during exercise in COPD: A comparison of dye dilution and impedance cardiography

INTRODUCTION

Impedance cardiography (IC) derived from morphological analysis of the thoracic impedance signal is now commonly used for noninvasive assessment of cardiac output (CO) at rest and during exercise. However, in Chronic Obstructive Pulmonary Disease (COPD), conflicting findings put its accuracy into question.

OBJECTIVES

We therefore compared concurrent CO measurements captured by IC (PhysioFlow: CO_IC ) and by the indocyanine green dye dilution method (CO_DD ) in patients with COPD.

METHODS

Fifty paired CO measurements were concurrently obtained using the two methods from 10 patients (FEV₁ : 50.5 ± 17.5% predicted) at rest and during cycling at 25%, 50%, 75% and 100% peak work rate.

RESULTS

From rest to peak exercise CO_IC and CO_DD were strongly correlated (r = 0.986, P < 0.001). The mean absolute and percentage differences between CO_IC and CO_DD were 1.08 L/min (limits of agreement (LoA): 0.05-2.11 L/min) and 18 ± 2%, respectively, with IC yielding systematically higher values. Bland-Altman analysis indicated that during exercise only 7 of the 50 paired measurements differed by more than 20%. When data were expressed as changes from rest, correlations and agreement between the two methods remained strong over the entire exercise range (r = 0.974, P < 0.001, with no significant difference: 0.19 L/min; LoA: -0.76 to 1.15 L/min). Oxygen uptake (VO₂ ) and CO_DD were linearly related: r = 0.893 (P < 0.001), CO_DD = 5.94 × VO₂ + 2.27 L/min. Similar results were obtained for VO₂ and CO_IC (r = 0.885, P < 0.001, CO_IC = 6.00 × VO₂ + 3.30 L/min).

CONCLUSIONS

These findings suggest that IC provides an acceptable CO measurement from rest to peak cycling exercise in patients with COPD.

Non-invasive cardiac output evaluation in postoperative cardiac surgery patients, using a new prolonged expiration-based technique

The gold standard methods to measure cardiac output (CO) are invasive and expose the patient to high risks of various complications. The aim of this study is to assess an innovative non-invasive method for CO monitoring in mechanically ventilated patients after cardiac surgery and its agreement with values obtained by thermodilution technique. Continuous monitoring of respiratory gas concentrations and airflow allows the estimation of CO through a newly developed algorithm derived from a modified version of the Fick equation. It consists of two phases: the first involves measurements during steady breathing state, and the second starts when a sudden perturbation into the carbon dioxide elimination process is introduced by a prolonged expiration. This prospective clinical study involved thirty-five adult patients, undergone cardiac surgery. The measurements were performed in curarized and haemodynamically stable patients, during the post-surgery recovery in intensive care unit. The study protocol, which lasted 1 h for each patient, consisted of 20 measurements obtained by prolonged expiration-based method and 10 by thermodilution. The estimation of CO using the proposed method (COK) agreed with the thermodilution (COT) as demonstrated by: a low mean bias between COK and COT considering all patients (i.e., -0.11 L min(-1)); a best fitting line having slope = 0.98, r = 0.81, p < 0.0001; the lower and upper limits of agreement were -0.77 and +0.54 L min(-1), respectively. COK shows a mean percentage error of 34 %. In stable mechanically ventilated patients, undergone cardiac surgery, the proposed method is reliable if compared to the thermodilution. Considering the non-invasivity of the technique, further evaluations of its performances are encouraged.

Non-Invasive Cardiac Output Measurement using a Fast Mixing Box to Measure Carbon Dioxide Elimination

This study investigated the accuracy of a new technique for measuring cardiac output using the derivative Fick principle based on the ratio of change in the partial pressures of end-tidal and mixed expired carbon dioxide produced by short periods of partial rebreathing.

A prospective clinical study involving 24 patients following cardiopulmonary bypass for coronary artery bypass grafting or valvular surgery was undertaken in the intensive care unit of a university-affiliated hospital.

Haemodynamic measurements were performed after admission to the intensive care unit. Cardiac output was measured simultaneously by bolus pulmonary artery thermodilution and by a noninvasive carbon dioxide partial rebreathing technique.

Cardiac output measurement using the new technique demonstrated a significant but consistent underestimate, with a bias of -0.60 ± 0.87 l/min.

This new adaptation of the partial rebreathing technique is reliable in measuring cardiac output in postoperative patients. Reasons for the consistent discrepancy between thermodilution and partial rebreathing techniques are discussed.

Usefulness of real-time three-dimensional echocardiography for reliable measurement of cardiac output in patients with ischemic or idiopathic dilated cardiomyopathy

The determination of stroke volume (SV) is a potentially important application of real-time 3-dimensional echocardiography (RT3DE). SV measurements by thermodilution were compared with values obtained using transthoracic RT3DE in a sequential cohort of patients who underwent assessment for potential cardiac transplantation. There was a strong correlation between echocardiographically derived SV and catheterization data (r = 0.95, n = 14). On average, RT3DE appeared to underestimate SV by 7.5 ml (SD = 5.8) or 17% (SD = 12%). A role for RT3DE in the measurement of SV in severe heart failure is suggested.

Three-dimensional echocardiographic determination of cardiac output at rest and under dobutamine stress: comparison with thermodilution measurements in the ischemic pig model

Determination of cardiac output is a potentially important clinical application of three-dimensional (3-D) echocardiography since it could replace invasive measurements with the Swan-Ganz-catheter. To date, there are no studies available to determine whether cardiac output measured by thermodilution can be predicted reliably under changing hemodynamic conditions. Fifteen pigs with ischemic myocardium were examined under four hemodynamic conditions at rest and under pharmacological stress with 5, 10, and 20 microg/kg/min dobutamine. The 3-D datasets were recorded by means of transesophageal echocardiography. The endocardial definition was enhanced by administering the contrast agent FS069 (Optison). Cardiac output was calculated as the product of stroke volume (end-diastolic - end-systolic volume) and heart rate. The invasive measurements were performed with a continuous thermodilution system. In general, there was moderate correlation between 3-D echocardiography and thermodilution(r = 0.72, P < 0.001). At rest, the 3-D echocardiographic measurements were slightly but significantly lower than the invasive measurements (mean difference 0.6 +/- 0.5L/min,P < 0.001). Under stress with 5, 10, and 20 microg/kg/min dobutamine, there was a marked increase in the deviation (1.3 +/- 0.5L/min,P < 0.001; 1.6 +/- 0.7 L/min,P < 0.001; and 2.1 +/- 1.1L/min,P < 0.001, respectively). The deviation was based on two factors: (1). Under stress, the decreasing number of frames per cardiac cycle acquired with 3-D echocardiography led to imprecise recording of end-diastolic and end-systolic volumes, and thus to an underestimation of cardiac output. At least 30 frames per cardiac cycle are needed to eliminate this effect. (2). There is a systematic difference between 3-D echocardiographic and invasive measurements, which is independent of the imaging rate. This is based on an overestimation of the true values by thermodilution. In conclusion, cardiac output can be determined correctly by 3-D echocardiography for normal heart rates at rest. At elevated heart rates, the temporal resolution of 3-D systems currently available is not adequate for reliable determination. In performing and evaluating future clinical comparative studies, the systematic difference between 3-D echocardiography and thermodilution, based on overestimation by thermodilution, must be taken into account.

Comparison of cardiac output measured by two automated methods of CO2 rebreathing

PURPOSE

The aim of the present study was to investigate the reproducibility of the exponential method of CO2 rebreathing with the use of automated curve fitting and to determine whether this method is superior to the equilibrium method in terms of reproducibility and clinical practicability.

METHODS

Repeated measurements of cardiac output were performed using the automated equilibrium and exponential methods. These measurements were compared in 12 healthy male subjects at rest and during incremental exercise tests.

RESULTS

Estimated cardiac output was not significantly different between duplicate measurements at rest nor at any level of exercise with either method. At rest the exponential method showed a tendency toward larger variability than the equilibrium method. The exponential method produced significantly higher (P < or = 0.001) estimates at rest (averaging up to 9.8 L x min(-1)) compared with the equilibrium method (averaging up to 6.5 L x min(-1)). Reproducibility improved for both methods with increasing workloads, and a second measurement at rest also seemed more reproducible and valid than the first. During exercise, both methods produced comparable values for cardiac output, and highly significant relations between cardiac output and oxygen uptake were observed for both methods (ranging from r2 = 0.79 to r2 = 0.88, P < or = 0.001). The equilibrium method produced unpleasant side effects more frequently (75% vs 21%, P < or = 0.001) compared with the exponential method and lead more subjects to premature interruption of the exercise test because of the rebreathing maneuver (42% vs 17%, P = 0.058).

CONCLUSIONS

Automated curve fitting for the exponential method gave reproducible and valid results during submaximal and maximal exercise but not at rest. The equilibrium method on the other hand interfered with exercise. Therefore, the equilibrium method is recommended at rest and at lower levels of exercise and the exponential method at higher intensities.

Reliability and validity of measures of cardiac output during incremental to maximal aerobic exercise. Part I: Conventional techniques

The assessment of cardiac function, particularly cardiac output (Q) during heavy exercise is essential for the evaluation of cardiovascular factors that might limit oxygen transport. A series of invasive and noninvasive techniques has been developed for the assessment and monitoring of Q during resting and submaximal exercise conditions. However, very few techniques have been found to give accurate and reliable determinations of Q during vigorous to maximum exercise. For exercise physiologists and sport cardiologists, maximal exercise data are of primary importance. The 'gold standard' measures of cardiac function are considered to be the direct Fick and dye-dilution methods. These have been widely shown to give accurate and reliable determinations of Q during resting and submaximal exercise conditions; however, their use during maximal exercise conditions is debatable due to the inherent risks involved with each and their increasing inaccuracy during the later stages of vigorous exercise. Thermodilution has also been considered to be a relatively good method for the determination of Q during rest and exercise conditions, but recent authors have questioned its use due to the nature of the measure and its inaccuracy during strenuous exercise. Various noninvasive measures of cardiac function have been developed to overcome the problems associated with the 'gold standard' measures. The first part of this article discusses conventional techniques used in exercise physiology settings. The majority of these provide accurate and reliable determinations of Q during rest and submaximal exercise. However, very few techniques are suitable for maximal exercise conditions. Perhaps only the foreign gas rebreathe using acetylene (C2H2) meets all the criteria of being noninvasive, simple to use, reliable over repeated measurements, accurate and useful during maximal exercise.

Non-invasive measurement of cardiac output: whole-body impedance cardiography in simultaneous comparison with thermodilution and direct oxygen Fick methods

OBJECTIVE

To determine the reliability of whole-body impedance cardiography (ICGWB), with electrodes attached to wrists and ankles, in the measurement of cardiac output (CO) on the basis of simultaneous comparison with thermodilution (TD) and direct oxygen Fick (Fick) methods.

DESIGN

Prospective clinical study.

SETTING

A surgical intensive care unit at a university hospital.

PATIENTS

Thirty consecutive subjects undergoing a coronary artery bypass surgery were investigated preoperatively.

MEASUREMENTS

ICGWB derived CO was measured simultaneously with the TD and Fick methods to establish the biases and limits of agreement (LA) between the methods.

RESULTS

The results obtained by ICGWB and the invasive methods showed good agreement. The bias and LA between COTD and COICG were 0.00 l/min: 1.37 and 1.37 l/min, respectively, and were close to those obtained between COTD and COFICK, 0.32 l/min; 1.74 and -1.10 l/min. The bias and LA between the COFICK and COICG were -0.32 l/min; -2.24 and 1.60 l/min respectively. The repeatability value of consecutive single measurements for ICGWB (RVICG = 0.57 l/min) was much better than for the TD method (RVTD = 1.10 l/min).

CONCLUSION

There was close agreement between the results of the three methods in the measurement of CO. In sedated preoperative patients the accuracy of ICGWB is within clinically acceptable limits and its repeatability is excellent. ICGWB provides a useful alternative to the TD and Fick methods in cases where the pressures supplied by the pulmonary artery catheter are not essential.

Noninvasive determination of cardiac output in a model of acute lung injury

OBJECTIVE

To examine the utility of single breath CO2 analysis as a noninvasive measure of cardiac output in a model of acute lung injury.

SETTING

An animal laboratory in a university-affiliated medical center.

DESIGN

A prospective, animal cohort study comparing 21 parameters derived from single breath CO2 analysis with cardiac output determined by an ultrasonic flow probe.

SUBJECTS

Six adult sheep with saline lavage-induced acute lung injury.

INTERVENTIONS

Animals were treated with repetitive saline lavage to achieve a uniform degree of acute lung injury (PaO2 of < 100 torr [< 13.32 kPa] on an FIO2 of 1.0). Cardiac output was manipulated by successive injections of an hydraulic constrictor placed around the inferior vena cava and measured using an ultrasonic flow probe. Twenty-one derived components of the CO2 expirogram were evaluated as predictors of cardiac output.

MEASUREMENTS AND MAIN RESULTS

Thirty-eight measurements of cardiac output were available for comparison with derived variables from the CO2 expirogam. Stepwise linear regression identified four variables for the equation predicting cardiac output: a) PaO2/FIO2 ratio; b) the angle between the slope lines for phases II and III divided by the tidal volume; c) mixed expired CO2 tension; and d) physiologic deadspace to tidal volume ratio. The multivariate equation was highly statistically significant and explained 80% of the variance (adjusted R2 = .80, p < .0001). The blas and precision of the calculated cardiac output were .00 and .38, respectively. The mean percent difference for the cardiac output estimates derived from the single breath CO2 analysis station was -0.01%.

CONCLUSIONS

Our results indicate that changes in cardiac output can be determined using components of the CO2 expirogram with a high degree of reliability in animals with induced acute lung injury. Specifically, the use of four parameters derived from a plot of expired CO2 concentration vs. expired volume predict changes in cardiac output in adult sheep with induced lung injury with an adjusted coefficient of determination of .80. Prospective application of this technology in the clinical setting with the rapidly changing physiology that is characteristic of the acutely ill patient will be essential in determining the clinical usefulness of single breath CO2 analysis as a noninvasive measure of cardiac output.

Partial carbon dioxide rebreathing: a reliable technique for noninvasive measurement of nonshunted pulmonary capillary blood flow

OBJECTIVE

To determine the validity and clinical utility of the partial CO2 rebreathing technique for measurement of nonshunted pulmonary capillary blood flow and cardiac output.

DESIGN

Prospective, controlled animal laboratory investigation and clinical trial.

SETTINGS

Animal research facility and intensive care unit of a university hospital.

SUBJECTS

Fifteen adult sheep, weighting 58 to 78 kg.

PATIENTS

Mechanically ventilated patients with different underlying diseases (n = 12) and with adult respiratory distress syndrome (ARDS) (n = 8).

INTERVENTIONS

CO2 elimination rate (VCO2) was measured breath-by-breath with a system developed for the study and also by gas collection (validation procedure in patients with different underlying diseases). Partial CO2 rebreathing maneuvers, cardiac output by thermodilution, and blood gas analysis were performed in sheep with lung atelectasis and in patients with ARDS.

MEASUREMENTS AND MAIN RESULTS

The degree of correlation between VCO2 measured with the system developed and gas collection was very good (r2 = .95, p < .0001), and bias and precision calculations (1 +/- 9 mL/min) showed close agreement between methods. The overall degree of correlation between partial CO2 rebreathing measurements and cardiac output was moderate (r2 = .54, p < .0001), the noninvasive method tending to underestimate cardiac output, as shown by bias and precision calculations (-1.69 +/- 1.90 L/min). In contrast, the overall degree of correlation between partial CO2 rebreathing measurements and nonshunted pulmonary capillary blood flow was good (r2 = .73, p < .0001). Bias and precision calculations (0.25 +/- 0.83 L/min) showed a tendency for the partial CO2 rebreathing technique to slightly overestimate pulmonary capillary blood flow. Variance differences between partial CO2 rebreathing measurements and cardiac output could be mostly explained by intrapulmonary right-to-left shunt fraction (r2 = .51, p < .0001).

CONCLUSIONS

Our results support the use of the system developed for breath-by-breath VCO2 measurements. The lack of agreement between partial CO2 rebreathing measurements and cardiac output was mostly explained by intrapulmonary right-to-left shunt, suggesting that this technique may not be appropriate for monitoring cardiac output in patients with increased venous admixture. In contrast, our results demonstrate that the partial CO2 rebreathing technique is reliable for measurement of the effective nonshunted pulmonary capillary blood flow. This technique may prove useful to guide ventilatory therapy adjustments in an attempt to optimize nonshunted pulmonary capillary blood flow.

Measurement of cardiac output in standing horses by Doppler echocardiography and thermodilution

Measurement of cardiac output by Doppler echocardiography were compared to simultaneous measurements by thermodilution in 9 conscious horses. In the Doppler technique, mean blood flow velocities for estimation of cardiac output were recorded from the aorta and pulmonary artery. The flow area of each vessel was calculated from the vessel diameter, measured from a 2-dimensional ultrasound image. Differences in the site and method of measuring the vessel diameter altered the estimation of cardiac output by the Doppler method. Cardiac output was modified by the i.v. infusion of 4 micrograms/kg bwt/min dopamine and 4 micrograms/kg bwt/min dobutamine and by the i.v. administration of 10 micrograms/kg bwt detomidine and 20 micrograms/kg bwt butorphanol. Doppler measurements of cardiac output correlated closely with measurement by thermodilution. Measurements from the aortic outflow correlated more closely with thermodilution, than those from the pulmonary artery (r = 0.89 and r = 0.77, respectively). Doppler measurements when the mean flow velocity was recorded from the aorta and the flow area was measured from the ascending aorta using the leading edge method. There was no significant bias between the 2 techniques when Doppler flow velocities were recorded by this method and the limits of agreement were narrow (+/- 12.26 l/min). The differences between the 2 methods increased with increasing cardiac output. Doppler echocardiography is a safe noninvasive method of measuring cardiac output in horses. The agreement between Doppler echocardiography and thermodilution in this study is similar to that reported in man and is similar to that reported between thermodilution and other techniques in man.

Noninvasive determination of cardiac output using single breath CO2 analysis

OBJECTIVE

To examine the utility of single breath CO2 analysis as a noninvasive measure of cardiac output.

SETTING

An animal laboratory in a university-affiliated medical center.

DESIGN

A prospective, animal cohort study comparing 21 parameters derived from single breath CO2 analysis with cardiac output determined by an ultrasonic flow probe.

SUBJECTS

Six healthy adult sheep.

METHODS

The single breath CO2 analysis station consists of a mainstream capnometer, a variable orifice pneumotachometer, a signal processor, and computer software with capability for both on- and off-line data analysis. Twenty-one derived components of the CO2 expirogram were evaluated as predictors of cardiac output. Cardiac output was manipulated by successive injections of a hydraulic constrictor placed around the inferior vena cava.

MEASUREMENTS AND MAIN RESULTS

Thirty-four measurements of cardiac output were available for comparison with derived variables from the CO2 expirogram. Stepwise linear regression identified two variables that were most predictive of cardiac output: a) the angle between the slope lines for phase II and III of the CO2 expirogram divided by the volume of CO2 per breath (angle/mL CO2); and b) the slope of phase II. The multivariate equation was highly statistically significant and explained 94% of the variance (adjusted r2 = .94, p < .0001). The bias and precision of the calculated cardiac output were .00 and .23, respectively. The mean percent difference for the cardiac output estimate derived from the single breath CO2 analysis station was 0.36%.

CONCLUSIONS

Our data indicate that analysis of the CO2 expirogram can yield accurate information about the cardiovascular system. Specifically, two variables derived from a plot of expired CO2 concentration vs. expired volume predict changes in cardiac output in healthy adult sheep with an adjusted coefficient of determination of .94. Prospective application of this technology in the setting of lung injury and rapidly changing physiology will be essential in determining the clinical usefulness of the technique.

Comparison of cardiac output measurement techniques: thermodilution, Doppler, CO2-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.

Haemodynamic and hormonal response to a stream of cooled air

Many patients with angina note that their symptoms deteriorate in cold weather, although the precise physiological mechanism that explains this remains unclear. Exposure of the face to cool winds may be a contributory factor. The cardiovascular and hormonal response to a localised stream of room (22 degrees C) and cold (4 degrees C) air during submaximal treadmill exercise was therefore studied in nine normal subjects. Cardiac output and respiratory gases were measured with a mass spectrometer, using the indirect Fick principle. Blood samples were taken for plasma noradrenaline. A localised stream of air at 5 m.s-1 produced significant cardiovascular effects at rest, some of which persisted during exercise. In response to cold air, stroke volume, cardiac output, blood pressure and oxygen uptake increased (all P < 0.05). There was a trend towards a reduction in heart rate at rest and increase in plasma noradrenaline. Room air caused a reduction in blood pressure (P = 0.01) but stroke volumes and oxygen uptake were unchanged. The results of this study demonstrate significant cardiovascular effects of a cooled air facial stimulus at rest and during exercise. They may, in part, explain the effects of cold winds on patients with angina.

Felodipine, metoprolol and their combination compared with placebo in isolated systolic hypertension in the elderly

This study compared with placebo the efficacy and tolerability of optimised doses of felodipine 5-20 mg daily, metoprolol 50-200 mg daily and their combination in subjects 60 years or over with isolated systolic hypertension. The study employed a randomised double-blind crossover design with allocation of treatment order within subjects by Latin squares. For each subject, after a single-blind run-in placebo phase, there were four randomised treatment phases each of six weeks duration, with a dose titration step at three weeks if necessary. Twenty-eight subjects entered the randomised phases of the study and twenty-one completed all four phases--13 male, 8 female (ages: median 71, range 59-85 years). At the end of both the felodipine and metoprolol phases systolic and diastolic pressure were reduced at 2 hours postdose compared with the placebo phase (p < 0.001), the blood pressure reduction with felodipine (-40/-20 mmHg) being greater than that with metoprolol (-15/-9 mmHg) (p < 0.01). Immediately predose (12 hours postdose) there was a persisting reduction of supine systolic blood pressure (-17 mmHg) with felodipine (p < 0.001), but there was no significant effect of metoprolol. At both measurement times the two drugs when in combination had an additive effect on blood pressure. There was a 20% increase in reported symptoms during each of the active treatment phases. Four subjects withdrew during the randomised phases because of probable drug-related adverse events and six subjects required dosage reductions during the felodipine or combination phases.(ABSTRACT TRUNCATED AT 250 WORDS)

Improved ventilatory response to exercise after cardioversion of chronic atrial fibrillation to sinus rhythm

The purpose of this study was to assess hemodynamic and respiratory measures of submaximal and maximal exercise performance in patients with chronic atrial fibrillation, before and one month after cardioversion to sinus rhythm. Restoration of sinus rhythm (n = 16) produced significant reductions in resting and exercise heart rates, 14 percent to 20 percent (p < 0.01). Due to a proportionately larger increase in stroke volume, cardiac output increased by 9 percent during low-level exercise (p < 0.01) and by 7 percent during exercise above the anaerobic threshold (p < 0.05). Minute ventilation was reduced by 7 percent during low-level exercise (p < 0.01) and by 9 percent above the anaerobic threshold (p < 0.05). The ratio between minute ventilation and carbon dioxide elimination was significantly reduced (p < 0.01). Maximum oxygen uptake (+8 percent; p < 0.01) and maximal tolerated work load (+6 percent; p < 0.05) increased. Hemodynamic changes during exercise were similar in patients with (n = 7) or without (n = 9) disopyramide prophylaxis. Restoration of sinus rhythm induced improvement in hemodynamics and in efficiency of ventilation, thereby reducing the ventilatory demand during submaximal exercise.

Validity of cardiac output measurement by computer-averaged impedance cardiography, and comparison with simultaneous thermodilution determinations

The accuracy and reproducibility of noninvasive cardiac output determinations by computer-averaged impedance cardiography were compared with those of simultaneously performed thermodilution cardiac output. In all, 43 patients (14 men and 29 women = 201 pairs) were studied by simultaneously performed impedance and thermal determinations. Individual impedance values correlated with paired thermodilution determinations (r = 0.75; p less than 0.0001). Each patient's average thermodilution values correlated with the average impedance values (r = 0.86; p less than 0.0001). Mean thermodilution output was 4.6 +/- 1.37 liters/min. Mean impedance output was 4.5 +/- 1.27 liter/min Reproducibility was comparable for impedance (0.0059 +/- 0.639) and thermodilution cardiac output (0.023 +/- 0.556). There was high agreement between methods by plot of the difference against mean of the 2 methods. Impedance cardiac output values agree and correlate highly with quality-controlled thermodilution outputs across a wide range of clinical conditions and hemodynamic values.

Determination of cardiac output by an angle and diameter independent dual beam Doppler technique in critically ill infants

OBJECTIVE

To compare cardiac output measurements in critically ill infants by the dual beam Doppler and thermodilution techniques.

DESIGN

Prospective direct comparison of the two techniques. For statistical evaluation one randomly assigned paired measurement of every patient was used.

SETTING

Paediatric intensive care unit in a university hospital.

PATIENTS

18 infants after open heart surgery aged 4-25 months (weight 4-10 kg).

INTERVENTIONS

Cardiac output measurements by dual beam Doppler and thermodilution techniques were performed within 10 minutes of each other and without knowledge of the results of the other methods. Multiple measurements were performed on some patients with a pharmacological or electrophysiological intervention or with a minimum of six hours between each pair of measurements.

MEASUREMENTS AND MAIN RESULTS

Three patients were excluded because of an inadequate Doppler signal or a significant residual shunt. Cardiac output measurements ranged from 0.4 to 2.2 l/min for the thermodilution technique and from 0.5 to 2.1 l/min for the dual beam Doppler technique. Agreement between both methods was acceptable. The mean difference between the two methods was 0.026 l/min with two standard deviations ranging from -0.20 to 0.26 l/min.

CONCLUSION

The dual beam Doppler technique was shown to have promise for the non-invasive determination of cardiac output in critically ill infants.