electrolyte free water clearance
DESCRIPTION
this is an 18 slide subset of the way too long lecture on sodium called Hyponatremia DreadnaughtTRANSCRIPT
True hyponatremia
Hyponatremia occurs when water intake exceeds water excretion.
True hyponatremia
Hyponatremia does not occur when sodium excretion exceeds sodium intake.
Negative salt balance causes hypovolemia
If a person drinks more water than the kidney is capable of clearing the excess water will dilute the plasma.
Causes of hyponatremia: Increased intake
To exceed the maximal renal clearance of water an adult needs to drink about 18 liters a day.
clearing
Water clearance
Clearance as a general concept: The clearance of any substance is the volume
of blood cleared of that substance in a set unit of time.
Water clearance Total water clearance is equal to urine output. Not a
useful concept.
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CX =UX ×V
PX
Free water clearance and soup Imagine urine divided
into two components A solute component
containing all of the solute at the same osmolality as plasma.
Loss of this component does not change plasma osmolality
Ladle of soup A free water component
providing the balance of the volume.
Loss of this solute free water will change serum osmolality.
Boiling off water from soup
In regards to sodium all that matters is the free water component
Free water clearance
0.5 liter free water
1 liter 142 mOsm/Kg
0.5 liter 284 mOsm/Kg
Solute component (plasma osmolality 284 mOsm/kg)
Use clearance to calculate the osmolar clearance
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Cosm =Uosm ×V
Posm
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Cosm =142 ×1
284
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Cosm = 0.5
Free water clearance
1 liter 142 mOsm/Kg
0.5 liter 284 mOsm/Kg
Solute component (Solute Clearance)
??0.5 liter Zero mOsm/Kg
Free water component (Free water Clearance)The free water
component equals urine volume minus the solute component
Free water clearance
0.5 liter 568 mOsm/Kg
1 liter 284 mOsm/Kg
-0.5 liter 568 mOsm/Kg
Free water clearance
1 liter 284 mOsm/Kg
0.5 liter 568 mOsm/Kg
Solute component (plasma osmolality 284 mOsm/kg)
Use clearance to calculate the osmolar clearance
€
Cosm =Uosm ×V
Posm
€
Cosm =568 × 0.5
284
€
Cosm =1.0
Free water clearance
Solute component (Solute Clearance)
??Free water component (Free water Clearance)
0.5 liter 568 mOsm/Kg
– 0.5 liter Zero mOsm/Kg
1 liter 284 mOsm/Kg
Free water clearance: Implications
Dilute urine Solute free water
Dilute urine increases serum osmolality
Concentrated urine Negative free water
Concentrated urine de-creases serum osmolality
Na+
Na+
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CtH2O=V
CtH2O=Cosm +CH2O
Substitute V for CtH2O
V =Cosm +CH2O
CH2O=V −Cosm
CH2O=V −
Uosm ×V
Posm
⎡
⎣ ⎢
⎤
⎦ ⎥
CH2O=V × 1−
Uosm
Posm
⎡
⎣ ⎢
⎤
⎦ ⎥
Free water clearance: The math
Free water clearance: Math Examples
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CH2O=V × 1−
Uosm
Posm
⎡
⎣ ⎢
⎤
⎦ ⎥
CH2O=1000 × 1−
50
285
⎡ ⎣ ⎢
⎤ ⎦ ⎥
CH2O= 825ml
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CH2O=V × 1−
Uosm
Posm
⎡
⎣ ⎢
⎤
⎦ ⎥
CH2O=1000 × 1−
1200
285
⎡ ⎣ ⎢
⎤ ⎦ ⎥
CH2O= −3211ml
Electrolyte free water clearance
Osmolality doesn’t cause problems, rather tonicity causes changes in cell volume which cause clinical syndromes.
So free water clearance must be refined to measure clinically significant changes in tonicity.
Electrolyte free water clearance
Osmotically active particles (those that contribute to tonicity): Sodium Potassium Albumin, calcium and others
Sodium is the dominant osmotically active solute of serum to the point that others can be ignored.
Urine has a significant potassium content so in urine sodium and potassium are equal partners in determining urinary tonicity.
Electrolyte free water clearance
Convert the free water clearance calculation to electrolyte free water clearance Substitute urine osmolality with the sum urine
Na + K Substitute serum osmolality with serum sodium
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CH2O=V × 1−
Uosm
Posm
⎡
⎣ ⎢
⎤
⎦ ⎥
Free water clearance
€
CEFW =V × 1−UNa +UK
PNa
⎡
⎣ ⎢
⎤
⎦ ⎥
Electrolyte free water clearance
Electrolyte free water clearance: CHF vs. SIADH Heart Failure
Urine Osmolality: 800 Serum Osmolality: 270 Urine Volume: 800 Urine Na: 5 Urine K: 40 Serum Na: 125
SIADH Urine Osmolality: 800 Serum Osmolality: 270 Urine Volume: 800 Urine Na: 125 Urine K: 40 Serum Na: 125
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CH2O=V × 1−
Uosm
Posm
⎡
⎣ ⎢
⎤
⎦ ⎥
CH2O= 800 × 1−
800
270
⎡ ⎣ ⎢
⎤ ⎦ ⎥
CH2O= −1570ml
€
CH2O=V × 1−
Uosm
Posm
⎡
⎣ ⎢
⎤
⎦ ⎥
CH2O= 800 × 1−
800
270
⎡ ⎣ ⎢
⎤ ⎦ ⎥
CH2O= −1570ml
€
CEFW =V × 1−UNa +UK
PNa
⎡
⎣ ⎢
⎤
⎦ ⎥
CEFW = 800 × 1−5 + 40
125
⎡ ⎣ ⎢
⎤ ⎦ ⎥
CEFW = 512
€
CEFW =V × 1−UNa +UK
PNa
⎡
⎣ ⎢
⎤
⎦ ⎥
CEFW = 800 × 1−125 + 40
125
⎡ ⎣ ⎢
⎤ ⎦ ⎥
CEFW = −256
Etiology of hyponatremia
Hyponatremia occurs when water intake exceeds water excretion.
Hyponatremia occurs when water intake exceeds electrolyte free water clearance.
Ingestion > EFW clearance