Saturday, July 25, 2009

Things have been coming in pairs: electrolyte free water and hypernatremia

First we had the highest creatinine followed by the lowest creatinine.

Then we had a case of hyponatremia/SIADH that we evaluated with the concept electrolyte free water and now we have a case of hypernatremia that we also evaluated with electrolyte free water.

I have a special fondness for dysnatremia formulas that work with either hyper- or hyponatremia because there is an elegance in using a model that works at both extremes.

Both the change of sodium formula and electrolyte free water calculation work as well with hypernatremia as they do with hyponatremia.

The case: 56 year old African American nursing home resident with a history of bipolar disease. She presents with altered mental status and initial labs reveal acute kidney injury and hypernatremia.

Body weight 70 kg
Na 177
Cr 4.18

On exam the patient had obvious hypovolemia and the elevated sodium reveals dehydration. The primary team appropriately uses half normal saline to correct both of these deficiencies.

The sodium came down to 167 the following day and after that they have been unable to further correct it. The patient remained in the ICU for 4 days prior to us being consulted for persistant hypernatremia. The creatinine rapidly corrected over four days to 1.6.
Additional data:
Admission urine sp grav 1.011
Admission urine Na 10
Admission urine osmolality 330
On the fourth day of the admission the urine output was 1,500 over the prior 8 hours

The primary team had been using the water deficit formula to estimate the amount of fluid to give the patient to correct the sodium over 2 days:

The water deficit equation asks how much water will be needed to dilute all the solutes to some ideal. In many books and programs the ideal sodium is fixed at 140 mmol/L. I usually use 145 mmol/L, which is the least amount of change in sodium to get a normal sodium. I do this because the downside of correcting sodium is all on the side over correction and inducing cerebral edema.

The equations often wont let you determine the % body water and fill in 60%. This is a large source of error because in the United States we're all fat and fat people are relatively anhydrous. Also, since typical internal medicine patient is about 100, and old people are likewise anhydrous 60% is a over estimation of total body water. My fellow estimated the total body water to be 35 liters.

The calculated free water water deficit is 7.7 liters. I usually administer half of that in the first 24 hours unless the sodium is very high, as in this case, and then I would have given about a third of the volume in the first day and correct the sodium over three days. I shoot for a change of about 12 mmol/l per day.

The equation worked well initially with the sodium going from 177 to the mid 160's but after that they stalled. The reason the sodium stopped improving was that they cured the patient of her renal failure and the urine output increased. Neither the team nor the water deficit formula accounted for this.

We can account for the urine output by replacing the electrolyte free water clearance:

So one would have to add 2.8 liters of electrolyte free water to the free water deficit calculation to account for urinary loss of electrolyte free water. If one were to recalculate the water formula using the partially corrected sodium of 167 you get:

If you plan to give half that in the first day that is 2.65 liters. Compare that to the electrolyte free water loss of 2.8 liters and it immediately becomes obvious why the sodium remained stable for days.

Teaser: An elevated electrolyte free water clearance in the presence of hypernatremia is presumptive evidence of diabetes insipidis. I will save that discussion for my next post.
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