16. Nutrition

The first step is to determine when a patient would benefit from supplemental nutrition (either enteral or parenteral).

The following are the broad groups of patients who may benefit from supplemental nutrition.

Those with pre-existing malnourishment or severe weight loss (>15%)

Examples include the elderly, patients with advanced cancer, malignancies that result in decreased oral intake (e.g., oropharyngeal cancer, esophageal cancer), or chronic inflammatory states like Crohn’s disease.

Those subjected to significant inflammatory injury such as severe trauma, burns >20% total body surface area (TBSA), pancreatitis, and sepsis.

Those who will be or have been without nutrition for 7 days or more.

Patients unable to take any nutrition independently (e.g., intubated patients), who are not anticipated to do so within 24 to 72 hours.

Determining Nutritional Requirements

Once a patient has been deemed appropriate for nutritional supplementation, the nutritional needs must be calculated.

Calculating basal energy expenditure

First, estimate the total number of calories required on a daily basis.

Calculate the patient’s basal energy expenditure (BEE), which is a measure of basal metabolism in a rested and fasting state.

The traditional approach is to calculate the BEE using the Harrison-Benedict equation.

Men = 66.5 + (13.7 × wt) + (5.0 × ht) − (6.76 × age)

Women = 65.5 + (9.6 × wt) + (1.85 × ht) − (4.68 × age)

An alternative method of calculating the BEE is to simply multiply the patient’s weight in kilograms by 25

BEE = 25 kcal/kg × Wt (kg).

Calculating total caloric requirement

Once BEE has been calculated, multiply the baseline energy expenditure by a stress factor so as to estimate total caloric needs.

Mild stress is adjusted by 20% (i.e., BEE × 1.2), moderate stress by 40%, and severe stress can be 60% or higher.

In extreme cases, such as those patients with large burns (i.e., >50% TBSA), the stress factor can be 100% (i.e., double the BEE).

Almost every patient in an ICU will be under some degree of stress.

These estimates are generally adequately for estimating energy needs in clinical practice, but may not be accurate in the setting of hypermetabolism.

In such cases, indirect calorimetry can provide a quantitative measure of energy needs.

Indirect calorimetry calculates the total number of calories needed for baseline energy needs plus the thermal effect of food.

This is known as the resting energy expenditure (REE).

REE calculation is accomplished by measuring the subject’s oxygen uptake, CO₂ production, and applying the following formula:

REE = (3.9 × VO₂) + (1.1 × VCO₂) − 61

Limitations of indirect calorimetry include the need for specialized equipment and specially trained personnel, and the need for a FiO₂ < 50%.

Sources of calories

In addition to calculating the total number of calories, it is important to assure that patients receive these calories from appropriate sources, namely carbohydrates, lipids, and protein.

Each gram of carbohydrate contains 3.4 kcal/g of energy, lipids have 9 kcal/g and proteins comprise 4 kcal/g.

Protein: protein requirements typically start at 0.8 to 1.0 g/kg/day.

This number is also subject to adjustment by the stress factors discussed above.

Protein requirements can be quantitatively determined by measuring the nitrogen balance.

This requires measuring the quantity of protein that the patient is receiving per day, and measuring the nitrogen loss using a 24-hour urine collection.

Nitrogen balance is calculated using the following formula:

N₂ balance = N₂ intake − N₂ output

N₂ balance = (Protein intake (g)/6.25) − (Urine Urea Nitrogen + 4)

A reasonable goal is a positive nitrogen balance of 4 to 6 g/day.

Lipid: the lipid content in enteral formulas is usually limited to 30% or less because at higher levels it can promote diarrhea.

Higher lipid content may be considered in patients with CO₂ retention, as lipid metabolism generates less CO₂ per unit energy than carbohydrate metabolism.

Carbohydrate: the remaining calories should be composed of carbohydrate.

This typically consists of 40% to 50% of total calories.

Other components of nutritional support such as vitamin deficiencies and immunonutrition are discussed in the sections Feeding and Deficiency States.

Methods of Nutritional Support

Enteral nutrition

The next important consideration is the method by which nutrition is provided. There are two major methods: enteral or parenteral feeding.

Enteral nutrition should be utilized unless there is a contraindication; this is also discussed in the Section Feeding.

Enteral nutrition formulations may vary in order to address a patient’s underlying condition.

Examples include:

Lowering carbohydrate levels so as to decrease CO₂ in the setting of pulmonary disease

Lowering protein and electrolyte levels and total volume in renal failure

Lowering carbohydrate levels in diabetes

Lowering protein and high branched chain amino acid levels to avoid encephalopathy in liver failure

Elemental formulations for small bowel feeding

Others vary the amount of protein, quantity of fiber, or osmolarity

Enteral feeding can be delivered via several methods.

A common way is via a nasoenteric route through a weighted feeding tube.

Usually this tube terminates in the stomach, but its distal end may be varied based on underlying condition or injury.

There is inadequate evidence to demonstrate that the risk of aspiration is less with post-pyloric feeding compared to nasogastric feeding.

Parenteral nutrition

In cases where the gastrointestinal tract cannot be used, intravenous nutrition is indicated.

Peripheral parenteral nutrition provides a small number of total calories, but can be given through a peripheral vein.

More commonly utilized is total parenteral nutrition (TPN) which can provide total caloric requirements.

TPN requires a central venous catheter since its high osmolality can result in venous sclerosis, especially of smaller peripheral veins.

Determining the formulation of TPN solution requires utilization of the methods described earlier in this section.

First, the total calories are calculated followed by total protein requirements, both in grams/day and calories/day.

The protein calories are subtracted from the total calories leaving the combined total carbohydrate and lipid calories.

Typically, these remaining calories are split with 70% of the non-protein calories being carbohydrates and 30% or less as lipids.

It is important to consider lipid sources from medications (such as propofol) and account for them in these calculations.

Below is an example illustrating these calculations in an 80-kg patient with severe septic shock and an estimated stress factor of 50%:

Determine total calories: 25 kcal/kg/day × 80 kg = 2,000 kcal/day

Adjust for stress response: 2,000 kcal/day × 1.5 = 3,000 kcal/day (with stress factor)

Calculate total protein:

Stress factor for protein: 0.8–1.0 g/kg × 1.5 = 1.2–1.5 g/kg/day

Calories from protein (using 1.5 g for ease of calculation):

80 kg × 1.5 g/kg/day = 120 g protein daily

120 g protein × 4 kcal/g = 480 protein calories

Calculate carbohydrate and lipid calories:

Subtract protein from total calories:

3,000 kcal–480 kcal= 2,520 kcal carbohydrates + lipids

Carbohydrate calories:

2,520 × 0.7 = 1,764 kcal proteins

Lipid calories:

2,520–1,764=736 kcal lipids

Calculate grams of carbohydrate and lipids:

Carbohydrate: 1,764 kcal/3.4 kcal/g = 518.8 g carbohydrates daily

Lipids: 746 kcal × 9 kcal/g = 82.8 g lipids daily

Therefore, this patient would require a total 120 g of protein, 518.8 g of carbohydrates, and 82.8 g of lipid each day for total nutritional support.

These goals are accomplished using standard concentrations of carbohydrates, lipids, and proteins.

When TPN is started, it usually begins at half the total goal so as to reduce the risk of infusion complications.

The infusion is advanced to total calorie goals over the subsequent few days.

Daily electrolytes should be monitored and TPN composition modified for any specific requirements.

Infusion complications can include:

Hyperglycemia

Hypophosphatemia (associated with refeeding syndrome)

Fatty liver

Hypercapnea (2° elevated CO₂ from carbohydrate production)

The small bowel undergoes atrophy from disuse raising concerns for bacterial translocation.

Patients receiving nutritional support from TPN should be transitioned to enteral feeding whenever possible.

Considerations during the Initiation of Nutrition

Aspiration

There are some important issues that may arise during initiation of feeding.

One of the most common concerns with enteral nutrition is aspiration.

In addition to assuring proper tube placement, other steps taken to reduce that risk may include elevating the head of bed 30 to 45 degrees, distal placement of the feeding tube (beyond ligament of Treitz), delivering continuous enteral feeding, and adding promotility agents.

Such agents include prokinetic substances like metroclopramide and erythromycin or narcotic antagonists like alvimopan and naloxone.

Refeeding

Refeeding syndrome can occur when severely malnourished patients begin to receive nutritional supplementation.

Overfeeding

The saying that “the enemy of good is better,” can also be applied to nutritional supplementation.

Overfeeding can increase CO₂ production and increase respiratory demands for patients, potentially resulting in intubation or contributing to difficulty weaning from the ventilator.

Overfeeding of carbohydrates or lipids for prolonged periods of time can result in fatty deposition in the liver and hepatic failure.

This can be monitored by checking liver function tests.

Significantly elevated protein delivery can result in elevated blood urea nitrogen (BUN), especially when BUN levels rise to above 30% of baseline.

At extreme levels, this can pose challenges balancing protein needs with the potential for dialysis.

Assessment of Nutritional Support

Regardless of the administration method (enteral or parenteral), the next important consideration is to assess the adequacy of nutritional support.

One simple approach is to weigh the patient daily or weekly.

While simple, this approach has limited use in the ICU given the potential for large fluid shifts, and patients’ inability to stand on scales, forcing reliance on bed weights that may not be precise.

Another method to assess the adequacy of nutritional support is through laboratory assessment.

Serum albumin is a good surrogate measure of outpatient nutritional status and preoperative hypoalbuminemia is strongly associated with postoperative complications and mortality, but albumin takes 21 days to turnover in the body and serum levels may be affected (lowered) by acute inflammation.

Therefore, serum albumin is not optimal for following nutritional status on a short-term basis.

Assessment of nutritional status on a short-term basis relies on other markers.

Prealbumin (transthyretin) is used quite frequently because its turnover in the body occurs every 4 to 7 days.

Thus, it can be followed on a weekly or bi-weekly basis.

Serum prealbumin levels are also lowered by acute inflammation, and some authors recommend sending C-reactive protein levels at the same time as prealbumin assessment to attempt to account for this.

Another short-acting protein that can be used is retinol binding protein as its half-life is ~12 hours.

In addition to laboratory values, another way to assess the adequacy of nutritional therapy is through indirect calorimetry measuring both REE and the respiratory quotient.

The respiratory quotient is calculated through indirect calorimetry and is obtained with the following equation:

Respiratory Quotient (RQ) = (CO₂ eliminated)/ (O₂ produced)

The ideal respiratory quotient is 1.

When patients are being overfed, it is greater than 1.

For pure protein metabolism, the RQ=0.8 and with fat metabolism alone the RQ=0.7.

When the RQ is <0.7, the patient is undergoing starvation.

RQ values between these ranges represent combined metabolism.

For example, a value of 0.9 is a combination of carbohydrate and protein metabolism.

SUGGESTED READINGS

Deane A, Chapman MJ, Fraser RJ, Horowitz M. Bench-to-bedside review: the gut as an endocrine organ in the critically ill. Crit Care. 2010;14(5):228.

Deane AM, Summers MJ, Zaknic AV, et al. Exogenous glucagon-like peptide-1 attenuates the glycaemic response to postpyloric nutrient infusion in critically ill patients with type-2 diabetes. Crit Care. 2011;15(1):R35.

Gibbs J, Cull W, Henderson W, Daley J, Hur K, Khuri SF. Preoperative serum albumin level as a predictor of operative mortality and morbidity: results from the National VA Surgical Risk Study. Arch Surg. 1999;134(1):36-42.

Herndon DN, Hart DW, Wolf SE, Chinkes DL, Wolfe RR. Reversal of catabolism by beta-blockade after severe burns. N Engl J Med. 2001;345(17):1223-1229.

Heyland DK, Dhaliwal R, Drover JW, Gramlich L, Dodek P, Canadian Critical Care Clinical Practice Guidelines Committee. Canadian clinical practice guidelines for nutrition support in mechanically ventilated, critically ill adult patients. J Parenter Enteral Nutr. 2003;27(5):355-373.

Khan LU, Ahmed J, Khan S, Macfie J. Refeeding syndrome: a literature review. Gastroenterol Res Pract. 2011;pii:410971.

Marik PE, Zaloga GP. Immunonutrition in critically ill patients: a systematic review and analysis of the literature. Intensive Care Med. 2008;34:1980-1990.

Marino PL. The ICU Book. 2nd ed. Lippincott Williams & Wilkins. Chapters 46-48. 1998.

NICE-SUGAR Study Investigators, Finfer S, Chittock DR, et al. Intensive versus conventional glucose control in critically ill patients. N Engl J Med. 2009;360(13):1283-1297.

Norbury WB, Herndon DN. 3rd ed. Saunders. Chapter 31: Modulation of the hypermetabolic response after burn injury. In: Herndon DN, ed. Total Burn Care. 3rd ed. 2007.

Pontes-Arruda, A, Aragão, AMA, Albuquerque, JD. Effects of enteral feeding with eicosapentaenoic acid, [gamma]-linolenic acid, and antioxidants in mechanically ventilated patients with severe sepsis and septic shock. Crit Care Med. 2006;34(9):2325-2333.

Saffle JR, Graves C. 3rd ed. Saunders. Chapter 30: Nutritional Support of the burned patient. In: Herndon DN, ed. Total Burn Care. 3rd ed. 2007.

Sailhamer E, Alam H. In: Bigatello LM, senior ed. Critical Care Handbook of the Massachusetts General Hospital, 8th ed. Lippincott Williams & Wilkins. 2010:177-186, Chapter 11.

16.2

Stress Hormone Response

John O. Hwabejire and Hasan B. Alam

Introduction

Definition of stress: a variety of conditions or insults whose common denominators include:

Distortion of the body’s homeostasis

Threat to the body’s safety and well-being

Increase in the demands placed on the body’s metabolic machinery

Examples: trauma, surgery, sepsis, burns, prolonged starvation, strenuous exercise, hypotension, and critical illness (e.g., acute myocardial infarction, diabetic ketoacidosis)