Increased lipid fuel dependence in the critically ill septic patient

The effect of sepsis in modifying post-surgical fuel utilization in critically ill patients was determined from 374 observations (246 septic [S] and 128 nonseptic [N] in 12 intubated ICU patients studied serially. Patients received TPN (values/24 hrs: Septic, N2, 9.1 +/- 2.2 gm; glucose, 543 +/- 211 kcal/m2, Nonseptic, N2, 8.3 +/- 3.6 gm; glucose, 550 +/- 346 kcal/m2). In some periods, intravenous lipid (L) was given to raise total caloric intake to 826 +/- 223 kcal/ 24 hr/m2. The VO2, VCO2, respiratory rate, minute volume, and blood gas levels were measured, and respiratory quotient (RQ) and metabolic rate (MR) computed. Statistics were performed by 2-way ANOVA and analysis of covariance. Without lipid, mean VCO2 for S (126 ml/min/m2) and N (128 ml/min/m2) were not significantly different, but VO2 in S (146 ml/min/m2) and N (132 ml/min/m2), and the RQ values S (0.88) and N (0.97), were different (p less than 0.0001). In 360 studies RQ was shown to be increased by the total caloric intake, but reduced in the presence of sepsis: RQ = 0.00014 (kcal/m2) - 0.09 (sepsis effect + 0.878 N = 360; r2 = 0.304; F2,357 = 78; p less than 0.0001; but both administered glucose and lipid calories contribute to the RQ in sepsis: RQ = 0.00017 (glucose kcal/m2) + 0.266 X 10(-3) (lipid kcal/m2) + 0.732 n = 114; r2 = 0.260; F2,111 = 19.5; p 0.0001. Sepsis increased VO2 with little change in VCO2, thus RQ fell, suggesting increased use of lipid fuels for oxidation. During hypercaloric lipid infusion in septic patients (SL) VO2 and VCO2 increased but VO2 was still greater, so RQ remained low (SL RQ = 0.89). As sepsis worsened VO2 remained high but VCO2 fell producing RQ less than 0.8, while plasma glucose levels were increased. These data suggest that septic patients are more dependent than nonseptics on lipid fuels for oxidative metabolism, and that IV lipids can be used to increase oxidative metabolism in sepsis at a time when glucose metabolism appears reduced.