Emily mentioned in her recent post on the Rule of 6s that potassium administration needs to be taken into account when correcting the plasma sodium in cases of hyponatremia.
Why is this the case?
When potassium is given most of it will move into to cells (remember that 98% of total body potassium resides here.) As this occurs, electroneutrality is maintained in three ways, all of which act to raise the plasma sodium concentration.
1. Intracellular sodium exits cells directly increasing the plasma sodium concentration.
2. Extracellular Cl will enter cells along with potassium. The addition of intracellular KCl raises intracellular osmolality causing water to move into cells. The loss of extracellular water causes the plasma sodium concentration to rise.
3. Intracellular protons exit cells as potassium enters. Proton movement is osmotically neutral as they combine both intra and extracellularly with buffers. This is in contrast to potassium, which raises intracellular osmolality and causes water to move into cells. This loss of extracellular water again causes the plasma sodium concentration to rise.
The Practical Point: If solute is part of your hyponatremia treatment plan and you need to additionally replete potassium remember that each mEq of potassium given will act on the plasma sodium like a mEq of sodium.