Thermodilution cardiac output. Cold vs room temperature injectate and the importance of measuring the injectate temperature in the right atrium

Pressure–flow responses to exercise in aortic stenosis, mitral regurgitation and diastolic dysfunction

Background

Haemodynamic exercise testing is important for evaluating patients with dyspnoea on exertion and preserved ejection fraction. Despite very different pathologies, patients with pressure (aortic stenosis (AS)) and volume (mitral regurgitation (MR)) overload and diastolic dysfunction after recent acute myocardial infarction (AMI) reach similar filling pressure levels with exercise. The pressure–flow relationships (the association between change in cardiac output (∆CO) and change in pulmonary arterial wedge pressure (∆PAWP) may provide insight into haemodynamic adaptation to exercise in these groups.

Methods and results

One hundred sixty-eight subjects aged >50 years with a left ventricular ejection fraction of ≥50% underwent invasive exercise testing. They were enrolled in four different studies: AS (40 patients), AMI (52 patients), MR (43 patients) and 33 healthy subjects. Haemodynamic data were measured at rest, at 25 W, 75 W and at peak exercise. In all groups, PAWP increased with exercise. The greatest increase was observed in patients with AMI (from 12.7±3.9 mm Hg to 33.1±8.2 mm Hg, p<0.0001) and patients with AS (from 11.8±3.9 mm Hg to 31.4±6.1 mm Hg, p<0.0001), and the smallest was observed in healthy subjects (from 8.3±2.4 mm Hg to 21.1±7.5 mm Hg, p<0.0001). In all groups, the relative pressure increase was greatest at the beginning of the exercise. CO increased most in healthy patients (from 5.3±1.1 to 16.0±3.0 L/min, p<0.0001) and least in patients with AS (from 5.3±1.2 L/min to 12.4±2.6 L/min, p<0.0001). The pressure–flow relationships (∆PAWP/∆CO) and differed among groups (p=0.02). In all groups, the pressure–flow relationship was steepest in the initial phase of the exercise test. The AMI and AS groups (2.3±1.2 mm Hg/L/min and 3.0±1.3 mm Hg/L/min, AMI and AS, respectively) had the largest overall pressure–flow relationship; the healthy group had the smallest initially and at peak exercise (1.3±1.1 mm Hg/L/min) followed by MR group (1.9±1.4 mm Hg/L/min).

Conclusion

The pressure–flow relationship was steepest in the initial phase of the exercise test in all groups. The pressure–flow relationship differs between groups.

Trial registration numbers

NCT01974557, NCT01046838, NCT02961647 and NCT02395107.

Noninvasive Assessment of Cardiac Output: Accuracy and Precision of the Closed-Circuit Acetylene Rebreathing Technique for Cardiac Output Measurement

Background

Accurate assessment of cardiac output is critical to the diagnosis and management of various cardiac disease states; however, clinical standards of direct Fick and thermodilution are invasive. Noninvasive alternatives, such as closed‐circuit acetylene (C₂H₂) rebreathing, warrant validation.

Methods and Results

We analyzed 10 clinical studies and all available cardiopulmonary stress tests performed in our laboratory that included a rebreathing method and direct Fick or thermodilution. Studies included healthy individuals and patients with clinical disease. Simultaneous cardiac output measurements were obtained under normovolemic, hypovolemic, and hypervolemic conditions, along with submaximal and maximal exercise. A total of 3198 measurements in 519 patients were analyzed (mean age, 59 years; 48% women). The C₂H₂ method was more precise than thermodilution in healthy individuals with half the typical error (TE; 0.34 L/min [r=0.92] and coefficient of variation, 7.2%) versus thermodilution (TE=0.67 [r=0.70] and coefficient of variation, 13.2%). In healthy individuals during supine rest and upright exercise, C₂H₂ correlated well with thermodilution (supine: r=0.84, TE=1.02; exercise: r=0.82, TE=2.36). In patients with clinical disease during supine rest, C₂H₂ correlated with thermodilution (r=0.85, TE=1.43). C₂H₂ was similar to thermodilution and nitrous oxide (N₂O) rebreathing technique compared with Fick in healthy adults (C₂H₂ rest: r=0.85, TE=0.84; C₂H₂ exercise: r=0.87, TE=2.39; thermodilution rest: r=0.72, TE=1.11; thermodilution exercise: r=0.73, TE=2.87; N₂O rest: r=0.82, TE=0.94; N₂O exercise: r=0.84, TE=2.18). The accuracy of the C₂H₂ and N₂O methods was excellent (r=0.99, TE=0.58).

Conclusions

The C₂H₂ rebreathing method is more precise than, and as accurate as, the thermodilution method in a variety of patients, with accuracy similar to an N₂O rebreathing method approved by the US Food and Drug Administration.

The Precision of Pulmonary Artery Catheter Bolus Thermodilution Cardiac Output Measurements Varies With the Clinical Situation

OBJECTIVE

To investigate the effects of ventilatory mode, injectate temperature, and clinical situation on the precision of cardiac output measurements.

DESIGN

Randomized, prospective observational study.

SETTING

Single university hospital.

PARTICIPANTS

Forty patients undergoing planned cardiac surgery, receiving a pulmonary artery catheter according to institutional routine.

INTERVENTIONS

Cardiac output was measured at 4 predefined time points during the perioperative patient course, twice during controlled and twice during spontaneous ventilation, using 2 blocks of 8 measurement replications with cold and tepid injectate in random order.

MEASUREMENTS AND MAIN RESULTS

The data were analyzed using a hierarchical linear mixed model. Clinical precision was determined as half the width of the 95% confidence interval for the underlying true value. The single-measurement precision measured in 2 different clinical situations for each temperature/ventilation combination was 8% to 10%, 11% to 13%, 13% to 15%, and 23% to 24% in controlled ventilation with cold injectate, controlled ventilation with tepid injectate, spontaneous breathing with cold injectate, and spontaneous breathing with tepid injectate, respectively. Tables are provided for the number of replications needed to achieve a certain precision and for how to identify significant changes in cardiac output.

CONCLUSIONS

Clinical precision of cardiac output measurements is reduced significantly during spontaneous relative to controlled ventilation. The differences in precision between repeated measurement series within the temperature/ventilation combinations indicate influence of other situation-specific factors not related to ventilatory mode. Compared with tepid injectate in patients breathing spontaneously, the precision is 3-fold better with cold injectate and controlled ventilation.

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.

Iced vs room-temperature injectates for cardiac index measurement during hypothermia and normothermia

BACKGROUND

Few data are available on the accuracy of thermodilution measurements of cardiac index with room-temperature injectates and a closed delivery system in patients with low cardiac indexes and/or hypothermic body temperatures.

OBJECTIVE

To compare iced and room-temperature injectates for thermodilution measurement of cardiac index in postoperative cardiac surgical patients during hypothermia and normothermia.

METHODS

In a convenience sample of cardiac surgical patients in a cardiac recovery unit, cardiac index was measured with both room-temperature and iced injectates during hypothermia (< or =36.0 degrees C) and normothermia (> or =36.1 degrees C and < or =38.0 degrees C). Device bias and precision were calculated and graphed by using the Bland-Altman method. A Student t test was used to determine differences between cardiac indexes by injectate temperature.

RESULTS

A total of 38 patients were studied. Mean bias and precision for room-temperature and iced injectates in all patients were 0.11 (SD, 0.27) during hypothermia and -0.03 (SD, 0.21) during normothermia. In hypothermic patients, cardiac index differed significantly between room-temperature and iced injectates (t(1,37) = 2.41, P = .02). Cardiac index measurements did not differ between room-temperature and iced injectates in normothermic patients (P = .33).

CONCLUSIONS

Although significant differences in thermodilution cardiac index were found between room-temperature and iced injectates during hypothermic body temperatures, these differences were small (mean, <0.11). These findings add to the results of the few studies on accuracy of room-temperature injectates for thermodilution measurement of cardiac index.

Precision of bolus thermodilution cardiac output measurements in patients with atrial fibrillation

BACKGROUND

The precision of bolus thermodilution cardiac output measurements in patients with atrial fibrillation (AF) has not previously been determined. A priori we suspected that the precision would be lower in patients with AF than in patients with sinus rhythm (SR). Consequently, we also determined if the precision could be improved by injecting the thermal indicator into the right ventricle instead of the right atrium.

METHODS

Cardiac output was determined as the average result of four injections of 10 ml of iced saline. Replicate measurements were performed with thermal indicator injections into the right atrium and ventricle. The coefficients of variation and the precisions were calculated.

RESULTS

In the 25 patients with AF, mean cardiac output was 3.96 l min(-1) (range 2.4-7.4), the coefficient of variation 0.073 (95% CI +/- 0.011), and the precision 0.38 l min(-1) (95% CI +/- 0.14) with injection into the right atrium. In the 25 patients with SR, mean cardiac output was 4.73 l min(-1) (range 2.4-7.3), the coefficient of variation 0.047(95% CI +/- 0.006), and the precision 0.38 l min(-1) (95% CI +/- 0.14). In both groups, an agreement analysis demonstrated that the injection of indicator into the right ventricle resulted in a significantly higher cardiac output [AF+0.25 (95% CI +/- 0.15) l min(-1), SR+0.29 ( +/- 0.20) l min(-1)].

CONCLUSION

The coefficient of variation for cardiac output determinations is 55% higher in patients with AF. Two measurements, separated by time or intervention, must differ by 15% in AF patients and 9% in SR patients before one can be 95% confident that a real change has taken place.

Thermodilution cardiac output--are three injections enough?

BACKGROUND

Bolus thermodilution cardiac output measurements have been a mainstay in clinical monitoring of critically ill patients for more than 30 years. Usually the results of an arbitrarily chosen number (1-6) of thermal indicator injections are averaged to increase the reliability of the measurement. The number of injections needed to achieve a given level of precision has, however, not previously been systematically investigated.

METHODS AND RESULTS

In 80 hemodynamically stable patients cardiac output was determined as the average of eight injections of 10 ml of iced saline. From the 638 measurements we examined the relationship between the number of thermal indicator injections and the precision of the resulting cardiac output estimate. Furthermore, the association between the number of injections and the least detectable difference among two sets of measurements was established.

CONCLUSION

The current study shows that one needs to average the results of four injections to be 95% confident that the result is within 5% of the 'true' cardiac output and that two series of four measurements have to differ by at least 7% before one can be sure (95%) that a change in cardiac function has taken place.