You have a patient with hyponatremia and you’ve determined that salt tabs and normal saline just won’t do, they need some 3% hypertonic saline. Great! You go to write the order…how many cc’s should they get? At what rate? Is there a better way of calculating it rather than just saying, give 50 cc x1 and hope for the best?
As it turns out, giving 50 cc boluses and checking Na every 2 hours is an accepted form of empiric treatment. But if that makes you uneasy, there is a more mathematical solution. (Reference)
Example: a 60 kg woman with an Na of 108. Your goal is to increase her Na by 4 mmol over the next 4 hours.
So far, your answer should be: the predicted change in Na will be 13.06 for every 1 L 3% NS. Therefore, to increase Na from 108 to 112 over four hours:
=0.294 L or 294 mL. However, because you want to run this over 4 hours, 294/4=73.5 mL/hour. And that’s your hypertonic saline infusion rate!
Remember your Na correction goals:
- 2-4 mEq in the first 2-4 hours (if you go over that, consider giving some free water back)
- <8-10 mEq over 24 hours
- You can stop being aggressive about correction around the time Na >125 mmol/L