Nutrition in ICU: 
During critical illness, catabolism (breakdown of muscle protein, fat and other complex molecules) occurs faster than anabolism. The major goal of nutritional support during this period of acute illness was to ensure the body has adequate energy and nutrients available to slow down this process of fat and muscle loss. Loss of lean muscle mass during acute illness has been associated with worsened outcomes including prolonged mechanical ventilation and debility. 
Providing enteral nutrition early to people with critical illness may reduce their infection risk, as compared to delaying enteral nutrition or not providing any. Enteral nutrition should be provided within 48 hours to people with critical illness who are not at high risk for bowel ischemia. 
Trophic feedsMost critically ill patients with impaired gut motility can tolerate “trophic” enteral feedings (tube feeds provided at 10 mL/hour or so) during critical illness. Enteral nutrition within 48 hours in critically ill patients might reduce the risk of hospital-acquired infection (nosocomial infection), compared to providing no nutrition or delaying enteral nutrition. However, it’s okay not to reach “caloric goals” for at least 7 days in nourished critically ill patients. Trophic (trickle) feedings continually stimulate the gut, keeping it healthy and reducing the risk for infection by bacterial translocation.  
In patients who were previously adequately nourished, providing minimal calories (trophic feedings) enterally for up to 7 days led to equivalent outcomes to more aggressive feeding, in mechanically ventilated critically ill patients (EDEN trial). They randomized 1000 patients newly diagnosed with ALI or ARDS, without obvious malnutrition, to receive either trophic feedings(20 or full enteric feedings(80 for 6 days. After that, they all received full enteric feedings, if they were still ventilated. Despite the full enteric feeding group receiving many more calories, there were no differences in important clinical outcomes (ventilator-free days at 28 days, 60 day mortality, or infections). 
A 2011 randomized trial suggested feeding critically ill patients below caloric goals might improve survival. Eligible patients were randomly assigned to permissive underfeeding or target feeding groups (caloric goal: 60–70% vs. 90–100%) with either intensive glucose control or conventional glucose control (target blood glucose: 4.4–6.1 vs. 10–11.1 mmol/L). Hospital mortality was lower in the permissive underfeeding group than in the target group (30.0% compared with 42.5%; No significant differences in outcomes were observed between the glucose control groups (Am J Clin Nutr 2011;93:569–77.
Gastric residuals: There was no benefit seen from monitoring gastric volume in ventilated patients on tube feedings. It’s been assumed that delayed gastric emptying, resulting in a stomach full of liquid nutrition, predisposes patients to have aspiration events and develop ventilator associated pneumonia (VAP). Therefore, expert bodies recommend we regularly “check residuals” on these patients, aspirating their stomach contents to see how much tube feeding is pooling there, and stopping the feeding if the volume seems high. The practical result is that patients often don’t receive their goal caloric intake, because tube feedings are so often stopped for these so-called “high residuals.” Prokinetic drugs like metoclopramide are also often given in these situations. 
In a study, authors found that not monitoring gastric volumes was noninferior to monitoring them, in its effect on the development of ventilator associated pneumonia. Investigators randomized mechanically ventilated patients to undergo either gastric volume residual checks every 6 hours, with tube feeds pausing for regurgitation, vomiting, or residual volumes >250 mL; or no checks. In the “no check” control group, feedings were still paused for regurgitation or vomiting. There was no difference in the development of VAP between the groups. There were also no differences in secondary outcomes such as infections, ICU stay, mortality, or ventilator days. However, significantly more patients in the “no checks” group got 100% of their intended calories. (JAMA. 2013;309(3):249-256) 
Another trial, REGANE study, suggests residual volumes up to 500 mL are safely tolerated by patients. (Intensive Care Med. 2010 Aug;36(8):1386-93 
Early vs. Late TPN: Many critically ill patients have reduced gut motility and fail to tolerate enteral feedings in the amounts calculated to meet their theoretical caloric needs. For these patients, there appears to be no benefit to starting total parenteral nutrition in the first week after impaired gut motility occurs, and doing so may increase the risk for nosocomial infection. Providing no nutritional support or dextrose infusions are as good or better than early TPN for critically ill patients who can’t tolerate tube feedings. 
A huge trial (n=4,640) showed no benefit and potential harm from providing TPN at day 3, rather than day 8 of failing enteric feeds. (N Engl J Med 2011; 365:506-517).
For patients whose caloric goals can’t be met through enteric feedings, U.S. and Canadian guidelines recommend delaying TPN for 7 days, while European guidelines recommend starting TPN within 2 days.  
Nasogastric vs. post-pyloric feeding:  
Nasojejunal tube placement for enteral feedings doesn’t provide a detectable benefit in most critically ill patients, even those with clearly impaired gastric motility. Nasogastric tube feedings seem to achieve similar caloric goals, if pro-motility agents like metoclopramide and erythromycin are used. 
In a study, Authors randomized 181 mechanically ventilated patients who had high gastric “residuals” of > 150 mL aspirated while receiving their initial nasogastric tube feeds (<72 hours), to either receive continued tube feedings through a nasogastric tube, or placement of a post-pyloric nasojejunal tube for ongoing enteric nutrition. Erythromycin and metoclopramide (Reglan) were given to the patients in the nasogastric tube feeding group who developed “high residuals,” to stimulate gastric emptying and gut motility.  
There was no difference between groups in the percentage of intended calories delivered. There was no difference in secondary outcomes, either, including ventilator-associated pneumonia (VAP), ICU length of stay, duration of mechanical ventilation, in-hospital mortality, vomiting, abdominal distention, or diarrhea. (Crit Care Med. 2012 Aug;40(8):2342-8) 

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