SM 202: Hyponatremia

Question 1

What are effective osmols?

Answer 1

Na, K, Glucose: molecules trapped on one side of the cell membrane, changes of these effect transmembrane water flow

Question 2

Equation for serum osmolarity

Answer 2

(2*Na) + (Glucose/18) + (BUN/2.8) = serum osmolarity

Question 3

Where is ADH released? What are the stimuli for ADH release?

Answer 3

ADH produced in organum vasculosum of lamina terminalis (OVLT) & sub-fornical organ (SFO)
ADH released from MnPO (signal median preoptic nucleus) and PVH (hypothalamus) all within 3rd cerebral ventricle

Stimuli: osmotic: hypertonicity and Ang II (volume depletion)
Non-osmotic stimuli: volume depletion, nausea, pain, sedation, drugs (key for SIADH)

Question 4

Equation for free water clearance in urine

Answer 4

CefH2O = (Urine Volume)*[1 - (UNa - UK)/SNa]

Where Urine Volume = Excreted Solute/Urine OsM

Excreted Solute ~600mOsm on average

Question 5

What are the causes of isotonic hyponatremia and hypertonic hyponatremia

Answer 5

Isotonic hyponatremia = artifactual = elevated proteins/lipids = less plasma water = na concentration looks low, but it is normal (hypertriglyceridemia)

Hypertonic hyponatremia = translocational = addition of new ineffective osmols (mannitol, glucose) draws more water into ECF = Na conc looks low

Key = Na Conc =/= Total Body Na!!

Question 6

What are the symptoms of hypotonic hyponatremia?

How does the brain adapt to hypotonic hyponatremia?

Answer 6

Nausea, Fatigue, headache, lethargy, somnolence, coma, seizures (all due to cell swelling)
Brain: Secretes solutes into ECF to reduce swelling to compensate - risk of replenishing ECF solutes too quickly is OSMOTIC DEMYELINATION - brain shrinks = permanent damage

Question 7

What volume states can hypotonicity occur? What is the requirement for Hypotonicity?

Answer 7

Hypotonicity can occur at ANY volume: depletion, euvolemia, hypervolemia

Requirement: water intake must EXCEED free water clearance (can never secrete more Na than water)

Question 8

Causes of hypotonic hyponatremia with hypovolemia. What is the mechanism/findings?

Answer 8

Ex: Addison’s Disease, Vomiting, Diarrhea, Diuretics, Exercise/Sweating
Low EABV = activate AngII and Aldo and ADH = more Na and Water Reabsorption = causes oliguria
Patient replaces fluids with hypotonic ones, causing hyponatremia

Question 9

Causes of hypotonic hyponatremia with hypervolemia. What is the mechanism/findings?

Answer 9

Ex: Edema due to cirrhosis, nephrosis, heart failure

Perceived low EABV triggers Na/H20 Reclamation and Oliguria, patient replaces lost fluids with hypotonic ones

Question 10

Causes of hypotonic hyponatremia with euvolemia. What is the mechanism/findings?

Answer 10

1. Psychogenic Polydipsia (potomania): people drink more pure water than they excrete (potomania: drinking water beyond thirst while crash dieting) - volume intake suppresses ADH - high water clearance but higher water intake
2. Thiazide Induced: requires some degree of polydipsia, unknown mechanism but some people experience more water reabsorption and less free water clearance
3. SIADH: syndrome of inappropriate ADH secretion - Uosm is inappropriate for Posm (more concentrated), patients cannot excrete much of a water load

Question 11

Treatment for hyponatremia

Answer 11

1. safest = water restriction - no water in and lose water through insensible losses
2. Volume deplete = give IV saline (0.9% = 154 mEq Na/L) until euvolemic
3. Edema = control water intake and tx primary disease
4. Oral V2R antagonists to raise tonicity in SIADH patients - very EXPENSIVE!
5. SIADH: avoid normal saline (they’ll just excrete the Na), unless coupled with loop diuretics (kidney dumps more solute = more water with it)

Question 12

Equation to calculate change in SNa upon administering saline

Answer 12

0.9% saline: 154mEq/L Na
with potassium: 40mEq/L K

Change in SNa = (infused Na + infused K - SNa)/(TBW + 1)
Should keep Change in SNa < 8mEq to reduce risk of osmotic demyelination