SM 202 Hyponatremia Flashcards
What is tonicity?
A term that refers to the volume behavior of cells in a solution, such as expansion in hypotonic and contraction in hypertonic solutions
What is the requirement for a solute to be an effective osmol?
Solutes that are effective osmols are trapped on one side of the cell and therefore effect transmembrane water flow
What are common effective osmoles?
Na + K due to the Na/K ATPase
Glucose because it gets trapped inside of cells via phosphorylation
What common solutes are not effective osmols?
Urea and Ethanol because they can cross the membrane
What is the utility of serum Sodium?
Serum Sodium is a surrogate marker of tonicity
How rapidly is the response to changes in tonicity?
Changes in tonicity rapidly cause changes in cell shape which lead to cell toxicity
What cell type is resistant to changes in tonicity?
Neurons - because they can export or import ions to compensate for changes in ions/tonicity
What is the difference between tonicity and serum osmolarity?
Tonicity is “guessed” at while serum osmolarity is measured
What are the units on serum osmolarity?
mMol/L or mOsm/L or mEq/L
How do ineffective mOsms effect osmolarity?
Can’t get rid of the ineffective mOsms so they raise Osmolarity
What causes hypertonicity and hypernatremia, broadly speaking?
Low water such as from dehydration raises effective Sodium concentration
What causes hypotonicity and hyponatremia, broadly speaking?
Excessive water such as from H2O intoxication
How does total body Na related to Effective Arterial Blood Volume?
Total blood Na contributes to EABV, with 15% on the baroreceptor side
What happens in response to a true decrease in EABV?
Volume depletion such as orthostasis
What happens in response to a perceived decrease in EABV?
Edema, perceived EABV as low despite a normal EABV
What neurosignaling effects EABV?
Adrenergic signaling (norepi and epi) Aldosterone release (Na reclamation from principal cells) ADH (Water reclamation from principal cells)
What do osmoreceptors respond to?
Osmoreceptors respond to changes in plasma tonicity due to stretch deformations
Describe the neural pathway for ADH release?
Brain osmoreceptors and AngII receptors in the third ventricle detect changes tonicity
The OVLT and SFO in the 3rd cerebral ventricle signal the MnPO and Hypothalamus to release ADH from the Posterior Pituitary
Why is the Third Ventricle the site for osmoreceptors?
The third ventricle has a more permeable BBB which allows it to sample blood and determine tonicity
Do osmotic or non-osmotic stimuli release ADH?
Both! Osmotic stimuli drive ADH release primarily but severe volume depletion can also stimulate ADH
How is it that Na reabsorption is separate from water handling?
Most of the Sodium is absorbed iso-osmotically in the PCT, while the water is absorbed or released into the urine at the collecting duct
What determines whether or not water is absorbed in the collecting tubule?
ADH binding to the V2R inserts Aquaporins into the Collecting Duct to determine whether or not it gets absorbed
What is required for ADH to reclaim H2O in the collecting duct?
ADH requires the medullary concentration gradient setup by NKKC in the TALH to reabsorb water and draw it out of the urine
What are insensible losses and how do they effect the kidney?
Insensible losses are losses of water due to respiration and sweating, which force to kidney to retain some water
What is the equation for electrolyte-free H2O clearance?
Solute Excretion/Urine Osmolarity * (1 - (UrineNa + UrineK)/SerumNa)
How do the Clearance of Electrolytes and Clearance of Free-H2O relate to Urine Volume?
Urine Volume = Electrolyte-free H2O Clearance + Electrolyte Clearance
How does Electrolyte free clearance vary with urine osmolarity?
Electrolyte free clearance decreases with urine osmolarity
More of the urine output contains solute and less contains free water, because the urine is more concentrated at higher osmolarity
How does urine concentration vary with solute excretion?
At higher solute excretions, urine concentration increases
What are the 3 broad categories of hyponatremia?
Isotonic Hyponatremia
Hypotonic Hyponatremia
Hypertonic Hyponatremia
Which form of hyponatremia is fake?
Isotonic Hyponatremia, aka artificial hyponatremia
What conditions can cause Isotonic Hyponatremia?
Elevated proteins or fatty acids
Why is Isotonic Hyponatremia fake?
In Isotonic Hyponatremia, the protein/lipid fraction of blood goes up so the portion of blood composed of water goes down, lowering the concentration of sodium measured in the tube, even if the body sodium concentration is normal; hence, fake hyponatremia
What causes Hypotonic Hyponatremia?
Decreased water excretion leading to more water retention and lower effective sodium concentration = hyponatremia
What causes Hypertonic Hyponatremia?
Adding impermeable solutes to the ECF, where Sodium is found, draws water into the ECF, lowering the effective concentration of Sodium = hyponatremia
What symptoms accompany hyponatremia and why?
Seizures, nasuea, fatigue all due to cell swelling in the setting of hyponatremia
What determines the severity of symptoms in hyponatremia?
Symptom severity is determined by how rapidly the hyponatremia sets in, with more rapid onset causing more severe symptoms
How does the brain respond to hyponatremia?
Hyponatremia = less sodium in the ECF which lowers osmolarity and favors water influx into neurons, causing the brain to swell until it can compensate
What blood volumes does hypotonicity occur at?
Hypotonicity can occur at any blood volume
Can hyponatremia arise from the kidney excreting more sodium than water?
Never
Can hyponatremia arise from the kidney failing to excrete the water it takes in?
Yes, hyponatremia results if water intake exceeds the free water clearance
What are the 3 types of hypotonic hyponatremia?
Volume depletion, euvolemic, and edema
Low, normal, and high volume states
How do diuretics and vomiting/diarrhea cause hyponatremia?
Diuretics and vomiting/diarrhea cause hypotonic hyponatremia through volume depletion
How do psychogenic polydipsia and thiazide-induced SIADH induce hyponatremia?
Psychogenic polydipsia and thiazide-induced SIADH cause euvolemic hypotonic hyponatremia
Thiazides cause water retention for unknown reasons
Patient drinks too much water
Both involve elevated water and normal total body sodium
How do cirrhosis, nephrosis, and heart failure induce hyponatremia?
Cirrhosis, nephrosis, and heart failure cause excessive water retention leading to hypotonic hyponatremia
How does volume depletion lead to hypotonic hyponatremia?
Reduced EABV activates the RAAS system and constricts renal arteries via Angiotensin II
Angiotensin II promotes the activity of the Na/H and Na/Cl transporters leading to more Na reabsorption
AngII promotes Ald which leads to more Na reabsorption
Person then drinks water to lower the Na concentration further
How does edema lead to hypotonic hyponatremia?
Edema involves blood pooling in the veins and lowers the perceived EABV
Low EABV leads to activation of the RAAS system and further promotes water reabsorption
What causes psychogenic polydipsia leading to hyponatremia?
Patient consumes large amounts of water in excess of the kidney’s ability to release water, leading to water retention and hyponatremia
What is a normal urine volume?
There isn’t one - it’s set by the amount needed to excrete solutes from diet and metabolism
What is SIADH?
Syndrome of Inappropriate ADH secretion
Patients with SIADH cannot excrete a water load due to overproduction of ADH leading to high urine sodium concentration and low plasma osmolarity
How should you treat hypotoninc hyponatremia?
Water restriction
Administer normal saline if the patient is volume depleted
Treat a primary cause if possible
How should SIADH be treated?
ADH V2 Receptor anagonists
When should 3% Saline be administered?
Use 3% saline (higher than the normal 0.9%) to treat cerebral edema from water overload, such as during SIADH
