Hyponatremia Flashcards
1
Q
Inappropriate ADH secretion vs DI
A
- Syndrome of inap. ADH secretion (SI ADH) cause serum osm to be low and Uosm to be high (reabsorbs water, concentrates urine)
- DI is opposite, the serum osm is high (or normal) and the Uosm is low (can’t reabsorb water- either central or nephrogenic deficit)
- Hyponatremia in part due to excess free water intake
- Hypernatremia in part due to no free water intake
2
Q
Calculating serum osmolality
A
- Sosm: (2Na) + (BUN/3) + (glc/20)
- If it goes above 285 the osmoreceptors will stimulate ADH secretion
3
Q
Hyponatremia (Sna < 135 mEq/L)
A
- 3 different types depending on Sosm
- Hyperosmolar (>290 Sosm)
- Normal Sosm (pseudohyponatremia)
- Hypoosmolar (<275 Sosm)
4
Q
Pseudohyponatremia
A
- Due to serum dilution, Na is not actually low, but there is less of it since the % of water in the serum is low
- This occurs when there is hyperproteinemia or hyperlipidemia, which increases the protein or lipid phase of plasma and reduces the amount of water in plasma
- Since there is less water there is less Na dissolved in a given volume and the lab reads hyponatremic
- But within the portion of water present the concentration of Na is normal
5
Q
Hyperosmolar hyponatremia 1
A
- Sosm is high due to presence of osmotically active solutes (glc, mannitol, ionic contrast) which shifts water from ICF to ECF and dilutes out the Na
- With hyperglycemia, water is pulled out of the cell into the ECF and thus serum Na shows a low value
- However, the Na amount is not actually low, it is just diluted (still hyponatremia since concentration is low)
6
Q
Hyperosmolar hyponatremia 2
A
-We correct for this by calculating a corrected Sna value (the value that Sna would be if the glc were corrected and the water went back into the cell)
This is done by adding 1.6mEq/L to the Sna for every 100mg glc over 100mg/dL
7
Q
ADH response to hyperosmolar hyponatremia
A
-ADH levels will be high, since the body is trying to reabsorb water and bring down the Sosm
8
Q
Hypoosmolar hyponatremia 1
A
- Water intake is greater than water excretion
- Normally this will decrease ADH levels to near zero, but some pts will have abnormally elevated ADH levels, which are contributing to the low Sosm
- Psychogenic polydipsia can cause hypoosmolar hyponatremia b/c there is excess water intake
- There is low ADH since the body has so much water it doesn’t want to reabsorb anymore (appropriately dilute urine)
9
Q
Hypoosmolar hyponatremia 2
A
- Can also be due to inability to excrete free water (inappropriate water retention)
- Inappropriately high ADH levels due to:
- Decreased effective circulating volume (arterial blood) from CHF or liver failure
- Hypovolemia (bleeding, diuresis, vomiting, diarrhea)
- Drugs (narcotics)
- Pain, nausea, and vomiting
10
Q
Hypoosmolar hyponatremia 3
A
- SIADH (Dx of exclusion) often caused by lung and CNS lesions/tumors can lead to euvolemic hypoosmolar hyponatremia
- Impaired urinary diluting ability (renal failure) and nephrotic syndrome
- Multifactorial, including hormonal mechanisms
11
Q
Hypoosmolar hyponatremia based on volume state
A
- 3 different subset of hypoosmolat hyponatremia: hypovolemic, euvolemic, and hypervolemic
- Hypovolemic pts will have dry mucous membranes, skin turgor, other hypovolemia findings
- Euvolemic pts will have no findings (no edema, dryness)
- Hypervolemic pts will have edema
- Within each one there are various causes, but for hypovolemia they can be either extra renal or renal
12
Q
Hypovolemic hypoosmolar hyponatremia
A
- There will be elevated ADH levels since there is low ECFV (pt is dehydrated)
- Can be due to sweating, vomiting, blood loss, renal causes
- If Una is low (kidneys reabsorbing Na like they should be) the problem is extra renal
- If Una is high (kidneys excreting Na) the problem is renal, as in diuretics
13
Q
Euvolemic hypoosmolar hyponatremia
A
- Can be due to a number of things:
- Polydipsia (will have low ADH, low Uosm)
- Hormone problems: pituitary, thryroid, or adrenal insufficiency (leads to high Una, high Uosm)
- Or SIADH (requires euvolemic hypoosmolar hyponatremia plus high Una plus high Uosm)
- Important note: main factor in determining Uosm is not Na, but is urea
14
Q
Causes of SIADH
A
- Durgs
- Post op: pain, nausea, narcotics
- Ectopic production by tumors (small cell CA of lung)
- Pulm disease (pneumonia or TB)
- Neuropsychiatric d/o: CVA, neoplasm, psychosis
15
Q
Hypervolemic hypoosmolar hyponatremia
A
- There is Na and H2O retention, but more H2O retention than Na retention
- This causes dilution of Na so much so to cause hyponatremia
- Can either be due to CHF, liver failure, or nephrotic syndrome (proteins being filtered by glom)
- These causes will result in a low Una (kidney is reabsorbing Na since it thinks it has low volume due to the underlying disease)
- The other cause is renal failure, which presents as a high Una, since the kidney cannot reabsorb Na
16
Q
Clinical manifestations of hyponatremia
A
- Severe Sx when Sna falls below 120-125
- Lethargy
- Seizures
- Coma/death
- Due to decrease in Sosm thus shift of H2O from ECF to ICF and resulting cerebral edema
17
Q
Calculating free water excess
A
- Exact same as calculating free water deficit (just switch the numerator)
- TBW x (140-Sna/140)
18
Q
Rx of hyponatremia
A
- Rx underlying causes
- Water restriction
- Hypertonic saline for serious CNS Sx
- Furosemide: inhibits urinary concentrating ability, and add NaCl
- More specific Rx: based on isotonic, hypertonic, or hypotonic
19
Q
Specific Rxs
A
- Isotonic hyponatremia: no Rx
- Hypertonic: Rx cause such as hyperglycemia
- Hypovolemic hypoosmolar hyponatremia: give volume (saline, blood)
- Hypervolemic hypoosmolar
hyponatremia: Rx the liver failure/CHF and limit Na/H2O intake and use furosemide - Euvolemic hyponatremia: Rx underlying cause, restrict free water, and furosemide
- No survival benefit for V2 receptor blockers
20
Q
Rate correction of hyponatremia
A
- Acute and Sx hyponatremia: do not exceed 10mEq/L increase in 1st day to avoid brain shrinkage (H2O moves from inside to outside the cells when serum Na is replenished)
- This can lead to osmotic demyelination syndrome
- Chronic and ASx: increase by .5 mEq/L per hour
- Acuity more important than severity
21
Q
Osmotic demyelination syndrome
A
- Central and extrapontine myelinolysis
- Risk factors: excessive rate of correction of serum Na after being hyponatremic
- Also: malnutrition, and alcoholism, severe liver disease, hypoxia
- Presentation: dysphagia, quadriparesis, locked in syndrome
- Can be permanent or fatal
22
Q
Key points
A
- Hyponatremia is caused by water excess, likely due to increase in ADH and/or water intake
- Rate of decrease in serum Na best predictor of Sx
- Rapid correction of hyponatremia may induce osmotic demyelination
