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Author Topic: Correcting hyponatremia with normal saline??

Posts: 219
Post Correcting hyponatremia with normal saline??
on: September 6, 2012, 09:22

I'm in the MICU this month and I was wondering if I could ask you a few questions about the management of hyponatremia. I had a patient with previous hx hyponatremia, questionable etiology of SIADH vs. psychogenic polydipsia. He presented to the ED with mild confusion (oriented x2) s/p fall and was found to have a Na of 109. I was instructed by my senior to start him on normal saline at 100cc/hr. I didn't think the patient had an appropriate response to this treatment regimen, so I proceeded to calculate what I think the rate of normal saline infusion should be. Here's what I did:

Change in serum Na per L infused = [(Infusate Na) - (Serum Na)] / (TBW + 1); this patient weigh 62kg and I elected to use normal saline

Change in serum Na per L infused = [(154 - 109)]/ 38.2 = 1.176 mEq of change in serum Na if pt was infused with 1 L of normal saline

With the recommendation that Na correction be about 0.5 mEq/ L / hour, I concluded that the bag of normal saline solution should be running at approx 500cc/hr:
If one L of normal saline will raise the serum sodium by 1.176, then 500cc of saline should raise it by 0.588, right?

So,thereotically, if we continue to infuse at the 500cc/hr rate, he would reach a goal serum Na of 120 in 18-19 hours.

(His serum osmolality was 230, urine osmolality 368, euvolemic and hemodynamically stable, so dx of euvolemic hyponatremia 2/2 questionable SIADH, beer potomania (he has hx of alcohol dependence vs. abuse), or malnutrition.)

My questions are: Did I do the math correctly and used the right equations? Is it reasonable to give a hyponatremic patient with Na of 109 a liter of normal saline at 500cc/hr, for 18-19 hrs? The rate seems high to me and is certainly much higher than what my senior suggested. When I asked him about how he came up with 100cc/hr, he didn't really give me a good answer. I didn't see the nephrologist who was consulted on this case and he didn't explain anything over the phone. My calculations seem correct, albeit I rounded the numbers here and there. I did a search on Nephrology On Demand. I came up with a NEJM article on Hyponatremia that suggested giving hypertonic saline in the first 24 hrs, but I was unsure about that recommendation. This topic seems like such a simple problem to solve on paper and so easy to understand when you were explaining to me last year.

What actually happened to the patient:
- He was started on the normal saline drip at 100cc/hr. He was then transferred to the MICU. He started with a Na of 109. Four hours later it dropped to 108, four hours after that it dropped to 107. At this point, NS was switched to hypertonic saline infusing at 30cc/hr. Na increased to 111 after 3 hrs, and then 114 three hours after that. It has been over 24 hrs since the start of his treatment in the ED and his Na remains 114. I was confused with this management plan. (The reason why he was transferred to the MICU and not gen med floors was because there were concerns about whether his Na would be managed correctly.)

My ultimate question is, if you were taking care of this patient, at what rate would you infuse and with what type of crystalloid? I want to be prepared the next time I see a hyponatremic patient and I want to be able to defend my decision when challenged by my senior.

I know you are very busy, so I will patiently wait for your answer. In the meantime, I will be looking for articles/videos on the Nephrology On Demand website on the topic of hyponatremia.


Posts: 37
Post Re: Correcting hyponatremia with normal saline??
on: September 6, 2012, 09:48

Great questions. Unfortunately, it seems you did a lot of mathematical calculations for no good reason. Allow me to go comment on your question point-by-point.

1. The initial diagnosis was either SIADH or primary polydipsia. Given that the UOSM was greater than the SOMS (368 versus 238), the diagnosis of primary polydipsia cannot be true. If one were drinking to excess (or, "out drinking" their kidneys), the UOSM would be very very low (much lower than the SOSM).

2. 0.9% normal saline was the incorrect fluid to use in this case. The amount of Na in NS is 154 meq/l. That means the osmolarity of NS is 308 (154 x 2). However, the patient's UOSM is 368 (higher than 308). In other words, you are administering a fluid (normal saline) that is more DILUTE than the urine generated by the patient. The patient's kidneys are seeing normal saline not as "salt and water" but as "water alone". Thus, you are basically administering water to a patient with SIADH.

3. In order to pick the correct crystalloid, you must find one whose osmolarity is greater than UOSM of the patient. That way, you can be sure you are infusing a solution that is more CONCENTRATED than the urine being produced by the SIADH-patient. Hypertonic saline, whose Osmolarity is 514 meq/l, would have been the best first choice.

4. Adjusting the rate of NS administration should not change the amount of water and salt you are giving to the patient. That's because the ratio is always 154 meq Na per liter of normal saline. All the rate does is allow you to give a set amount of salt quickly, but it doesn't change the total amount that you would give after a 1 liter infusion of normal saline.

5. Beer potomania would be a little hard to diagnose if the UOSM is 368. It would suggest that he is eating something and therefore has some osmoles in his urine.

I hope this helps.

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