TABLE 1.

Sodium Definitions

Sodium-Related Parameter	Definition
[Na]first	The first serum sodium measurement within 24 hr of ICU admission.
Δ48 hr-[Na]	The difference between the mean of serum sodium measurements 24–48 hr after ICU admission and the first serum sodium measurement within 24 hr of ICU admission (Δ48 hr-[Na] = mean [Na]24–48 hr-[Na]1).
[Na]max	The highest serum sodium measurement during ICU admission.
V-[Na]max	Velocity of reaching the highest serum sodium measurement during ICU admission.
Vmax-Δ[Na]	Maximum velocity of change in serum sodium in the first 48 hr of ICU admission. Calculated as the difference in serum sodium at admission and a sodium measurement within 48 hr after admission, divided by the time between these two measurements. As patients can have several serum sodium measurements within 48 hr, Vmax-Δ[Na] is the highest velocity of all pairs of measurements in a patient. Only if sodium measurements were taken more than 2 hr apart, velocity was calculated.