Giving normal saline for SIADH

First question: will giving normal saline even help? Normal saline has an osmolality of 308 mEq (Na+Cl). In hypovolemic and some cases of euvolemic hyponatremia, it can help raise Na appropriately. If the urine osmolality is too high, giving normal saline will actually worsen hyponatremia because it will be like giving a more dilute fluid. 

Example: You have a man with suspected SIADH (he’s a smoker and has a new diagnosis of lung cancer). His Na is 120. His serum osm is 265 and his urine osm is 320. Your total free water loss is -37.5 mL, which is to say, if you give him 1 L NS, you will make zero difference. 

Let’s say the same man has a urine osm of 100. Your total free water loss is +2.08 L. If you give him 1 L NS, you will dramatically increase his Na. 

Let’s say the same man has a urine osm of 600. Now your total free water loss is -486 mL, which means if you give him 1 L NS, you will actually cause him to gain free water and worsen his Na. 

*It’s technically serum “osmolality” 
**D5 1/2NS osmolarity is often listed around 405, but it’s really less than that because dextrose moves more freely across cell membranes, and what I care about is the Na component

This chart is also very helpful for understanding how fluids are selected in common scenarios. 

Second question: How much normal saline should I give? The equation below is extrapolated from a calculation for how much hypertonic saline to give (see this post)

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