Nutritional Support of the Burn Patient

Transcription

1 Nutritional Support of the Burn Patient Objectives To understand the principles of normal nutrient utilization and the abnormalities caused by burn injury To be able to assess nutrient needs To be able to understand and implement nutritional support

2 Defining the nutrient needs of the hypermetabolic catabolic burn patient Measuring caloric needs using Indirect Calorimetry Table of Contents I. Indications for Nutritional Support II. Principle of Nutrient Utilization III. Assessment of Nutritional Status IV. Assessment of Nutritional Needs V. Method of Delivery of Nutritional Support VI. Summary VII. References

3 I. INDICATIONS FOR NUTRITIONAL SUPPORT As presented in the section on Metabolic Changes in the Burn Patient it is clear that the stress response needs to be treated. One of the key anabolic strategies, to a catabolic state, is nutritional support. A major burn patient needs to have good nutrition initiated to avoid excessive lean mass loss and resulting weakness. Immediate nutritional support is indicated to manage the severe stress response because of the marked catabolism which will rapidly ensue. Immediate nutrition is also indicated for the patient already malnourished. The degree of nutritional support corresponds with burn size. The presence of a smoke inhalation injury further increases the metabolic demands. The increased protein, energy and micronutrient demands need to be met before complications caused by loss of body weight, mainly lean mass, and protein energy malnutrition develop Because the maladaptive hormonal environment and inflammatory focus will persist, nutrition will attenuate but not eradicate the stress response as discussed in the metabolism section. Controlling the underlying stressors is still necessary for nutritional therapy to obtain optimal beneficial effects. The addition of anabolic activity is needed to decrease the utilization of protein for fuel, the metabolic abnormality in nutrient partitioning caused by stress. II. PRINCIPLES OF NUTRIENT UTILIZATION Adequate nutritional support, that is, adequate provisions of energy and protein substrate, is an essential aspect of the management of the surgical patient. The use of nutrients by the cell for energy and tissue synthesis requires an adequate delivery of oxygen to the cells. Maintenance of tissue, oxygen, delivery, and hemodynamic stability are essential for nutritional management. Energy requirements are defined in terms of the calorie. One calorie is the energy required to increase the temperature of 1 gram of water by 1 degree centrigrade. The calorie used to define human metabolic needs is actually a kilocalorie (1000 calories). Protein requirements are defined in terms of nitrogen needs, nitrogen making up about 15 percent of a protein. The respiratory quotient (RQ) is the ratio of carbon dioxide production during metabolism to the amount of oxygen consumed. RQ equals CO 2 produced (Moles) /O 2 consumed (Moles). The RQ value for carbohydrate is 1.0, for fat 0.7 and for protein 0.8: The grams of nitrogen in protein = grams of protein/6.25. The calorie: nitrogen ratio is the number of calories per gram of nitrogen. Nutrient Calories/gm Carbohydrates 4 (3.4 hydrated) Fat 10 Protein 4 A. Carbohydrate Metabolism

4 Carbohydrate is the primary fuel in man accounting for 60-70% of calories. In its anhydrous form 1 gram of carbohydrate generates 4 calories. However, carbohydrate is stored and used, for example, in solution in its hydrated form from which 3.4 calories is generated per gram metabolized. Insulin is required for glucose to enter the cell to be used for energy. Only small amounts of glucose are stored. About 150 grams can be stored in the adult liver and 300 grams in muscle as glycogen, which can be rapidly mobilized to glucose for when needed. Glucose requires phosphorylation to initially enter the cell, and it cannot subsequently leave that cell. The total number of stored glucose calories is less than 1500 in the normal adult. Excess glucose cannot be stored beyond that which is needed for energy and a small amount of glucogen which tissues can use. Excess glucose is rapidly conversed to fat. This process requires energy, and the respiratory quotient exceeds 1.0, resulting in a marked excess I carbon dioxide production. Glucose + O 2 = CO 2 + Energy (RQ=1.0) Glucose (excess) + Energy = Fat + CO 2 (RQ=8) Carbohydrate cycling occurs with the stress response, via activation of the Cori cycle. Carbohydrate to lactate and back to carbohydrate. This process yields only a fraction of the ATP produced with the complete burning of carbohydrates to CO 2 and water. In addition, the hormonal stimulus for gluconeogenesis exceeds demands and a relative insulin resistant state is present due to elevated anti-insulin hormone activity. Providing sufficient carbohydrate (60% of total calories) is still necessary to attenuate the drive to breakdown protein with amino acids used to make glucose. B. Fat Metabolism Fat is used fuel in unstressed man, accounting for about percent of the calories, depending on the diet. Fat is used for 85-90% of fuel in starvation. Fat is composed of triglycerides, which in turn comprise glycerol and fatty acids. The glycerol is burned like carbohydrate, while the fatty acids enter the Krebs cycle via a process called beta-oxidation whereby two carbon segments are cleaved off. Ketones are then produced. The ketones enter the Krebs cycle as acetylcoenzyme A, generating energy at a respiratory quotient of 0.7 with 9 to 10 calories per gram of fat. Increased ketosis signals the predominant use of fat through ketones for energy. This situation is seen in the starved surgical patients with a lack of available carbohydrate where ketones become the predominant fuel. With a large carbohydrate load, fat utilization is depressed. In addition to the availability of carbohydrate, fat breakdown is dependent on the endocrine environment. Insulin decreases the utilization for energy in preference to carbohydrate while catecholamines, and growth hormone increase fat breakdown. The number of available calories in stored fat in the normal adult with 10 kg of fat is about 100,000 about 100 times more than that in stored carbohydrate. With the stress response fat is not used to decrease amino acid use for energy. Instead there is an increase in the cycling of fatty acid to fat and back. However, fat may provide up to 50% of fuel in the absence of adequate carbohydrates. Ketosis is not usually seen in the stressed surgical patient reflecting the decreased fat utilization compared to that seen in starvation. C. Protein Metabolism In normal man, almost all the energy comes from carbohydrate and fat as long as they are present in sufficient amounts in the diet. With a carbohydrate or fat deficit or a protein excess, the amino acids are metabolized to produce calories. Approximately 4 calories are generated per gram of protein.

5 Normally for every 300 calories, 1 gram of nitrogen (6.25 grams of protein) is ingested in the normal diet, the majority of the nitrogen being used for protein synthesis. With surgical trauma, or infection and its altered endocrine and inflammatory environment, more than 20 percent of the total energy comes from the use of protein as fuel. The RQ value for protein is 0.8. The three branched chain amino acids- valine, leucine, and isoleucine-can be utilized directly by tissues for energy via the Krebs cycle and mitochondria. The remaining amino acids require deamination in the liver with conversion to keto-acids which can be transported to all tissues for burning using the Krebs cycle. Protein synthesis is vital in the maintenance of the integrity of all cells, in particular those that have a rapid protein turnover. Deamination of amino acids in the liver leads to liver urea production. Adequate renal clearance is necessary to avoid a rapid increase in the blood urea concentration, especially during a catabolic state. The rate of protein synthesis depends on the available amino acid substrate, tissue demands and the hormonal environment, especially the activity of anabolic hormones. Sufficient protein intake in the form of amino acids is necessary to keep up with necessary new protein formation and net losses seen in the surgical patient. In a normal uninjured man this usually requires about 1 gram of nitrogen (6.25 grams of protein) for every 300 calories less than 1 gram of protein per kilogram of body weight. With increased tissue demands and the increased use of protein for energy, as seen in injured patients, protein needs, increase to approximately 1.5 grams per kilogram of body weight, with a ratio of calories to nitrogen of 100:1 to as low as 80:1. Growth hormone, androgens, and insulin increase protein synthesis, whereas glucocorticoids and the catecholamines increase protein breakdown. D. Micronutrient Metabolism Micronutrients are essential for cellular function. They are called nutrients because of their key role in metabolism, but these of their key role in metabolism, but these compounds and elements also are involved in many other aspects of homeostasis, including wound healing, antioxidant protection, and immune function. The term micro is used because of the extremely small amounts found in the circulation tissues. Their concentration is critical to cellular function. Micronutrients usually are divided into the organic compounds (vitamins) and inorganic compounds (trace minerals). The microminerals are used in a variety of metabolic pathways often used as cofactors for enzymatic reactions. Both need to be provided and both are utilized and lost in increased quantities to metabolic response to stress seen in the surgical patient. Deficiency states therefore can occur easily. Because measurement of adequate levels is difficult, if not impossible, prevention of a deficiency often is accomplished only by increasing intake. Vitamins are organic substances that are essential in humans for growth and homeostasis A few of their characteristics are: Essential organic micronutrients involved in fundamental body functions Supplied mainly by food Each vitamin has multiple, unrelated functions No chemical relationship among the group The term vitamin was first used by a biochemist who discovered one of the first of these essential elements, which happened to be an amine, thus vitamin. Vitamins are found in very small

6 quantities in the body. Each has a name defined by a letter as well as a chemical name. These compounds play a key role in metabolism, growth, and homeostasis and therefore are especially important in surgical critical illness, in which hypermetabolism, healing, and immune function are so important for survival. A deficiency state, which can occur readily, clearly will amplify the magnitude of disease. It is important to point out that trace elements are absorbed from food and mineralized water, which means an intact, functioning gut and food intake are critical. Tube feeding solutions and TPN contain insufficient quantities of these nutrients for the critically ill. The addition of trace elements in increased amounts to enteral and parenteral feeding regimens is of major importance in managing the surgical patient considering their importance in metabolism, healing, and immune defenses. III. ASSESSMENT OF NUTRITIONAL STATUS Not all surgical patients require immediate nutritional support. However, any catabolic patient described in the table or with pre-existing weight loss and malnutrition requires early nutritional intervention. Assessment begins with determining the current nutritional and metabolic status of the patient and then determining the status of the surgical illness or injury. Both factors dictate the timing of support as well as the quantity. As stated, the presence of a pre-existing protein energy malnutrition in the now stressed patient necessitates beginning nutritional support. Assessment of Needs Current Status Status of surgical illness or burn injury A. Current Status There are a number of risk factors for a pre-existing or evolving (PEM) protein energy malnutrition.

7 Risk Factors for Protein Energy Malnutrition Elderly, disabled Chronic illness Cancer Presence of a catabolic state Presence of a wound Assessment is based on a history of past and current nutritional intake as well as a history or evidence of a recent involuntary weight loss. As described, a weight loss of over 10% of normal weight over 6 months, or a 5% decrease in 30 days, is a good marker for malnutrition. Findings on physical exam of impaired nutrition include wasting, weakness, delayed wound healing and CNS depression. However, obvious physical findings will not be present with early malnutrition. Current Status History of past and present nutrient intake Physical exam Evidence of significant involuntary weight loss Biochemical markers

8 Is this patient malnourished? Biochemical markers are often more sensitive. Albumin levels have been frequently used. However, it is not a very sensitive marker. The half-life of albumin is over 30 days so it takes a long time to see a change. In addition, albumin synthesis is decreased with onset of the stress response after burns. Acute phase protein synthesis increases and albumin decreases unrelated to nutritional status. Pre-albumin (transthyretin) levels are much more sensitive as the half-life is only a few days. This protein is not an albumin precursor. A value 15mg/dl reflects early malnutrition and the need for nutritional support. Physical Findings Wasting, weakness CNS depression Dementia, glossitis Delayed wound healing

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