Lab

Total Body Sodium Deficit

search

Total Body Sodium Deficit, Total Body Water, Total Body Water Excess, Free Water Deficit, Sodium Infusion Rate in Hyponatremia

  • Calculation
  • Normal Total Body Water (TBW)
  1. Normal TBW (L) = fractionWater * wtKg
  2. Where wtKg = Weight in kg
  3. Where fractionWater
    1. Child: 0.6 for males, 0.6 for females
    2. Adult: 0.6 for males, 0.5 for females
    3. Elderly: 0.5 for males, 0.45 for females
  • Calculation
  • Current Total Body Water (TBW)
  1. Current TBW = Normal TBW * (140 / sNa)
  2. Where 140 mEq/L is the normal Serum Sodium, and sNa is the current Serum Sodium
  3. Adjust measured Serum Sodium (sNa) for Hyperglycemia
    1. See Corrected Serum Sodium for Glucose
  1. Free Water Deficit (FWD in L) = (Normal TBW) - (Current TBW)
  2. Free Water Deficit (FWD in L) = (Normal TBW) * (sNa - 140) / 140
  3. Free Water Deficit (FWD in L) = (Normal TBW) * (sNa/140 - 1)
    1. Where 140 mEq/L is the normal Serum Sodium, and sNa is the current Serum Sodium
  4. Adjustments to Total Body Water
    1. Decrease Total Body Water by 10% in Hypernatremia (multiply TBW * 0.9)
    2. Some equations use 0.5 * LbmKg as TBW for both men and women in Hypernatremia
  • Calculation
  • Total Body Sodium Deficit or Water Excess (Hyponatremia)
  1. Excess Total Body Water (TBW)
    1. Excess TBW = Current TBW - Normal TBW
  2. Sodium delivered by various crystalloid solutions (ivNa)
    1. Hypertonic Saline (3% NaCl): 513 meq/L
    2. Normal Saline or NS (0.9% NaCl): 154 meq/L
    3. Lactated Ringers or LR: 130 meq/L
    4. 1/2 Normal Saline or 1/2NS (0.45% NaCl): 77 meq/L
  3. Total Body Sodium Deficit (NaDeficit)
    1. Sodium deficit (meq) = Normal TBW * (140 - sNa)
    2. Where 140 mEq/L is the normal or desired Serum Sodium, and sNa is the current Serum Sodium
  4. IV Fluid rate (IVrate) to achieve a given rise in Sodium per hour (sNaChangePerH )
    1. IVrate = (1000 * sNaChangePerH * (TBWnorm + 1)) / (ivNa - sNa)
    2. where sNaChangePerH = Desired change per hour (typically <0.25 to <0.50 meq/h)
    3. where TBWnorm = Normal Total Body Water (see above for calculation)
    4. where ivNa = Sodium per Liter of IV fluid (see above)
    5. where sNa = Serum Sodium
  5. Serum Sodium change per liter of IV fluids (sNaChangePerL)
    1. sNaChangePerL = (ivNa - sNa) / (TBWnorm + 1)
    2. where TBWnorm = Normal Total Body Water (see above for calculation)
    3. where ivNa = Sodium per Liter of IV fluid (see above)
    4. where sNa = Serum Sodium
  • References
  1. Marino (2014) ICU Book, Wolters Kluwer, p. 653-72
  2. Rose (1989) Clinical Physiology of Acid Base and Electrolytes, p. 816-19
  3. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]