Should you use NS, LR, or 1/2 NS as Maintenance fluids?
Serum osmolality = 2 (Na+) + Glucose/18 + BUN/2.8
Notice that sodium plays a bigger part in determining your osmolality than glucose. That’s why D5 1⁄2 NS is not an isotonic solution. As such, D5 1⁄2 NS is NOT appropriate for most medical patients who are hypovolemic. Such patients need isotonic fluids (normal saline or Lactated Ringers). “Also, remember that dextrose gets almost immediately metabolized to water and CO2 when it enters the circulation so it is not osmotically active for too long.”
When it comes to IV fluids, there are a few basic scenarios.
- The hypovolemic patient (e.g. patient with sepsis, pneumonia, intractable nausea/vomiting, etc).
- The hypervolemic patient (e.g. CHF, cirrhosis, renal failure, etc).
- The euvolemic patient who will be NPO (or take inadequate po)
- The euvolemic patient who can take PO and has been admitted for an elective reason (e.g. to expedite workup for cancer)
- The patient who needs free water, e.g. in hypernatremia.
NB: For the hypervolemic patient, avoid fluids at all cost.
Also, patients who are healthy, can take po, and are admitted for an elective reason don’t need fluids.
Pre-procedure or NPO patient who is euvolemic: treat this patient like a surgical patient if the patient is going to be NPO for longer than 6-12 hours. Give D51⁄2 NS. The reason for giving dextrose (D5) is to prevent catabolism.
| For Volume Replacement (Hypovolemic Patient) |
For Maintenance | For Free Water Replacement |
| Lactated Ringer’s (LR) | 1/2 NS | Free water (orally, via NGT, via PEG tube) |
| Normal Saline (NS) | D5 1/2 NS | D5W through IV |
| NS | ||
| Even 1/4 NS or D5 1/4 NS works for maintenance fluids | ||
| Comments | ||
| if you are planning to give more than 3-4 liters of normal saline, switch to LR because of the risk of “expansion acidosis”. | ||
“Because lack of potassium is rare, there is no RDA or RNI for this mineral. However, it is thought that 1600 to 2000 mg (40 to 50 milliequivalents [mEq]) per day for adults is adequate.” Mayo Clinic.
When a patient is NPO and is on fluids, putting 20 KCL in it will give them about 40-50 per day. So put KCL patients that are NPO.
The timing of one-half isotonic saline therapy may also be influenced by potassium balance. “Potassium repletion affects the saline solution that is given, since potassium is as osmotically active as sodium. Thus, concurrent potassium replacement may be another indication for the use of one-half isotonic saline.” Kitabchi et al. UTD
“Although isotonic saline is often the initial replacement fluid used in treating diabetic ketoacidosis or nonketotic hyperglycemia, the addition of potassium will make this a hypertonic fluid (since potassium is as osmotically active as sodium), thereby delaying reversal of the hyperosmolality. Thus, 40 to 60 meq of potassium per liter in one-half isotonic saline is preferred.” Mount et al. UTD
Sources for further reading