There have been many times over the years when a PCP has asked me “how many milligrams of (__) per day is this patient receiving in their TPN. Generally, it is a questions about sodium or potassium, but sometimes they want to know about phosphorus or magnesium. Electrolytes in TPN are generally expressed in mEq per liter and per day.
I have yet to find a better resource to carry with me than the one below. As you can tell it is quite old, it is from the early 1990’s and was most likely typewritten. I always carry it as a reference in my notebook and review it several times per year. It is easier for me to reference this than doing an internet search. This graphic includes TPN specific electrolytes with their atomic weights and valences and covers the conversion examples of milligrams to milliequivalent and visa versa.
One could call this a vintage chart!
As you can also see, I have written to the side that the atomic weight of sodium is in fact 23, because sometimes the lighting isn’t good and the small 3 looks like an 8.
A standard TPN that is infusing 2 liters per day will usually provide approximately 64 mEq of K per day, most PCPs will understand this amount because an oral renal diet will generally restrict K to 60 mEq per day or less. It is safe to say that most TPN preparations provide the low end of K.
When discussing sodium, most PCPs will reference milligrams. One reason may be that they are familiar with cardiac diets restricting sodium to 2000 milligrams per day. When asked “how much sodium is the patient receiving per day in the TPN, you can answer with mEq, or quickly convert to milligrams by using the above calculations.
Standard TPN that is infusing 2 liters per day will generally provide approximately 70 mEq per day. You simply take the mEq per day and multiply by 23. 70mEq X 23 = 1610 milligrams per day.
Sodium levels are generally affected by the amount of free water the patient is receiving or is deficient in. The decision to increase sodium in the TPN is generally a discussion with the PCP and pharmacy.
A patient that has a lower sodium level for several days and cannot receive oral Na supplementation, I will generally increase the amount of sodium in the TPN by 50-60%. I have found that increasing Na from 35 mEq per liter to <=50 mEq per liter does nothing for the sodium level. Sometimes, patients need a lot of sodium, especially if they are losing fluids from drains or tubes.