Thermodilution cardiac output--a systematic error

Effect of thermodilution injectate volume and temperature on the accuracy and precision of cardiac output measurements for healthy anesthetized horses

OBJECTIVE

To compare the accuracy and precision of cardiac output (CO) measurements derived from 4 thermodilution protocols that used different injectate temperatures and volumes in healthy adult horses.

ANIMALS

8 healthy adult horses.

PROCEDURES

Horses were anesthetized and instrumented with Swan-Ganz catheters. The CO was derived from each of 4 thermodilution protocols (IV injection of physiologic saline [0.9% NaCl] solution chilled to < 5 °C at volumes of 1 mL/15 kg of body weight [protocol A; control], 1 mL/25 kg [protocol B], and 1 mL/35 kg [protocol C] or maintained at 17 °C at a volume of 1 mL/15 kg [protocol D]) 3 times during each of 5 measurement cycles, with a 30-minute interval between cycles. During each measurement cycle, protocol A was performed first, and protocols B, C, and D were performed in a randomized order. Mean CO and within-subject variance in CO were compared among the 4 protocols.

RESULTS

Mean CO did not differ significantly among the 4 protocols. The within-subject variance for CO measurements derived from protocols C and D, but not protocol B, was significantly greater than that for protocol A (control).

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that, in healthy adult horses, decreasing the thermodilution injectate volume to 1 mL/25 kg from the recommended volume of 1 mL/15 kg did not adversely affect the accuracy or precision of CO measurements. However, use of smaller injectate volumes or use of injectate at approximately room temperature is not recommended owing to a clinically unacceptable increase in CO measurement variability.

In Vivo Validation of the M-COVX® Metabolic Monitor in Patients under Anaesthesia

A practical method of breath-by-breath monitoring of metabolic gas exchange has been developed by GE Healthcare/Datex Ohmeda and incorporated into existing anaesthetic and critical care monitoring systems (M-COVX®). This device relates flow measurements made at the mouth by pneumotachograph to measurements of inspired and expired gas composition by matching the two waveforms thereby allowing continuous, breath-by-breath monitoring of an intubated patient's oxygen uptake and carbon dioxide production. Given that there is a paucity of data comparing this new device against methods more widely used clinically, we tested the device on 11 patients undergoing cardiopulmonary bypass surgery. Using a standard anaesthetic machine (Datex Ohmeda Excel 210 SE) with a semi-closed circle absorber system, oxygen uptake was measured at the mouth continuously throughout the operation at approximately six-second intervals. The data were compared against the reverse Fick method and against standard indirect calorimetry using the Haldane transformation. When compared to the calculated reverse Fick oxygen uptake, a mean difference of +16.5% was found pre-bypass and +9.9% post-bypass, consistent with uptake of oxygen by lung tissue, which is not taken into account by the reverse Fick method. Measurements made comparing the M-COVX metabolic monitor against standard Haldane showed a mean difference of +5.1% pre-bypass and –2.1% post-bypass. Given the ease with which this device can be incorporated into existing anaesthetic monitoring systems and its accuracy in measuring oxygen uptake, the M-COVX module is an attractive addition to existing perioperative monitoring.

Diastolic Function and Peripheral Venous Pressure as Indices for Fluid Responsiveness in Cardiac Surgical Patients

OBJECTIVE

Identifying fluid responsiveness is critical to optimizing perfusion while preventing fluid overload. An experimental study of hypovolemic shock resuscitation showed the importance of ventricular compliance and peripheral venous pressure (PVP) on fluid responsiveness. The authors tested the hypothesis that reduced ventricular compliance measured using transesophageal echocardiography results in decreased fluid responsiveness after a fluid bolus.

DESIGN

Prospective observational study.

SETTING

Two-center, university hospital study.

PARTICIPANTS

The study comprised 29 patients undergoing elective coronary revascularization.

INTERVENTION

Albumin 5%, 7 mL/kg, was infused over 10 minutes to characterize fluid responders (>15% increase in stroke volume) from nonresponders.

MEASUREMENTS AND MAIN RESULTS

Invasive hemodynamics and the ratio of mitral inflow velocity (E-wave)/annular relaxation (e'), or E/e' ratio, were measured using transesophageal echocardiography to assess left ventricular (LV) compliance at baseline and after albumin infusion. Fifteen patients were classified as responders and 14 as nonresponders. The E/e' ratio in responders was 7.4 ± 1.9 at baseline and 7.1 ± 1.8 after bolus. In contrast, E/e' was significantly higher in nonresponders at baseline (10.7 ± 4.6; p = 0.04) and further increased after bolus (12.6 ± 5.5; p = 0.002). PVP was significantly greater in the nonresponders at baseline (14 ± 4 mmHg v 11 ± 3 mmHg; p = 0.02) and increased in both groups after albumin infusion. Fluid responsiveness was tested using the area under the receiver operating characteristic curve and was 0.74 for the E/e' ratio (95% confidence interval 0.55-0.93; p = 0.029) and 0.72 for the PVP (95% confidence interval 0.52-0.92; p = 0.058).

CONCLUSION

Fluid responders had normal LV compliance and lower PVP at baseline. In contrast, nonresponders had reduced LV compliance, which worsened after fluid bolus. E/e,' more than PVP, may be a useful clinical index to predict fluid responsiveness.

Comparison of cardiac output measured by use of computed tomography and thermodilution in dogs

OBJECTIVE To compare cardiac output (CO) measured by use of CT coronary angiography and thermodilution (criterion-referenced standard) at various CO values, record adverse effects, and determine the time needed to measure CO. ANIMALS 5 healthy purpose-bred Beagles (2 males and 3 females). PROCEDURES A prospective nonrandomized crossover study was conducted. Dogs were premedicated with butorphanol tartrate (0.2 mg•kg^-1, IM). Anesthesia was induced by IV administration of etomidate (1 to 2 mg•kg^-1) and midazolam (0.25 mg•kg^-1). Orotracheal intubation was performed, and anesthesia was maintained by administration of isoflurane. The CO was determined by use of thermodilution and by use of CT at 3 CO values. Dobutamine was infused at various rates to obtain the 3 CO values. RESULTS 13 values were obtained and analyzed. The mean ± SD difference between methods was 0.09 ± 0.71 L•min^-1 (95% confidence interval [CI], 0.52 to -0.34 L•min^-1). Only 1 of 13 values was located on the 100% agreement line (ie, 0 line), 7 of 13 values were located within the 95% CI, and 5 of 13 values were outside the 95% CI. CONCLUSIONS AND CLINICAL RELEVANCE For this study, there was poor agreement between the 2 methods. The 95% CI interval was 0.52 to -0.34 L•min^-1, and 5 of 13 values were outside the 95% CI. Therefore, results for the CT method appeared to be inappropriate for use in making clinical decisions.

Determination of the precision error of the pulmonary artery thermodilution catheter using an in vitro continuous flow test rig

BACKGROUND

Thermodilution cardiac output using a pulmonary artery catheter is the reference method against which all new methods of cardiac output measurement are judged. However, thermodilution lacks precision and has a quoted precision error of ± 20%. There is uncertainty about its true precision and this causes difficulty when validating new cardiac output technology. Our aim in this investigation was to determine the current precision error of thermodilution measurements.

METHODS

A test rig through which water circulated at different constant rates with ports to insert catheters into a flow chamber was assembled. Flow rate was measured by an externally placed transonic flowprobe and meter. The meter was calibrated by timed filling of a cylinder. Arrow and Edwards 7Fr thermodilution catheters, connected to a Siemens SC9000 cardiac output monitor, were tested. Thermodilution readings were made by injecting 5 mL of ice-cold water. Precision error was divided into random and systematic components, which were determined separately. Between-readings (random) variability was determined for each catheter by taking sets of 10 readings at different flow rates. Coefficient of variation (CV) was calculated for each set and averaged. Between-catheter systems (systematic) variability was derived by plotting calibration lines for sets of catheters. Slopes were used to estimate the systematic component. Performances of 3 cardiac output monitors were compared: Siemens SC9000, Siemens Sirecust 1261, and Philips MP50.

RESULTS

Five Arrow and 5 Edwards catheters were tested using the Siemens SC9000 monitor. Flow rates between 0.7 and 7.0 L/min were studied. The CV (random error) for Arrow was 5.4% and for Edwards was 4.8%. The random precision error was ± 10.0% (95% confidence limits). CV (systematic error) was 5.8% and 6.0%, respectively. The systematic precision error was ± 11.6%. The total precision error of a single thermodilution reading was ± 15.3% and ± 13.0% for triplicate readings. Precision error increased by 45% when using the Sirecust monitor and 100% when using the Philips monitor.

CONCLUSION

In vitro testing of pulmonary artery catheters enabled us to measure both the random and systematic error components of thermodilution cardiac output measurement, and thus calculate the precision error. Using the Siemens monitor, we established a precision error of ± 15.3% for single and ± 13.0% for triplicate reading, which was similar to the previous estimate of ± 20%. However, this precision error was significantly worsened by using the Sirecust and Philips monitors. Clinicians should recognize that the precision error of thermodilution cardiac output is dependent on the selection of catheter and monitor model.

Simultaneous determination of the accuracy and precision of closed-circuit cardiac output rebreathing techniques

Foreign and soluble gas rebreathing methods are attractive for determining cardiac output (Q(c)) because they incur less risk than traditional invasive methods such as direct Fick and thermodilution. We compared simultaneously obtained Q(c) measurements during rest and exercise to assess the accuracy and precision of several rebreathing methods. Q(c) measurements were obtained during rest (supine and standing) and stationary cycling (submaximal and maximal) in 13 men and 1 woman (age: 24 +/- 7 yr; height: 178 +/- 5 cm; weight: 78 +/- 13 kg; Vo(2max): 45.1 +/- 9.4 ml.kg(-1).min(-1); mean +/- SD) using one-N(2)O, four-C(2)H(2), one-CO(2) (single-step) rebreathing technique, and two criterion methods (direct Fick and thermodilution). CO(2) rebreathing overestimated Q(c) compared with the criterion methods (supine: 8.1 +/- 2.0 vs. 6.4 +/- 1.6 and 7.2 +/- 1.2 l/min, respectively; maximal exercise: 27.0 +/- 6.0 vs. 24.0 +/- 3.9 and 23.3 +/- 3.8 l/min). C(2)H(2) and N(2)O rebreathing techniques tended to underestimate Q(c) (range: 6.6-7.3 l/min for supine rest; range: 16.0-19.1 l/min for maximal exercise). Bartlett's test indicated variance heterogeneity among the methods (P < 0.05), where CO(2) rebreathing consistently demonstrated larger variance. At rest, most means from the noninvasive techniques were +/-10% of direct Fick and thermodilution. During exercise, all methods fell outside the +/-10% range, except for CO(2) rebreathing. Thus the CO(2) rebreathing method was accurate over a wider range (rest through maximal exercise), but was less precise. We conclude that foreign gas rebreathing can provide reasonable Q(c) estimates with fewer repeat trials during resting conditions. During exercise, these methods remain precise but tend to underestimate Q(c). Single-step CO(2) rebreathing may be successfully employed over a wider range but with more measurements needed to overcome the larger variability.

The level of cardiac output affects the relationship and agreement between pulmonary artery and transpulmonary aortic thermodilution measurements in an animal model

OBJECTIVE

The authors investigated the relationship between pulmonary artery and transpulmonary aortic thermodilution cardiac output measurements under conditions of increasing cardiac output (CO).

DESIGN

Animal study with repeated simultaneous measurements comparing 2 cardiac output measurement techniques.

SETTING

Experimental animal facility of a university hospital.

PARTICIPANTS

Ten female pigs.

INTERVENTIONS

In anesthetized pigs, an aortic thermistor catheter and a pulmonary artery catheter (PAC) were inserted. Then dobutamine was infused under continuous cardiac output (CCO) monitoring to target different levels of CO. After each L/min increase of CCO simultaneous aortic and PAC thermodilution, CO measurements were performed by using a bolus injection of cooled normal saline and the amount of thermal indicator loss (TL) was calculated.

MEASUREMENTS AND MAIN RESULTS

Pooled analysis of CO data with the method of Bland and Altman showed that aortic thermodilution CO was higher than PAC thermodilution CO with a bias of 3.8% +/- 11.1%. The range of TL was 30.4% to -10.1%. Differential analysis according to the range of CO revealed that, in each animal under conditions of low CO, aortic thermodilution CO was higher than PAC thermodilution CO, whereas results were inverse under conditions of high CO.

CONCLUSIONS

The authors concluded that the amount of CO differentially affects the relationship between aortic and PAC thermodilution CO. TL and recirculation may be the explanation for this finding.

Non-invasive metabolic monitoring of patients under anaesthesia by continuous indirect calorimetry—an in vivo trial of a new method

BACKGROUND

Oxygen uptake is an important form of metabolic monitoring for patients under anaesthesia. In critically ill patients oxygen uptake has been shown to provide valuable clinical information in directed therapy and acts as a useful monitor of cardiovascular dysfunction. A new method of continuous real time monitoring of metabolic gas exchange was tested in patients during anaesthesia.

METHODS

Using a standard anaesthetic machine with attached semi-closed circle absorber system, oxygen uptake was measured continuously throughout surgery in 30 patients undergoing cardiopulmonary bypass surgery and compared with paired measurements made with the reverse Fick method. The method is an indirect calorimetry technique which uses fresh gas rotameters for control, regulation and measurement of the gas flows into the system, with continuous sampling of mixed exhaust gas.

RESULTS

When compared with the reverse Fick method the oxygen uptake showed a mean difference (and sd) of 20.7 ml min(-1) or 12.1% (25.3 ml min(-1)) pre-bypass and 13.9 ml min(-1) or 8.1% (27.0 ml min(-1)) post-bypass. This bias is consistent with previous studies comparing oxygen uptake measured at the mouth against oxygen uptake by reverse Fick, which have shown a difference of approximately 10-15% accounted for by the consumption of oxygen by lung tissue.

CONCLUSIONS

As the method allows continuous measurement of gas exchange and can be adapted to a modern anaesthetic workstation it is an attractive method for use in clinical setting.

Continuous measurement of cardiac output by inert gas throughflow: comparison with thermodilution

OBJECTIVE

The throughflow method is a new technique for continuous and minimally invasive measurement of cardiac output by the Fick principle, which uses ventilation of the 2 lungs with unequal inspired gas concentrations by means of a double-lumen endobronchial tube. It exploits steady-state gas exchange and thus permits rapid repetition of measurement.

DESIGN

Comparison of paired measurements by the throughflow method using N(2)O exchange with bolus thermodilution.

SETTING

Departments of anesthesiology in 2 university teaching hospitals.

PARTICIPANTS

Nine patients undergoing cardiac surgery in the precardiopulmonary bypass period.

INTERVENTIONS

Patients intubated with a double-lumen endobronchial tube were ventilated with 45% nitrous oxide (N(2)O) to the left lung (zero to the right lung). Arterial blood gas samples were taken to measure alveolar deadspace to allow correction for the alveolar-arterial N(2)O difference and to correct for the presence of unmeasured shunt perfusion.

MEASUREMENTS AND MAIN RESULTS

Throughflow measurements correlated with thermodilution (r = 0.719, p < 0.05) with a mean bias of -0.208 L/min (-5.2%). The standard error of the bias was 0.060 L/min, with 95% confidence limits for the bias of -0.088 L/min and -0.328 L/min. The limits of agreement between the 2 methods were +0.960 L/min and -1.376 L/min.

CONCLUSIONS

The throughflow method showed good agreement with thermodilution. It permits continuous cardiac output measurement without the need for sampling of mixed venous blood, using techniques of lung isolation, which are readily available in clinical anesthetic practice.

Oxygen transport and acid-base balance during exercise in the tammar wallaby

Rates of oxygen consumption (V(O)2), body temperatures and pulmonary blood temperatures, blood gases and blood pH were measured for seven 4.9+/-0.8 (SE) kg tammar wallabies (Macropus eugenii) during rest and during treadmill hopping. For animals resting on the treadmill V(O)2 averaged 0.030+/-0.003 L min(-1). During hopping V(O)2 increased linearly with speed up to 2.5 m sec(-1). Above 2.5 m sec(-1) V(O)2 was independent of hopping speed and averaged 0.340+/-0.004 L min(-1). At rest, rectal temperatures and pulmonary blood temperatures averaged 36 degrees C. During treadmill hopping, rectal temperatures and pulmonary blood temperatures increased similarly, to 39 degrees C. The Pv(CO)2 increased and pHv decreased in proportion to the increased V(O)2. The Pa(CO)2 and pHa were not significantly changed from values for animals resting on the treadmill. Cardiac output (Vb) averaged 0.97+/-0.04 L min(-1) when the wallabies were at rest on the treadmill and increased linearly with treadmill speeds up to 2.5 m sec(-1). Above 2.5 m sec(-1) Vb was independent of hopping speed and averaged 2.9+/-0.04 L min(-1). When data for all speeds were combined, Vb increased linearly with V(O)2. Thus, in spite of their unique mode of locomotion wallabies have maintained relationships between pulmonary ventilation and V(O)2 and between Vb and V(O)2 that are similar to those reported for eutherian mammals.

Improved accuracy and precision of thermodilution cardiac output measurement using a dual thermistor catheter system

OBJECTIVES

To assess whether thermodilution cardiac output determination based on measurement of injectate temperature in vivo leads to more accurate and precise estimates and to study the influence of chilled injectate on test performance.

BACKGROUND

Cardiac output measurement via right heart catheterization is used extensively for hemodynamic evaluation in a variety of diagnostic, perioperative and critical care settings. Maximizing accuracy is essential for optimal patient care.

METHODS

This prospective study of 960 thermodilution cardiac output measurements was conducted using conventional and dual thermistor techniques. Specialized dual thermistor right heart catheters were constructed using a second thermistor positioned to measure injectate temperature in vivo just prior to entry into the right atrium. To eliminate interinjection variability, a custom set-up was developed that permitted output measurement using both techniques simultaneously. Both ambient temperature injections and cooled injections were investigated.

RESULTS

The dual thermistor technique demonstrated significantly less measurement variability than the conventional technique for both ambient temperature (precision = 0.41 vs. 0.55 L/min, p < 0.001) and cooled (precision = 0.35 vs. 0.43 L/min, p = 0.01) injections. Similarly, the average range of cardiac output values obtained during five sequential injections in each patient was less using the dual thermistor approach (1.05 vs. 1.55 L/min, p < 0.001). The use of cooled injectate reduced the mean error of the dual thermistor technique but actually increased the mean error of the conventional technique. Even with ambient temperature injections, injectate warming during catheter transit varied considerably and unpredictably from injection to injection (2 SD range = -0.22 to 5.74 degrees C). Conventional ambient temperature and cooled measurements significantly overestimated Fick cardiac output measurements by 0.32 and 0.50 L/min, respectively (p < 0.001). In contrast, dual thermistor measurements were statistically similar (-0.08 and -0.08 L/min, p = 0.34) to Fick measurements.

CONCLUSIONS

This new dual thermistor approach results in a significant improvement in both precision and accuracy of thermodilution cardiac output measurement.

Effect of injectate temperature and thermistor position on reproducibility of thermodilution cardiac output determinations

STUDY OBJECTIVE

The purpose of this study was to evaluate the effect of thermistor position with varying injectate temperatures on the reproducibility of thermodilution cardiac output determination. The key hypothesis to be tested was that the positioning of the proximal thermistor at the right atrial port would improve the reproducibility of thermodilution cardiac output determination, independent of injectate temperature.

DESIGN

Prospective randomized trial.

SETTING

The study was performed in the cardiac catheterization laboratory of the West Haven Veterans Affairs Medical Center.

PARTICIPANTS

Twenty consecutive patients undergoing right and left heart catheterizations were enrolled in the study.

INTERVENTIONS

Each patient underwent triplicate determination of thermodilution cardiac output measurements under four experimental conditions: (1) ambient or room temperature injectate using an external thermistor in the injectate reservoir; (2) iced injectate using an external thermistor; (3) room temperature injectate using an internal right atrium (RA) thermistor; and (4) iced injectate using an RA thermistor. Reproducibility was assessed by the coefficient of variation (CV) and standard error of the mean percent (SEM%), of the triplicate measurements.

MEASUREMENTS AND RESULTS

Using an internal RA thermistor improved the reproducibility of cardiac output determinations independent of injectate temperature. Using room temperature injectate, the CV was 12.8 percent using an external thermistor and 7.9 percent using an internal RA thermistor (p < 0.05). Using iced injectate, the CV was 10.2 percent using an external thermistor and 5.5 percent using an internal RA thermistor (p < 0.05).

CONCLUSIONS

Reproducibility of thermodilution cardiac output determinations is improved when injectate temperature is measured internally, at the RA, as opposed to externally in the reservoir. This has clinical implications for determining significant changes in serial cardiac output determinations.

Continuous Fick cardiac output measurement during exercise by monitoring of mixed venous oxygen saturation and oxygen uptake

To determine whether continuous Fick cardiac output measurement is applicable to exercise testing, cardiac output data obtained by the continuous Fick method (Qcf) during exercise were compared with data obtained by the thermodilution method (Qth). Seventeen patients with old myocardial infarction underwent a 1-min or 3-min incremental exercise test (protocols 1 and 2, respectively). During exercise, the oxygen consumption (VO2), arterial oxygen saturation (SaO2), and mixed venous oxygen saturation (SvO2) were monitored continuously. Qcf was calculated at 12-s intervals by the Fick equation. The SaO2 remained almost constant during exercise. The SvO2 showed four characteristic phases during exercise protocol 1. SvO2 values changed rapidly in phases 2 and 4, but only slightly during phase 3. In exercise protocol 2, SvO2 almost reached a steady-state by the end of each stage. The correlation between Qcf and Qth was good in protocol 1 (r = 0.86), except in phases 2 and 4, and was also good in protocol 2 (r = 0.80). We conclude that the continuous Fick method may be applicable for determining the cardiac output during exercise provided that the variation in SvO2 is slight.

Errors in the measurement of cardiac output by thermodilution

Cardiac output (CO) determination by thermodilution, which was introduced by Fegler in 1954, has gained wide acceptance in clinical medicine and animal experiments because it has several advantages over other methods with respect to simplicity, accuracy, reproducibility, repeated measurements at short intervals, and because there is no need for blood withdrawal. However, errors in determination of CO by thermodilution may be introduced by technical factors and the patients' pathological conditions. The current review summarizes these issues and provides our recommendations, based on the medical literature published between 1954-1992. To obtain more reproducible and accurate CO values by thermodilution, one should make several determinations (1) by using 10 ml injectate at room temperature for adults and 0.15 ml.kg-1 injectate for infants and children; (2) at evenly spaced intervals of the ventilation cycle; (3) when rapid intravenous fluid administration is discontinued; (4) by observing thermodilution curves so that baseline pulmonary artery temperature drift or the existence of intra- and extracardiac shunts are noticed. Finally, CO determination by thermodilution may be unreliable or impossible in patients with low CO states and tricuspid or pulmonary regurgitation. Since non-invasive CO monitoring has not replaced CO determination by thermodilution, intimate knowledge of this method is crucial for anaesthetists to prevent errors in the management of patients.

Frequently repeated Fick cardiac output measurements during anesthesia

A computer-based system was developed for monitoring cardiac output using the Fick principle during general anesthesia. The variables of the oxygen-consumption Fick equation were measured using the following system: oxygen uptake by an originally developed respiratory gas monitoring system, arteriovenous oxygen saturation difference by pulse and fiberoptic oximetry, and hemoglobin concentration by an in vitro oximeter. Fick cardiac output and systemic vascular resistance were calculated every 30 seconds. Fick cardiac output was compared with thermodilution cardiac output in 11 anesthetized patients. A total of 208 corresponding cardiac output measurements showed a range of 2 to 9 L.min-1. The correlation coefficient between the thermodilution and Fick cardiac outputs was 0.961, with a regression equation of Fick cardiac output = 1.058 thermodilution cardiac output - 0.359. The difference between the thermodilution and Fick cardiac outputs was 0.103 +/- 0.395. The Fick cardiac output was significantly lower than the thermodilution cardiac output, especially in the low flow range. We demonstrated that this new monitoring system was clinically feasible and sufficiently accurate, under the limited circumstances of our study. The integration of routinely used equipment has made possible a frequently repeatable method for estimating cardiac output in patients.

Thermodilution cardiac output measurements during conventional and high-frequency ventilation

The thermodilution method for cardiac output determinations correlates well with Fick and dye dilution methods. Experimental work with thermodilution techniques has shown that individual measurements of right heart cardiac output during conventional ventilation vary throughout the respiratory cycle. The aims of this study were to compare thermodilution cardiac output determinations made at a fixed point (zero end-expiratory pressure [ZEEP]) with those made randomly throughout the respiratory cycle during conventional controlled positive pressure ventilation (CPPV) and high-frequency jet ventilation (HFJV) with up to 10 cm H2O positive endexpiratory pressure (PEEP). There were no statistically significant differences between the cardiac output determinations made at ZEEP and randomly in the ventilation cycle in any group and all correlations were significant. The clinical implications of these results are discussed, and it is concluded that it is not necessary to time the measurements of thermodilution cardiac output determinations during CPPV or HFJV with up to 10 cm H2O of PEEP.

Investigation of a continuous heating/cooling technique for cardiac output measurement

Cardiac output is frequently measured to assess patient hemodynamic status in the operating room and intensive care unit. Current research for measuring cardiac output includes continuous sinusoidal heating and synchronous detection of thermal signals. This technique is limited by maximum heating element temperatures and background thermal noise. A continuous heating and cooling technique was investigated in vitro to determine if greater thermal signal magnitudes could be obtained. A fast responding thermistor was employed to measure consecutive ejected temperature plateaus in the thermal signal. A flow bath and mechanical ventricle were used to simulate the cardiovascular system. A thermoelectric module was used to apply heating and cooling energy to the flow stream. Trials encompassing a range of input power, input frequency, and flow rate were conducted. By alternating heating and cooling, thermal signal magnitude can be increased when compared to continuous heating alone. However, the increase was not sufficient to allow for recording in all patients over the expected normal range of cardiac output. Consecutive ejected temperature plateaus were also measured on the thermal signal and ejection fraction calculations were made.

Effects of tricuspid regurgitation on thermodilution cardiac output: studies in an animal model

The thermal dilution technique (TD) of measuring cardiac output (Q) has been assumed to be inaccurate when the tricuspid valve is regurgitant (TR). The indicator transit time from the right atrium to pulmonary artery (PA) is prolonged and may increase indicator loss to the heart and adjacent tissue. We surgically created TR in four anaesthetized dogs and compared simultaneous cardiac output measurement made with an implanted PA flow probe with intermittent TD cardiac output measurements. We found an excellent correlation (r = 0.98) between the techniques in the normal heart when Q was increased by 100-150 per cent of control values using an intravenous dobutamine infusion. After TR was produced by incorporating the tricuspid valve leaflet within a suture, Q decreased. Dextran 40 was then infused and Q increased significantly. The rate at which Q increased was greater with the TD technique than with the PA flow probe; however, a significant (r = 0.85) linear relationship was still present with TR. The altered thermal waveform detected by the PA thermistor in TR was characterized by a lower peak amplitude and a slower return to baseline. Within the limitations of an animal model, our data suggest that TD cardiac output may be more accurate than previously assumed, particularly in low output states.

Cardiac output measurements. A review of current techniques and research

Cardiac output is the volume of blood ejected by the heart per unit time. It is a useful measurement in that it can be used to evaluate overall cardiac status in both critically ill patients and patients with suspected cardiovascular disease. An ideal cardiac output measurement system would have automated continuous output capability, be minimally invasive, accurate, fast, small, low cost and clinically adaptable. This paper presents a theoretical and practical description of the variety of clinical techniques in use today and lists their advantages and shortcomings with respect to the ideal system. Included are the Fick method, indicator dilution techniques, velocity measurements and transthoracic impedance and combined Doppler ultrasound as noninvasive techniques. In addition, several experimental methods are described along with their desirable features and possible constraints. These include intravascular heating/recording, thermistor tracking of cardiac output, ejection fraction measurements and magnetic susceptibility plethysmography.

Method of assessing the reproducibility of blood flow measurement: factors influencing the performance of thermodilution cardiac output computers

Measurements of blood flow by three different makes of thermodilution cardiac output computer in an artificial circulation were analysed by linear regression against absolute flow measured by timed blood volume collection. For each computer the horizontal distance between the 95% confidence limits for a single prediction was calculated at a standard flow rate of 5 litres per minute. This measurement represents the range of flow rates that could give rise to an identical measurement and provides a summary of the reproducibility of the computer's results and its ability to detect a change of flow rate. This measurement was used to evaluate the effect on each computer's performance of pulsatile or continuous flow, injectate volume, and injectate temperature. With continuous flow the optimum results were 1.8, 0.85, and 0.85 litres per minute and with pulsatile flow they were 1.3, 1.05, and 1.65 litres per minute. There was generally a deterioration in performance when pulsatile flow was evaluated. Under the conditions of the experiment optimum performance in both flow modes was obtained with 5 ml of ice cold injectate, but these findings cannot necessarily be extrapolated to the clinical situation. With pulsatile flow the overall range of blood flows that could give rise to identical measurements were for each computer 2.0, 1.5, and 3.1 litres per minute, corresponding to 40, 30, and 62% changes of the standard flow rate of 5 litres per minute.

A comparison of methods of cardiac output measurement

Cardiac output measurements determined by dye dilution, iced-injectate thermodilution and room temperature thermodilution were compared in man in order to assess the random error of each method and to examine the systematic error of both thermodilution methods in comparison with dye dilution. Results showed that random error was greatest with room temperature thermodilution and least using iced thermodilution. Iced thermodilution correlated well with dye dilution, tending to overestimate cardiac output only at low flows. Room temperature thermodilution, however, overestimated cardiac output by up to 25% in the clinically important range and more so at low cardiac output.

An evaluation of thermodilution cardiac output measurement using the Swan-Ganz catheter

Errors in thermodilution cardiac output measurement were quantitated to determine the order of accuracy of routine measurements performed by unskilled personnel. In vitro and in vivo studies were undertaken to examine factors affecting the volume and temperature of the injectate, catheter thermistor and computer performance, effect of respiration, use of cold (0-4 degrees C) versus ambient temperature (20-25 degrees C) injectate, and the interpretation of measurements. Ambient temperature injectate incurred unacceptably large errors; cold injectate (injections were timed with respiration) produced variations in performance by equipment and personnel which accounted for only 2% of the variation between successive measurements. Real changes in cardiac output and inherent variability of the downslope of the thermal curve, necessitating an empirically based calculation, account for up to 10% variation between successive measurements. When cold injectate was used, and the average of three corrected measurements taken, thermodilution cardiac output measurements were within 10% of a simultaneous dye dilution measurement.