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Following critical injury and severe sepsis there are increases in energy expenditure and massive changes in body composition.
Changes in Energy Expenditure
Resting energy expenditure (REE) rises to 55% above predicted and remains elevated for 3 weeks or more. Total energy expenditure is 1.1 x REE early on but rises to 1.6 x REE during the second week after admission to hospital.
Changes in Body Weight and Body Water
Most of the changes in body weight can be accounted for by changes in body water particularly changes in extracellular water. At the end of the period of haemodynamic instability (defined as the time when no further colloid infusion or increasing inotropic support is required) there is a net accumulation of more than 10 liters of extracellular water which is excreted at a variable rate depending to some extent on the age of the patient. Elderly patients may take more than 3 weeks to correct this over expansion of extracellular water whereas in younger patients it is corrected in half this time.
Changes in Body Protein and Skeletal Muscle
Over the first 21 days after onset trauma patients lose 16% of their total body protein; in patients with severe sepsis about 13% is lost. Around two thirds of this protein loss comes from skeletal muscle.
Changes in Body Fat Mass and Energy Balance
Changes in body fat mass reflect energy balance. Body fat is oxidized when energy intake is insufficient. If energy intake equals total energy expenditure then body fat is not oxidized and body fat mass is preserved.
Cellular Composition
Cellular composition can to some extent be preserved with state the art metabolic and nutritional care. At the end of the period of haemodynamic instability intracellular potassium is low (~130 mmol/1 cf. a normal value of 150 mmol/1) but with modern forms of nutritional support further deterioration does not occur.
Nutritional Requirements
Total Energy Requirements
Total energy requirements in critical illness vary over time. In both trauma and sepsis about 30 kcal /kg/day should be prescribed to meet energy needs during the first week after admission to hospital. During the second week energy requirements are greater for both trauma (~52kcal/kg/day) and sepsis (~46kcal/kg/day). It is difficult to meet energy needs in these patients with glucose alone without pharmacological doses of insulin and close monitoring. Although insulin promotes clearance of glucose from plasma it does not increase glucose oxidation. Hyperglycaemia, fatty liver. and increased heat production are likely to result. Equicaloric mixtures of glucose and fat can be given simply and safely and when given in this way lipaemia and hyperglycaemia are uncommon.
Protein Requirements
Protein requirements in critically ill patients have been difficult to determine and depend to some extent on extra losses from drains. into collections or from the gastrointestinal tract. A recent study of changes in total body protein with differing protein intakes (1.0g/kg/day; 1.3g/kg/day and 1.6g/kg/day ) in 21 critically ill patients showed that protein intakes greater than 1.3g/kg/day were not associated with increased protein retention.
In conclusion our work and that of others has shown that critical illness is associated with hypermetabolism, lipolysis, proteolysis and extracellular water gain. We have for the first time quantified these changes and shown that they are more prolonged and greater than expected. We have also shown that state of the art management is able to meet energy requirements, prevent lipolysis and avoid further deterioration in cellular composition. We have confirmed the fact that there is a level of protein intake above which further utilization does not occur. Our work further highlights, however, how important it is to focus on research designed to prevent the massive loss of skeletal muscle protein that occurs in association with multiple injury and severe sepsis. New strategies with potential to do this include the administration of growth hormone, the provision of conditionally essential amino acids (glutamine), and the use of diets enriched with arginine, nucleotides, and n-3 fatty acids. Inhibition of the production or actions of factors that signal the increase in proteolysis or to stimulate cellular processes that conserve body proteins are possible new approaches.
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