Measurement of cardiac output by thermal dilution

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.

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.

Cardiovascular response to acute hypoxia in double-muscled calves

Hypoxic-induced pulmonary hypertension is known to be intensive in the bovine species and sometimes leads to pathological cardiac repercussions. On the other hand, doubled-muscled cattle are predisposed to develop hypoxaemia during exercise and with respiratory diseases. Therefore the purpose of this study was to investigate the cardiovascular response to acute hypoxia in double-muscled calves compared with calves of standard conformation. Pulmonary arterial pressure, electrocardiogram and blood temperature were simultaneously recorded, arterial blood was sampled for blood gas analysis and cardiac output was determined in six Friesian calves and six double-muscled calves of the Belgian White and Blue breed(BWB) when breathing air (fractional inspiratory oxygen concentration [FIO2]: 21 per cent) and when breathing a hypoxic gas mixture (FIO2: 10 per cent). All the absolute values of the measured parameters were significantly (P less than or equal to 0.001) different between the two breeds, except heart rate and arterial blood gas values. The pattern of hypoxic-induced decrease in arterial PO2 was similar in the two breeds of calves, suggesting that the pulmonary exchange capacities during hypoxia are no less efficient in double-muscled calves than in calves of standard conformation. Similarly, the percentage of variation of the mean pulmonary arterial pressure from its normoxic to its hypoxic value was the same in the two breeds of calves, suggesting that double-muscled calves are not predisposed to develop a more precocious or more intense pulmonary hypertension for a given level of hypoxaemia. The significantly smaller normoxic and hypoxic cardiac index and stroke index found in BWB compared with Friesian calves was interpreted as a less efficient cardiac function in double-muscled subjects.

Continuous determination of cardiac output using a flow-directed Doppler pulmonary artery catheter

A newly developed, flow-directed, Doppler pulmonary artery catheter that uses multiple ultrasonic transducers to measure instantaneous and continuous cardiac output was evaluated in 20 patients undergoing cardiac and vascular surgical procedures. Cardiac output was determined using the product of the average velocity and the area of the main pulmonary artery. Pulmonary artery area was obtained from measurements of diameter via ultrasound transit time, and average velocity of blood flow was determined from the Doppler shift frequency. Two hundred thirty-eight simultaneous Doppler catheter and thermodilution cardiac output measurements were obtained preoperatively, intraoperatively, and during postoperative recovery. Catheter indwelling time varied from 18 through 94 hours (mean +/- SD, 40 +/- 19 hours) with 2 to 26 (mean +/- SD, 12 +/- 6) sets of triplicate cardiac output measurements obtained per patient. Doppler catheter cardiac output correlated well with thermodilution (r = 0.76, slope or m = 0.87, and SEE = 0.05 with P = 0.0001) and mean predictive error (bias) appeared clinically insignificant (bias +/- SD, -0.13 +/- 0.79 L/min). Accurate, continuous monitoring of instantaneous and mean cardiac output appears possible with use of this Doppler pulmonary artery catheter system.

Validation of the thermodilution technique for the estimation of the cardiac output in the unsedated calf

The purpose of this study was to establish a standardized protocol in order to obtain accurate and reproducible cardiac output (CO) measurements by the thermodilution technique in the unsedated calf. In 5 healthy calves, the effect of various temperatures, volumes, rates, means and sites of injection of the thermal indicator were tested for their effect on accuracy and reproducibility of CO measurement by the thermodilution technique, the Fick method being used as the standard technique. Five milliliters of an iced 5% dextrose solution per 100 kg body weight constituted the correct amount of thermal indicator. Manual injection of the indicator bolus and/or injection of this bolus through the proximal lumen of a thermodilution Swan-Ganz catheter gave poor results of CO measurements by the thermodilution technique. On the other hand, results of CO measurements were highly reliable when the bolus injection was performed by an ECG-synchronized injection pump and/or through a right atrial catheter with a larger diameter and a shorter length than the classical Swan-Ganz catheter. It was concluded that an iced 5% solution of dextrose (5 ml/100 kg body weight) injected through a right atrial catheter by an ECG-synchronized pump gives the most accurate and reproducible CO measurements by the thermodilution technique in unsedated calves.

Instantaneous and continuous cardiac output in humans obtained with a Doppler pulmonary artery catheter

A new Doppler pulmonary artery catheter was used to measure instantaneous and continuous cardiac output in both an in vitro model and in 44 patients undergoing cardiac catheterization. Cardiac output was calculated with use of the Doppler catheter-determined instantaneous space-average velocity and the ultrasonically determined instantaneous vessel area. Doppler flow and thermodilution were compared with electromagnetic flow in the in vitro model and with Fick cardiac output in patients. Doppler catheter-determined flow was highly predictive of electro-magnetic flow in the pulsatile flow model (r = 0.99, slope [m] = 1.01 and SEE = 0.05) and appeared comparable to thermodilution measurements (r = 1.00, m = 1.03 and SEE = 0.02). In patients undergoing cardiac catheterization, Doppler catheter-determined cardiac output appeared to modestly underestimate Fick cardiac output (r = 0.82, m = 0.80 and SEE = 0.09; mean error +/- SEM = -0.26 +/- 0.14 liters/min). However, predictive accuracy was comparable to simultaneously obtained thermodilution measurements (r = 0.85, m = 1.07 and SEE = 0.10; mean error +/- SEM = 0.61 +/- 0.16 liters/min). This new Doppler catheter system utilizes multiple ultrasound transducers to provide angle-independent measurements of vessel diameter and instantaneous velocity within the main pulmonary artery, resulting in a more accurate assessment of Doppler-derived cardiac output. In addition, useful information concerning hemodynamic variables such as peak flow, acceleration, deceleration, stroke work and pulmonary impedance may be derived.

Comparison of iced and room temperature injectate for thermodilution cardiac output

Cardiac output estimation by thermodilution is carried out using room temperature or iced injectate, but the accuracy and variability of the two methods is not well documented. Room temperature and iced injectate were compared in 21 patients undergoing diagnostic cardiac catheterization. Dextrose injectate (10 ml) was administered in prefilled syringes left to stand either in iced water or in room air. Four injections were made sequentially with room temperature and iced injectate. Cardiac output by room temperature and iced injectate were not significantly different (4.70 +/- 1.22 for room temperature and 4.90 +/- 1.37 for iced injectate, n = 21, P = 0.155). There was a significant difference in the variance of the estimations by the two methods (room temperature = 0.296, iced = 0.120, P less than 0.005). From this variance the calculated number of measurements needed to estimate cardiac output to +/- 0.5 L/min with 95% confidence is seven for room temperature and four for iced injectate. For five patients with cardiac output less than 4.00 L/min with room temperature injectate, cardiac output with iced injectate was significantly higher (3.33 +/- 0.34 for room temperature vs. 3.69 +/- 0.49 for iced injectate, P = 0.05). Thus room temperature injectate generally gives a satisfactory cardiac output estimation but with significantly greater variability than iced injectate. Sample size for accurate cardiac output estimation must be greater with room temperature injectate. Iced injectate may over-estimate output when cardiac output is low.

Instantaneous and continuous cardiac output obtained with a Doppler pulmonary artery catheter

A newly developed, flow-directed, Doppler pulmonary artery catheter, capable of measuring instantaneous and continuous cardiac output, was evaluated in both an in vitro pump model and an animal model. Quantitative flow was calculated with use of the instantaneous, space-average velocity (obtained from the velocity profile) and the instantaneous area (obtained from the vessel diameter) and compared with electromagnetic flow. Additionally, simultaneous thermodilution flow measurements were obtained. Doppler catheter-determined flow was highly predictive of electromagnetic flow in both continuous and pulsatile pump models (r2 = 0.98, slope or m = 1.04, SEE = 0.44; and r2 = 0.97, m = 1.04 and SEE = 0.33, respectively). Thermodilution was less predictive and appeared to underestimate electromagnetic flow in both the continuous and the pulsatile model (r2 = 0.99, m = 0.91, SEE = 0.20 and r2 = 0.95, m = 0.84 and SEE = 0.34, respectively). In the animal model, Doppler catheter-determined cardiac output appeared to modestly underestimate electromagnetic flow (r2 = 0.80, m = 0.87, SEE = 0.61). However, Doppler determinations of flow remained more accurate than did simultaneous thermodilution measurements (r2 = 0.73, m = 0.79, SEE = 0.72). Accurate, continuous and instantaneous cardiac output measurements appear possible with use of a flow-directed, Doppler pulmonary artery catheter. This catheter system also provides instantaneous diameter measurements and mapping of instantaneous velocity profiles within the main pulmonary artery and may lead to more accurate Doppler-derived assessment of cardiac output in humans.

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.

Instantaneous aortic blood flow measurement with range-gated Doppler flowmeter in anesthetized rat

The availability of a range-gated Doppler flowmeter system enabled us to construct miniaturized probes using piezoelectric crystals that emit an 8 MHz signal and receive the reflected sound waves from passing blood cells. The finished flow probe are 4 mm long and 3 mm in external diameter with lumen diameter appropriate to be placed around the ascending aorta in the rat. The accuracy of the Doppler method in measuring cardiac output in the rat was established by the demonstration of a significant correlation between cardiac output simultaneously measured from ultrasonic (Qd) and thermodilution (Qt) procedures in anesthetized Wistar rats (Qd = 0.55, Qt - 6.67 cm3/min, r = 0.69, p less than 0.001). An average Qd determination is 52.5% +/- 16% of average Qt. From aortic blood pressure and phasic aortic blood velocity, we have compared hemodynamic and cardiovascular functions in 15 spontaneously hypertensive rats and in 15 control Wistar-Kyoto rats under pentobarbital sodium anesthesia. The present study demonstrates that phasic aortic blood flow can be quantified easily and accurately in anesthetized rats by using a range-gated Doppler flowmeter and an implantable perivascular flow probe, and should provide a relatively simply method for investigating hemodynamic characteristics in models of disease such as ventricular hypertrophy and hypertension.

Cardiac output by thermodilution technique. Effect of injectate's volume and temperature on accuracy and reproducibility in the critically Ill patient

We compared determinations of cardiac output using various combinations of injectate volumes and temperatures to results obtained with 10 ml of iced (0 degrees C) injectate (standard technique) in 33 critically ill patients. The use of a 10-ml injectate at room temperature resulted in comparable reproducibility (12.7 vs 10.8 percent; not significant) and a small and nonsignificant error (-0.013 +/- 0.543 L/min). Five milliliters at room temperature resulted in markedly decreased reproducibility (17.9 vs 8.9 percent; p less than 0.05); however, the error associated with the technique was still not significant (0.136 +/- 0.829 L/min). When a 5-ml injectate at 0 degrees C was used, a reproducibility comparable to that of 10 ml at 0 degrees C was obtained (12.3 vs 7.5 percent; not significant). The results underestimated values obtained with 10 ml at 0 degrees C (-0.360 +/- 0.857 L/minute); however, the difference did not reach statistical significance. The use of 3 ml at 0 degrees C was associated with a substantial increase in variability, with a coefficient of variation of 32.0 percent (10.4 percent for 10 ml of iced injectate; p less than 0.01); however, the differences between the average value of cardiac output obtained with this technique and the standard technique were only minimal (error, -0.063 +/- 0.455; not significant). We reached the following conclusions: (1) the use of 10 ml at room temperature and 5 ml at 0 degrees C as the indicator for thermodilution determinations of cardiac output results in small and insignificant differences in reproducibility and accuracy from the standard technique; (2) five milliliters at room temperature and 3 ml at 0 degrees C are associated with markedly decreased reproducibility; however, the error in values for cardiac output obtained with these techniques is not statistically significant; and (3) the ability to use injectate at room temperature and in small volume should substantially simplify the technique, lowering its cost, and should prevent volume overloading.

Saline and thermal dilution flow measurements: evaluation by means of an open hydraulic model

A simple open hydraulic model was constructed with the main objective of making comparative evaluations of flow rate measurements, with saline and thermal dilution. In the range of 0.5 to 3.5 l/min, it was found that saline indicator gave a slight underestimation (in the order of 2%) while the thermal indicator showed a minor overestimation of approximately 1%. These values wee taken from the regression equations F (saline) = 0.945 F (real) + 0.0351 and F (thermo) = 1.020 F (real) - 0.0144, both in l/min. Correlations were higher than 0.99 in the two cases. Considering the overall error range, the thermodilution produced a value approximately twice that of saline (-21 to 25% versus -12 to +11%). However, this is not necessarily extendable to living organisms. The model was found useful for test and calibration of detecting cells and for teaching purposes.

Two-dimensional echocardiographic assessment of left ventricular stroke volume: experimental correlation with thermodilution and cineangiography in normal and ischemic states

Left ventricular stroke volumes derived by two-dimensional echocardiography (2D echo) were compared with thermodilution and cineangiography measurements in closed-chest dogs before andone hour after proximal LAD occlusion. Stroke volume was calculated from end-diastolic and end-systolic volumes reconstructed by two models: 1) Simpson's rule employing left ventricular length and five short-axis cross-sectional areas; 2) a simplified volume formula (V = 5/6 area . length), utilizing a single short-axis area at either the mitral valve or midpapillary muscle level. The comprehensive Simpson reconstruction yielded a good correlation of 2D echo stroke volume against thermodilution (r = 0.89) over a range of normal (N = 14) and ischemic (N = 8) states. The simplified formula provided a satisfactory correlation (r = .90, N = 22) when using the midpapillary cross-section, which encompassed the induced ischemic dys-synergy. In contrast, when using the mitral valve level cross-section above the site of ventricular asymmetry, there was no significant statistical correlation. Comparison of cineangiography with 2D echo volume reconstruction based on the simplified formula with the midpapillary muscle level section yielded good correlations for stroke volume (r = 0.87) and ejection fraction (r = 0.97). Intraobserver and interobserver variability of duplicate echo stroke volume measurements was 8% and 10%, respectively. We conclude that 2D echocardiography in dogs permits quantitation of left ventricular stroke volume in normal and ischemic states.

Determination of cardiac output by thermodilution during hypothermia

The thermodilution method for estimating cardiac output was compared with the electromagnetic flowmeter technique in 10 mongrel dogs at normothermia and during surface-induced deep hypothermia. Thermodilution curves obtained during cooling or rewarming must be corrected for the baseline drift caused by changing core temperature. At normothermia, the correlation coefficient between the two methods was 0.96 and the reproducibility of the thermodilution technique was 5 percent. Comparable correlation was present during hypothermia. Curves corrected for baseline drift resulted in significantly different output values from those derived from uncorrected curves (p < 0.05). The thermodilution method is valid at low body temperatures. Clinical confirmation of these results, particularly during open heart surgery in infants, is warranted.

Determination of catheter wall heat transfer in cardiac output measurement by thermodilution

1. A model system of the thermal dilution technique was used to determine in vitro the loss of thermal indicator across different sizes of polyethylene catheters. 2. Thermal indicator loss (caloric gain by the indicator fluid or caloric loss by the catheter) per unit length of catheters (H/L) increased as a function of the outer radius (OR) of the catheters. Linear regression analysis gave the following equation: H/L=0.337OR+0.073 (r=0.88, P<0.05). 3. Thermal indicator loss per unit surface area (H/SA) decreased almost linearly with the following regression equation: H/SA=-1.860 OR+0.364 (r=0.97, P<0.97, P<0.01). 4. Thermal indicator loss per unit volume contained in the catheter (H/vol) decreased exponentially according to the following equation: H/vol =89.le-25.4(OR) (r=0.97, P<0.01). 5. This study indicates that there is a measurable loss of thermal indicator during injection corresponding to a loss between 9 and 16% (for a 11.5 cm length of PE 50 and PE 240) of the required calories for warming the injectate to 37 degrees C. Data is provided for correction of heat loss across the wall of a given length and diameter of polyethylene tubing.

The applicability of the thermodilution method for determination of pulmonary blood flow and pulmonary vascular resistance in infants and children with ventricular septal defects

The thermodilution method is accepted for measuring cardiac output in adults without shunts. It has been shown to work in animals the same size as newborn infants and has recently been used in children with congenital heart defects. We have proved its clinical applicability in determining the pulmonary blood flow, in twenty-five infants and children with VSD. The pulmonary vascular resistance can be calculated more accurately than by the Fick principle with an assumed O2 consumption, which has often had to be done in earlier studies.

[Determination of cardiac output by means of an automatized thermal dilution technique (author's transl)]

Cardiac output was measured in 55 patients by the direct Fick method and an automatized thermodilution method. The results showed a good correlation (correlation factor r = 0.87, regression equation y = 0.91 x + 0.59). For the thermodilution method ice-cold normal saline solution as indicator substance was injected into the right atrium. The temperature-time-curve was measured by a thermistor located in the pulmonary artery. The area of indicator dilution was determined by electronic integration, the calculated cardiac output appeared as 1/min in digital display.