SM 202a - Hyponatremia Flashcards
What are the three major categories of hypotonic hyponatremia?
- 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
If volume and osmolality are both low, how (in general) will the body react?
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
What are the major causes of hypotonic hyponatremia with volume depletion?
- Diarrhea + Vomiting
- Diuretics
- Cerebral salt wasting
Basically: volume depletion leads to stimulation of RAAs, which promotes Na+ reabsorption and leads to thirst and ADH release. If a patient takes in hyptonic fluids, the kidney will work to reabsorb as much fluid as possible at the cost of osmolality -> hypotnatremia
What is the appropriate treatment for hypotonic hyponatremia?
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
What electrolyte abnormality is associated with oxytocin?
Hyponatremia
- Oxytocin acts like ADH
- Normal solute excretion + abnormal H2O retention
- -> Hyponatremia
What are the major causes of hypotonic hyponatremia due with Edema?
- Congestive HF
- Cirrhosis
- Nephrotic syndrome
Basically: Redistribution of ECF from plasma interstitium leads to perceived volume depletion. This leads to stimulation of RAAs, which promotes Na+ reabsorption and leads to thirst and ADH release. If a patient takes in hyptonic fluids, the kidney will work to reabsorb as much fluid as possible at the cost of osmolality -> hypotnatremia
When you see a low serum Na level, you know there is an excess of _____________
When you see a low serum Na level, you know there is an excess of water
What are the major causes of euvolemic hyponatremia?
- Adrenal insufficiency
- No aldosterone = cannot reabsorb Na+
- Too much fluid intake
- SIADH
- ADH secreted even when there is no physiologic reason -> H2O retention
- Hypothyroidism
What urine osmolality indicates that ADH is present?
Urine osm >100 indicates ADH is present
What is the major cause of hypotonic hyponatremia?
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
Will a patient with low effective circulating volume (ex: CHF) be more likely to develop hyponatremia or hypernatremia?
Why?
Hyponatremia
- Low effective circulating volume = stimulation of ADH
- -> Increased retention of water via ADH
- -> Increased thirst
This will replete water but not solutes
Hypovolemia is caused by a deficit of ________
Hypovolemia is caused by a deficit of Na+
Which electrolyte is a surrogate marker for tonicity?
Serum Na+
What is the equation for electrolyte-free water clearance?
Urine volume = (solute exretion / average daily urine osm)
So
CEFH2O = Urine volume * (1 - [UNa + UK] / [PNa] )
What are the signs of SIADH?
- Urine is concentrated, but serum osmolality is low
- ADH is secreted inappropriately (ex: when serum osm is low)
- -> Cannot excrete H2O load
Reset osmostat looks similar, but the patient is able to excrete the H2O load
Which neurosignalling mechanisms are active in the maintenence of Na+/H2O balance?
- Adrenergic hormones
- Norepinephrine and epinephrine
- Aldosterone
- Na+ reclamation
- Release triggered by hypovolemia
- ADH
- H2O reclamation
- Release triggered by increased osmolality/tonicity
- Alternate pathway triggers ADH release due to hypovolemia, nausea, and pain
What is psychogenic polydipsia?
What electrolyte abnormality is it likely to lead to?
Very excessive water intake
- Usually in the setting of schizophrenia or OCD
- Intake acutely exceeds mechanical limits of the kidneys
- -> Hyponatremia
- Causes hyponatremic siezure when sever
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?
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
What is pseudo-hyponatremia?
What causes it?
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
How does the brain respond to hypotonic hyponatremia?
When the brain is in a hypotonic environment…
- Water gain
- Loss of Na+, K+, Cl-
- Loss of organic osmolytes
Proper therapy = slow (<8 mEq/L/Day) correction of the hypotonic state
(if it is too fast, osmotic demyelination occurs)
What is potomania?
What electrolyte abnormality can it lead to?
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
What is the difference in management of patients with low circulating volume due to CHF vs due to diarrhea?
- CHF (edema)
- Acute diuretics
- Chronic water restriction + chronic diuretics
- Optimize cardiac function to improve renal perfusion
- Diarrhea
- Give saline
Describe the mechanism of thiazide-induced hyponatremia
Thiazides increase water reabsorption in the intermedullary collecting duct for unclear reasons in some people
Usually requires some degree of polydipsia
