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

Abstract

Importance: Thermodilution (Td) and estimated oxygen uptake Fick (eFick) methods are widely used to measure cardiac output (CO). They are often used interchangeably to make critical clinical decisions, yet few studies have compared these approaches as applied in medical practice.

Objectives: To assess agreement between Td and eFick CO and to compare how well these methods predict mortality.

Design, setting, and participants: This investigation was a retrospective cohort study with up to 1 year of follow-up. The study used data from the Veterans Affairs Clinical Assessment, Reporting, and Tracking (VA CART) program. The findings were corroborated in a cohort of patients cared for at Vanderbilt University, an academic referral center. Participants were more than 15 000 adults who underwent right heart catheterization, including 12 232 in the Veterans Affairs cohort between October 1, 2007, and September 30, 2013, and 3391 in the Vanderbilt cohort between January 1, 1998, and December 31, 2014.

Exposures: A single cardiac catheterization was performed on each patient with CO estimated by both Td and eFick methods. Cardiac output was indexed to body surface area (cardiac index [CI]) for all analyses.

Main outcomes and measures: All-cause mortality over 90 days and 1 year after catheterization.

Results: Among 12 232 VA patients (mean [SD] age, 66.4 [9.9] years; 3.3% female) who underwent right heart catheterization in this cohort study, Td and eFick CI estimates correlated modestly (r = 0.65). There was minimal mean difference (eFick minus Td = -0.02 L/min/m2, or -0.4%) but wide 95% limits of agreement between methods (-1.3 to 1.3 L/min/m2, or -50.1% to 49.4%). Estimates differed by greater than 20% for 38.1% of patients. Low Td CI (<2.2 L/min/m2 compared with normal CI of 2.2-4.0 L/min/m2) more strongly predicted mortality than low eFick CI at 90 days (Td hazard ratio [HR], 1.71; 95% CI, 1.47-1.99; χ2 = 49.5 vs eFick HR, 1.42; 95% CI, 1.22-1.64; χ2 = 20.7) and 1 year (Td HR, 1.53; 95% CI, 1.39-1.69; χ2 = 71.5 vs eFick HR, 1.35; 1.22-1.49; χ2 = 35.2). Patients with a normal CI by both methods had 12.3% 1-year mortality. There was no significant additional risk for patients with a normal Td CI but a low eFick CI (12.9%, P = .51), whereas a low Td CI but normal eFick CI was associated with higher mortality (15.4%, P = .001). The results from the Vanderbilt cohort were similar in the context of a more balanced sex distribution (46.6% female).

Conclusions and relevance: There is only modest agreement between Td and eFick CI estimates. Thermodilution CI better predicts mortality and should be favored over eFick in clinical practice.

Figure 1.. Distribution of Differences and Bland-Altman Analysis of Thermodilution (Td) vs Estimated Oxygen Uptake Fick (eFick) Cardiac Index (CI) Estimates in the Veterans Affairs Clinical Assessment, Reporting, and Tracking (VA CART) Cohort

Shown are the proportions of patients with a given absolute difference between Td and eFick estimates (A). Shown is a Bland-Altman plot of the mean of the Td and eFick CI estimates on the x-axis against the difference (eFick minus Td) on the y-axis (B). Shown are the percentage differences between Td and eFick estimates (C).There was no substantial systematic difference between Td and eFick, but the 95% limits of agreement were wide (−1.31 to 1.27 L/min/m², or −50.1% to 49.4%) (D).

Figure 2.. Cumulative Mortality Through 90 Days and 1-Year Follow-up, Classified by Normal and Low Thermodilution (Td) and Estimated Oxygen Uptake Fick (eFick) Cardiac Index Categories

A, Shown are the cumulative proportions of patients who had died by 90 days and 1 year after catheterization, stratified by cardiac index classification in the Veterans Affairs Clinical Assessment, Reporting, and Tracking (VA CART) cohort. All patients who survived had at least 1 year of follow-up. Patients with concordantly low cardiac index estimates by both Td and eFick had the highest mortality risk. Those with either normal cardiac index by both methods or isolated low eFick cardiac index had the lowest risk. The incidence of death was intermediate for patients with low cardiac index by Td but normal eFick cardiac index. B, Shown are the corresponding Vanderbilt cohort results. The findings are qualitatively equivalent (eg, isolated low Td was associated with worse prognosis than isolated low eFick). However, in this cohort, there was marginally increased risk of death for those with isolated low eFick cardiac index relative to normal cardiac index by both methods.

Figure 3.. Cumulative Mortality Through 1-Year Follow-up, Stratified by Estimated Oxygen Uptake Fick (eFick) and Thermodilution (Td) Cardiac Index (CI)

Patients with low CI (<2.2 L/min/m²) in the VA cohort had a higher incidence of death than those with normal CI, whether estimated by eFick (A) or Td (B). However, the relative hazard for mortality and model fit was better for Td CI. C and D, Equivalent plots are presented for the Vanderbilt cohort, stratifying CI by lowest tertile vs second and third tertiles. The solid and dotted line in each plot represents the cumulative mortality for those with low and normal CI, respectively. Gray area reflects the 95% confidence interval.