Fluids, Electrolytes & Nutrition: Nutritional Assessment

Calorimetry

Direct Calorimetry (Whole-Room Calorimeter)

  • Direct Measurement of the Heat Generated by the Body to Determine Energy Expenditure
  • Gold Standard for Measuring Metabolic Rate
    • Does Not Provide Any Information About the Nature of Substrates Being Oxidizes to Generate Energy
  • Requires Placing the Whole Body into a Large Chamber for Measurement
  • Not Practical in the Hospital Setting – Large, Expensive & Complex Engineering

Indirect Calorimetry (Metabolic Cart)

  • Measures Oxygen Consumed & Carbon Dioxide Produced to Estimate Energy Expenditure
  • Gold Standard for Determining In-Hospital Energy Requirements
  • Respiratory Quotient (RQ) = CO2 Produced / O2 Consumed
    • Measure of Energy Expenditure
    • Values:
      • RQ > 1.0: Overfeeding (Lipogenesis)
        • Management: Decrease Carbohydrates & Calorie Intake
      • RQ < 0.7: Starving (Ketosis)
        • Management: Increase Carbohydrates & Calorie Intake
    • Values of Pure Utilization:
      • Carbohydrate/Glucose = 1.0
      • Protein = 0.8
      • Fat = 0.7
  • Use Has No Proven Benefit to Morbidity or Mortality

Serum Proteins

Serum Protein Half-Life

  • Used as Markers of Nutritional Status
  • *Values Can be Significantly Decreased During Inflammatory States & May Not Accurately Represent Nutritional Status

Half-Lives

  • Prealbumin: 2 Days
    • Best Protein Indicator of Acute Nutritional Status
  • Transferrin: 1 Week
  • Albumin: 2-3 Weeks
    • Deficiency (< 3.0 g/dL) is a Strong Risk for Morbidity and Mortality

Nitrogen Balance

Nitrogen Balance

  • Nitrogen Balance = Nitrogen Intake – Nitrogen Output
    • Balance = (Protein/6.25) – (UUN + 4)
  • Nitrogen Intake = Protein (g) / 6.25
  • Nitrogen Output = Total Urine Nitrogen (TUN) + Insensible Loss
    • Values:
      • Insensible Loss (In Feces & Sweat) = 2 g (Estimate)
      • Urinary Urea Nitrogen (UUN): Measured in Lab
      • Total Urine Nitrogen (TUN) = UUN + 2
        • 2: Estimate for Non-Urea Urinary Nitrogen Content
    • Estimates:
      • Nitrogen Output = TUN + 2
      • Nitrogen Output = UUN + 4

Interpretation

  • Positive Balance: Anabolism
    • Ex: Growth, Wound Healing, Pregnancy
  • Negative Balance: Catabolism
    • Ex: Burns, Tissue Injury, Fever, Periods of Fasting

Limitations

  • Overestimates Output
  • Underestimates Input
  • Inaccurate in Liver or Renal Failure