By Elaine Chen, MD
Assistant Professor, Department of Internal Medicine, Division of Pulmonary and Critical Care Medicine, Section of Palliative Medicine, Rush University Medical Center, Chicago
Dr. Chen reports no financial relationships relevant to this field of study.
Enteral nutrition (EN), defined as any method of feeding that uses the gastrointestinal (GI) tract (including oral feeding), usually refers to the delivery of nourishment to the GI tract through a tube. Nutrition is of utmost importance for patients suffering from a critical illness, and EN is a mainstay of nutrition in the ICU. Malnutrition and nutritional risk are common in patients admitted to the ICU. The presence of critical illness causes the body to enter a catabolic state, putting patients at risk of development or worsening of malnutrition. The Society of Critical Care Medicine and the American Society for Parenteral and Enteral Nutrition have published and revised joint guidelines to offer evidence-based recommendations for how best to feed critically ill patients. The body of evidence continues growing, with an update to prior guidelines from 2009,1,2 published in 2016 both in Critical Care Medicine and the Journal of Parenteral and Enteral Nutrition.3,4 Many recommendations are based on expert consensus or “low-quality” evidence, which speaks more to the challenges in performing high-quality studies on extremely varied critically ill populations rather than the quality of the guidelines and should not deter clinicians from adhering to the guidelines.
CONSIDERATION OF ENTERAL NUTRITION
EN should be considered in any patient admitted to the ICU with nutritional risk. A full assessment of nutritional risk should be performed on all patients admitted to the ICU using a validated assessment tool that accounts for disease severity, such as the NRS 2002 or the NUTRIC.5 The NUTRIC score includes age, APACHE II, SOFA, number of comorbidities, days from hospital to ICU admission, and IL-6 (if available). Note that traditional serum markers of nutrition, such as albumin, pre-albumin, and transferrin, are not used for nutrition assessment in the ICU because of the acute phase response of these values.6 When patients can meet their nutritional requirements through oral intake, EN is not needed. However, if oral nutrition is contraindicated or insufficient, EN support is the first line of nutrition that should be considered in the critically ill patient. In the ICU, there are many situations in which EN is contraindicated, such as hemodynamic instability or an inability to use the GI tract (e.g., obstruction, bleeding, ischemia, perforation, and malabsorption). In these cases, many other factors must be evaluated in considering whether and when to start parenteral nutrition.
INITIATION OF ENTERAL NUTRITION
Generally, EN should be initiated as soon as possible into the course of critical illness through an acceptable route of access. In patients with low nutritional risk, if oral intake cannot be initiated immediately, waiting up to seven days prior to initiation is acceptable. However, in patients at moderate to high nutritional risk, initiation of EN should begin as soon as 24-48 hours after admission to the ICU or after oral intake is deemed unsuitable. Early initiation of enteral feeds has benefits beyond improving nutritional status compared with withholding or delaying EN. These benefits may exist even with minimal volumes, also known as trophic feeds. By maintaining villous height and supporting immunocytes in the gut-associated lymphoid tissue, EN helps maintain the functional integrity of the gut and modulate the systemic stress immune responses.7 A meta-analysis of 21 randomized, controlled trials showed that early EN was associated with a significant reduction in mortality (relative risk [RR] = 0.70; 95% confidence interval [CI], 0.49-1.00; P = 0.05) and infectious morbidity (RR = 0.74; 95% CI, 0.58-0.93; P = 0.01) compared with delayed EN.3
To initiate EN, a route of access must be established. In most critically ill patients, EN access in the stomach via orogastric (OG) or nasogastric (NG) tube is acceptable. However, small bowel enteral access is indicated in those critically ill patients at high risk for aspiration or with documented intolerance to gastric EN. There is conflicting evidence in the literature, but recent aggregate data show a decrease in risk of pneumonia in small bowel EN but no difference in mortality or length of stay compared with gastric EN.3 Evidence for adequate bowel function such as passage of flatus/stool or presence of bowel sounds are not required prior to initiation of EN. Bowel sounds are indicative of contractility but not mucosal integrity, barrier function, or absorptive capacity. Hypoactive or absent bowel sounds are associated with worsened prognosis and higher mortality.8
SELECTION OF ENTERAL NUTRITION
Selection of EN formula and dosage is complex and should be optimized together with a registered dietitian. There are both general guidelines as well as specific recommendations for different ICU populations. In selecting a formula from the myriad available, first, of course, is to know what is available in your hospital. In general, standard formulas, which are isotonic or near isotonic with 1-1.5 kcal/mL, are recommended for initiation of EN in the ICU. Sometimes, fiber is added to EN formulas to promote bowel regularity or treat diarrhea. It is not recommended to use fiber-containing formulas prophylactically in the adult critically ill patient; in fact, fiber-containing formulas should be avoided in patients at high risk for bowel ischemia or severe dysmotility. However, fiber-containing formulas can be considered in the presence of persistent diarrhea. Assessment of the patient’s digestive and absorptive capabilities of the patient also can aid with formula selection. If a patient experiences malabsorption or maldigestion, a peptide-based elemental formula can be considered. If the patient is fluid-restricted, such as in a volume-overloaded state or with renal failure, a concentrated 2 kcal/mL formula should be considered. There are many specialty type formulas available, such as immune-modulating formulations made with arginine, eicosapentaenoic acid, docosahexaenoic acid, glutamine, and nucleic acid, or immune-enhancing formulations with anti-inflammatory lipid profiles and antioxidants. Data are not yet strong enough to recommend their routine use in the ICU.
DOSING OF ENTERAL NUTRITION
The dose for EN includes both goal calories and rate of delivery. Indirect calorimetry (IC) is suggested as the best mode for assessing energy requirements, but may not always be available or accurate. IC measures the change in concentration of pulmonary gas to determine the resting metabolic rate, resting energy expenditure, and respiratory quotient. Although it is considered the gold standard measurement for energy expenditure, it is expensive, labor-intensive, and requires repeated measurements. Factors in the critically ill patient such as air leaks, chest tubes, supplemental oxygen, ventilator settings, renal replacement therapy, anesthesia, or excessive movement may affect the accuracy of IC.9 In the absence of accurate IC, a simplistic weight-based equation is used to determine energy requirements at 25-30 kcal/kg/day. In obese, underweight, and volume-overloaded patients, this also would be inaccurate, and adjustments are made based on body mass index (BMI). Generally, one should aim to provide more than 50-65% of goal calories, with protein of 1.2-2.0 g/kg of actual body weight, per day.3 In most critically ill patients, continuous infusion EN will lead to less intolerance than bolus gastric EN. Trophic feeds are defined as 10-20 mL/hour, 10-20 kcal/hour, or up to 500 kcal/day. Trophic feeds are intended to prevent mucosal atrophy and maintain gut integrity in low- to moderate-risk patients, but they may be insufficient to achieve the nutritional goals desired for EN therapy in high-risk patients.
MONITORING OF ENTERAL NUTRITION
After initiation of EN, periodic monitoring is important to evaluate for tolerance and adequacy of EN. Signs of intolerance may include vomiting, abdominal distension or discomfort, high NG output, high gastric residual volumes (GRVs), diarrhea, reduced flatus or stool, or abnormal radiographs. Traditionally, GRVs of 200-250 mL were considered threshold levels to temporarily stop EN. GRVs no longer are recommended for routine monitoring of tolerance. If used, EN should be held only if GRV is > 500 mL in the absence of other signs of intolerance. Instead, careful physical exam, review of abdominal radiographs, and evaluation of clinical risk factors for aspiration are recommended for monitoring critically ill patients receiving EN.
For the purposes of this review, there is a focus on three special populations that are common in the medical ICU setting.
SHOCK AND SEPSIS
Shock and sepsis are common conditions in the ICU, often requiring aggressive fluid resuscitation and vasopressors. It is recommended that EN be initiated within 24-48 hours of presentation in this population once hemodynamically stable, as early EN is associated with decreased hospital mortality in those on vasopressors.10 EN should be withheld when vasopressors are initiated or escalated, but may be provided once patients are on “chronic, stable, low” doses of vasopressors.3 Specifically in septic shock, hemodynamic stability is defined as adequate perfusion pressure, stable doses of vasoactive drugs, stabilized or decreasing levels of lactate and metabolic acidosis, and mean arterial pressure (MAP) ≥ 60 mmHg. Risks of using the GI tract in shock include subclinical ischemia and reperfusion injury involving the intestinal microcirculation, and, very rarely, ischemic bowel. Patients on vasopressors should be monitored very closely for early signs of gut ischemia. If signs exist, feeding should be held. Trophic feeds, at 10-20 kcal/hour or up to 500 cal/day are recommended for the initial phase of sepsis, and it may be most appropriate and optimal to maintain trophic rates for the first 24-48 hours after initiation of feeds.11 Special formulations have been considered in shock and sepsis. There was previous excitement that immune-modulating formulas (which may contain glutamine, antioxidants, trace elements, butyrate, and/or arginine) or immune-enhancing formulas (which may contain omega-3 fatty acids, gamma-linolenic acid, and/or antioxidants) may improve outcomes and decrease length of stay (LOS) or organ dysfunction.12 However, follow-up randomized, controlled trials have failed to show a clear benefit in a medical ICU setting; thus, they currently are not recommended. Selenium and zinc have been studied for their antioxidant properties with some promise, but there are no formal recommendations for their use yet because of conflicting studies.
Patients in the ICU may develop respiratory failure requiring invasive or noninvasive ventilation support. First, safety of feeding must be considered; it is difficult to safely deliver EN to patients on noninvasive ventilation. Patients on noninvasive ventilation receiving EN via NG or OG access demonstrate higher rates of mucus plugging, aspiration pneumonia, and airway complications.13 With invasive mechanical ventilation, gastric access produces the dual benefit of GI decompression and nutritional access. High-fat/low-carbohydrate formulations have been proposed to reduce the duration of mechanical ventilation in patients with respiratory failure compared to standard formulas, but the small trial that showed initial promise was unable to be reproduced in a larger study.14 To optimize volume status in patients with respiratory failure, fluid-restricted, energy-dense EN formulations with 1.5-2 kcal/mL should be considered. Any patient with acute respiratory distress syndrome who is expected to require mechanical ventilation for at least 72 hours should start EN. Whether mechanically ventilated patients should be prescribed trophic feeds or full feeds is under investigation. In 2012, the EDEN study compared one week of trophic feeds with full EN and showed a lower incidence of GI intolerance in the trophic group, who also did not experience worse outcomes compared with those who received full feeds; these were studies of patients who exhibited lower nutrition risk.15 In contrast, the INTACT study published in 2015 showed that providing intensive nutrition (> 75% of estimated energy needs) led to higher mortality than standard nutrition; the standard nutrition group received approximately 55% of estimated energy.16 For now, aiming for 50-65% of energy needs may be an appropriate goal in ARDS as we await further studies.
Obesity is common in the ICU, with a prevalence of up to 75%.17 Although obese patients would seem to possess an excess nutritional reserve, critically ill obese patients are nonetheless at risk for malnutrition. In fact, 57% of hospitalized adults with a BMI > 25 kg/m2 show evidence of malnutrition. Obese patients are more likely than lean subjects to demonstrate difficulty using fuel and to lose their lean body mass from protein metabolism. There is a U-shaped mortality curve for BMI, with highest mortality in those with Class III obesity (BMI > 40 kg/m2) as well as those with a BMI < 25 kg/m2 (underweight or normal weight). Those with moderate obesity, with a BMI of 30-40 kg/m2, actually demonstrate the lowest mortality rate. This is called the obesity paradox.18 Use of high-protein, hypocaloric feeding is recommended to preserve lean body mass, mobilize adipose stores, and minimize the metabolic complications of overfeeding. If IC is used, the EN regimen should target 65-70% of energy requirements for all classes of obesity. If IC is not used, dosing for BMI of 30-50 kg/m2 should be 11-14 kcal/kg actual body weight, and and for BMI > 50 kg/m2, dosing should be 22-25 kcal/kg ideal body weight. Protein recommendations are based on ideal body weight and should be provided at about 2.0 g/kg/day for BMI of 30-40 kg/m2 and 2.5 g/kg/day for BMI > 40 kg/m2.19
EN support is an important and complex component in the management of critically ill patients. (See Table 1.) EN should be considered early in those who cannot maintain their nutritional needs via oral feeding, ideally started within 24 to 48 hours of presentation in the ICU or when hemodynamics stabilize. Providers should consult a nutritionist to assist with nutrition risk stratification and selection and dosing of formula. Clinical conditions, such as shock and sepsis, respiratory failure, malabsorption, malnutrition, and obesity, may alter the nutritional need. (See Table 2.) There are more risks associated with overfeeding than underfeeding, and even trophic feeds produce significant benefit. Evidence is constantly evolving regarding best practices in the nutritional management of the critically ill patient, so stay alert for updated guidelines.
- McClave SA, Martindale RG, Vanek VW, et al. Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient: Society of Critical Care Medicine (SCCM) and American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.). JPEN J Parenter Enteral Nutr 2009;33:277-316.
- Martindale RG, McClave SA, Vanek VW, et al. Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient: Society of Critical Care Medicine and American Society for Parenteral and Enteral Nutrition: Executive Summary. Crit Care Med 2009;37:1757-1761.
- McClave SA, Taylor BE, Martindale RG, et al. Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient: Society of Critical Care Medicine (SCCM) and American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.). JPEN J Parenter Enteral Nutr 2016;40:159-211.
- Taylor BE, McClave SA, Martindale RG, et al. Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient: Society of Critical Care Medicine (SCCM) and American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.). Crit Care Med 2016;44:390-438.
- Heyland DK, Dhaliwal R, Jian X, Day AG. Identifying critically ill patients who benefit the most from nutrition therapy: The development and initial validation of a novel risk assessment tool. Crit Care 2011;15: R268.
- Davis DJ, Sowa D, Keim KS, et al. The use of prealbumin and C-reactive protein for monitoring nutrition support in adult patients receiving enteral nutrition in an urban medical center. JPEN J Parenter Enteral Nutr 2012;36:197-204.
- Kang W, Kudsk KA. Is there evidence that the gut contributes to mucosal immunity in humans? JPEN J Parenter Enteral Nutr 2007;31:246-258.
- Reintam A, Parm P, Kitus R, et al. Gastrointestinal symptoms in intensive care patients. Acta Anaesthesiol Scand 2009;53:318-324.
- Schlein KM, Coulter SP. Best practices for determining resting energy expenditure in critically ill adults. Nutr Clin Pract 2014;29:44-55.
- Khalid I, Doshi P, DiGiovine B. Early enteral nutrition and outcomes of critically ill patients treated with vasopressors and mechanical ventilation. Am J Crit Care 2010;19:261-268.
- Levy MM, Artigas A, Phillips GS, et al. Outcomes of the surviving sepsis campaign in intensive care units in the USA and Europe: A prospective cohort study. Lancet Infect Dis 2012;12: 919-924.
- Beale RJ, Sherry T, Lei K, et al. Early enteral supplementation with key pharmaconutrients improves sequential organ failure assessment score in critically ill patients with sepsis: Outcome of a randomized, controlled, double-blind trial. Crit Care Med 2008;36:131-144.
- Kogo M, Nagata K, Morimoto T, et al. Enteral nutrition is a risk factor for airway complications in subjects undergoing noninvasive ventilator for acute respiratory failure. Resp Care 2017;62:459-468.
- Mesejo A, Acosta JA, Ortega C, et al. Comparison of a high-protein disease-specific enteral formula with a high-protein enteral formula in hyperglycemic critically ill patients. Clin Nutr 2003;22:295-305.
- National Heart, Lung, and Blood Institute Acute Respiratory Distress Syndrome (ARDS) Clinical Trials Network; Rice TW, Wheeler AP, Thompson BT, et al. Initial trophic vs full enteral feeding in patients with acute lung injury: The EDEN randomized trial. JAMA 2012;307:795-803.
- Braunschweig CA, Sheean PM, Peterson SJ, et al. Intensive nutrition in acute lung injury: A clinical trial (INTACT). JPEN J Parenter Enteral Nutr 2015;39:13-20.
- Dennis DM, Trevenen M. Prevalence of obesity in an intensive care unit patient population. Intensive Crit Care Nurs 2016;35:52-56.
- Valentijn TM, Galal W, Tjeertes EK, et al. The obesity paradox in the surgical population. Surgeon 2013;11:169-176.
- McClave SA, Kushner R, Van Way CW 3rd, et al. Nutrition therapy of the severely obese, critically ill patient: Summation of conclusions and recommendations. JPEN J Parenter Enteral Nutr 2011;35:88S-96S.