Electrolytes

What is the difference between osmolality and tonicity? What's an example of where they might differ?

Osmolality = measure of number of particles in a solution. Tonicity = measure of pull across a membrane. Urea, for example, easily passes through the cell membrane and thus contributes to osmolality but not tonicity.

Hyponatremia

Hyponatremia diagnostic walk through https://reblocke.github.io/files/Tutorials/Hyponatremia.html

DDAVP Clamp

Consider, change in sodium for a given amount of fluid should be:

(Infusate Na - Serum Na) / (TBW + Amnt Infused)

TBW = 0.6 per kg in men (0.5 if elderly). 0.5 per kg in women (0.45 if elderly)

This is SMALL - over-correction in hypovolemic hyponatremia is due to repletion of volume -> ADH secretion (appropriate ADH to maintain volume at the cost of tonicity) stops and there is a huge UOP of low salt urine.

DDAVP (ADH) is given back to stop this (to avoid overcorrection) by impeding the bodies ability to excrete water.

Free water Clearance

CH2O = (total solute excreted / urine osms) * (1 - uosm/posm)

Hypernatremia

Free water needed = Free water deficit (yesterday) + Free water to be lost during repletion (0 for 1L UOP, .5 * next 2L, then add all of subsequent liters)

Free water losses are greatest in DI, thus this is more important for that situation than hypernatremia due to insufficient access to water.

What is it about situations where no access to fluids leads to hypernatremia vs hyponatremia? (sweat / insensible loss, sodium inputs e.g. in the ICU)