Diluted and Undiluted Enteral Nutrition

Adult patients after elective major abdominal surgeries who are planned to be admitted to the Intensive Care Unit (ICU) can be included in the trial.

Each patient will be fed via the gastrointestinal tract. Half of the patients will receive enteral nutrition (EN) with additional fluids, and the rest will receive undiluted EN.

The primary aim of this study is feeding intolerance assessment in both groups of patients.

Approximately 50 % of the intensive care unit (ICU) population has feeding intolerance (FI), which includes nausea, vomiting, diarrhea, and others. Some studies suggest that FI can be alleviated in patients fed with supplemental parenteral nutrition (PN).

Adult patients after elective major abdominal surgeries who are planned to be admitted to the ICU can be included in the trial.

After the ICU admission, the patient will be stabilized, including warming, correction of water, electrolyte, and acid-base disorders, and blood transfusion if required. The fluid therapy will be monitored using the transpulmonary dilution technique.

Then, an attending physician will contact an investigator. The investigator will decide about the randomization (no contraindication). The investigators plan to maintain fluid therapy with continuous Glucose-Na-K Baxter 50 mg/ml solution for infusion (GNAK). GNAK will be administered in the same flow as EN, enterally or intravenously (i.v.).

Patients will be randomized to one of two studied groups:

* Continuous EN will be administered solely to the GI tract in the first group. The same dose of GNAK will be given i.v. (IVF group).
* In the second group, GNAK will be administered enterally with EN, a routine practice in our department (ENF group).

The attending physician will correct all fluid disturbances with balanced fluids or blood products according to laboratory tests and hemodynamic monitoring. GNAK will only be given as maintenance fluid with EN.

The primary outcome of our study will be feeding intolerance (FI).

FI is a composite outcome consisting of at least one of the following:

* Incidents of nausea and vomiting (nausea measured with a 4-point verbal descriptive scale (0=no nausea, 1=mild, 2=moderate, 3=severe))
* Incidents of diarrhea (≥ three loose stools per day)
* Increased gastric residual volume (\> 500 ml of gastric aspirate/ 6 hours). Only in patients after lower GI tract surgeries (with intact stomach and gastric feeding)
* Achieving target EN on day three and later: 80% of protein requirements according to ESPEN (1.3/kg of ideal body weight (patients BMI \< 30) or adjusted body weight, BMI ≥ 30)
* Administration of prokinetic agents. Starting with both erythromycin (125mg twice daily enterally) and metoclopramide (10mg three times per day i.v.)

Secondary outcomes (routinely performed procedures):

* PN requirements (days of support, grams of proteins, extra protein calories per day, contribution of PN in total nutrition)
* Insulin consumption (units per day and total per stay)
* Electrolyte supplementation (potassium, phosphorus, calcium, and magnesium in mmol/ stay)
* Enteral access obstruction (rinsing with fluid, need for replacement) per stay
* Intraabdominal pressure (twice daily)
* Sequential Organ Failure Assessment Score (twice daily)
* Fluid balance: additional fluids given intravenously during ICU stay
* Blood product transfusion
* Acute kidney injury, according to KDIGO definition
* Usage of vasoactive drugs: cumulative dose per stay
* Hemodynamic parameters, measured at least twice per day, such as stroke volume variation, pulse pressure variation, cardiac output, global end-diastolic volume, systemic vascular resistance, and extravascular lung water
* Laboratory tests, including lactate, electrolytes, arterial blood gas analysis, coagulation, total blood count
* Infections during the stay (site, antibiotics requirements)
* Mechanical ventilation time (hours)
* ICU stay (days)
* Hospital stay (days)
* Hospital mortality
* Intestinal fatty-acid binding protein (I-FABP) collection from blood and urine once daily during ICU stay
* Serum zonulin on the 1st day, 4th day, and at ICU discharge
* Serum ketones collection at ICU admission, 4th day, and discharge
* Gut microbiome collection at ICU admission and discharge

Follow-up:

• Quality of recovery - phone interview 30 days after randomization

Eligibility Criteria:
Inclusion Criteria:

* Scheduled for major abdominal surgery requiring ICU admission
* Having access to the GI tract (gastric or jejunal)
* Planned to be fed enterally

Exclusion Criteria:

* Patients unable to give informed consent
* After emergency surgeries
* Without access to the GI tract
* Individuals with contraindications to EN, such as short bowel syndrome, uncompensated shock, acidosis (pH \< 7.1; lactate \> 5 mmol/l), bleeding from the upper GI tract, obstruction, intestinal ischemia, abdominal compartment syndrome
* Patients with symptomatic gastro-esophageal reflux
* Expected ICU stay \< 3 days
* Pregnancy and lactation