Hyponatremia

Question 0

Formula for calculated plasma osmolality?

Answer 0

2*Na + Glu/18 + BUN/2.8

Question 1

Basic cause of hyponatremia?

Answer 1

Water intake > water excretion.

Question 2

Formula for calculated tonicity?

Answer 2

2*Na + Glu/18

notably, this tonicity doesn’t include urea because urea freely diffuses across cell membranes

Question 3

How do people with hyponatremia die?

Answer 3

Brain swelling -> herniation.

this spring, I might have even been able to describe the symptoms of this…

Question 5

When the serum is hypotonic and brain cells start swelling, how do they initially protect themselves?

Answer 5

Brain cells will pump out osmolytes to try to mitigate swelling.

Question 6

What accounts for reduced water excretion when EABV is low?

Answer 6

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.

Question 7

Review: How does ADH increase the permeability of the collecting duct?

Answer 7

Increasing Aquaporin-2 levels.

Question 8

Formula for Free H2O clearance?

Answer 8

Free H2O clearance = UrineVolume * [ 1 - (Urine Na + K)/(Plasma Na + K) ]

(If this is confusing, take a look at Dr. Berns’ email.)

Question 9

4 “non-physiologic” stimulators of ADH?

Meaning… not a stimulus that ADH is trying to reverse, per se.

Answer 9

Pain
Nausea
Hypoxia
Drugs (esp. morphine)

Question 10

2 physiologic stimulators of ADH release?

Answer 10

Low BP
Low EABV
(recall that these aren’t the same thing)

Question 11

If you see low serum [Na+] and you’re concerned from PE about hyponatremia, what should you assess?

Answer 11

Assess serum tonicity.

Question 12

If serum [Na+] is low, and serum tonicity is low, what is your next step?

Answer 12

Assess urine tonicity: If it’s low (100mOsm, there are a few possibilities.

Question 13

If serum [Na+] is low, serum tonicity is low, and urine tonicity is > 100mOsm, what should you next assess?

Answer 13

Assess volume: Hypervolemic, Hypovolemic, and Euvolemic Hyponatremia have different causes and treatments.

Question 14

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?

Answer 14

Hypovolemic hyponatremia - ADH has been activated to preserve EABV, but the extra water retention results in hyponatremia.
Give saline.

Question 15

What are the 2 major factors controlling water excretion?

Answer 15

Effective arterial blood volume (EABV) - other lectures call this similar names.
ADH.

Question 16

If you see “small cell lung cancer” in a question you should suspect….

Answer 16

Syndrome of Inappropriate ADH (SIADH).

Small cell lung cancer can secrete ADH -> hyponatremia.

Question 17

Does SIADH cause hypo, hyper, or euvolemic hyponatremia?

Answer 17

Euvolemic.

Question 18

Treatment for SIADH (assuming it’s not really severe)?

Answer 18

Water restriction. (or if really severe, hypertonic saline…)
Treat underlying cause.

Question 19

4 causes of SIADH?

Answer 19

Tumors making ADH (eg. small cell lung cancer).
Drugs.
Pulmonary processes (eg. pneumonia)
Genetic.

Question 20

4 etiologies of Euvolemic Hypotonic Hyponatremia?

Answer 20

SIADH.
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)

Question 21

What lab values would be consistent with SIADH as a cause of hyponatremia?

Answer 21

UOsm > 100. (urine is inappropriately concentrated)
UNa > 40.
Low serum uric acid…

Question 22

What are 3 etiologies of hypervolemic hypotonic hyponatremia?
What’s the unifying physiological perturbation?

Answer 22

CHF
Liver disease
Nephrosis

Reduced EABV -> poor renal perfusion and increased ADH

Question 23

The appropriate treatment for hyponatremia depends on…

Answer 23

The severity of symptoms, primarily. It also depends on the underlying cause.

Question 24

What should be the target serum [Na+] when correcting osmolality?
How quickly (in mEq/L/hr) should you do it?

Answer 24

Don’t correct to higher than 130mEq/L.

Go slow: don’t exceed 0.5mEq/L/hr

Question 25

What terrible outcome are you trying to avoid by not correcting hyponatremia too quickly?

Answer 25

Central Pontine Myelinolysis

which results in permanent damage ranging from loss of fine motor control to locked-in syndrome

Question 26

Are ADH antagonists clinically useful?

Answer 26

Not yet… studies show increased serum Na+, but the patients didn’t improve.

Question 27

What would you do if you had a patient with [Na+] of 110mEq/L?

Answer 27

This is crazy, life-threateningly low.

Give hypertonic saline to get Na+ to a more safe range of 120mEq/L-ish before addressing the underlying cause.