Flashcards in Hyponatremia Deck (27)
Basic cause of hyponatremia?
Water intake > water excretion.
Formula for calculated plasma osmolality?
2*Na + Glu/18 + BUN/2.8
Formula for calculated tonicity?
2*Na + Glu/18
(notably, this tonicity doesn't include urea because urea freely diffuses across cell membranes)
How do people with hyponatremia die?
Brain swelling -> herniation.
(this spring, I might have even been able to describe the symptoms of this...)
When the serum is hypotonic and brain cells start swelling, how do they initially protect themselves?
Brain cells will pump out osmolytes to try to mitigate swelling.
What accounts for reduced water excretion when EABV is low?
Way more water is absorbed in the proximal tubule and thin descending limb.
ADH plays a role, but very little water even gets to the collecting duct.
Review: How does ADH increase the permeability of the collecting duct?
Increasing Aquaporin-2 levels.
Formula for Free H2O clearance?
Free H2O clearance = UrineVolume * [ 1 - (Urine Na + K)/(Plasma Na + K) ]
(If this is confusing, take a look at Dr. Berns' email.)
4 "non-physiologic" stimulators of ADH?
(Meaning... not a stimulus that ADH is trying to reverse, per se.)
Drugs (esp. morphine)
2 physiologic stimulators of ADH release?
(recall that these aren't the same thing)
If you see low serum [Na+] and you're concerned from PE about hyponatremia, what should you assess?
Assess serum tonicity.
If serum [Na+] is low, and serum tonicity is low, what is your next step?
Assess urine tonicity: If it's low (100mOsm, there are a few possibilities.
If serum [Na+] is low, serum tonicity is low, and urine tonicity is > 100mOsm, what should you next assess?
Assess volume: Hypervolemic, Hypovolemic, and Euvolemic Hyponatremia have different causes and treatments.
Say a vampire sucks some of your blood out, but you rehydrate with water and juice; but then you have a headache and orthostasis.
The serum [Na+] is low, and the urine osmolality is 300... what's going on?
How would you treat this situation?
Hypovolemic hyponatremia - ADH has been activated to preserve EABV, but the extra water retention results in hyponatremia.
What are the 2 major factors controlling water excretion?
Effective arterial blood volume (EABV) - other lectures call this similar names.
If you see "small cell lung cancer" in a question you should suspect....
Syndrome of Inappropriate ADH (SIADH).
Small cell lung cancer can secrete ADH -> hyponatremia.
Does SIADH cause hypo, hyper, or euvolemic hyponatremia?
Treatment for SIADH (assuming it's not really severe)?
Water restriction. (or if really severe, hypertonic saline...)
Treat underlying cause.
4 causes of SIADH?
Tumors making ADH (eg. small cell lung cancer).
Pulmonary processes (eg. pneumonia)
4 etiologies of Euvolemic Hypotonic Hyponatremia?
Thiazides (due to subclinical reduction in EABV).
Glucocorticoid deficiency (causes increased ADH).
Hypothyroidism (low EABV due to poor pump function).
(low EABV stimulates ADH... so these all ultimately are mediated by increased ADH)
What lab values would be consistent with SIADH as a cause of hyponatremia?
UOsm > 100. (urine is inappropriately concentrated)
UNa > 40.
Low serum uric acid...
What are 3 etiologies of hypervolemic hypotonic hyponatremia?
What's the unifying physiological perturbation?
Reduced EABV -> poor renal perfusion and increased ADH
The appropriate treatment for hyponatremia depends on...
The severity of symptoms, primarily. It also depends on the underlying cause.
What should be the target serum [Na+] when correcting osmolality?
How quickly (in mEq/L/hr) should you do it?
Don't correct to higher than 130mEq/L.
Go slow: don't exceed 0.5mEq/L/hr
What terrible outcome are you trying to avoid by not correcting hyponatremia too quickly?
Central Pontine Myelinolysis
(which results in permanent damage ranging from loss of fine motor control to locked-in syndrome)
Are ADH antagonists clinically useful?
Not yet... studies show increased serum Na+, but the patients didn't improve.