Oxygen consumption calculated from the Fick equation has limited utility

To determine if oxygen consumption (VO2) calculated using the Fick relationship (calculated VO2) determines total body VO2 accurately and precisely enough to employ this method during clinical assessment of oxygen transport.

Methods comparison, using repeated measures during four physiologic states: normal heart/normal lungs, heart failure/normal lungs, normal heart/acute lung injury, heart failure/acute lung injury.

University research laboratory.

Thirteen adult Yucatan pigs.

Oleic acid-induced acute lung injury; heart failure was induced with a continuous infusion of esmolol.

Calculated VO2 was determined by multiplying thermodilution cardiac output by the arterialvenous oxygen content difference in anesthetized, spontaneously breathing animals. Conditions were tightly controlled so that calculated VO2 would be as accurate as possible. "True" VO2 was measured simultaneously with a water-sealed spirometer (spirometry VO2). Calculated VO2 and spirometry VO2 were determined and analyzed during the four physiologic states listed above. Pooled data also were evaluated. Mean spirometry VO2 and calculated VO2 differed significantly during all four physiologic states and when data were pooled (spirometry VO2 273 +/- 70, calculated VO2 178 +/- 58 mL/min; p < .01). Calculated VO2 consistently underestimated spirometry VO2, as demonstrated by the large, positive bias in pooled data (95 +/- 59 mL of oxygen/min) and in the four physiologic states. Linear regression of data from all four states yielded slopes that were indistinguishable from 1, but y intercepts that varied from -152 to +182. For pooled data, the following equation was used: calculated VO2 = 0.5 x (spirometry VO2 + 46); r2 = .35. Precision in pooled data was 22% of the mean spirometry VO2. Data analysis for the four physiologic states demonstrated results similar to those results obtained when data were pooled.

Even in a tightly controlled, clinical simulation in the laboratory, calculated VO2 from the Fick relationship systematically underestimated VO2 measured with a water-sealed spirometer. If true VO2 changes, the magnitude and direction of change will be reflected by calculated VO2 but with approximately 20% error in the absolute value. Heart failure, acute lung injury, and their combination did not affect the accuracy of calculated VO2. Therefore, calculating VO2 using the Fick relationship is too inaccurate to be used for research purposes. Because assessment of the directional change of VO2 may be clinically useful, calculated VO2 can be employed with discretion during clinical oxygen transport evaluation, bearing in mind the calculation's inherent imprecision.