The metabolic effects of thermal injury

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Abstract

Major thermal injury is associated with extreme hypermetabolism and catabolism as the principal metabolic manifestations encountered following successful resuscitation from the shock phase of the burn injury. Substrate and hormonal measurements, indirect calorimetry, and nitrogen balance are biochemical metabolic parameters which are useful and more readily available biochemical parameters worthy of serial assessment for the metabolic management of burn patients. However, the application of stable isotopes with gas chromatography/mass spectroscopy and more recently, new immunoassays for growth factors and cytokines has increased our understanding of the metabolic manifestations of severe trauma. The metabolic response to injury in burn patients is biphasic wherein the initial ebb phase is followed by a hypermetabolic and catabolic flow phase of injury. The increased oxygen consumption/metabolic rate is in part fuelled by evaporative heat loss from wounds of trauma victims, but likely also by a direct central effect of inflammation upon the hypothalamus. Although carbohydrates in the form of glucose appear to be an important fuel source following injury, a maximum of 5-6 mg/kg/min only is beneficial. Burn patients have accelerated gluconeogenesis, glucose oxidation, and plasma clearance of glucose. Additionally, considerable futile cycling of carbohydrate intermediates occurs which includes anaerobic lactate metabolism and Cori cycle activity arising from wound metabolism of glucose and other substrates. Similarly, accelerated lipolysis and futile fatty acid cycling occurs following burn injury. However, recent evidence suggests that lipids in the diet of burned and other injured patients serve not only as an energy source, but also as an important immunomodulator of prostaglandin metabolism and other immune responses. Amino acid metabolism in burn patients is characterized by increased oxidation, urea synthesis, and protein breakdown which is prolonged and difficult to reduce with current nutritional therapy. However, the current goal of nutritional support is to optimize protein synthesis. Specific unique requirements may exist for supplemental glutamine and arginine following burn injury but further research is needed before enhanced branched chain amino acids supplements can be recommended for burn patients. Recent research investigations have revealed the importance of enteral feeding to enhance mucosal defense against gut bacteria and endotoxin. Similarly, research has demonstrated that many of the metabolic perturbations of burns and sepsis may be due, at least in part, to inflammatory cytokines. Investigation of their pathogenesis and mechanism of action both at a tissue and a cellular level offer important prospects for improved understanding and therapeutic control of the metabolic disorders of burn patients.

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