Low respiratory quotient correlates with high mortality in patients undergoing mechanical ventilation

Abstract

Objective: Oxygen consumption (VO₂), carbon dioxide generation (VCO₂), and respiratory quotient (RQ), which is the ratio of VO₂ to VCO₂, are critical indicators of human metabolism. To seek a link between the patient's metabolism and pathophysiology of critical illness, we investigated the correlation of these values with mortality in critical care patients.

Methods: This was a prospective, observational study conducted at a suburban, quaternary care teaching hospital. Age 18 years or older healthy volunteers and patients who underwent mechanical ventilation were enrolled. A high-fidelity automation device, which accuracy is equivalent to the gold standard Douglas Bag technique, was used to measure VO₂, VCO₂, and RQ at a wide range of fraction of inspired oxygen (F_IO₂).

Results: We included a total of 21 subjects including 8 post-cardiothoracic surgery patients, 7 intensive care patients, 3 patients from the emergency room, and 3 healthy volunteers. This study included 10 critical care patients, whose metabolic measurements were performed in the ER and ICU, and 6 died. VO₂, VCO₂, and RQ of survivors were 282 +/- 95 mL/min, 202 +/- 81 mL/min, and 0.70 +/- 0.10, and those of non-survivors were 240 +/- 87 mL/min, 140 +/- 66 mL/min, and 0.57 +/- 0.08 (p = 0.34, p = 0.10, and p < 0.01), respectively. The difference of RQ was statistically significant (p < 0.01) and it remained significant when the subjects with F_IO₂ < 0.5 were excluded (p < 0.05).

Conclusions: Low RQ correlated with high mortality, which may potentially indicate a decompensation of the oxygen metabolism in critically ill patients.

Keywords: Carbon dioxide generation; Douglas bag; Indirect calorimetry; Oxygen consumption; Respiratory quotient.