Electrolyte disorders Flashcards
How should you approach hypernatremia?
If pt has polyuria, consider diabetes insipidis
If the pt has low urine output, evaluate urine osmolality.
If osmlality is >400, they have low fluid intake or excess loss
If osmolality is
How is a water deficit calculated?
Total body water X (Na/140 - 1)
this is equal to 0.6* mass in kg * (Na/140 - 1)
How is the water deficit used to treat a patient with hypernatremia?
- First 24 hrs: maintenance fluids PLUS half of free water deficit
- Give rest of the free water deficit over the next 1-2 days (in addition to maintenance fluids)
- Monitor closely to avoid excessive Na correction- you don’t want to reduce Na by more than 12 mEq/day maximum
Note that free water may be slightly artificially high because of fluid loss which translates into high total body water
What are the 2 types of diabetes insipidus?
Antidiuretic hormone not doing it’s job!
- Central diabetes insipidus: fialure of posterior pituitary to secrete ADH. May be idiopathic or due to cerebral trauma, pituitary tumor, hypoxic encephalopathy, malnutrition
- Nephrogenic: Kidneys don’t respond to ADH. May be hereditary or due to lithium toxicity, hypercalcemia, or hypokalemia
What is a water deprivation test for diabetes insipidis?
- 2 to 3 hrs of water deprivation, followed by ADH administration.
Normal response: no change in urine osmolality; also seen in nephrogenic type
Increased urine osmolality after ADH administration suggests central type
Treatment of diabetes insipidis
Central: desmopressin
Nephrogenic: salt restriction, increase water intake. Thiazide diuretics may help
What is pseudohyponatremia?
aka iso-osmotic hyponatremia (with serum osms of 280-295)
this is due to extreme hyperlipidemia or hyperproteinemia
-It is an artifact -it is NOT REAL
What are causes of hyperosmotic hyponatremia? What formula should I know?
basically, something ELSE in the blood is pulling out a lot of water from the ICF. Your sodium stores are normal, but your concentration appears low because of the osmotic effect of that other substance, which is usually glucose.
To calculate: Ad 1.6 mEq of Na for every 100 mg/dL of glucose that exceeds 100.
What needs to be working in order to excrete excess water?
- No ADH
- You must have a working DCT
- Enough fluid to be delivered distally
What is a good approach to hypoosmotic hyponatremia?
- Determine volume status
a) hypovolemic (vomit, diarrhea, burn, possibly thiazides): Urine sodium will be 1%)
b) hypervolemic: effective circulating volume is depleted. consider cirrhosis, nephrotic syndrome, CHF. FENa will be 1%.
FENa >1 also seen with hypothyroidism, adrenal insufficiency, kidney failure
What are underlying causes of SIADH?
-CNS bleeds, surgery, pneumonia, asthma, empyema, TB, pneumothorax, sarcoid, HIV, small cell lung cancer, SSRI, cyclophosphamide, major surgery
How fast can you correct hyponatremia?
no faster than 12 mEq/day
What are the key labs in SIADH?
urine osmolarity > 100, urine Na > 20, normal renal, adrenal, thyroid studies
What is the tx fo SIADH?
fluid restriction, loop diurectics and hypertonic saline if symptomatic, demeclocycline to maintain normal Na levels
What is pseudohypokalemia?
due to red cell hemolysis that occurs after blood has been drawn. for this reason, K should be measured right away in drawn blood and increased serum K should be confirmed with a repeat blood sample
