Hypernatremia

Clinical Scenario:  A 66-year-old male with a past medical history of traumatic brain injury and frequent urinary tract infections sent from his nursing home with hypernatremia. On a routine blood draw he was found to have hypernatremia to 160. His mental status is unchanged. How do you correct his hypernatremia?

Hypernatremia is caused by

1. Excessive water loss: gastrointestinal tract (vomiting or diarrhea), sweating, or urination, renal, third spacing (loss of hypotonic fluid)
   • Signs: dry mucous membranes, decreased skin turgor, orthostatic changes
2. Hypertonic sodium gain (iatrogenic)
   • Often causes acute hypernatremia

Steps to correct

STEP 1: Calculate water deficit

• TBW = lean body weight x %
  • Young: 60% male or 50% female
  • Elderly: 50% male or 45% female
• Calculate water deficit

STEP 2: Choose rate of correction

1. • Acute hypernatremia (<48 hours)
     • Goal to lower acutely to 145mmol/L within 24 hours
   • Chronic hypernatremia (>48 hours)
     • Goal lower maximum 10mEq/L in 24 hours (0.5mEq/L/hour)
     • The rate should not exceed this to avoid cerebral edema
     • Re-check sodium every 2 hours

Step 3: Calculate Fluid Repletion

• Acute (< 48 hours)
  • Hourly infusion rate (mL/hour) > water deficit in mL/24 hours
• Chronic (> 48 hours)

1. Pick infusate based on volume status. More hypotonic solutions require less volume.

• Free water via oral or peg tube if mentating well
• D5, 1/4 NS, or 1/2 NS if unable to take by mouth
• Only use normal saline if the patient is hypotensive, orthostatic, or with frank hemodynamic compromise.

2. Calculate change in serum sodium

3.  Calculate the infusion rate

• Infusion rate (L/d) = Target change in serum Na / Calculated change in serum Na = 10mEq/L / Calculated change in serum Na
• Account for insensible losses by adding 1 L/day and you get the infusion volume/day

Back to the case… this was CHRONIC hypernatremia

Step 1:

• TBW = lean body weight x 50% = 70kg x 50% = 35kg
• Water deficit = TBW x (Serum [Na]/140 – 1) = 35 x (160/140 – 1) = 5 L

Step 2:

• Goal: 0.5mEq/L/hour or 10mEq/L/day

Step 3: In this case we will use 0.45% NaCl as the patient is moderately hypovolemic

• Change in serum Na = (infusate Na – serum Na)/(TBW +1) = (77-160)/35 + 1) = -2.3 mmol
• 1/2NS infusion rate = 10mEq/L / 2.3 mEq = (4.34 L + 1L)/24 hours = 220 mL/hour

Or… alternatively you can use MDCalc …Free Water Deficit or Sodium Correction

References/Additional resources (Accessed 12/30/16)

1. Adrogue HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499.
2. If you prefer podcast form http://emcrit.org/podcasts/hypernatremia/
3. Free water deficit calculator http://www.mdcalc.com/free-water-deficit-hypernatremia/
4. Sodium correction in hyper/hyponatremia http://www.mdcalc.com/sodium-correction-rate-hyponatremia-hypernatremia/