Medical Nutrition Therapy in Complex and Critically Ill Patients
Critical care nutrition centers on timing, hemodynamics, and substrate dose rather than on reaching calculated goals quickly. The exam tests whether you can withhold, start, or advance feeding in shock, titrate protein through organ failure, and recognize when standard equations mislead.
The metabolic response
- The ebb phase (first 24 hours) features hypovolemia and reduced metabolic rate; the flow phase (days 1–3 and beyond) features hypermetabolism, insulin resistance, and accelerated proteolysis that diet cannot reverse.
- Early full feeding does not correct this catabolism. Permissive underfeeding (roughly 15–20 kcal/kg) and trophic feeding paradigms have not shown benefit from early full-goal energy in acute respiratory distress syndrome or mixed ICU cohorts.
Core rules
- ESPEN advises withholding enteral nutrition during uncontrolled shock until vasopressor requirements are decreasing and lactate trends are improving; resume at trophic rates.
- Start enteral nutrition within 24–72 hours in patients who cannot eat; Surviving Sepsis favors early enteral nutrition over parenteral nutrition.
- Protein: 1.2–2.0 g/kg/day in most ICU adults, up to 2.0–2.5 g/kg in burns and polytrauma, and as high as 2.5 g/kg with continuous renal replacement therapy, which removes amino acids.
- Glucose target 140–180 mg/dL with insulin infusion; avoid intensive control near 110 mg/dL.
- Account for propofol at 1.1 kcal/mL when sedation is continuous.
- In extracorporeal membrane oxygenation, oxygenator gas exchange invalidates respiratory quotient interpretation, though energy expenditure can still be measured.
Worked example
A 45-year-old woman with septic shock is on norepinephrine 0.3 mcg/kg/min with a lactate of 4 mmol/L. Enteral nutrition is deferred and she receives dextrose-containing maintenance fluid only. Twelve hours later, pressors are halved and lactate falls to 1.8 mmol/L. Trophic enteral feeding begins at 10–20 mL/h, advances over 48–72 hours toward 20 kcal/kg and 1.5 g/kg protein, and is held for rising lactate or new vasopressor escalation.
Traps
Starting full-goal feeds on admission day in shock; using the Curreri burn formula uncritically because it overestimates in modern practice; forgetting that continuous renal replacement therapy increases protein needs; and missing refeeding risk in the patient who ate poorly for ten days before admission (thiamine 100 mg before feeding, then daily for several days).