Nutritional Support in the ICU
By Richard J. Wall, MD, MPH, Pulmonary Critical Care & Sleep Disorders Medicine, Southlake Clinic, Valley Medical Center, Renton, WA, is Associate Editor for Critical Care Alert.
Dr. Wall reports no financial relationships relevant to this field of study.
Nutritional support is an essential component of caring for critically ill patients. Historically, nutritional support in the critically ill patient was viewed as an adjunctive modality for supplying exogenous fuel to the patient during their illness. The goals were mostly to avoid malnutrition, immune suppression, and metabolic derangements. More recently, there has been a paradigm shift. Nutritional support is now considered a therapy, with goals of attenuating metabolic stress responses, preventing apoptosis (cell death), reducing oxidative stress in other organs, and modulating the body’s immune response.
We often forget that the gut is the largest immune organ in the body. The gastrointestinal (GI) tract is an entry point to the external environment, and the presence (or absence) of nutrition has a direct effect on immune function. Systemic immunity is directly influenced by gut epithelial integrity, splanchnic blood flow, and bacterial flora colonizing the intestinal lumen. In this article, I will review general issues relating to nutritional support in ICU patients.
Enteral vs Parenteral
Enteral nutritional (EN) is the preferred route whenever feasible. EN maintains the functional and structural integrity of the intestinal epithelium by stimulating contractility and releasing trophic substances such as bile salts, gastrin, bombesin, and motilin.1 Gut feeding releases secretory IgA, which coats bacteria and prevents them from adhering to the epithelial wall. The contractility from feeding subsequently sweeps bacteria downstream, thereby controlling bacterial numbers and reducing translocation. EN also stimulates blood flow to gut-associated lymphoid tissue (GALT), which in turn supports the education of naive CD4 helper lymphocytes that migrate to other organs such as the lungs, liver, and kidneys.
Parenteral nutrition (PN) does not produce the same effects as EN. Critically ill patients without luminal nutrition lose both functional and structural integrity of the intestinal epithelium. Reduced contractility leads to bacterial overgrowth, emergence of virulent luminal organisms, and intestinal cell apoptosis. The apoptosis creates structural defects that allow bacterial products to aggravate the immune system. In turn, activated macrophages prime neutrophils, which then travel to distant organ sites and cause further oxidative stress. In other words, the absence of luminal food raises the overall systemic inflammatory response.
Timing of Nutrition
Early EN can improve patient outcomes and reduce ICU length of stay. In a meta-analysis of 14 prospective, randomized controlled trials (RCTs), early EN started within 48 hours after admission was associated with a 24% reduction in infectious complications (P = 0.04) and a 32% reduction in mortality (P = 0.06), as compared with feedings delayed until after 48 hours.2 Lewis et al conducted a meta-analysis of 12 RCTs in patients undergoing major elective surgery or needing surgical critical care.3 Patients randomized on the operating table to early tube feedings had a 28% lower rate of infections and a nearly 1-day reduction in hospital length of stay (P = 0.0001), as compared to patients receiving no nutrition. Recent guidelines from the Society of Critical Care Medicine and American Society for Parenteral and Enteral Nutrition suggest that enteral feeding routinely be started “early” (within 24-48 hours following admission).4
Patients in Shock
Many clinicians withhold EN if patients are hemodynamically unstable or on vasopressors. The rationale is that nutrient delivery to the gut in the setting of diminished cardiac output can cause intestinal ischemia and small bowel necrosis. In reality, ischemic bowel is a rare complication of EN, occurring in < 1% of cases. In addition, most cases of ischemic bowel were reported in the past with use of surgical jejunostomy tubes, not the more commonly used gastric tubes.
Guidelines now advise that EN be cautiously provided to patients who are adequately resuscitated and on stable low vasopressor doses. EN can be to either the stomach or small bowel.4 EN infused directly into the small bowel, however, should be held until the patient is normotensive (mean arterial pressure > 60 mmHg). In general, when feeding a patient with recent hemodynamic instability, start with a low rate, progress slowly to goal, and use an isotonic non-fiber containing formula. Monitor for signs of intolerance that might indicate early gut ischemia (e.g., abdominal distension, increasing residual volumes, increasing metabolic acidosis and/or base deficit).
Bowel Sounds and Flatus
In ICU patients, the presence of bowel sounds and passage of flatus or stool are not required to initiate EN. Bowel sounds only indicate contractility and do not necessarily reflect mucosal integrity, barrier function, or absorptive capacity. Studies describe success rates of reaching nutrition goals within the first 72 hours ranging from 30-85%, but when enteral feeding protocols are followed, success rates are typically 70-85%. Numerous studies, mostly in surgical patients, have reported feasibility and safety of early enteral feeding with the first 2 days. Bowel sounds and flatus were not strong factors in these high success rates.
Gastric vs Post-Pyloric
EN can be achieved via either an oral or nasal enteric tube positioned in the stomach or small intestine. Gastric tubes are usually easier to place. Most institutions confirm correct position by portable radiograph. Other verification methods include auscultation and aspiration of gastric contents. However, studies have shown that auscultation alone is ineffective, and measuring pH of aspirated gastric contents is unreliable because of widespread use of acid-reducing medications. In fact, no single non-radiographic method exists that can reliably differentiate between respiratory, esophageal, gastric, and small bowel placement of blindly inserted feeding tubes in the fed or unfed state.5 Sometimes, a combination of non-radiographic techniques can be used successfully.
Numerous studies have evaluated gastric vs jejunal feeding in various ICU populations. Although there is probably less gastroesophageal reflux with small bowel feeding, studies have not consistently found that this translates to a reduction in ventilator-associated pneumonia (VAP). Of the three meta-analyses looking at this subject,6-8 only one showed a significant reduction in VAP (RR = 0.76; 95% confidence interval, 0.59-0.99; P = 0.04).8 Furthermore, this effect was driven largely by a single study, and when the study was removed from the meta-analysis, the difference was no longer significant. No study has ever shown a difference in mortality between gastric vs post-pyloric feeding.
Achieving Success with Enteral Nutrition
For various reasons, patients tend to receive only 50% of their target EN calories.9 Providers tend to under-order calories, and feedings are held 20% of the day in the average ICU patient. Many of these stoppages are avoidable. Common reasons for cessation include “NPO after midnight” orders, elevated gastric residual volumes, and tube displacement.
Every ICU should have a formal protocol for ensuring adequate nutritional support, and daily ICU multidisciplinary rounds should include a registered dietician whenever possible. Evidence-based nutritional protocols increase the overall percentage of calories provided. Protocols standardize delivery of nutritional support, promote earlier initiation of feeding, improve patient tolerance, and minimize unnecessary stoppages. Numerous protocols are available online. A recent publication from Harborview Medical Center in Seattle outlines its current approach.10
Gastric residual volumes do not correlate well with gastric emptying, risk of aspiration, or incidence of pneumonia. Studies have showed that raising the gastric residual volume cutoff value from 50-150 mL up to 250-500 mL does not increase the risk for regurgitation, aspiration, or pneumonia.11 Gastric residual volumes < 200 mL should rarely prompt cessation of feeds. Volumes of 200-500 mL should raise concern, but the feeds should not automatically stop if there are no other signs of intolerance. Rather, one should implement measures to reduce aspiration and continue to monitor closely. Unless contraindicated, all intubated patients receiving EN should have their head of bed elevated to 30-45 degrees regardless.
Prokinetic motility agents such as metoclopramide have been shown to improve gastric emptying and EN tolerance, but they have not changed other outcomes such as pneumonia or mortality. Other steps to decrease aspiration risk and pneumonia are probably more effective, including chlorhexidine mouthwash twice a day, minimizing sedation and analgesia when possible, minimizing transport out of the ICU for tests and procedures, and moving the patient to a unit with a lower patient:nurse ratio.
There are now a myriad of immune-modulating enteral formulations and supplements. Several meta-analyses suggest that the proper use of these formulations is associated with significant reductions in duration of mechanical ventilation, infectious complications, and hospital length of stay.12 None of the studies have showed these products reduce mortality, however. In addition, the benefits are mainly apparent in postoperative and not medical ICU patients. In fact, the agents seem to have the greatest effect if given in the preoperative period.
Prior studies had suggested that patients with sepsis and acute lung injury (ALI) benefited from supplementation with omega-3 fatty acids. The recent OMEGA study, a double-blind, placebo-controlled RCT, seems to have tempered that excitement.13 The study enrolled 272 patients within 48 hours of developing ALI, and randomized them to twice-daily placebo vs omega-3 fatty acids, gamma-linolenic acid, and antioxidants. The study was stopped early for futility because the patients receiving the supplements had fewer ventilator-free days (14 vs 17.2; P = 0.02) and fewer ICU-free days (14 vs 16.7; P = 0.04). Patients in the supplement group also had fewer non-pulmonary organ failure-free days and higher hospital mortality. The authors concluded that the supplements did not improve the measured outcomes. It remains unclear whether they were actually harmful.
A detailed discussion of PN is beyond the scope of this article, but I will offer a few thoughts. As mentioned above, the enteral route is always preferable to PN. If the gut works, use it. Several studies, including two meta-analyses,14,15 have shown a trend toward greater complications with PN as compared to EN. Explanations for this adverse signal include immune suppression and hyperglycemia associated with PN. Either way, most experts agree that PN should be conservatively withheld for at least 7-14 days while all enteral options are exhausted. There is one exception: patients with protein-calorie malnutrition on admission who are unable to receive or tolerate EN. In this group, early PN might be of benefit. In all other patients, the harms of early PN seem to outweigh the benefits.
In critically ill patients, nutritional support is no longer just about delivering calories but also an important therapeutic modality. Whenever possible, use of the enteral route is always preferable to the parenteral route. EN confers a variety of immune benefits that affect non-GI organs and it affects the body’s systemic inflammatory response. Given the complexity of nutritional support, every ICU should include a registered dietician in the care team, and clinicians should care for their patients using formalized nutrition protocols and evidence-based guidelines.
- McClave SA, Heyland DK. The physiologic response and associated clinical benefits from provision of early enteral nutrition. Nutr Clin Pract 2009;24:305-315.
- Heyland DK, et al. Canadian clinical practice guidelines for nutrition support in mechanically ventilated, critically ill adult patients. JPEN J Parenter Enteral Nutr 2003;27:355-373.
- Lewis SJ, et al. Early enteral feeding versus “nil by mouth” after gastrointestinal surgery: Systematic review and meta-analysis of controlled trials. BMJ 2001;323:773-776.
- 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 and American Society for Parenteral and Enteral Nutrition: Executive Summary. Crit Care Med 2009;37:1757-1761.
- Metheny NA, Meert KL. Monitoring feeding tube placement. Nutr Clin Pract 2004;19:487-495.
- Ho KM, et al. A comparison of early gastric and post-pyloric feeding in critically ill patients: A meta-analysis. Intensive Care Med 2006;32:639-649.
- Marik PE, Zaloga GP. Gastric versus post-pyloric feeding: A systematic review. Crit Care 2003;7:R46-51.
- Heyland DK, et al. Optimizing the benefits and minimizing the risks of enteral nutrition in the critically ill: Role of small bowel feeding. JPEN J Parenter Enteral Nutr 2002;26(6 Suppl):S51-55; discussion S56-7.
- McClave SA, et al. Enteral tube feeding in the intensive care unit: Factors impeding adequate delivery. Crit Care Med 1999;27: 1252-1256.
- Bailey N, et al. New paradigm in nutrition support: Using evidence to drive practice. Crit Care Nurs Q 2012;35:255-267.
- Montejo JC, et al. Gastric residual volume during enteral nutrition in ICU patients: The REGANE study. Intensive Care Med 2010;36:1386-1393.
- Heyland DK, et al. Should immunonutrition become routine in critically ill patients? A systematic review of the evidence. JAMA 2001;286:944-953.
- Rice TW, et al. Enteral omega-3 fatty acid, gamma-linolenic acid, and antioxidant supplementation in acute lung injury. JAMA 2011;306:1574-1581.
- Braunschweig CL, et al. Enteral compared with parenteral nutrition: A meta-analysis. Am J Clin Nutr 2001;74:534-542.
- Heyland DK, et al. Total parenteral nutrition in the critically ill patient: A meta-analysis. JAMA 1998;280:2013-2019.