Oxygen consumption in critically ill patients: the relation between calculation by Fick's principle and measurement by gas-mixing chamber indirect calorimetry

Background

The accurate calculation of Oxygen consumption (VO₂) using the Fick equation require accurate measurement of cardiac output (Q), usually by thermodilutuion, Oxygen content in arterial blood (CaO₂) and Oxygen content in venous blood (CvO₂). Each of these measurements can carry random errors and proportional errors and this well result in a significant final error in the calculated VO₂ (cVO₂). Gas mixing chamber indirect calorimeter has been an extremely valuable tool in measuring VO₂ in critically ill patients (mVO₂).

Objectives

To compare cVO₂ and mVO₂ in different groups of critically ill patients.

Subjects and methods

The study was conducted on 31 critically ill patients admitted to the department of critical care medicine, Cairo University, 25 males and 6 females, mean age 56.2 ± 10.4 (range: 37–75), 11 patients with congestive heart failure (CHF) (class III & IV according to NYHA), 10 males and 1 female, mean age 53.4 ± 5.9 (range: 45–63), 10 patients with acute respiratory failure, 8 males and 2 females mean age 63.7 ± 7.97 (range: 50–75), 10 patients with sepsis or septic shock, 7 males & 3 females, mean age 52.2 ± 12.9 (range: 37–69). Each was subjected to VO₂ measurement utilizing Fick's equation; cVO₂ = Q (CaO₂ — CvO₂), the average of three double checked readings over 1 hour is determined. Concomitant measurements utilizing gas mixing chamber indirect calorimeter is carried out, the average of sixty readings over the same hour is recorded (mVO₂).

Results

Oxygen consumption measured by Fick's method was comparable with that measured utilizing gas mixing chamber indirect calorimetry with no significant difference between means in the whole population (162.3 ± 44.7 vs 165.0 ± 33.4 ml/min/m², respectively, n = 61), in CHF (176.4 ± 57.4 vs 177.4 ± 22.1 ml/min/m², respectively, n = 20), in RF (143.8 ± 40 vs 147.2 ± 30.7 ml/min/m², respectively, n = 17) and in sepsis and septic shock (161.9 ± 35.3 vs 170.0 ± 39.9 ml/min/m², respectively, n = 24). However, correlation was present only in the whole population (r = 0.3, P < 0.05) and in patients with congestive heart failure (r = 0.4, P < 0.05).

Conclusion

Gas mixing chamber indirect calorimetery is a non-invasive, safe method for measuring Oxygen consumption in critically ill patients. Oxygen consumption measured by Fick's method is comparable with that measured utilizing gas mixing chamber indirect calorimetry, however correlation was absent in patients with respiratory failure, sepsis and septic shock.