The w/u of hyponatremia involves first determining plasma osmolality. secondly, you need to understand the pt's volume status. in this case we were not given plasma osm's. I would order that. the na, bun, glucose calcuation may or may not tell the whole story. there could be lipoproteins, paraproteins causing a normal plasma osmolality, etc. Assuming the pt.'s plasma osmolality is low, we then look at volume status. In this case, they appear to be volume overloaded, before we even start the case. By definition then, this is one of three things, nephrotic syndrome, cirrhosis, or chf. with the pulmonary findings, chf is highest on the differential. if the pt. had started euvolemic, then we have SIADH and it's mimcs (with thyroid dz, chronic HCTZ, etc, etc, in that differential). If the pt. is volume depleted, then they may simply be appropriately holding onto water.
Let's assume we have SIADH, then regardless of how the fluid is administered....be it isotonic IV fluids or worse yet, hypotonic (LR), or worst, free water by mouth or D5 etc)...they will drop their sodium. Indeed as mil points out, it's because they will create a concentrated urine given the ADH effect on the collecting duct. they key is recognizing that this can happen with our routine IVF.
Generally when pt.'s are chronically hyponatremic and symptom free, or Na >120 they do not need rapid correction. If there are symptoms (seizure, obtunation etc.) then the treatment calls for 3% saline with the goal to raise the sodium by no more than 12meq per 24 hours. This usually works out to be a 50cc bolus followed by 15cc/hr of hypertonic saline. If the potassium is also low, then you can often treat both problems simply by giving runs of K. As the K+ is taken up by the cells, water goes with it, thus dropping the serum Na. You need to then be careful you don't overshoot with the K, especially if your are also giving hot salt.
Would I do the case? No. It's elective, and I'd be concerned about my ability to monitor the electrolyte situation effectively. This is a problem that needs to be fixed slowly.
Central Pontine Myelinolysis occurs 4-5 days later, so you won't really know that you f'd up until it's well too late.
BNE