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Lack of Agreement Between Thermodilution and Fick Cardiac Output in Critically Ill Patients^*

^* From the Division of Critical Care Medicine (Drs. Dhingra, Fenwick, Chittock, and Ronco), Vancouver Hospital and Health Sciences Center, and St. Paul’s Hospital (Dr. Walley), University of British Columbia, Vancouver, BC.

Correspondence to: Vinay K. Dhingra, MD, Critical Care Medicine, 360 Echelon Building, Vancouver Hospital and Health Sciences Center, 855 West 12th Ave, Vancouver, BC, Canada V5Z 1M9; e-mail: vdhingra{at}vanhosp.bc.ca

Study objectives: Individual comparison of cardiac output via intermittent thermodilution and Fick technique over a wide range of cardiac outputs.

Design: Prospective clinical investigation.

Setting: Multidisciplinary ICUs of two teaching hospitals in Vancouver, British Columbia.

Participants: Eighteen critically ill patients who had pulmonary and systemic arterial catheters and in whom active support was being withdrawn.

Interventions: Measurement of thermodilution cardiac output and calculation of Fick cardiac output while support was withdrawn. Active support was withdrawn in a three-step process: removal of vasopressors followed by decrease in fraction of inspired oxygen to 0.21, and finally removal of mechanical ventilation.

Measurements and results: Simultaneous Fick and thermodilution cardiac outputs were obtained over a wide range. Fick calculated cardiac outputs were obtained using the Fick equation with oxygen uptake (O₂) being measured with indirect calorimetry. O₂ determinations were made using five measurements over 5 min, with the mean being used for subsequent analysis. Thermodilution cardiac outputs were determined by the mean of five measurements, with the first being discarded. Coefficient of variation was calculated for the O₂ and thermodilution cardiac outputs. One hundred thirty-six simultaneous cardiac outputs were obtained in 18 patients with a mean APACHE (acute physiology and chronic health evaluation) II score of 25.5. The range of cardiac outputs was 1.39 to 16.95 L/min. Linear regression analysis found a good correlation of the data sets, with an R of 0.85. Bias and precision calculations found a bias of - 0.17 L/min with the upper and lower limits of agreement being 2.96 L/min and - 3.30 L/min, respectively. In patients with high cardiac outputs (> 7 L/min), the bias was - 1.90 with the limits of agreement being 1.87 L/min and - 5.67 L/min. The coefficient of variation for O₂ was 4.6% and for thermodilution cardiac output was 7.75%.

Conclusions: There was good consistency of each of the measurements with a low coefficient of variation. The bias for the whole group was small, but the limits of agreement extended into a clinically relevant area, resulting in a lack of agreement. In patients with high cardiac outputs, the Fick tended to consistently produce higher cardiac outputs compared to thermodilution, suggesting a systematic error.

Key Words: bias • cardiac output • critically ill • Fick • indirect calorimetry • precision • regression analysis • thermodilution

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