Thermodilution Assessment of Cardiac Index in Patients With Tricuspid Regurgitation

Abstract

Background. Disparities between thermodilution (TD) and Fick measurements of cardiac index (CI) are common in real-world clinical practice. Published studies about the effect of tricuspid regurgitation (TR) on TD are small and describe conflicting results. We tested the correlation between TD and Fick across a wide range of TR severity, in a larger group of patients undergoing right heart catheterization (RHC). We aimed to determine if TD is an acceptable alternative to Fick in patients with TR in clinical practice. Methods. We retrospectively evaluated patients undergoing RHC at a single center over a 10-month period, and included those with recent (<90 days) echocardiograms. TD was measured during RHC and Fick was calculated using estimated oxygen consumption. The primary outcome was the correlation between TD and Fick CIs. We performed regression modeling to evaluate predictors of the difference between TD and Fick. Results. A total of 349 patients were included, 40% of whom had at least moderate TR. The correlation between TD and Fick was strong (r=0.765) and did not significantly differ in those with none to mild TR (r=0.73) and those with moderate to severe TR (r=0.80). Atrial fibrillation or atrial flutter was the only variable significantly associated with the difference between CI by Fick and TD (P=.04). Conclusion. The correlation between TD and Fick was strong and unaffected by TR severity.

J INVASIVE CARDIOL 2023;35(3):E122-E125. Epub 2023 January 6.

Key words: atrial fibrillation, thermodilution

Accurate measurement of cardiac index (CI) is essential to the proper diagnosis, classification, and management of a variety of cardiovascular diseases. During cardiac catheterization, CI can be measured using either thermodilution (TD) or the Fick equation (Fick), but TD is typically a better predictor of outcomes.¹ This is especially true when estimated oxygen consumption is used to calculate the Fick CI, rather than directly measuring it.² TD has no such limitations, but it has been suggested that it is inaccurate in the setting of tricuspid regurgitation (TR). The literature, made up mostly of small studies, shows wide variability. Some studies suggest that CIs measured by TD and Fick are similar when TR is present,^3-7 while others report that CI assessed by TD is falsely low^8-10 or falsely high¹¹ in the setting of TR. The purpose of our study was to test the concordance of TD and Fick across a range of TR severity to more definitively determine whether TR impacts the accuracy of TD in real-world practice.

Methods

We retrospectively evaluated patients who underwent right heart catheterization at our medical center over a 10-month period. At least 3 thermodilution measurements were obtained on each patient and averaged to determine the CI. We used the Dehmer equation to estimate oxygen consumption and calculate the Fick CI.¹² Patients with a transthoracic echocardiogram within 90 days prior to right heart catheterization were included. Echocardiograms with moderate or severe TR were reviewed for accuracy. We excluded patients with end-stage renal disease, shunts, congenital heart defects, mechanical circulatory support, and prior tricuspid valve surgery. Clinical, echocardiographic, and hemodynamic characteristics were also collected. Each patient was included only once. The primary outcome of interest was the correlation between Fick and TD in patients with TR.

Patients were stratified by TR severity according to the American Society for Echocardiography 2017 guidelines.¹³ Continuous variables are presented as mean ± standard deviation and were compared with a 1-way analysis of variance test. Categorical variables were compared using the Chi-square test. We evaluated the correlation between TD and Fick CI by calculating the Pearson correlation coefficient. We performed regression modeling to evaluate predictors of the difference between TD and Fick. All tests of statistical significance were 2-sided, and P<.05 was considered significant. Data were analyzed using SPSS, version 26 (IBM, Inc). This study was approved by the institutional review board at Virginia Commonwealth University. Informed consent was not required.

Results

We included 349 patients, with an average age of 58 years and 46% female. Clinical, echocardiographic, and hemodynamic data are presented in Table 1, stratified by TR severity. Moderate or greater TR was present in 40% of patients. Patients with moderate or severe TR had more atrial arrhythmias and heart failure, as well as lower weight, sodium, and estimated glomerular filtration rate. They also had lower left ventricular ejection fraction, lower CIs, and higher right and left heart filling pressures.

Overall, the correlation between CI by TD and Fick was strong (r=0.765), with an average difference of 0.39 ± 0.36 L/min/m². This correlation was similar in patients with none to mild TR (r=0.73) and those with moderate to severe TR (r=0.80). The absolute difference between the Fick and TD CIs was not significantly different between groups with varying degrees of TR (P=.15). In a regression analysis, the presence of atrial fibrillation or atrial flutter was the only clinical or hemodynamic variable significantly associated with the difference between CI by Fick and TD (P=.04).

Discussion

We evaluated the correlation between TD and Fick measurement of CI in patients with varying TR severity undergoing right heart catheterization. The correlation between TD and Fick was strong and unaffected by TR severity. As expected, patients with more severe TR had lower CIs and clinical characteristics consistent with more advanced heart failure.

Previous studies in human and animal models evaluating the effects of TR on measurements of cardiac output have yielded conflicting results. Two small prospective studies, each with approximately 30 patients, demonstrated that more severe TR, measured by TR jet length,  was associated with underestimation of cardiac output by TD.^8,10 Several other prospective studies, some of which were performed including pulmonary hypertension patients, showed no significant difference between the TD and Fick methods.^4,6,7 In a cohort of pulmonary hypertension patients, TD produced significantly higher cardiac output measurements than the indirect Fick method.¹¹ Animal studies in canine hearts with surgically created TR have also revealed conflicting results.^5,9 In the largest study on this topic, a retrospective examination of 198 pulmonary hypertension patients, the authors found a significant discrepancy between TD and Fick that was independent of TR severity.³ Our study is the largest to date on this topic and adds to the growing body of evidence that TD is reliable in patients with TR in real-world clinical practice. Given the limitations of the Fick equation, especially when oxygen consumption is estimated, our findings should give physicians confidence when using TD for decision making in this population.

Tricuspid regurgitation is increasingly recognized as independently associated with increased morbidity and mortality.^14,15 With new treatment options for tricuspid disease being developed, the ability to reliably evaluate the hemodynamics of these patients will only gain importance.^16,17 Our study suggests that TD can be a reliable tool when doing so.

Study limitations. Our study has limitations, including its retrospective design at a single institution. The severity of TR documented on transthoracic echocardiogram could have changed prior to right heart catheterization with changes in volume status or initiation of guideline directed medical therapy for heart failure.

Conclusion

The correlation between TD and Fick measurements of CI was strong and unaffected by TR severity in patients undergoing right heart catheterization. 

Affiliations and Disclosures

From ¹Section of Cardiology, Virginia Tech Carilion School of Medicine, Roanoke, Virginia; and ²Division of Cardiology, ³Department of Internal Medicine, Virginia Commonwealth University Health System, Richmond, Virginia.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The authors report no conflicts of interest regarding the content herein.

Manuscript accepted October 13, 2022.

Address for correspondence: Zachary Gertz, MD, Department of Internal Medicine, Division of Cardiology, Pauley Heart Center, Virginia Commonwealth University, 1200 East Broad Street, Box 980036, Richmond, VA 23298. Email: zachary.gertz@vcuhealth.org

References

1. Opotowsky AR, Hess E, Maron BA, et al. Thermodilution vs estimated Fick cardiac output measurement in clinical practice: an analysis of mortality from the Veterans Affairs Clinical Assessment, Reporting, and Tracking (VA CART) program and Vanderbilt University. JAMA Cardiol. 2017;2(10):1090-1099. doi:10.1001/jamacardio.2017.2945

2. Gertz ZM, McCauley BD, Raina A, et al. Estimation of oxygen consumption in elderly patients with aortic stenosis. Catheter Cardiovasc Interv. 2014;83(1):E128-E133. Epub 2013 Jul 2. doi:10.1002/ccd.25018

3. Fares WH, Blanchard SK, Stouffer GA, et al. Thermodilution and Fick cardiac outputs differ: impact on pulmonary hypertension evaluation. Can Respir J. 2012;19(4):261-266. doi:10.1155/2012/261793

4. Hoeper MM, Maier R, Tongers J, et al. Determination of cardiac output by the Fick method, thermodilution, and acetylene rebreathing in pulmonary hypertension. Am J Respir Crit Care Med. 1999;160(2):535-541. doi:10.1164/ajrccm.160.2.9811062

5. Kashtan HI, Maitland A, Salerno TA, Lichtenstein SV, Byrick RJ. Effects of tricuspid regurgitation on thermodilution cardiac output: studies in an animal model. Can J Anaesth. 1987;34(3 Pt 1):246-251. doi:10.1007/BF03015161

6. Konishi T, Nakamura Y, Morii I, Himura Y, Kumada T, Kawai C. Comparison of thermodilution and Fick methods for measurement of cardiac output in tricuspid regurgitation. Am J Cardiol. 1992;70(4):538-539. doi:10.1016/0002-9149(92)91205-i

7. Hamilton MA, Stevenson LW, Woo M, Child JS, Tillisch JH. Effect of tricuspid regurgitation on the reliability of the thermodilution cardiac output technique in congestive heart failure. Am J Cardiol. 1989;64(14):945-948. doi:10.1016/0002-9149(89)90851-5

8. Balik M, Pachl J, Hendl J. Effect of the degree of tricuspid regurgitation on cardiac output measurements by thermodilution. Intensive Care Med. 2002;28(8):1117-1121. Epub 2002 Jun 15. doi:10.1007/s00134-002-1352-0

9. Boerboom LE, Kinney TE, Olinger GN, Hoffmann RG. Validity of cardiac output measurement by the thermodilution method in the presence of acute tricuspid regurgitation. J Thorac Cardiovasc Surg. 1993;106(4):636-642.

10. Cigarroa RG, Lange RA, Williams RH, Bedotto JB, Hillis LD. Underestimation of cardiac output by thermodilution in patients with tricuspid regurgitation. Am J Med. 1989;86(4):417-420. doi:10.1016/0002-9343(89)90339-2

11. Alkhodair A, Tsang MYC, Cairns JA, et al. Comparison of thermodilution and indirect Fick cardiac outputs in pulmonary hypertension. Int J Cardiol. 2018;258:228-231. Epub 2018 Feb 2. doi:10.1016/j.ijcard.2018.01.076

12. Dehmer GJ, Firth BG, Hillis LD. Oxygen consumption in adult patients during cardiac catheterization. Clin Cardiol. 1982;5(8):436-440. doi:10.1002/clc.4960050803

13. Zoghbi W, Adams D, Bonow R, et al. Recommendations for noninvasive evaluation of native valvular regurgitation: a report from the American Society of Echocardiography developed in collaboration with the Society for Cardiovascular Magnetic Resonance. J Am Soc Echocardiogr. 2017;30(4):303-371. Epub 2017 Mar 14. doi:10.1016/j.echo.2017.01.007

14. Chorin E, Rozenbaum Z, Topilsky Y, et al. Tricuspid regurgitation and long-term clinical outcomes. Eur Heart J Cardiovasc Imaging. 2020;21(2):157-165. doi:10.1093/ehjci/jez216

15. Topilsky Y, Maltais S, Medina Inojosa J, et al. Burden of tricuspid regurgitation in patients diagnosed in the community setting. JACC Cardiovasc Imaging. 2019;12(3):433-442. Epub 2018 Aug 15. doi:10.1016/j.jcmg.2018.06.014

16. Asmarats L, Taramasso M, Rodés-Cabau J. Tricuspid valve disease: diagnosis, prognosis and management of a rapidly evolving field. Nat Rev Cardiol. 2019;16(9):538-554. doi:10.1038/s41569-019-0186-1

17. Lurz P, Stephan von Bardeleben R, Weber M, et al. Transcatheter edge-to-edge repair for treatment of tricuspid regurgitation. J Am Coll Cardiol. 2021;77(3):229-239. doi:10.1016/j.jacc.2020.11.038