Abstract
Total parenteral nutrition (TPN) is now widely available, but there is relatively little data relating to the optimal substrate support in different types of patients. In critically ill patients who require parenteral nutrition, underlying metabolic alterations will influence the capacity for substrate utilization; in them, fat emulsion may serve as a useful energy source but may also have deleterious effects. The intravenous fat tolerance test has been widely used as a measure of the capacity of the organism to clear exogenous fat from the bloodstream. We have devised and employed a three-stage lipid clearance test using Intralipid-10% as a substrate. The rates of infusion are such that First order kinetics are followed during the first two stages yielding fractional removal rates, and zero order kinetics are followed during the third infusion yielding maximal clearance rates. Six injured and 6 infected patients displayed a greater capacity for lipid clearance than 13 normal subjects. Four injured patients who also had received multiple transfusions failed to show this response. Fractional removal rates were influenced by injury and infection to a greater extent than maximal clearance rates. Models are presented utilizing enzyme saturation kinetics and treating endogenous triglyceride as a competitive inhibitor (for the enzyme-substrate system of lipoprotein lipase-Intralipid triglyceride). The importance of employing enzyme saturation kinetics in the interpretation of lipid clearance tests is noted. The calculated apparent Michaelis-Menten constant for Intralipid triglyceride during the lipid clearance test is 521 ± 38 (SEM) μmol/liter. In vitro systems utilizing Intralipid triglyceride as a substrate have yielded values in this range.
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