A number of factors impede the delivery of EN in the critical care setting; these include gastrointestinal intolerance, under-prescribing to meet daily requirements, frequent interruptions for procedures, and technical issues with tube placement and maintaining patency [68]. Monitoring gastric residual volumes (GRV) contributes to these factors, yet volumes do not correlate well with incidence of pneumonia [69], measures of gastric emptying, or to the incidence of regurgitation and aspiration [70,71]. However, few studies have highlighted the difficulty of obtaining an accurate GRV due to feeding tube tip location, patient position, and type of tube [69]. Several high quality studies have demonstrated that raising the cutoff value for GRV from a lower number of 50–150 mL to a higher number of 250–500 mL does not increase risk for regurgitation, aspiration, or pneumonia [70,71]. A lower cutoff value for GRV does not protect the patient from complications, often leads to inappropriate cessation, and may adversely affect outcome through reduced volume of EN infused [72]. Gastric residual volumes in the range of 200–500 mL should raise concern and lead to the implementation of measures to reduce risk of aspiration, but automatic cessation of feeding should not occur for GRV < 500 mL in the absence of other signs of intolerance [12,69]. Metheny et al (2012) conducted a survey in which more than 97% of nurses responded that they assessed intolerance by measuring GRV; the most frequently cited threshold levels for interrupting feedings were 200 mL and 250 mL [73]. While threshold levels varied widely, only 12.6% of the nurse respondents reported allowing GRV up to 500 mL before interrupting feedings. While monitoring GRV is unnecessary with small bowel feeding, the location of the feeding tube tip should be questioned if gastric contents are obtained from a small bowel tube. The use of GRV as a parameter for trending may also yield important information regarding tolerance of feeding when the patient is unable to communicate abdominal discomfort. Other objective measures to use in the assessment of tolerance include an abdominal exam with documentation of changes in bowel sounds, expanding girth, tenderness or firmness on palpation, increasing nasogastric output, and vomiting [12,68]. If there are indications of intolerance, it is appropriate to divert the tip of the feeding tube into the distal small bowel as discussed previously.
Trophic vs Full Feeding
For the patient with low nutrition risk, there is a lack of convincing data to support an aggressive approach to feeding, either EN or PN, in the first week of critical illness [7]. In recent years, results of several trials suggest early goal-directed feeding in this population may cause net harm with increased morbidity and mortality. When discussing recent controversies in critical care nutrition, one must mention the two schools of thought when it comes to full versus limited energy provision in the first week following ICU admission. Studies in animals and humans have shown a trophic effect of enteral nutrients on the integrity of the gut mucosa, a finding that has provided the rationale for instituting enteral nutrition early during critical illness [15]. However, the inability to provide enteral nutrition early may be a marker of the severity of illness (ie, patients who can be fed enterally are less ill than those who cannot) rather than a mediator of complications and poor outcomes. Compher et al (2017) stated that greater nutritional intake is associated with lower mortality and faster time to discharge alive in high-risk, chronic patients but does not seem to be significant in nutritionally low-risk patients [74]. The findings of the EPaNIC and EDEN trials raised concern that targeting goals that meet full energy needs early in critical illness does not provide benefit and may cause harm in some populations or settings [32,75]. The EDEN trial [32] left us believing that trophic feeding at 10–20 mL/hr may be just as effective as any feeding in the first few days of critical illness striving for 15% to 20% of daily goal calories. After establishing tolerance, advancing daily intake to > 50% to 65% of goal calories, and up to 80% for the highest risk patients, may be required to prevent intestinal permeability and achieve positive clinical outcomes [33].
The systematic review and meta-analysis performed by Al-Dorzi et al (2016) adds further evidence for judicious advancement of EN for critically ill patients [76]. The authors reported finding no association between the dose of caloric intake and hospital mortality. Furthermore, a lower caloric intake resulted in lower risk of bloodstream infections and the need for renal replacement therapy (in 5 of the 21 trials only). As with many other meta-analyses, the authors reported that their results are most assuredly impacted by the heterogeneity in design, feeding route, and dose prescribed and delivered [16,76,77]. Other recent trials such as Arabi et al (2015) that enrolled 894 patients with different feeding targets further confirmed that there is no difference in outcome between groups when it comes to moderate (40% to 60% of goal) vs high (70% to 100% of goal) energy intake, infection rates, or 90-day mortality. The authors summarized their findings saying feeding closer to target is associated with better outcomes compared with severe underfeeding [78]. This adds to the controversy when considering the findings of still other RCTs or meta-analyses that evaluated minimal or trophic feeding versus standard feeding rates [9,46,77]. The meta-analysis performed by Marik and Hooper concluded that there were no differences in the risk of acquired infections, hospital mortality, ICU LOS, or ventilator-free days whether patients received intentional hypocaloric or normocaloric nutrition support [9]. Similarly, there was no significant difference in overall mortality between the underfeeding and full-feeding groups (OR, 0.94; 95% CI, 0.74–1.19; I2 = 26.6%; P = 0.61) in the meta-analysis done by Choi et al (2015), although only 4 trials were included to ensure homogeneity of the population and the intervention [77]. Furthermore, the hospital LOS and ICU LOS did not differ between the 2 groups, nor did any other secondary clinical outcome, leading the authors to conclude that calorie intake of the initial EN support for ICU patients had no bearing on relevant outcomes.
Recent studies have attempted to correlate caloric intake and patient outcomes without success; achieving 100% of caloric goal has not favorably impacted morbidity and mortality. Evidence suggests that intake greater than 65% to 70% of daily caloric requirement in the first 7 to 10 days of ICU stay may be associated with poorer outcomes, particularly when parenteral nutrition is used to supplement intake to achieve the caloric target [33–35].
