Base deficit formula: Base deficit = kg × deviation from normal HCO3 (24 mEq/L) × 0.3. What extracellular fluid fraction is used in this calculation?

Base deficit formula: Base deficit = kg × deviation from normal HCO3 (24 mEq/L) × 0.3. What extracellular fluid fraction is used in this calculation?

• A. 0.25
• B. 0.3
• C. 0.4
• D. 0.5

Answer: (B)

Base deficit estimates how much bicarbonate must be added to correct metabolic acidosis by assuming that bicarbonate distributes mainly in the extracellular fluid. To turn the per-liter deficit into a total amount of bicarbonate, you multiply by the extracellular fluid volume per kilogram of body weight. In this calculation, that distribution volume is taken as 0.3 L per kilogram, so the equation becomes base deficit = body weight (kg) × (24 − HCO3) × 0.3. That 0.3 L/kg represents the extracellular fluid space used for distributing bicarbonate in this context. For example, with a 70 kg patient and an HCO3 of 18 mEq/L, the deficit would be 70 × (24−18) × 0.3 = 126 mEq. Therefore, the extracellular fluid fraction used is 0.3.

Base deficit estimates how much bicarbonate must be added to correct metabolic acidosis by assuming that bicarbonate distributes mainly in the extracellular fluid. To turn the per-liter deficit into a total amount of bicarbonate, you multiply by the extracellular fluid volume per kilogram of body weight. In this calculation, that distribution volume is taken as 0.3 L per kilogram, so the equation becomes base deficit = body weight (kg) × (24 − HCO3) × 0.3. That 0.3 L/kg represents the extracellular fluid space used for distributing bicarbonate in this context. For example, with a 70 kg patient and an HCO3 of 18 mEq/L, the deficit would be 70 × (24−18) × 0.3 = 126 mEq. Therefore, the extracellular fluid fraction used is 0.3.