respiratory failure and fatty deposition in the liver and other organs.
A patient’s response to nutritional support is altered by the underlying stress of illness. Care needs to be taken to ensure that metabolic derangements such as hyperglycemia or hypertriglyceridemia are not precipitated or worsened.
Micronutrients including minerals, vitamins, and trace elements are essential components of nutritional therapy in critically ill patients. Their deficiency can present with systemic disorders.
Enteral nutrition
Nutrition provided via the gastrointestinal tract is the preferred route for nutritional support. Its benefits include a more physiologic preparation, less cost, and fewer associated metabolic complications.
Enteral nutrition allows for preservation of mucosal integrity and stimulation of protective gut functions including immunomodulatory and endocrine effects.
Enteral nutrition has associated potential risks of aspiration, malabsorption, gut ischemia, and variable tolerance to administered feeds.
In the absence of contraindications, enteral nutrition should be started within 48–72 hours of ICU admission. Early feeding is associated with lower gut permeability, smaller energy deficits, and concomitant reductions in morbidity and mortality.
For initiating enteral feeds a temporary enteral access is secured via a nasogastric feeding tube. In patients with anticipated prolonged inability to swallow, a percutaneous endoscopic gastrostomy (PEG) tube can be utilized.
In patients with gastric distension, poor motility, or concern for aspiration, post pyloric feeding tubes can be used instead of gastric feeding tubes.
Enteral nutrition formulation
Enteral formulas are premixed solutions with a fixed ratio of non‐protein calories to protein. They are designed to provide an energy density of 1–1.5 kcal/mL of solution. The total volume of feed over 24 hours is calculated by dividing the calories required (25 kcal/kg/day) by the energy density of the feeding.
The protein component in formulary feeds can be either whole protein (soy/casein), peptides, or elemental amino acids depending on the patient’s ability to digest and absorp protein.
The use of supplements such as arginine, glutamine, antioxidants, and omega‐3 polyunsaturated fatty acids has been studied with the aim of reducing inflammation and modulating immune response. While animal and in vitro studies have been promising, there is currently inadequate clinical evidence to recommend their widespread adoption.
Volume loss and hypernatremia can be corrected by supplying free water along with the enteral feeds.
Total parenteral nutrition
TPN formulations are designed to intravenously provide all necessary macronutrients and micronutrients through one solution. It enables consistent and predictable provision of nutritional intake to the patient.
Macronutrients include carbohydrates and lipids which serve as the primary source for calories. Protein in the TPN solution is designed to meet the body’s requirement for increased synthetic function as well as a source for energy.
Micronutrients include electrolytes (sodium, potassium, magnesium, calcium, chloride, selenium, chromium), buffers (phosphate, acetate), and other compounds (vitamins, insulin, famotidine).
The total volume of the formulation is adjusted in accordance with the patient’s volume status as well as to meet their daily fluid requirements.
TPN formulations need to be compounded daily to avoid degradation and microbial contamination.
Indications for TPN
Diseases of the gastrointestinal system that preclude all enteral feeding for prolonged periods of time:Trauma, bowel perforations, surgical resections, fistulae, and diversions.Inflammatory disorders including severe necrotizing pancreatitis, appendiceal abscess, and gangrenous cholecystitis.Immune disorders including bowel transplant rejection and graft versus host disease.Infections including severe Clostridium difficile infections, infected peritonitis, and intra‐abdominal abscesses.Malabsorptive disorders including short gut syndrome and inflammatory bowel disease.Neoplastic disorders including obstructive tumor growth, radiation‐ and chemotherapy‐induced mucositis, fistulae, and strictures.Vascular disorders including bleeding and ischemic bowel disease.
Disease conditions that preclude adequate intake of enteral nutrition:Profoundly cachectic patients with poor nutritional reserves.Inability to obtain or maintain enteral access.Enteral nutritional meeting <50% of nutritional requirements.Worsening signs of gastrointestinal tolerance.
Continuation of ongoing parenteral nutrition.
Energy and macronutrient dosing calculations
Daily energy requirements are calculated based on the patient’s ideal body weight (IBW) (Table. 7.1). Adjusted body weight is used if the patient’s weight exceeds IBW by 20%. If the patient’s weight is less than 90% of IBW then the actual weight is utilized.
The estimated daily energy consumption used to determine total energy requirements is 25 kcal/kg/day. In obese patients with a body mass index (BMI) >30, energy requirements are decreased to 22 kcal/kg/day.
The amount of protein is determined by factors including severity of illness, surgical trauma, and organ system failure (Table 7.2). This allows the protein intake to be matched to the body’s synthetic requirements. At the same time, the calories contributed by the proteins are counted towards the total energy requirements, to minimize the risk of overfeeding.
Half the total caloric requirements are supplied by carbohydrate. The remaining half of caloric requirements is divided between lipids and proteins in a ratio determined by the patient’s protein requirement. The amount of each component is calculated using the energy equations (Tables 7.3 and 7.4).
Table 7.1 Determining patients’ ideal body weight (IBW).*
| Males IBW = 48 kg for the first 5 feet of height + 2.7 kg for each inch taller |
| Females IBW = 45.5 kg for the first 5 feet of height + 2.2 kg for each inch taller |
| For patients with actual weight greater than 1.2 times IBW the adjusted body weight (ABW) is used ABW= ([actual weight – IBW] × 0.25) + IBW |
| For patients with actual weight less than 0.9 times IBW the actual body weight is utilized |
*
