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## Total Body Sodium Deficit

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

- See Also
- Indications
- Calculation: Normal Total Body Water (TBW)
- Normal TBW (L) = fractionWater * wtKg
- Where wtKg = Weight in kg
- Where fractionWater
- Child: 0.6 for males, 0.6 for females
- Adult: 0.6 for males, 0.5 for females
- Elderly: 0.5 for males, 0.45 for females

- Calculation: Current Total Body Water (TBW)
- Current TBW = Normal TBW * (140 / sNa)
- Where 140 mEq/L is the normal Serum Sodium, and sNa is the current Serum Sodium
- Adjust measured Serum Sodium (sNa) for Hyperglycemia

- Calculation: Free Water Deficit (Hypernatremia)
- Free Water Deficit (FWD in L) = (Normal TBW) - (Current TBW)
- Free Water Deficit (FWD in L) = (Normal TBW) * (sNa - 140) / 140
- Free Water Deficit (FWD in L) = (Normal TBW) * (sNa/140 - 1)
- Where 140 mEq/L is the normal Serum Sodium, and sNa is the current Serum Sodium

- Adjustments to Total Body Water
- Decrease Total Body Water by 10% in Hypernatremia (multiply TBW * 0.9)
- Some equations use 0.5 * LbmKg as TBW for both men and women in Hypernatremia

- Calculation: Total Body Sodium Deficit or Water Excess (Hyponatremia)
- Excess Total Body Water (TBW)
- Excess TBW = Current TBW - Normal TBW

- Sodium delivered by various crystalloid solutions (ivNa)
- Hypertonic Saline (3% NaCl): 513 meq/L
- Normal Saline or NS (0.9% NaCl): 154 meq/L
- Lactated Ringers or LR: 130 meq/L
- 1/2 Normal Saline or 1/2NS (0.45% NaCl): 77 meq/L

- Total Body Sodium Deficit (NaDeficit)
- Sodium deficit (meq) = Normal TBW * (140 - sNa)
- Where 140 mEq/L is the normal or desired Serum Sodium, and sNa is the current Serum Sodium

- IV Fluid rate (IVrate) to achieve a given rise in Sodium per hour (sNaChangePerH )
- IVrate = (1000 * sNaChangePerH * (TBWnorm + 1)) / (ivNa - sNa)
- where sNaChangePerH = Desired change per hour (typically <0.25 to <0.50 meq/h)
- where TBWnorm = Normal Total Body Water (see above for calculation)
- where ivNa = Sodium per Liter of IV fluid (see above)
- where sNa = Serum Sodium

- Serum Sodium change per liter of IV fluids (sNaChangePerL)
- sNaChangePerL = (ivNa - sNa) / (TBWnorm + 1)
- where TBWnorm = Normal Total Body Water (see above for calculation)
- where ivNa = Sodium per Liter of IV fluid (see above)
- where sNa = Serum Sodium

- Excess Total Body Water (TBW)
- Resources
- Medscape Hyponatremia correction infusate rate

- References
- Marino (2014) ICU Book, Wolters Kluwer, p. 653-72
- Rose (1989) Clinical Physiology of Acid Base and Electrolytes, p. 816-19
- Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]