Clinical example:

• A 60-year-old gentleman (80 kg) presented to the ED with severe shortness of breath and was diagnosed with severe bilateral pneumonia and COPD exacerbation, he was intubated and placed on mechanical ventilation, these are his Na values:
  • In the ED: 143 (2.5 L of NS given as part of sepsis protocol)
  • Day1 morning’s labs: Na 145 (Nothing done)
  • Day 2 morning’s labs: Na 152 (D5W started at 75 ml/hr)
  • Day 3 morning’s labs: Na 155!! 
• What went wrong with this patient?
  • This patient should have received free water replacement from the get-go, he’s critically ill on mechanical ventilation so high insensible water losses, the patient also received a mildly hypertonic solution (NS).
  • Only on day 2, free water replacement started with D5W which was inadequate.
  • Worsening hypernatremia is the result.

Water losses

• Hypernatremia = water deficit in ECF or increased sodium load.
• Decreased Water in ECF:
  • Inadequate water intake: No access to water (unable to drink or no water to drink).
  • Water loss:
    • Sensible (renal or GI).
      • Renal: osmotic diuresis and DI
      • GI: Osmotic diarrhea
    • Insensible (Sweats, hyperventilation, fever, stool). Physiologic, estimated 0.5- 1L/day.
• High Sodium load (Hypertonic solution): NS, 3% NS, and Sodium bicarbonate.

Ongoing water losses

• Sensible + Insensible
• Insensible:
  • Obligatory water output from sweat and stool.
  • Should be replaced for NPO patients, otherwise, Hypernatremia will develop.
  • Estimated to be 0.5-1L/day.
• Sensible (Renal & Extra-renal)
  • Renal (Osmotic diuresis (Urine osmolality > 300 mosm/L), DI (Urine osmolality < 150 mosm/L))
    • Urine water loss  =  UV  x  (1 – ([UNa + UK] )/SNa))
  • GI (Diarrhea): Not possible to calculate

Water deficit calculation

• Should be added to ongoing water losses.
• (Free water deficit = 0.6 x body weight (kg) x ([Current Na/140] – 1). 
  • Use 0.5 in adult females and elderly males.
  • Use 0.45 in elderly females.
  • 0.6 in children”
  • Use lean body weight, not actual body weight.

• Assume total body water is 50% of lean body weight, the amount of water that is needed to drop sodium level by 1 meq is:
  • 0.5 x Wt (141/140 – 1) = 0.003 L x Wt.
  • 3 ml/kg is required to drop the sodium by 1 meq. 
  • This formula can be used in almost all patients with a small marginal error of 200-300 ml more or less.

Rate of correction.

10 meq/24H is the sweet spot for most hypernatremias except in cases of acute hyponatremia which is uncommon where the total water deficit should be replaced in 24 hours.

Clinical example

Let’s bring up the previous example

Lean body weight is roughly 60 kg (Actual body weight 80 kg).

3 x 60 = 180 ml of free water to drop his Na by 1 meq

180 x 10 = 1800 ml of free water to drop by 10 meq which is the optimal daily correction rate in hypernatremia with few exceptions.

The ongoing water losses = insensible losses ( No sensible losses) ≈ 500 ml/day.

So our patient should get 1800 + 500 = 2300 ml of free water in 24 hours = 2300/24 = 96 ml/hr.

Remember that you may not see the expected Na value on morning labs if <24 hours from the time free water is started.

Once Na is corrected (140) and water deficit replaced then only ongoing losses should be replaced daily.

Type of fluids used in Hypernatremia treatment

• Free water is given orally: only if adequate absorption is anticipated (The best route).
• D5W: 100% free water if no hyperglycemia. (maximum 150 ml/hr)
• 0.45 NS: 50% is free water and 50% is NS.
• Our patient can be treated like this:
  • 383 ml of free water orally every 6 hours or
  • 96 ml/hr of D5W IV or
  • 192 ml/hr of 0.54 NS.

Volume depletion and Hypernatremia

• Use isotonic solution if any hemodynamic instability then switch to hypotonic solutions
• Or use two intravenous solutions simultaneously, one for free water and the other for sodium with or without potassium as an isosmotic solution (in hypovolemia).

Acute hypernatremia

• Hypernatremia is acute if it has been present for 48 hours or less.
• Uncommon.
• Occurs in:
  • Patients with salt poisoning.
  • Patients with arginine vasopressin disorders (DI) who acutely lose the ability to replace their water losses (eg, a patient with an arginine vasopressin disorder who undergoes surgery and does not receive adequate intravenous water).
  • Patients being treated for severe hyperglycemia, whose water losses from glycosuria are not adequately replaced.

• In a patient with acute hypernatremia, the entire deficit is replaced in less than 24 hours.
• 3-6 ml/kg/hr of D5W (maximum 666 ml/hr)
• Once the water deficit is replaced, Hourly infusion rate (mL/hour) > Water deficit in mL  ÷  24 hours

Monitoring in acute hypernatremia

Every 2 hours in acute hypernatremia until Na level < 145.