Undiluted and Diluted Nutrition

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 vomiting * Administration of prokinetic agents. Starting with both erythromycin (125mg twice daily enterally) and metoclopramide (10mg three times per day i.v.) due to significant EN intolerance, i.e. ≥ 2 incidents of vomiting/24h; \> 500 mL of gastric volume/6h; presence of gastric contents/nutrition in the endotracheal tube due to regurgitation

Secondary outcomes (routinely performed procedures):

* Incidents of nausea (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) * 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 (daily) * Acute Physiology and Chronic Health Evaluation II (daily) * Fluid balance: additional fluids given intravenously during ICU stay * Blood products 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

Additional procedures:

* 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 (for 60 patients, 30 participants in each group)

Follow-up:

• Quality of recovery - phone interview 30 days after randomization

• Number of participants having nausea [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Number of participants having diarrhea [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Number of participants having increased gastric residual volume [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Number of participants in whom target EN will be achieved [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Days of support with PN [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Insulin consumption [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Electrolyte supplementation [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Intraabdominal pressure [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Sequential Organ Failure Assessment Score [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• APACHE II [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Fluid balance [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]
• Blood products transfusion [Time frame: From the date of randomization until the date of the ICU discharge or death but no longer than 8 weeks, whichever came first]