Whole-Body Co2 Production As An Index of the Metabolic Response To Sepsis

Whole-body carbon dioxide (CO₂) production (RaCO₂) is an index of substrate oxidation and energy expenditure; therefore, it may provide information about the metabolic response to sepsis. Using stable isotope techniques, we determined RaCO₂ and its relationship to protein and glucose metabolism in medical patients with sepsis and septic shock. Whole-body CO₂ production, an index of basal metabolic rate, was measured in 13 patients with sepsis or septic shock and 7 healthy controls using an i.v. infusion of ¹³C-sodium bicarbonate. Endogenous leucine flux, leucine oxidation, and nonoxidative disposal, indices of whole-body protein breakdown, catabolism, and synthesis, were measured with an infusion of 1-¹³C-leucine, and glucose production and clearance were measured with an infusion of ²H₂-glucose. There was no difference in mean RaCO₂ between the patients and controls, but the patients had a wider range of values. The four patients with the lowest RaCO₂ died. Protein breakdown and synthesis and glucose production were significantly faster in patients than in controls (P < 0.05). Whole-body CO₂ production was positively correlated with protein breakdown (P = 0.001), protein synthesis (P < 0.01), and glucose clearance (P = 0.01). Patients with low metabolic rates (mean - 2 SDs of controls) had slower protein breakdown and decreased glucose clearance compared with patients with high metabolic rates (mean + 2 SDs of controls). Septic patients were both hypometabolic and hypermetabolic. The correlation between RaCO₂ and protein breakdown and synthesis as well as glucose clearance suggests that RaCO₂ can provide information about substrate metabolism in septic patients. Because hypometabolism was associated with mortality and changes in protein and glucose metabolism in septic patients, it may be a useful clinical indicator of an inadequate metabolic response.