Hypernatremia

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    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

    free-water-deficit

    STEP 2: Choose rate of correction

      • 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

    Figure from: Adrogue HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499.

    Figure from: Adrogue HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499.

    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

    infusion-sodium

    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/

     

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