Height and Weight

A measured height and weight is preferred, but the clinician may ask the patient or caregiver for the height and weight. When the data are recorded, a notation should be made of the date and whether the height and weight were stated or measured. The height and weight can be evaluated to determine weight status by comparing actual body weight with ideal body weight. Table 21-1 shows healthy weights for adults. Ideal body weight may also be determined using the Hamwi formulas. To estimate ideal body weight in pounds, the following simple formulas (Hamwi method) can be used:

< ?xml:namespace prefix = "mml" />Men :106lb for the first5ft+6lb for each1in> 5ft

Women :100lb for the first5ft+5lb for each1in> 5ft

A more useful number, the body mass index (BMI), may also be calculated. This number is a handy tool for determining the category of body weight: underweight, healthy weight, overweight, obese, or morbidly obese (Figures 21-1 and 21-2). BMI numerically states the relationship of weight to height. The formula used to calculate BMI is as follows for measurements in kilograms and meters:

BMI=Actual body weight(kg)height2(m2)

The following formula is used when the weight and height measurements are in pounds and inches:

BMI=Actual body weight(lb)×703height2(in2)

An Internet calculator for BMI is available at: http://www.nhlbisupport.com/bmi/.

Body Composition

Other anthropometric measurements useful in nutrition assessment are arm muscle area (index for muscle) and skinfolds (measures of fat), bioelectric impedance analysis devices, and more sophisticated imaging technologies. These methods are generally expensive and time-consuming and are not relevant in the clinical setting but may be more useful in research settings.⁵

Arm Muscle Area

The triceps skinfold (TSF) measurement is used to calculate the arm muscle area (AMA). The result indicates muscle stores available for protein synthesis or energy needs. Changes over time in AMA show whether the patient has been deprived of protein or calories; AMA is one of the markers of nutrition status and can be a predictor of mortality.⁶ The formula to calculate AMA is as follows:

AMA=[MAC(cm)−(π×TSF cm)]2−10cm2(males)or6.5cm2(females)/4π

where MAC equals midarm circumference. A factor for sex is included for a corrected AMA formula because this accounts for bone area and provides a more accurate assessment of bone-free muscle area.⁷ Standards have been established for age groups throughout the life span.⁵

Albumin

Albumin constitutes most protein in plasma and is commonly measured at minimal cost. The half-life of albumin is 14 to 21 days, which reduces its usefulness for monitoring the effectiveness of nutrition in the critical care setting.⁸ The general availability and stability of albumin levels from day to day make it one of the most useful tests for assessing long-term trends (Table 21-2). It is important to determine an albumin value before the onset of disease or insult of injury. In combination with other findings, such as hair pluckability, edema, and poor wound healing, a serum albumin level less than 2.8 g/dl helps differentiate kwashiorkor from marasmus.⁹ Table 21-3 compares the two forms of PEM.^10,11

Transferrin

Transferrin is the transport protein for iron. It has a half-life of 8 to 10 days and is a better indicator of improved nutrition status than albumin. However, lack of iron influences transferrin values along with numerous other factors, including hepatic and renal disease, inflammation, and congestive heart failure.8,12

Transthyretin and Retinol-Binding Protein

Transthyretin, also called prealbumin, has a half-life of 2 to 3 days, and retinol-binding protein has a half-life of 12 hours. Each of these proteins responds to nutrition changes more quickly than either albumin or transferrin. However, numerous metabolic states, diseases, therapies, and infections influence the laboratory values.¹³

These two tests are more costly than testing for albumin. Levels of each protein are influenced by many factors other than nutrition status. Because these conditions are so common among critically ill patients, visceral protein markers are of limited usefulness for assessing nutrition deficiency and of greater usefulness in assessing the severity of illness and the risk for future malnutrition.¹⁴ Inflammatory metabolism causes a 25% decrease in the synthesis of these visceral proteins and causes lean body mass depletion and anorexia. It is important to evaluate their values with positive acute phase reactants (fibronectin, insulinlike growth hormone, C-reactive protein) where there is a reverse relationship.

Fibronectin, Insulinlike Growth Hormone, and C-Reactive Protein

Fibronectin, a glycoprotein with a half-life of 15 hours, plays an important role in wound healing. Because it is less affected by acute stress than other visceral proteins, it may become an important marker of nutrition status and an indicator of the efficacy of nutrition support.

Insulinlike growth hormone type 1, also called somatomedin C, is not routinely used in the clinical setting at this time in nutrition assessment. However, insulinlike growth hormone type 1 is used to measure protein status routinely in research settings. It may be more effective than other means in measuring nutrition status in the acute phase of stress, and its usefulness may increase.

C-reactive protein is an acute phase protein released with infection and inflammation. As transport proteins (albumin and prealbumin) decrease, levels of the acute phase proteins increase. Increased levels of C-reactive protein during stress, illness, and trauma have been linked to increased nutrition risk.¹²

Creatinine-Height Index

Because the rate of creatinine formation in skeletal muscle is constant, the amount of creatinine excreted in the urine every 24 hours reflects skeletal muscle mass. Predicted values are based on gender and height, with reference values of approximately 18 mg/kg body weight/day for women and approximately 23 mg/kg body weight/day for men. Values of 60% to 80% of predicted indicate a mild deficit of muscle mass, values of 40% to 60% of predicted indicate a moderate deficit, and values less than 40% of predicted indicate a severe depletion of muscle mass.⁵ Factors that influence creatinine excretion and complicate interpretation of this index include age, diet, exercise, stress, trauma, fever, and sepsis.¹²

Nitrogen Balance (Protein Catabolism)

Approximately 16% of protein is nitrogen, a useful factor in determining nitrogen balance. Because nitrogen is a major by-product of protein catabolism, its rate of urinary excretion can be used to assess protein adequacy. Nitrogen balance is calculated as follows:

Nitrogen balance=Nitrogen intake−Nitrogen losses

Nitrogen intake=Protein intake6.25

Nitrogen losses=UUN excretion in grams+3–5g(for insensible losses)

The conversion factor for dietary protein is 6.25 g of nitrogen per 1 g of protein. The amount of nitrogen excretion in the urine is typically measured as the 24-hour urinary urea nitrogen (UUN). Between 3 g/day and 5 g/day is added to the 24-hour urinary urea nitrogen for an estimate of the average daily unmeasured nitrogen lost through other sources (skin and gastrointestinal [GI] sloughing, hair loss, sweat, feces). Theoretically, by increasing exogenous protein, loss of endogenous protein is reduced. However, because of invalid 24-hour urine collections, alterations in renal or liver function, large immeasurable insensible losses (burns, high-output fistulas, wounds, or ostomies), and inflammatory conditions (generally occurring in critically ill patients), nitrogen balance calculations are inaccurate.

Immune Status

Impaired immunity (anergy) is a common finding in malnutrition, especially the kwashiorkor type. Two laboratory values, white blood cells and percentage of lymphocytes, have been used as measures of a compromised immune system. The result is often a reduction in lymphocytes, as measured by the total lymphocyte count (TLC): TLC = white blood cells × % lymphocytes. Many nonnutrition variables influence these laboratory values, and their usefulness in assessing nutrition status is questioned.¹⁵

Malnourished patients also may exhibit signs of depressed immunity when challenged by certain hypersensitivity skin tests. Because many disease states and drugs are associated with anergy, the value of this test also is limited.¹⁶

Twenty-four-Hour Recall

The 24-hour recall may be used in an interview in which the patient states the foods and the amount of each food consumed in the previous 24 hours. It is a straightforward commonly used technique that may easily be incorporated into a patient interview. Accuracy of the recall depends on the patient’s memory, the perception of serving size, and the skill of the interviewer to elicit complete information. Spreads, gravies, dressings, beverages, or snacks may be forgotten rendering a nutrient analysis inaccurate (i.e., especially total calories). Patients often report that the previous 24 hours were an aberration in their food intake.

Usual Intake Recall

To improve accuracy, patients may be asked to recall their usual daily food intake. However, neither the 24-hour recall nor usual intake recall method makes allowances for weekends, holidays, celebrations, or other major changes in food intake.

Food Diary or Food Record

The food diary or food record may be used to discover what a patient has been eating. In this case, the patient is asked to write down daily everything he or she has eaten. The patient may record food intake over an extended time, most frequently, a 3-day or 7-day period. Patients arrive with the dietary history in place rather than the interview time being used to reveal dietary practices. However, the act of recording everything eaten influences food choices, reducing the accuracy of a diet history.

Food Frequency Questionnaire

Patients may complete a multiple-item questionnaire choosing foods, serving size, and frequency in which the food was typically eaten to identify their food intake history. Such forms may cover a wide spectrum of foods or foods specifically of interest to current issues. Food frequency questionnaires list commonly used foods in typical serving sizes and often are linked to computer software for ease of analysis of the nutrient content of a patient’s diet. Accuracy of the nutrient analysis depends on the patient’s memory of foods eaten over an extended time and the patient’s perception of the serving size listed versus the serving size consumed.

Evaluation of Nutrition History

A 24-hour recall or usual recall, food diary or record, or a food frequency questionnaire may be evaluated for nutrient intake using the food guide pyramid My Pyramid, a nutrient analysis handbook, or a nutrient analysis software package.

Food Guide Pyramid

The U.S. Department of Agriculture and the U.S. Department of Health and Human Services have cooperated through the years to recommend balanced nutrition for U.S. citizens. The current graphic showing recommendations, My Pyramid,¹⁸ divides foods into groups based on the nutrient contribution of the grouping. For example, the grains group consists of breads, pasta, and rice, which all contribute B vitamins, folate, iron, and protein to the diet. The measure (ounces or cups) of servings is recommended for multiple calorie levels of each of the food groups: grains, protein, dairy, fruits, and vegetables. Consuming the recommended amounts of each should provide balanced, adequate nutrition for a healthy person (Table 21-4). The calorie level is based on age, gender, and activity level. The clinician or patient can go to http://mypyramid.gov to enter an individual’s data and be given the recommended calorie level. A comparison of a 24-hour recall with My Pyramid (Figure 21-3) gives an estimation of the adequacy of the patient’s diet.¹⁸ Individuals choosing meatless meals may select foods and number of servings from My Vegetarian Food Pyramid (Figure 21-4).¹⁹

TABLE 21-4

Dietary Guidelines for Americans—2005

Guideline	Application
Adequate nutrients with calorie needs	Consume a variety of nutrient-dense foods and beverages within and among the basic food groups while choosing foods that limit the intake of saturated and trans fats, cholesterol, added sugars, salt, and alcohol
	Meet recommended intakes within energy needs by adopting a balanced eating pattern, such as the USDA Food Guide or the DASH Eating Plan
Weight management	Maintain body weight in a healthy range, balance calories from food and beverage calories, and increase physical activity
	To prevent gradual weight gain, make small reductions in calorie intake and increase physical activity
Physical activity	Engage in regular physical activity
	Achieve physical fitness through cardiovascular conditioning, stretching, and resistance exercises
Food groups to encourage	Consume sufficient amounts of fruits and vegetables
	Choose a variety of fruits and vegetables every day
	Consume ≥3 oz equivalents of whole-grain products per day; half of the grains should be whole
	Consume 3 cups per day of fat-free or low-fat milk or equivalent milk products
Fats	Consume 10% of calories from saturated fats and <300 mg/day of cholesterol, and keep trans fats as low as possible
	Keep total fat intake between 20% and 35% of calories
	Choose lean, low-fat, or fat-free meat, poultry, dry beans, milk, and milk products
	Limit intake of fats and oils high in saturated and trans fats, and choose products low in such fats and oils
Carbohydrates	Choose fiber-rich fruits, vegetables, and whole grains often
	Choose and prepare foods and beverages with little added sugars
	Reduce incidence of dental caries by practicing good oral hygiene and consuming sugars and starches less often
Sodium and potassium	Consume <2300 mg of sodium per day (approximately 1 tsp of salt)
	Choose and prepare foods with little salt
	Consume potassium-rich foods (i.e., fruits and vegetables)
Alcoholic beverages	Alcohol should be consumed in moderation
	Pregnant and lactating women, individuals who cannot restrict their intake of alcohol, patients taking medications that interact with alcohol, and patients with specific medical conditions should not consume alcohol
Food safety	Clean your hands, food contact surfaces, and fruits and vegetables; meats and poultry should not be rinsed
	Separate raw, cooked, and ready-to-eat foods while shopping, preparing, or storing foods
	Cook foods to a safe temperature to kill microorganisms
	Refrigerate perishable food promptly, and defrost foods properly
	Avoid raw (unpasteurized) milk or any products made from unpasteurized milk, raw or partially cooked eggs or foods containing raw eggs, raw or undercooked meat and poultry, unpasteurized juices, and raw sprouts4

Compiled from U.S. Department of Agriculture and U.S. Department of Health and Human Services: Nutrition and your health: dietary guidelines for Americans, Washington, DC, 2005, U.S. Government Printing Office.

Children

The rapid growth seen in infancy slows during the toddler and school-age years. Nutrition must provide the nutrients for the steady growth of childhood. During these years, eating habits for a lifetime form, influenced by peers, parents, other relatives, television, and advertising. Children, especially an ill child, use food to manipulate parents and caregivers.²⁷

Adolescents

Adolescents continue to grow in spurts. Nutrient needs reflect growth and activity levels at this stage, which may be met by varying the number of servings of the same foods fed to the rest of the family. Food choices need to be based on the U.S. Dietary Guidelines and the Food Guide Pyramid, rather than on advertising or peer pressure. Food is an area where teens may express their growing independence by choosing new food patterns. Some patterns are based on needs or perceived needs, whereas others are based on image (e.g., pregnancy, muscle-building, calorie restricted or calorie excessive, fast foods, multiple snacks with skipped meals). Other adolescents choose vegetarian meal patterns.27,28

Adults

Previous adult nutrition recommendations aimed to decrease nutrient deficiencies. Beriberi, pellagra, and scurvy are no longer the primary health concerns of Americans. Heart disease, strokes, cancer, diabetes, and diseases that result from obesity drive the current recommendations. The U.S. Dietary Guidelines, the Food Guide Pyramid, and the food label together recommend meal patterns low in total fat, saturated fat, cholesterol, sodium, and sugar, while including abundant fruits, vegetables, and whole grains in food choices.4,18,19,29

Elderly Adults

The percentage of the U.S. population older than 65 years has increased from 4% in 1900 to 12.8% in 2000. This group of citizens is a very heterogeneous group ranging from very active individuals to institutionalized individuals. As the years pass, weight loss becomes a prime concern. Loss of total lean body mass (muscle) and an increase in total body fat secondary to a decrease in activity result in a decrease in the total calories needed. With the exception of increases from adult years in vitamins D and B₆ and minerals calcium and chromium,23–26 nutrient needs for elderly adults remain the same even though total calories have decreased. Often, elderly adults choose to consume fewer calories than previously consumed because of changes in the ability to taste, to smell, and to discern brightly colored foods; because of taste alterations from medications; and because of lack of interest in food procurement or food preparation. Socioeconomic issues may limit the ability to purchase food. In addition, the enjoyment of food decreases when eating alone.³⁰

Equipment and Technique

Good calorimetry results require extensive preparation. Box 21-4 outlines the key preparatory steps to be taken before testing.¹⁰ Indirect calorimetry can be performed with a Douglas bag, a Tissot spirometer, and CO₂ and O₂ gas analyzers. The patient’s expired gas is collected in the Douglas bag where it is sampled for O₂ and CO₂ concentrations; the Tissot spirometer measures expired volume. Commercially available metabolic carts are much easier to use. These automated systems either use a mixing chamber or perform breath-by-breath analysis. The breath-by-breath method provides real-time data, which may aid in ensuring optimal measurement conditions, particularly in mechanically ventilated patients.³⁶

Figure 21-5 shows the basic configuration for open-circuit indirect calorimetry during mechanical ventilation using a metabolic cart with mixing chamber. Gas sampled from the inspiratory limb of the ventilator circuit is assessed for fractional inspired oxygen (FiO₂) using a paramagnetic or zirconium oxide O₂ analyzer. Volume exhaled by the patient is measured using a flow transducer. The patient’s exhaled gas enters a mixing chamber, from which a sample is drawn to measure FeCO₂ (usually by infrared analysis) and FeO₂. Exhaled gas is returned to the ventilator after volume and gas concentration measurements. After all measurements are obtained, O₂ consumption, CO₂ production, and respiratory quotient (RQ) are computed using the equations shown in Box 21-5. All measurements must be corrected to standard temperature and pressure and dry conditions (STPD) before computation.³⁸ The values are used in the abbreviated Weir equation to determine REE:

REE=[(O2×3.9)+(CO2×1.1)]×1.44

Indirect calorimetry is more difficult to perform on spontaneously breathing patients, especially patients breathing supplemental O₂. Although a mouthpiece with nose clips or a mask can be used to collect expired gas, these items tend to alter the patient’s steady state and invalidate results.³⁶ Instead, most clinicians recommend using a plastic canopy that covers the patient’s head. Expired gases are cleared from the canopy by a preset flow of air; expired gas concentrations are sampled and corrected for the air dilution.

Because standard modes of O₂ therapy do not deliver a consistent FiO₂ to spontaneously breathing patients, special delivery systems must be used. To overcome this problem, the clinician can substitute a precise O₂ mixture for the air used to clear the canopy. Alternatively, one can place a large gas reservoir (e.g., a Douglas bag) between an O₂ flow source and the subject³⁶; this ensures that the FiO₂ remains stable throughout the test procedure.

Problems and Limitations

Indirect calorimetry is a technically complex procedure that requires rigorous attention to both instrument and procedure quality control. Regarding instrumentation, small errors in measurements can result in large errors in calculated O₂, CO₂, and energy expenditure. For this reason, the calorimeter’s gas analyzers and volume measurement device must be properly calibrated before each patient use. Gas analyzers should be accurate to the hundredth percent and linear over the clinical range of O₂ concentrations.38,39

Regarding procedure quality control, it is essential that measurements be made during steady-state conditions. Although proper patient preparation (see Box 21-4) is helpful in this regard, steady-state conditions can be confirmed only during the test procedure itself. A common standard for ensuring steady-state conditions is five consecutive 1-minute averages with a variability of 5% or less.³⁶

Perhaps the most significant problem in performing indirect calorimetry on mechanically ventilated patients is the presence of leaks (circuit, tracheal tube cuff, chest tubes).³³ Because any leak would invalidate test results, no procedure should begin until a leak-free patient-ventilator-calorimeter system is confirmed. Other sources of error during open-circuit indirect calorimetry of mechanically ventilated patients are listed in Box 21-6.³³

Interpretation and Use of Results

Results obtained from indirect calorimetry are used to assess metabolic status and to plan nutrition support. In regard to assessing metabolic status, the first step is to compare the REE obtained by calorimetry with the REE predicted by the Harris-Benedict equations. If the calorimetry REE is within 10% of the predicted value, the patient is considered normometabolic. Measured REEs greater than 10% above predicted values indicate a hypermetabolic state, whereas values less than 90% of predicted indicate hypometabolism.

The second step in metabolic assessment is to interpret the RQ. The RQ is the ratio of moles of CO₂ expired to moles of O₂ consumed. The RQ correlates to the ratio of percent calories provided to the percent calories required. Traditionally, the RQ has been used to determine substrate use, where carbohydrates have an RQ of 1.0, protein has an RQ of 0.82, and fat has an RQ of 0.7. Table 21-6 outlines the basic significance of the RQ relative to substrate use and traditional nutrition strategies.³⁶ The RQ has been shown more recently to have low sensitivity and reduced specificity in critically ill patients.³⁷ This finding limits the RQ as an indicator for substrate use. The RQ may be low in an overfed patient because of a shift in substrates secondary to inflammatory mediators. If the RQ is outside its physiologic range of 0.67 to 1.3, this should alert the clinician to assess the validity of the study.

Insufficient Energy Consumed

Malnutrition results from insufficient energy (calorie) intake over time. This insufficient intake leads to a state of impaired metabolism in which the intake of essential nutrients falls short of the body’s needs. Certain factors may place a patient at risk for malnutrition (Box 21-7).

Protein-Energy Malnutrition

Protein-energy malnutrition (PEM) has adverse effects on respiratory musculature and the immune response.¹⁵ PEM may be either primary or secondary. Primary PEM results from inadequate intake of calories or protein or both and is typically seen only in developing countries.¹⁰

Secondary PEM is due to underlying illness. Illness may cause (1) decreased caloric or protein intake (e.g., anorexia, dysphagia); (2) increased nutrient losses (e.g., malabsorption or diarrhea); and (3) increased nutrient demands (e.g., injury or infection).³² Secondary PEM may be present in 50% of hospital patients.

When PEM is due to inadequate nutrient intake or excessive loss, the body responds by decreasing its metabolic rate, ventilatory drive, thyroid function, and adrenergic activity.³⁴ As calorie intake decreases, energy for metabolic processes is initially supplied by converting liver glycogen stores into glucose (gluconeogenesis). However, liver reserves of glycogen are adequate for less than 1 day at rest and only a few hours during exercise.⁸ Thereafter, endogenous fat stores are mobilized in the form of free fatty acids (ketogenesis). When fat stores are depleted, nutrient needs must be met by catabolizing skeletal muscle protein. This type of PEM usually manifests as a gradual wasting process, as seen in patients with chronic diseases such as cancer and emphysema. The primary clinical sign is progressive weight loss.

When PEM is due to increased demand for nutrients, metabolism, thyroid function, and adrenergic activity all increase. Visceral protein levels tend to decrease early in the course of illness and are associated with impaired immunity.³⁴ This type of PEM typically occurs with acute catabolic disease, such as in sepsis, burns, or trauma. Weight loss and depletion of muscle and fat stores generally do not occur because of the rapidity of onset of the underlying disease.¹¹ The two types of PEM are often referred to as marasmus and kwashiorkor,^2,10,11 as previously described (see Table 21-3).

Micronutrient Malnutrition

The same problems that lead to PEM can produce deficiencies in micronutrients. Deficiencies of nutrients that are stored only in small amounts (e.g., water-soluble vitamins) or are lost through external secretions (e.g., zinc in diarrhea fluid or burn exudate) are quite common.¹⁰ Although the causes and results of micronutrient deficiencies are beyond the scope of this chapter, a few of the most common problems are described.

Signs of scurvy (vitamin C deficiency) may be observed in chronically ill patients and patients with alcoholism hospitalized for acute illnesses. Low folic acid blood levels are common wherever illness, alcoholism, or poverty is present. Alcoholism is also associated with thiamin deficiency. Zinc deficiencies can impair clotting, slow wound healing, and impair immunity. Magnesium deficiencies can result in cardiac, vascular, neurologic, and electrolyte abnormalities (hypocalcemia, hypokalemia) and in decreases in respiratory muscle strength. Hypophosphatemia is seen frequently with cachexia or alcoholism, especially in patients receiving intravenous glucose or taking antacids. Severe hypophosphatemia can result in decreased muscle strength and contractility and acute cardiopulmonary failure.

Protein

Amino acids or proteins are essential to maintaining or restoring lean body mass. Because illness usually increases protein catabolism and protein requirements, the Recommended Dietary Allowance (RDA) of 0.8 g/kg/day is generally insufficient for sick patients. Based on the assessment of the protein catabolism rate (see previous equations for nitrogen balance), protein intake may need to be doubled or tripled above the RDA (1.5 to 2.5 g/kg/day).15,40 Ideally, approximately 20% of a patient’s estimated calorie needs should be provided by protein. Higher percentages of protein may be needed in cachexic patients, critically ill patients, and patients with severe infections. However, whenever high protein intakes are given, the patient should be monitored for progressive azotemia (blood urea nitrogen >100 mg/dl).

Too much protein is harmful, especially for patients with limited pulmonary reserves. Excess protein can increase O₂ consumption, REE, minute ventilation, and central ventilatory drive.⁴¹ In addition, overzealous protein feeding may lead to symptoms such as dyspnea in patients with chronic pulmonary disease.

Carbohydrate

Adequate amounts of carbohydrates and fat help prevent protein catabolism. Glucose (dextrose) is the most commonly administered intravenous carbohydrate. For critically ill patients, 50% to 60% of the total daily calories can be in the form of simple carbohydrate.16,40,42 In an average-sized patient, daily glucose provision should be 300 to 400 g/day or less. Glucose blood levels should be monitored and maintained at less than 200 mg/dl.¹⁶

For patients with pulmonary disease or patients requiring mechanical ventilation, high carbohydrate loads can cause problems. High carbohydrate loads increase CO₂ production and the RQ, resulting in increased ventilatory demand, O₂ consumption, and work of breathing.⁴⁰ Although the observed effects are usually modest, some patients with limited functional reserves cannot tolerate these changes, resulting in development or worsening of ventilatory failure. More recent evidence indicates that this problem is probably more closely related to total calorie load (overfeeding) than to the proportion of carbohydrate in the diet.¹⁰ Based on this knowledge, overfeeding should be carefully avoided in patients with pulmonary disease and patients requiring mechanical ventilation.

Fat

The remaining calories (20% to 30%) should be provided from fat.39,40 In critically ill patients, current guidelines support increased levels of omega-3 fatty acid intake to modulate inflammation, to enhance immune response, and ultimately to support recovery.^41,42 Fat intakes greater than 50% of energy needs have been associated with fever, impaired immune function, liver dysfunction, and hypotension.

The initiation of nutrition support is determined by the patient’s nutrition status and the estimated length of time the patient will be unable to consume a diet by mouth to meet his or her nutrition needs. To ensure a satisfactory nutrition and metabolic response, early enteral nutrition begun within 24 to 48 hours provides significant benefits to critically ill patients, including reduced infectious complications and lengths of stay.⁴²

Enteral Feeding

Enteral feedings are the route of choice: “If the gut works, use it.” The enteral route is safer and cheaper to use than the parenteral route. Enteral feeding stimulates gut hormones, subjects nutrients to the absorptive and metabolic controls of the intestinal tract and liver, and produces less hyperglycemia (providing for better immune function) than the parenteral route. In addition, the buffering capacity of enteral feeding can improve resistance against stress ulcers. Finally, enteral feeding maintains a more normal intestinal mucosa than the parenteral route (the intestinal mucosa may undergo atrophy during parenteral nutrition).¹⁰

Parenteral Nutrition Support

When it is impossible to provide nutrition support through the GI tract, intravenous or parenteral nutrition support may be needed. Parenteral nutrition support can be administered through a peripheral or central vein. Ideally, the vascular access line should be isolated and maintained as a sterile route and not used for any other purpose. Because the volume and concentration of nutrients given through a small vein are limited, peripheral parenteral nutrition is generally considered only for short-term support. Mechanical, infectious, and metabolic complications have been reported in patients fed parenterally.⁵²