Determination of Cardiac Output by the Fick Method, Thermodilution, and Acetylene Rebreathing in Pulmonary Hypertension

Abstract

Assessment of cardiac output is an important part of the management of patients with pulmonary hypertension. The accuracy of the thermodilution technique in patients with low cardiac output or severe tricuspid regurgitation has been questioned. To address this issue, we simultaneously compared 105 cardiac output measurements by the Fick method and thermodilution in 35 patients with pulmonary hypertension. Moreover, we evaluated the acetylene rebreathing technique, a noninvasive method of determining cardiac output. The mean difference +/- 95% limit of agreement between thermodilution and the Fick method was +0.01 +/- 1.1 L/min. The mean difference +/- 95% limit of agreement between acetylene rebreathing and the Fick method was -0.23 +/- 1.14 L/min. Neither the mean agreement nor the 95% limits of agreement of both thermodilution and acetylene rebreathing with the Fick method were affected by the presence of low cardiac output or severe tricuspid regurgitation. We conclude that thermodilution and acetylene rebreathing are useful tools for assessing cardiac output in patients with pulmonary hypertension, even in the presence of low cardiac output or severe tricuspid regurgitation.

Noninvasive cardiac output measurements in patients with pulmonary hypertension

“…However, the wide limits of agreement as seen with Bland-Altman analyses led the authors of that study to conclude that TD and Fick should not be considered interchangeable. The findings in our study are similar to those by HOEPER et al [26], in that all three methods of CO correlated with one another and all methods reliably detected a change in CO from drug challenge. However, while all three methods showed acceptable overall agreement, the 95% limits of agreement by Bland-Altman analyses were also sufficiently wide that we too caution against assumption of interchangeability between the methods [30].…”

“…In this study, although the vast majority of patients carried a diagnosis of WHO group 1 PAH, we specifically included patients with PH from all five WHO groups, including those with lung disease, left ventricular diastolic dysfunction and obesity, to ensure the ability to apply NICOM across the entire PH spectrum. Also, similar to the findings of HOEPER et al [26], the presence of severe TR did not seem to influence the CO measurements, although this finding should be interpreted with caution in our study as this subgroup of patients was small (11.6%).…”

“…The lack of precision seen with TD also affects the usefulness of using TD as a reference technique when comparing with other methods of measurement of CO [22]. Some investigators have found TD to be less reliable in the setting of severe TR and/or low CO due to loss of indicator [23,24], although others have not found this to be true [25][26][27]. These conditions are particularly common in patients with PH.…”

“…The indirect Fick method also has limitations that may affect measurement accuracy, most notably the need to estimate V9O 2 , which when estimated erroneously will lead to an error of the same magnitude in the estimation of CO. Other factors that may affect the Fick method include errors in the measurement of oxygen saturations and haemoglobin levels and the variable influence of bronchial and thebesian venous drainage on the mixed venous saturation [26,28,29]. The indirect Fick method, however, is likely to be highly accurate and reliable in assessing percentage changes in CO in response to an acute intervention, since the only value that often changes in response to the intervention is the mixed venous saturation, and thus the other factors that go into the estimation will cancel out when calculating percentage changes in CO. HOEPER et al [26] compared CO measured by TD and by Fick in a cohort of patients with PAH and found that both methods correlated well, and that the presence of TR did not appear to influence the CO as determined by either method.…”

“…The indirect Fick method, however, is likely to be highly accurate and reliable in assessing percentage changes in CO in response to an acute intervention, since the only value that often changes in response to the intervention is the mixed venous saturation, and thus the other factors that go into the estimation will cancel out when calculating percentage changes in CO. HOEPER et al [26] compared CO measured by TD and by Fick in a cohort of patients with PAH and found that both methods correlated well, and that the presence of TR did not appear to influence the CO as determined by either method. However, the wide limits of agreement as seen with Bland-Altman analyses led the authors of that study to conclude that TD and Fick should not be considered interchangeable.…”