What is the role of parenteral nutrition in critical illness?
PN can be exclusive or supplemental (in a patient receiving EN). Historically, providers may have been reluctant to utilize PN for fear of infectious morbidity; however, contemporary pragmatic-design RCTs demonstrate safety with exclusive PN (Harvey SE et al. N Engl J Med. 2014;371:1673-84). When your patient has a contraindication for EN or does not tolerate it despite a trial of small bowel feeding, meta-analyses have shown a mortality benefit of early exclusive PN in malnourished patients, as compared with no nutrition (Braunschweig C et al. Am J Clin Nutr.2001;74:534-42).
As for supplemental PN (SPN), the 2016 ASPEN/SCCM guideline does not recommend it until day 7 in all critically ill patients, while the 2018 ESPEN guideline recommends its use on a case-by-case basis. Since, two trials inform SPN use. The EAT-ICU trial showed no difference in 6-month physical function between EN group and early-goal-directed nutritiongroup, which included SPN to achieve estimated energy requirement during the first week of critical illness (Allingstrup MJ et al. Intensive Care Med. 2017;43:1637-47). The TOP-UP trial compared EN alone with EN plus SPN in nutritionally high risk patients (ie, those who stand to have more complications as a result of undernutrition) and found those with a BMI < 25 kg/m2 and those with a NUTRIC score >5 who received supplemental PN atop EN had improved 30-day mortality, as compared with EN alone (Wischmeyer P et al. Crit Care. 2017;21:142). Mortality was a secondary outcome, and further study of supplemental PN in nutritionally high-risk patients is warranted. Until further data are available, supplemental PN should probably be restricted during the acute phase of critical illness.
Protein may be the important substrate
Proteolysis is the rule during critical illness, and amino acids are liberated from skeletal muscle breakdown. Using ultrasound, Puthucheary et al found a 17.7% reduction in rectus femoris cross-sectional area in 63 critically ill adults and identified muscle cellular infiltration at ICU day 10, suggesting critical illness leads to quantitative and qualitative muscle defects (Puthucheary Z et al. JAMA. 2013;15:1591-1600).
Since survivorship from critical illness is increasing, acquired loss of muscle mass may contribute to post-ICU physical functioning impairments. Thus, protein may be the most important substrate to deliver during critical illness. The 2016 ASPEN/SCCM guideline recommends 1.2 – 2.0 g/kg actual body weight (ABW)/day in nonobese critically ill patients.
Unfortunately, the optimal protein dose and the timing of intake are unknown. Observational studies suggest benefit with lower and higher doses, which creates equipoise for protein dose. The signal may be lost in heterogeneity, and observational data suggest higher protein dose may benefit patients with high nutritional risk. In terms of timing, one observational study found lower (<0.8 g/kg/d) protein dose before day 3 followed by higher (>0.8 g/kg/d) dose thereafter was associated with mortality benefit (Koekkoek WAC et al. Clin Nutr.2019;38:883-890).
Until stronger data are available to guide optimal protein dose and timing, it is reasonable to observe the 2016 ASPEN/SCCM guideline protein recommendation of at least 1.2 g/kg/day. The 2018 ESPEN guideline recommends a similar dose of 1.3 g/kg/day.