13CO2 recovery fraction in expired air of septic patients under mechanical ventilation

Abstract

The continuous intravenous administration of isotopic bicarbonate (NaH 13 CO 2 ) has been used for the determination of the retention of the 13 CO 2 fraction or the 13 CO 2 recovered in expired air. This determination is important for the calculation of substrate oxidation. The aim of the present study was to evaluate, in critically ill patients with sepsis under mechanical ventilation, the 13 CO 2 recovery fraction in expired air after continuous intravenous infusion of NaH 13 CO 2 (3.8 µmol/kg diluted in 0.9% saline in ddH 2 O). A prospective study was conducted on 10 patients with septic shock between the second and fifth day of sepsis evolution (APACHE II, 25.9 ± 7.4). Initially, baseline CO 2 was collected and indirect calorimetry was also performed. A primer of 5 mL NaH 13 CO 2 was administered followed by continuous infusion of 5 mL/h for 6 h. Six CO 2 production (VCO 2 ) measurements (30 min each) were made with a portable metabolic cart connected to a respirator and hourly samples of expired air were obtained using a 750-mL gas collecting bag attached to the outlet of the respirator. 13 CO 2 enrichment in expired air was determined with a mass spectrometer. The patients presented a mean value of VCO 2 of 182 ± 52 mL/min during the steady-state phase. The mean recovery fraction was 0.68 ± 0.06%, which is less than that reported in the literature (0.82 ± 0.03%). This suggests that the 13 CO 2 recovery fraction in septic patients following enteral feeding is incomplete, indicating retention of 13 CO 2 in the organism. The severity of septic shock in terms of the prognostic index APACHE II and the sepsis score was not associated with the 13 CO 2 recovery fraction in expired air.