There are a number of ways to grade the severity of SIADH. The most obvious is to look at how low the sodium is. The problem with this is that it largely depends on how much water a patient is drinking and is not solely dependent on the severity of the SIADH. A patient with mild SIADH that is started on hypotonic fluids will have a much lower sodium than a patient with severe hyponatremia who is adherent with her water restriction and urea tablets.
I assess the severity of SIADH by looking at the electrolyte content of the urine. Here is a doozy:
- Urine sodium: 134
- Urine Potassium: 62
- Urine osmolality: 777
But this strategy suffers from a similar limitation, the urine electrolyte are affected by factors beyond the severity of the SIADH. Patients with SIADH are in sodium balance, that means that all the sodium they ingest is excreted. Increased sodium intake will be reflected by increased urine sodium, the same goes for dietary potassium and urinary potassium. The above labs came after the patient was given isotonic saline for 24-hours. The serum sodium fell from 128 to 117 with saline.
The true measure of SIADH severity probably is simply the urine osmolality.
If the UNa+UK (134+62) > Serum Na (128) then the patient has a negative electrolyte free water clearance … no amount of water restriction will increase that serum Na+. Patient probably needs furosemide + NaCl tablets/IV 0.9 NS or tolvaptan. Also, as you point out, in SIADH, water homeostasis is altered but Na+ homeostasis is normal. For every liter of IV 0.9NS, patient will likely retain a significant amount of water (but not of Na+) and this will make serum Na+ worse. You can also predict 0.9 NS will make hyponatremia worse because UNa+UK (134 + 62)> 154 (mEq/L of Na in 1 L of 0.9 NS)
where did you get urea tablets?
I think they are only available in Europe