Table 1.

Comparison of EFW and tonicity balances when the PNa changesa

Na + K(mmol)Water(L)EFW(L)Therapy from Balances
EFWTonicity
Patient given 3 L of isotonic saline
    input45030+2 L+0 L water
    output150320 Na−300 mmol Na
    balance+3000−2
Same patient with no intravenous fluid administration, and urine output was unchanged
    input000+2 L+3 L water
    output150320 Na+150 mmol Na
    balance−150−3−2
  • a Two situations are described in which the plasma Na concentration (PNa) rose from 140 to 150 mM in a patient with 30 L of total body water (TBW); only the volume of isotonic saline infused in the two settings differed. In both settings, there is a negative balance of 2 L of electrolyte-free water (EFW). Nevertheless, the goals for correcting the hypernatremia become clear only after calculating a tonicity balance.