Review article

TBW

• Men 0.6 x weight
• Women 0.5 x weight
• Elderly have less

Hyponatremia is a disorder of water regulation. Most cases of hyponatremia are from problems in water excretions, kidney failure or CHF.

1. Pseudohyponatremia: serum Na is normal but serum osmolality is normal
   • Severe hypertriglyceridemia (TG’s in 1000’s)
   • Severe hypoproteinemia (such as in multiple myeloma)
2. Hyponatremia with hypertonicity
   • Severe hyperglycemia (DKA)
   • Mannitol
3. Hyponatremia with hypotonicity (“true” hyponatremia): Requires continued water intake
   • Renal failure
   • ECFV depletion (increased resorption of water)
   • Edematous states (increased resorption of water, e.g. CHF, liver failure)
   • Thiazide diuretics
   • SIADH
   • Endocrine (hypothyroidism, adrenal insufficiency)
   • Diminished solute intake: “tea and toast” diet, “beer potomania”

Workup

#1 FLUID RESTRICTION!!! to < 800 ml/24 hrs.  This will buy you some time (hours).

1. When faced with a low sodium value, first check a serum osmolality.
• If it’s low, you can immediately eliminate the first two above (pseudohyponatremia and hyponatremia with hypertonicity) and concentrate on the hyponatremia.
2. If you're not sure if the pt is dehydrated (vomiting) or SIADH, give slow NS.
• If the sodium corrects, then the problem was dehydration.
• If the sodium does not correct then the problem is SIADH
• In this case, calculate the sodium deficit
• Sodium deficit = ([Na]desired – [Na]measured ) x TBW
• 1L of NS has 154 mEq of Na
• 0.9 % to 3% = 3 1/3 and 154 x 3.33 = 513 1/3 So 1L 3% saline has 513 mEq Na

CHF, nephrotic syndrome, CRH, cirrhosis, thiazide diuretics Fig 3-2 Tx Loop diuretic causes more loss of water than Na, so give.