Hypernatremia

Hypernatremia – Rebecca Choudhury, Trevor Stevens

Background

  • Definition = Na+ >145; Hypernatremia is a lack of free water
  • Dehydration is both hypernatremia AND hypovolemia
  • New studies suggest there is no evidence that overcorrection of hypernatremia leads to adverse outcomes! Do not undertreat!
  • Common causes: Inadequate free water intake relative to solute intake (dementia, limited mobility, tube feeding/TPN (ICU), impaired thirst/adipsia from hypothalamic stroke)
  • Less common causes: Increased insensible losses, Upper GI losses, Na+overload (salt poisoning, iatrogenic from NS infusion, overcorrection)
    • Renal losses: osmotic diuresis (hyperglycemia, SGLT-2 inhibitors, urea, mannitol), central or nephrogenic diabetes insipidus

 

Presentation

  • Lethargy, irritability, confusion, esp. if chronic
  • Seizures, coma, hemorrhagic stroke, or subarachnoid hemorrhage in severe acute hypernatremia (typically >158)

 

Evaluation

  • If Na+ 145-150 and free water deficit < 3L, trial free water first, and if no improvement perform workup as below:
  • If Na+> 150, check serum osm, UA, urine Na+ and urine osm if etiology is unclear
      • UOsm >600, UNa+ low (typically <25): volume depletion
      • UOsm >600, UNa+ high (typically >100): Na+ overload
      • UOsm < SOsm (typically <300): DI
      • UOsm 300-600: osmotic diuresis and/or partial DI

 

Management

  • Step 1: Treat any underlying cause (vomiting, hyperglycemia, medications)
  • Step 2: Determine volume status: If severely volume deplete (i.e. hypotensive) the patient will need lactated ringers to restore volume first and then D5W to restore free water
  • Step 3: Estimate free water deficit (FWD) and replete as below:
    • Online calculators available (MDCalc): FWD = TBW x [(serum Na/140) - 1]
  • Step 4: determine if chronic or acute

 

Acute Hypernatremia

  • Correction for severe hypernatremia (<48 in duration, Na >158), Na+ <145 within 24h
  • Requires ICU level care: Trend Na+ q 2-3 hrs (plus glucose, if using D5W)
    • Once Na+ <145, can space to q6-8hrs
  • Start D5W at 3-6 cc/kg/hr (1/2 NS if pt is severely hyperglycemic), titrate to desired correction rate

 

Chronic Hypernatremia

  • Correction (>48h, most common scenario), aim for slow correction of 10 mEq/L per day
  • Use the free water deficit to guide your plan and total free water needs
  • Trend Na q8-12hrs until stable at desired correction rate, then can space to q12-24h
  • Start D5W and give based on free water deficit; for starting rate and length you can also use “Stone’s formula”: 2 x body weight in kg = rate [mL/hr], and 2 x change in Na+ desired = length [hrs] (e.g. if you want to drop Na+ by 6 then run for 12 hrs)
  • If severely hyperglycemic: Use 1/2NS at 1.35 mL/hr x patient’s weight in kg if pt
  • If mild and pt neurologically intact, may be done with oral or enteral (if feeding tube) water

 

Additional Information

  • Pts w/ suspected DI: Consult Nephrology (may require desmopressin or may receive desmopressin once stabilized to differentiate between central and nephrogenic DI)
  • Consider Urine volume an ongoing loss of free water, and add it to the D5W replacement
  • Pts with hypokalemia: giving K decreases total amount of free water you are giving the pt