7: Nutritional Support and Total Parenteral Nutrition
CHAPTER 7 Nutritional Support and Total Parenteral Nutrition
Rohit R. Gupta and Roopa Kohli‐Seth
Icahn School of Medicine at Mount Sinai, New York, NY, USA
Background
Patients in the ICU are faced with unique challenges predisposing them to a malnourished state. This includes systemic infections, pronounced blood loss, mechanical ventilator support, multiorgan failure, limitation to volitional intake, and prolonged bed rest.
Critically ill patients present with a hypermetabolic state, which is marked by increased energy requirements, simultaneous protein synthesis and breakdown, increased lipolysis, and increased insulin resistance.
There has been a direct link between poor nutritional status and worse hospital outcomes including wound healing, iatrogenic infections, prolonged ventilator dependence, renal insufficiency, and endocrine dysfunction.
These factors have to be kept in mind while assessing and supporting the patient’s nutritional needs. Enteral nutrition refers to the administration of nutrients through the gastrointestinal tract. Parenteral nutrition refers to the intravenous administration of nutrients.
Goals of nutrition in the critically ill
Caloric requirements during critical illness are increased. If these are unmet by the dietary intake it will lead to a catabolic state with breakdown of a patient’s protein and lipid reserves.
Overfeeding is as harmful as underfeeding as it leads to excess production of CO2, potentially exacerbating 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 7.1Determining 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
Table 7.3Determining caloric and macronutrient requirements (g/day): carbohydrates.
Total caloric requirement = [weight in kg] × 25 kcal/kg/day
½ × Total caloric requirements = carbohydrates calories ½ × Total caloric requirements = protein + lipid calories
Carbohydrates in grams = carbohydrate calories/calories per gram carbohydrate* = ½ × total caloric requirements/calories per gram carbohydrate = 0.5 × [weight in kg] × 25 kcal/3.4 kcal/g = [weight in kg] × 3.67 g/kg/day
Table 7.4Determining caloric and macronutrient requirements (g/day): lipids and proteins.
Lipid calories + protein calories = ½ × total caloric requirements
Protein calories = protein required (from Table 7.2) × calories per gram protein* = 1.5 g/kg/day × [weight in kg] (For critical illness) × 4 kcal/g
Lipids in grams = lipid calories/calories per gram lipid* = [(½ × total caloric requirements) – protein calories]/calories per gram lipid = [(½ × total caloric requirements) – protein calories]/10 kcal/g
Electrolytes are added to the TPN solution to maintain osmotic and electrolyte homeostasis. Sodium, potassium, magnesium, and calcium are added as either chloride, acetate, or phosphate salts.
Standard additions include thiamine, folate, multivitamin, and trace elements that include selenium, chromium, copper, and manganese.
Glycemic control is achieved by adding and titrating the amount of regular insulin in the TPN solution.
Certain other medications can be added to the TPN solutions depending on their solubility and stability. These include H2‐blockers and heparin.
A minimum of 150 mL of free water is required for dissolution of the additives in the TPN solution. This can be increased if the patient has additional free water deficits.
Administration
Parenteral nutrition needs to be infused via a secure central venous access. This avoids complications of phlebitis and injury from extravasation. To minimize infections we recommend maintaining one port dedicated to TPN infusion.
The total volume of TPN solution is infused at a fixed rate over a 24 hour period. In patients with severe cholestasis or hepatic dysfunction, TPN can be cycled over 12 hours instead.
While initiating TPN it is important to begin with a half strength solution to minimize complications such as electrolyte derangements, hyperglycemia, and refeeding syndrome. The solution can be advanced over 1–3 days if monitoring panels remain stable.
While weaning patients off TPN, the caloric strength of the solution should be reduced slowly by 50% before discontinuing the TPN completely.
Adaptation to special situations
Severe respiratory failure:
In patients with respiratory failure or significant ventilator dependence, care should be taken to avoid overfeeding patients. Overfeeding shifts the body into lipid synthesis with concomitant elevation in arterial CO2 levels potentially lengthening ventilator support duration.
For the production of the same amount of energy, the oxidation of lipid generates 25% less CO2 than carbohydrates. In patients with hypercapneic respiratory failure, a greater proportion of the caloric requirement should be met by lipids.
Renal failure:
Renal failure leads to metabolic acidosis secondary to accumulation of numerous organic acids and increased loss of bicarbonate. TPN orders should be modified to provide additional bicarbonate (as acetate) and to avoid iatrogenic hyperchloremic acidosis.
Continuous renal replacement therapies can result in up to 65 g/day loss of protein through the dialysate/ultrafiltrate process. Consequently, it is important to replenish the protein stores at a higher rate (≥1.5 g/kg/day).
Volume overload is a common complication of renal failure and has adverse impacts on other organ systems including the heart and the lung. Minimizing the total volume of the TPN solution by concentrating the elements can be helpful in preventing this from arising.
Electrolytes need to be carefully monitored in patients to avoid the risk of life‐threatening hyperkalemia and other electrolyte imbalances.
Liver failure:
Patients have poor intrinsic synthetic function and may require greater protein replacement.
Repletion of micronutrient reserves including water‐soluble vitamins requires special consideration.
The use of branched chain amino acids in patients with hepatic encephalopathy has yielded mixed results and cannot be recommended as standard practice.
If total bilirubin is >4, remove copper and manganese by omitting trace elements to avoid toxicities (these elements are dependent on bile for excretion).
Monitoring
Daily bedside clinical examination is important to assess vascular access site appearance, volume status, neurologic function, weight monitoring, and readiness for initiation of enteral nutrition.
Labs include daily monitoring of electrolytes, glucose, liver, and lipid panels.
For patients on prolonged TPN support, less frequently monitored parameters include TSH, PTH, vitamin D, transthyretin, and carnitine levels.
24 hour nitrogen balance has been a validated marker for improved outcomes. It is calculated by subtracting the total nitrogen removed (via urine and stool) from the total nitrogen consumed. Every gram of negative nitrogen balance reflects a loss of 6.25 g of protein or 30 g of muscle mass.
Indirect calorimetry allows for measurement of resting energy requirements and respiratory quotients (RQs) using measurements of oxygen consumed and carbon dioxide produced. While it requires considerable investment in specialized equipment and training, indirect calorimetry can also provide information on whether there is ongoing over‐ or underfeeding.
A RQ of <0.8 signifies a considerable catabolic state where fats and proteins are being utilized as the source of energy. Conversely, in patients with a RQ >1.0 this signifies overfeeding. The goal RQ for patients on TPN is between 0.8 and 1.0.
Complications
Catheter infections remain the major infectious complication of parenteral nutrition. Common pathogens include Staphylococcus epidermidis, Candida, and S. aureus. For patients on TPN, an established catheter monitoring and maintenance protocol is essential. In patients with suspected infection, replacement of the central catheter at a different site is necessary.
Hyperglycemia can occur as a result of increased parenteral carbohydrate availability along with concomitant insulin resistance. This can be avoided by gradual titration to the patient’s carbohydrate goal as well as regular glucose monitoring. Goal blood glucose should be between 140 and 180 mg/dL.
Hypertriglyceridemia can occur as a result of lipids infusion, the underlying stress hormone production, and renal insufficiency. In severe cases (triglycerides >1000 mg/dL) this can lead to precipitation of acute pancreatitis. Lipids should be removed from TPN if the plasma triglyceride concentration exceeds 400 mg/dL.
Hepatic steatosis and cholestasis are known complications of long‐term parenteral nutrition. More often seen in children, the exact etiology of fatty liver remains unclear. In the absence of enteral nutrition it is also common for development of cholestasis and biliary sludge. Early resumption of enteral feeding is the definitive treatment. Experimental use of ursodeoxycholic acid and cholecystokinin have shown mixed results.
Refeeding syndrome can occur when TPN is introduced to patients who are severely malnourished. A rapid intracellular shift of potassium phosphorous and magnesium can precipitate rhabdomyolysis heart failure and cardiac arrhythmias. Careful monitoring and timely replacement of electrolytes can help avoid this complication.
Intestinal mucosal atrophy can occur in the absence of enteral nutrition. There is risk for bacterial overgrowth and translocation of bacteria in the setting of mucosal denudation and impaired local immunity. The early initiation of trophic feeds helps reduce the risk of infection and translocation. The role of antibiotic agents such as neomycin to inhibit bacterial growth remains poorly defined.
Reading list
Hartl WH, Jauch KW, Parhofer K, Rittler P. Complications and monitoring – guidelines on parenteral nutrition, chapter 11. Ger Med Sci. 2009; 7:Doc17.
Neuman T, Kohli‐Seth R, Wilson S, Bassily‐Marcus A. Total parenteral nutrition in the ICU: the Mount Sinai Hospital experience. ICU Director 2010; 1(4):203–9.
Plauth M, et al. ESPEN Guidelines on Parenteral Nutrition: hepatology. Clin Nutr 2009; 28(4):436–44.
Singer P, Pichard C. Reconciling divergent results of the latest parenteral nutrition studies in the ICU. Curr Opin Clin Nutr Metab Care 2013; 16(2):187–93.
Thibault R, Pichard C. Parenteral nutrition. World Rev Nutr Diet 2013; 105:59–68.
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