Hyponatremia and ESRD

Hyponatremia can be seen in patients with end-stage renal disease (ESRD), often as a consequence of a patient’s increase in free water intake in the setting of the kidneys’ diminished ability to regulate sodium and water homeostasis. I recently received a question from a resident asking for some insight into the management of such patients.

To begin, it should be noted that uremic patients with chronic hyponatremia are thought to be protected from osmotic demyelination syndrome (ODS) after hemodialysis. In this situation, urea may act as an effective osmole, whereby the decline in blood urea nitrogen (BUN) levels during dialysis could offset serum hypertonicity.  Additionally, animal studies have shown that azotemic rats were protected from ODS due to a reaccumulation of organic osmolytes  such as myoinositol and taurine within two hours of correction of hyponatremia. 

Though rare, case reports of ODS in uremic patients with hyponatremia do exist. One case described a uremic patient who developed ODS after being initiated on hemodialysis with an initial serum Na of 100 meq/L that corrected to 121 meq/L after three hours against a dialysate sodium concentration of 140 meq/L. It is important to note that ESRD patients in other case reports who went on to develop ODS had other risk factors for the development of the disease, such malnutrition and chronic alcoholism.

I recently helped care for an ESRD patient who was admitted with probable sepsis. His serum sodium on admission was 122 meq/L.  Because we had no outside records available at the time, we assumed his hyponatremia had been present for at least 48 hours. The patient had no uremic symptoms and had no need for urgent small solute clearance or volume removal. Nephrology was consulted for routine dialysis needs.

In choosing our dialysis prescription, we attempted to limit the increase in sodium to no more than 1 meq/L/h, or 3 meq for a 3-hour treatment.  We elected to reduce the blood flow rate to 100 ml/min, use the lowest possible sodium concentration bath allowed in the dialysate (130 meq/L), and use a dialysate flow rate of 600 mL/min.  Cocurrent flows were not used.  After a 3-hour dialysis session, the patient’s serum sodium rose to 125 meq/L, and hourly measurements of serum sodium during dialysis revealed that the patient’s sodium had indeed risen by 1 meq/L/h.

Our calculation was derived from a helpful approach described by Wendland and Kaplan, which showed that the rate in rise of serum sodium could be estimated by the product of the concentration gradient between the patient’s sodium and the dialysate sodium multiplied by the clearance. To do this, we would begins with the following formula:

Change in total body Na = Clearance (L/h) * (dialysate Na – patient Na)

By minimizing the blood flow rate to 100 mL/min and maximizing the dialysate flow rate to 600 mL/min, we assumed that there would be near-total equilibration between the patient’s serum sodium and the sodium concentration of the dialysate bath.  Thus, clearance would be approximately equal to the blood flow rate.

Thus, using our patient as an example:

Change in total body Na = Blood flow rate (L/h) * (130 meq/L – 122 meq/L)

A blood flow rate of 100 mL/min = 6 L/h

Therefore, the change in total body Na after 1 hour of dialysis would be:

6 L/h * 8 meq/L = 48 meq/h

Our patient’s total body water was 48L.  If we add 48 meq Na to our patient’s initial total body Na of 5856 meq (48L * 122 meq/L), we obtain the new total body Na after 1 hour of dialysis: 5856 meq + 48 meq = 5904 meq.

Dividing this value by the patient’s total body water would give us the new serum sodium after 1 hour of dialysis, assuming no ultrafiltration or volume changes.

5904 meq/48L = 123 meq/L

Thus, the patient’s serum sodium would rise by approximately 1 meq/L after 1 hour.

It would be interesting to discuss other options are available for managing ESRD patients with chronic hyponatremia.  Assuming there is no urgent need for small solute clearance, would continuous renal replacement therapy be a safer option?  If uremia is indeed “protective” against ODS, should we avoid low dialysate flow rates and concurrent flows?  How would the prescription change if the patient were to need more aggressive small solute clearance?

It seems that there is more than one way to manage these patients, and factors such as co-morbidities and electrolyte abnormalities need to be taken into consideration when formulating a dialysis prescription.

Posted by Devika Nair, MD

Nephrology Fellow, Vanderbilt