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Correlation between oxygen consumption calculated using Fick's method and measured with indirect calorimetry in
Flávio Marson1, Maria Auxiliadora Martins, Francisco Antonio Coletto
1Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brasil.
Objective:
To compare the oxygen consumption index measured by using indirect calorimetry (VO2I Delta) with a portable metabolic cart and calculated according to Fick's principle (VO2 I Fick) in critically ill patients.
Methods:
Fourteen patients (10 men and 4 women, mean age 39.4 +/- 5.4 years) were analyzed, 5 of them trauma victims and 9 sepsis victims. The following mean scores were obtained for these patients: APACHE II = 21.3+/-1.8, ISS = 24.8+/-6, and sepsis score = 19.6+/-2.3. The mortality risk (odds ratio), calculated from APACHE II, was 41.9+/-7.1%. All patients underwent mechanical ventilation and invasive hemodynamic monitoring with a Swan-Ganz catheter. VO2 was obtained using the 2 methods (VO2I Delta and VO2I Fick) at 4 different times (T1-T4).
Results:
A good correlation was found between the 2 methods (r=0.77) for the mean of the 4 serial measurements. No statistically significant differences were observed between indirect calorimetry and Fick's equation at T1 (VO2I Delta = 138+/-28 and VO2I Fick = 59+/-38 mL.min-2.m-2, P=0.10) and T3 (VO2I Delta = 144+/-26 and VO2I Fick = 158+/-35 mL.min-2.m-2, P=0.14), but a significant difference was observed at T2 (VO2I Delta = 141+/-27 and VO2I Fick = 155+/-26 mL.min-2.m-2, P=0.03) and T4 (VO2I Delta = 145+/-24 and VO2I Fick = 162+/-26 mL.min-2.m-2, P=0.01).
Conclusion:
We may state that indirect calorimetry can be used for oxygen consumption analysis in critically ill patients and is as efficient as Fick's reverse equation, with the benefit of being a noninvasive and risk-free procedure.