Metabolic complications of parenteral nutrition

Metabolic complications of total parenteral nutrition can occur in several ways. Some of them, such as vitamin deficiencies, are easily foreseen and so avoided. Others can arise from the administration of excess calories, particularly in the form of glucose. Normally excess glucose inhibits fat oxidation and promotes lipid synthesis, so that the respiratory quotient rises about 1.0. In seriously ill, septic patients the main fuel is fat and its oxidation is not inhibited by excess glucose. Large amounts of O2 are required and large amounts of CO2 produced. By measuring the patient's resting energy expenditure, respiratory quotient and ventilatory equivalents one can assess his caloric requirements and whether or not he will have difficulty excreting the CO2 produced. If the stress on the pulmonary system appears too great CO2 production can be decreased by increasing the proportion of calories given as fat.