OXYGEN-CONSUMPTION CALCULATED FROM THE FICK EQUATION HAS LIMITED UTILITY

Objective: To determine if oxygen consumption (V over dot O-2) calcula ted using the Fick relationship (calculated V over dot O-2) determines total body V over dot O-2 accurately and precisely enough to employ t his method during clinical assessment of oxygen transport. Design: Met hods comparison, using repeated measures during four physiologic state s: normal heart/normal lungs, heart failure/normal lungs, normal heart /acute lung injury, heart failure/acute lung injury. Settings: Univers ity research laboratory. Subjects: Thirteen adult Yucatan pigs. Interv entions: Oleic acid-induced acute lung injury; heart failure was induc ed with a continuous infusion of esmolol. Measurements and Main Result s: Calculated V over dot O-2 was determined by multiplying thermodilut ion cardiac output by the arterial-venous oxygen content difference in anesthetized, spontaneously breathing animals. Conditions were tightl y controlled so that calculated V over dot O-2 would be as accurate as possible. ''True'' V over dot O-2 was measured simultaneously with a water sealed spirometer (spirometry V over dot O-2). Calculated V over dot O-2 and spirometry V over dot O-2 were determined and analyzed du ring the four physiologic states listed above. Pooled data also were e valuated. Mean spirometry V over dot O-2 and calculated V over dot O-2 differed significantly during all four physiologic states and when da ta were pooled (spirometry V over dot O-2 273 +/- 70, calculated V ove r dot O-2, 178 +/- 58 mL/min; p < .01). Calculated V over dot O-2 cons istently underestimated spirometry V over dot O-2, as demonstrated by the large, positive bias in pooled data (95 +/- 59 mt of oxygen/min) a nd in the four physiologic states. Linear regression of data from all four states yielded slopes that were indistinguishable from 1, but y i ntercepts that varied from -152 to +182. For pooled data, the followin g equation was used: calculated V over dot O-2 = 0.5 x (spirometry V o ver dot O-2 + 46); r(2) = .35. Precision in pooled data was 22% of the mean spirometry V over dot O-2. Data analysis for the four physiologi c states demonstrated results similar to those results obtained when d ata were pooled. Conclusions: Even in a tightly controlled, clinical s imulation in the laboratory, calculated V over dot O-2 from the Fick r elationship systematically underestimated V over dot O-2 measured with a water-sealed spirometer. If true V over dot O-2 changes, the magnit ude and direction of change will be reflected by calculated V over dot O-2 but with similar to 20% error in the absolute value. Heart failur e, acute lung injury, and their combination did not affect the accurac y of calculated V over dot O-2. Therefore, calculating V over dot O-2 using the Fick relationship is too inaccurate to be used for research purposes. Because assessment of the directional change of V over dot O -2 may be clinically useful, calculated V over dot O-2 can be employed with discretion during clinical oxygen transport evaluation, bearing in mind the calculation's inherent imprecision.