SM 202a - Hyponatremia

Question 1

What are the three major categories of hypotonic hyponatremia?

Answer 1

• Truly hypovolemic
  
  • Vomiting/diarrhea
  • Diuretic agents
  • Cerebral salt wasting
• Edema - Low EBV but not volume depleted
  
  • Congestive HF
  • Cirrhosis
  • Nephrotic syndrome
• Near euvolemic
  
  • Addison’s disease (Adrenal insufficiency)
  • Drinking too much
    
    • Polydipsia
    • Potomania
  • Hyperhyroidism
  • Renal failure
  • SIADH

Question 2

If volume and osmolality are both low, how (in general) will the body react?

Answer 2

The body’s #1 priority is restoring volume

• -> ADH secretion even though serum osmolality is low
• The most acute problem is hypovolemia; when volume is restored, a person with healthy kidneys and normal diet will be able to sort our the electrolytes

Question 3

What is the appropriate treatment for hypotonic hyponatremia?

Answer 3

Water restriction

• If volume depletion
  
  • Administer normal saline until euvolemic
• If edema
  
  • Do not administer saline
  • Give diuretics
  • Control H2O intake
  • Treat underling cause (nephrotic syndrome, CHF, cirrhosis)
  • 3% saline (slowly) if neurologic symptoms of cerebral edema
• If SIADH
  
  • ​Do not administer saline
  • The kidney will not be able to excrete the water due to ADH secretion

Question 4

What is the major cause of hypotonic hyponatremia?

Answer 4

Increased water intake + decreased water excretion

You know the patient took in too much water, but there are several reasons for not being able to excrete it

Not caused by excess Na+ excretion

Question 5

Will a patient with low effective circulating volume (ex: CHF) be more likely to develop hyponatremia or hypernatremia?

Why?

Answer 5

Hyponatremia

• Low effective circulating volume = stimulation of ADH
• -> Increased retention of water via ADH
• -> Increased thirst

This will replete water but not solutes

Question 6

What is the equation for electrolyte-free water clearance?

Answer 6

Urine volume = (solute exretion / average daily urine osm)

So

C_EFH2O = Urine volume * (1 - [U_Na + U_K] / [P_Na] )

Question 7

If a patient has high fluid intake and low urine output (in the setting of healthy kidneys), what is the most likely cause?

Which elecrolyte is most likey to be out of balance as a result?

Answer 7

Clinical picture: crash diet with high fluid intake

• The patient is taking in a lot of hyposmotic fluid and/or water and not enough solutes
• The body uses the availble solutes (Na+) to excrete the water, but runs out of solute
  
  • -> Hyponatremia
• No solute = cannot make urine = cannot excrete water ​

Treatment: Give saline (+/- K+ depending on level) OR do not give saline, restrict fluids, and feed the patient

Both methods work, but giving saline will restore electrolyte balance more quickly

Question 8

What is pseudo-hyponatremia?

What causes it?

Answer 8

Normal Na+ and Normal H2O, but test reads as a falsely low Na+ due to excess “other stuff” in the serum

• Triglycerides
• Proteins

May be indicative of underlying malignancy

Question 9

What is potomania?

What electrolyte abnormality can it lead to?

Answer 9

Excessive alcohol intake

Can lead to hyponatremia

Basically you are intaking solute-free fluids. You will excrete these fluids with salt, until you run out of salt and become hyponatremic

Question 10

What is the difference in management of patients with low circulating volume due to CHF vs due to diarrhea?

Answer 10

• CHF (edema)
  
  • Acute diuretics
  • Chronic water restriction + chronic diuretics
  • Optimize cardiac function to improve renal perfusion
• Diarrhea
  
  • Give saline