Metabolic Response

Increased glucose demand is initially met by gly-cogenolysis. When glycogen stores are exhausted, lipolysis and protein catabolism increase to supply gluconeogenic substrates. This hypermetabolic response is accompanied by increased cardiac output, increased oxygen consumption, and increased thermogenesis. The physical loss of skin cover has other major effects, including fluid loss, increased heat loss by evaporation, and loss of local immune function. Skin grafting will provide some cover for burned areas but the use of allografts increases the total area of damaged skin. Early excision and grafting is associated with increased survival in patients with more than 70% burns compared with conservative management. Donor split-skin graft sites heal within 7-14 days, unlike the burned area, which continues to make increased metabolic demands for weeks after the initial insult. Measurements of energy requirements take into account whole body metabolism and include any demands made by donor sites. Recent work suggests that early coverage with artificial 'skin' may reduce the hypercaloric requirements of the patient.

The sympathoadrenal axis is stimulated as a result of thermal injury, with increased plasma levels of epinephrine, norepinephrine, and cortisol. In addition, levels of both glucagon and insulin

Physical response Heat loss Fluid loss

Hormonal response Increased: Epinephrine Norepinephrine Cortisol Glucagon Insulin

WOUND

Metabolic response Increased glucose flux Glycogenolysis Gluconeogenesis Increased futile cycles Fat mobilization Skeletal muscle breakdown

Inflammatory response Leucocyte activation Cytokine production Growth factor production Chemotactic factor production

Figure 1 Schematic representation of the central nature of the burn wound in triggering physical, hormonal, metabolic, and inflammatory responses.

increase, although an apparent insulin resistance develops in peripheral tissues. There is an increase in core temperature, which appears to be mediated centrally by the hypothalamus in response to cyto-kine release, possibly interleukin-1 (IL-1). Plasma levels of glucose are maintained and may even increase, although glucose flux is greatly increased. Metabolic demands for glucose and amino acids increase and the body responds to meet these demands (Table 1). The degree of hypermetabolism and oxygen consumption are closely related to the extent and depth of burn injury. As a result, basal energy expenditure increases and is doubled for a 60% burn (Figure 2). Catecholamines augment

Table 1 Metabolic and circulatory responses to burn injury

Wound

Whole body

Damage

Increase in catecholamines, cortisol,

glucagon, insulin

Hepatic switch to synthesis of acute

phase proteins

Increased blood flow

Increased cardiac output

to the wound

Increased metabolism

Increased gluconeogenesis

of glucose

Increased free fatty acid flux

Increased oxygen consumption

Futile substrate cycling of carbohydrate

intermediates and fatty acids

Increased heat loss

Increased core temperature:

hypothalamic mediated

Attempted repair

Increased amino acid flux

Release of arginine and glutamine from

skeletal muscle

Increased nitrogen loss

Inflammatory

Inhibition of maximum inflammatory

response

response by cortisol

Cytokine and eicosanoids increase

inflammation

cardiac and circulatory performance, which increases blood flow to the wound. Liver and kidney blood flow also increase, with increased delivery of gluconeogenic precursors, increased glucose release into the circulation, and increased nitrogen clearance. The release of fat from adipose tissue is stimulated by catecholamines. In the liver, fat metabolism to glycerol and free fatty acids produces energy as long as an adequate glucose supply replenishes oxaloacetic acid for oxidation of acetyl CoA, the product of triacylglycerol oxidation.

I I Severe burn injury I I Severe bony injury

I I Severe burn injury I I Severe bony injury

Week 24

Week 10

a Week 5 e

Tim Week 4

Week 3 Week 2 Week 1

50 100% 150 200% Resting metabolism as % of normal value

Figure 2 The stress response to thermal injury is greater than that to any other insult. The resting energy requirements in burns patients are greater for longer than for any other injury. (Adapted with permission from Long CL, Schaffel N, Geiger JW et al. (1979) Metabolic response to injury and illness: Estimation of energy and protein needs from indirect calorimetry and nitrogen balance. Journal of Parenteral and Enteral Nutrition 3(6): 452-456.)

Week 24

Week 10

a Week 5 e

Tim Week 4

Week 3 Week 2 Week 1

50 100% 150 200% Resting metabolism as % of normal value

Figure 2 The stress response to thermal injury is greater than that to any other insult. The resting energy requirements in burns patients are greater for longer than for any other injury. (Adapted with permission from Long CL, Schaffel N, Geiger JW et al. (1979) Metabolic response to injury and illness: Estimation of energy and protein needs from indirect calorimetry and nitrogen balance. Journal of Parenteral and Enteral Nutrition 3(6): 452-456.)

Thus, both continued oxygen and glucose must be supplied to prevent ketoacidosis. Heat production and energy wastage occur as a result of a 2- or 3fold increase in futile cycling of substrates; glucose, pyruvate, and fructose-6-phosphate are all involved in these reactions.

Gluconeogenesis can occur only in the liver and is increased by catecholamines and glucagon. The plasma levels of gluconeogenic amino acids (alanine and glutamine) initially increase during the first 2 days, when glycogen is preferentially metabolized, but subsequently decrease. Days 4-7 are associated with a maximal decrease in plasma levels of gluco-neogenic amino acids, whereas muscle production and hepatic consumption are both increased.

Your Metabolism - What You Need To Know

Your Metabolism - What You Need To Know

If you have heard about metabolism, chances are it is in relation to weight loss. Metabolism is bigger than weight loss, though, as you will learn later on. It is about a healthier, better you. If you want to fire up your metabolism and do not have any idea how to do it, you have come to the right place. If you have tried to speed up your metabolism before but do not see visible results, you have also come to the right place.

Get My Free Ebook


Post a comment