[CO2 production and thermogenesis induced by enteral and parenteral nutrition]

The objectives of this study were to quantify, in critically ill patients, the thermogenesis induced by artificial nutrition (AN), when the caloric supply restores the energetic losses, and to evaluate the effects of the AN suppression on the production of CO2 (VCO2).

20 patients were studied, 14 with mechanical ventilation and 6 with spontaneous respiration, divided into two groups, depending on whether the AN was enteral (EN) or parenteral (PN). The VCO2 and the consumption of oxygen (VO2) were measured by means of the Douglas bag. The resting energy expenditure during the AN (REET) and 2 hours after discontinuing the AN (REE) were measured. The thermogenesis induced by the nutrition (TIN) was defined as the difference between REET and REE, expressed as a percentage of the caloric supply.

After discontinuing the AN, there were no statistically significant differences between EN and PN patients in the VCO2 decreases (p = 0.60) and the VO2 decreases (p = 0.78), and in the TIN percentage referred to the caloric supply (p = 0.82). After discontinuing the caloric supply, which was 1.12 times the REET, the VCO2 decreased 5 +/- 4.9% (Confidence interval (CI) of 95%, from 3% to 7%), the VO2 decreased 2 +/- 6.1% (CI of -0.36% to 5.32%), and the TIN represented 3 +/- 4.5% of the caloric supply (CI of 0.72% to 4.95%).

The results of this study indicate that the thermogenesis induced by artificial nutrition, when restores the energetic losses, is equivalent to 3% of the caloric supply. The discontinuation of the artificial nutrition induced a VCO2 decrease of 5% and thus this measure would probably not be useful for decreasing the ventilatory demand.