SM 214: Hypernatremia Flashcards
Equation for free water clearance
CefH2O = (Urine Volume)x(1 - (Uk + Una)/Sna)
Urine Volume = Solute Excretion Amount/Urine Conc.
What stimuli release ADH? Which one stimulates it more?
Stimulated by hypertonicity and low blood volume
ADH release more sensitive to hypertonicity than low blood volume
At less volume, ADH release more sensitive to hypertonicity
What is the definition of hypernatremia?
What are the symptoms of hypernatremia?
SNa > 145mEq/L
Sx: Thirst - Lethargy - Somnolence - Coma - Seizures
What is the brain’s response to hypernatremia?
Brain takes in solute to prevent cell shrinkage.
Upon volume restoration, risk of cerebral EDEMA (if SNa falls more than 8mEq/L/day)
What are the two mechanisms to hypertonic hypernatremia?
- High hypertonic salt intake
2. Persistent water loss not replaced by intake
Broad classes of DDx for Hypertonic Hypernatremia
- Hypertonic Na Gain
- Non-Polyuric Water Loss
- Polyuric Renal Water Loss
Mechanism and Cause of Hypertonic Na Gain (Hypertonic Hypernatremia)
high intake of hypertonic fluid = water shift from ICF to ECF = causes brain shrinkage, cerebral blood vessel tears, limbic demyelination, high EABV, acute pulm edema
Cause: Drink Sea Water/Salty feedings, receiving 3% NaCl, Primary Aldosteronism (lots of Na Reabsorption)
Mechanisms and Cause of Non-polyuric water loss causing hypertonic hypernatremia
Non-kidney hypotonic fluid loss
Primary hypodypsia: rare genetic variant and occurs in infirm elderly with less perceived thirst and no access to water
Mechanism: Water Loss > Water Gain - high insensible daily losses + fever/sweat
Cause: GI Loss (Vomit/non-secretory diarrhea)
May also have oliguria if volume deplete (due to ADH)
Mechanism of Polyuric Water Loss (Solute Diuresis) in Hypertonic Hypernatremia
Glycosuria: more water stays in tubule to hydrate non-reabsorbed glucose = decreases Na conc. = Less Na Reabsorption + more Na Excretion
Polyuria washes out interstitial gradient = ADH becomes less effective
Hypokalemia also accompanies this because ENaC activated to try to reclaim whatever Na it can
Hypotonic renal loss = low TBW = more thirst
Occurs with Glucose, mannitol, urea, diuretics, NaCl, NaH2CO3
Mechanisms of Polyuric Water Loss (Pure Water Loss) in Hypertonic Hypernatremia
- Central Diabetes Insipidus
Brain can’t make/release ADH
Cause: alcohol, pituitary tumors, post-brain surgery/brain trauma, genetic mutation in ADH - Nephorgenic Diabetes Insipidus
Kidneys can’t respond to ADH
Cause: hypercalcemia (binds CaSR = less NKCC activity = less Na reabsorption = less interstitial gradient); hypokalemia (less urea reabsorption in PT = less interstitial gradient); renal disease, drugs, genetic mutations (x-linked V2R, AD AQP2)
What is a water deprivation test good for?
Test: restrict water intake, measure urine volume/OsM and plasma OsM for 2-3 hrs
If Urine OsM stable despite rising Plasma Osm - ADH is not working!
Test part 2: give 4mcg desmopressin IV and follow urine response (volume/OsM)
Central Diabetes Insipidus: Urine OsM doubles! (kidneys are functional)
Nephrogenic Diabetes Insipidus: Urine OsM does not change (kidneys not working)
How to treat acute Na intoxication
Administer Water as D5W (SNa should not fall more than 8mEq/L/day)
Estimate target H2O deficit: TBW x ([SNa]/140 - 1)
How do you correct Na in High Serum Glucose?
SNa,correct = SNa + [(Glucose - 100)/100 x 2]
How do you treat hypernatremia due to hypotonic loss?
Sweating, GI loss, Solute Diuresis
Replace Na, K, H2O with NS or half normal saline
Change in SNa from 1 L infusion = (infused Na + infused K - SNa)/(TBW+1)
Don’t forget to correct for insensible losses and renal losses (CefH2O) - with D5W
How do you treat Central and Nephrogenic DI
Central DI: desmopressin
Nephrogenic: Low Na/protein diet, thiazide diuretics (lose Na), NSAIDs (because lowers prostaglandin synthesis, which competes with ADH)
