Indirect Calorimetry and Additives in a Burn Patient
A 44-year-old man (80 kg, 178 cm) with 40% TBSA flame burns is in the ICU on day 6 of mechanical ventilation. Indirect calorimetry reports VO2 320 mL/min, VCO2 205 mL/min, RQ 0.64, with FiO2 0.50, stable minute ventilation, and no chest tubes. Current nutrition: 5% dextrose at 100 mL/h, propofol at 20 mcg/kg/min (10 mg/mL emulsion, 1.1 kcal/mL), and a 1.5 kcal/mL enteral formula (68 g protein/L) at 40 mL/h, held three times in the past 24 hours for residuals of 250–300 mL. Measured REE is 1,780 kcal/day; the team's Harris-Benedict estimate with a 1.5 stress factor is 2,650 kcal/day. Triglycerides are 310 mg/dL (baseline 120), glucose 168 mg/dL on sliding-scale insulin, and he has gained 4 kg since admission.
1. What does the RQ of 0.64 tell you, and what is your first action?
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RQ = VCO2/VO2 = 205/320 = 0.64. Physiologic substrate oxidation spans about 0.70 (pure fat) to 1.00 (pure carbohydrate), so 0.64 is below the fat-oxidation floor and cannot be explained by diet composition alone. Two explanations compete: (a) net fat oxidation with gluconeogenesis/ketosis, and (b) technical artifact - an air leak around the endotracheal tube, a change in FiO2 or ventilator settings mid-test, condensed water in the sampling line, or hyperventilation blowing off CO2 (which falsely lowers RQ).
First action is to validate, not to change the feeding: confirm steady state (VO2 and VCO2 varying <5–10%), confirm FiO2 and minute ventilation unchanged during the run, check the line and cuff, and repeat in 30 minutes. This matters because acting on an artifact would mean chasing calories he does not need. If the value reproduces at FiO2 0.50 with a stable vent, treat it as fat-dominant oxidation with impaired glucose utilization (glucose 168 supports insulin resistance) and reassess protein and total intake rather than simply raising calories to the 2,650 kcal stress-factor estimate.
2. Calculate the actual 24-hour energy and protein delivery. What is your recommendation?
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Delivered intake:
- D5W 100 mL/h = 120 g dextrose/day → 120 × 3.4 = 408 kcal, 0 g protein.
- Propofol 20 mcg/kg/min × 80 kg = 1,600 mcg/min = 96 mg/h → 2,304 mg/day = 230 mL of 10% emulsion → ~253 kcal, ~23 g lipid, 0 g protein.
- Formula 40 mL/h × ~21 h (three holds) = 840 mL × 1.5 = 1,260 kcal; protein 0.84 L × 68 g/L = 57 g.
Total ≈ 1,920 kcal and 57 g protein (0.71 g/kg).
Calories are ~108% of the measured REE of 1,780 kcal - appropriate. Protein is the gap: 40% TBSA burns warrant roughly 1.5–2.0 g/kg, or 120–160 g/day. Propofol and dextrose must be counted and subtracted from the feeding budget; the 2,650 kcal equation estimate over-predicts in a ventilated patient and should be discarded. Recommend a protein-dense formula or modular protein to reach ~130 g, and address tolerance (prokinetic, post-pyloric access, or smaller-volume continuous feeding) rather than continuing to lose feeding hours to high residuals.
3. Triglycerides are 310 mg/dL. How do you manage the lipid load?
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Two lipid sources are running: ~23 g/day from the propofol emulsion and the enteral formula's fat. The response is not to reflexively stop enteral feeding - the patient is already protein-deficient.
- Confirm the number: verify fasting status and repeat the panel; rule out sepsis, worsening glycemic control (glucose 168), and renal or hepatic dysfunction as contributors before blaming the emulsion.
- Attack the modifiable source: propofol at 20 mcg/kg/min delivers ~2.3 g of drug and ~250 kcal of lipid daily that are easy to forget. Ask the intensivist whether sedation can be transitioned (e.g., to dexmedetomidine), which removes both the lipid load and ~250 unaccounted calories.
- If propofol must continue: monitor triglycerides every 48–72 hours, and hold exogenous lipid if triglycerides exceed ~400–500 mg/dL. Keep enteral protein and carbohydrate running.
- Document the reasoning - a TG of 310 is a signal to look for overfeeding or sepsis, not an automatic formula change.