Oxygen consumption calculated from the Fick equation has limited utility

M. Christine Stock*, Marqueta E. Ryan

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

27 Scopus citations

Abstract

Objective: To determine if oxygen consumption (V̇o2) calculated using the Fick relationship (calculated V̇o2) determines total body V̇o2 accurately and precisely enough to employ this method during clinical assessment of oxygen transport. Design: 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. Settings: University research laboratory. Subjects: Thirteen adult Yucatan pigs. Interventions: Oleic acid-induced acute lung injury; heart failure was induced with a continuous infusion of esmolol. Measurements and Main Results: Calculated V̇o2 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 V̇o2 would be as accurate as possible. "True" V̇o2 was measured simultaneously with a water-sealed spirometer (spirometry V̇o2). Calculated V̇o2 and spirometry V̇o2 were determined and analyzed during the four physiologic states listed above. Pooled data also were evaluated. Mean spirometry V̇o2 and calculated V̇o2 differed significantly during all four physiologic states and when data were pooled (spirometry V̇o2 273 ±70, calculated V̇o2 178 ± 58 mL/min; p < .01). Calculated V̇o2 consistently underestimated spirometry V̇o2, 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 V̇o2 = 0.5 × (spirometry V̇o2 + 46); r2 = .35. Precision in pooled data was 22% of the mean spirometry V̇o2. Data analysis for the four physiologic states demonstrated results similar to those results obtained when data were pooled. Conclusions: Even in a tightly controlled, clinical simulation in the laboratory, calculated V̇o2 from the Fick relationship systematically underestimated V̇o2 measured with a water-sealed spirometer. If true V̇o2 changes, the magnitude and direction of change will be reflected by calculated V̇o2 but with -20% error in the absolute value. Heart failure, acute lung injury, and their combination did not affect the accuracy of calculated V̇o2. Therefore, calculating V̇o2 using the Fick relationship is too inaccurate to be used for research purposes. Because assessment of the directional change of V̇o2 may be clinically useful, calculated V̇o2 can be employed with discretion during clinical oxygen transport evalua-tion, bearing in mind the calculation's inherent imprecision.

Original languageEnglish (US)
Pages (from-to)86-90
Number of pages5
JournalCritical Care Medicine
Volume24
Issue number1 SUPPL.
DOIs
StatePublished - Jan 1 1996

Keywords

  • Apparatus and instruments
  • Critical illness
  • Heart
  • Heart failure
  • Lungs
  • Monitoring
  • Oxygen
  • Oxygen consumption
  • Physiologic
  • Spirometry

ASJC Scopus subject areas

  • Critical Care and Intensive Care Medicine

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