Electrolyte Free Water Clearance

this is an 18 slide subset of the way too long lecture on sodium called Hyponatremia Dreadnaught

1. True hyponatremia
Hyponatremia occurs when water intake
exceeds water excretion.

2. True hyponatremia
Hyponatremia does not occur when
sodium excretion exceeds sodium intake.
Negative salt balance
causes hypovolemia

3. 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

4. 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.
 
C
X=
U
X
´V
P
X

5. 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

6. 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
 
C
osm=
U
osm
´V
P
osm
 
C
osm
=
142´1
284
 
C
osm
=0.5

7. 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

8. Free water clearance
0.5 liter 568 mOsm/Kg
1 liter 284 mOsm/Kg
-0.5 liter 568 mOsm/Kg

9. 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
 
C
osm=
U
osm
´V
P
osm
 
C
osm
=
568´0.5
284
 
C
osm
=1.0

10. 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

11. 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
+

12.  
C
tH
2O
=V
C
tH
2
O
=C
osm
+C
H
2
O
Substitute V for C
tH
2O
V=C
osm
+C
H
2
O
C
H
2
O=V-C
osm
C
H
2
O
=V-
U
osm´V
P
osm
é
ë
ê
ù
û
ú
C
H
2
O=V´1-
U
osm
P
osm
é
ë
ê
ù
û
ú
Free water clearance: The math

13. Free water clearance: Math
Examples
 
C
H
2
O
=V´1-
U
osm
P
osm
é
ë
ê
ù
û
ú
C
H
2O
=1000´1-
50
285
é
ë
ê
ù
û
ú
C
H
2
O
=825ml
 
C
H
2
O
=V´1-
U
osm
P
osm
é
ë
ê
ù
û
ú
C
H
2O
=1000´1-
1200
285
é
ë
ê
ù
û
ú
C
H
2
O
=-3211ml

14. 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.

15. 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.

16. 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

 
C
H
2O
=V´1-
U
osm
P
osm
é
ë
ê
ù
û
ú
Free water clearance

 
C
EFW
=V´1-
U
Na
+U
K
P
Na
é
ë
ê
ù
û
ú
Electrolyte free water clearance

17. 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
 
C
H
2O
=V´1-
U
osm
P
osm
é
ë
ê
ù
û
ú
C
H
2O
=800´1-
800
270
é
ë
ê
ù
û
ú
C
H
2
O
=-1570ml
 
C
H
2O
=V´1-
U
osm
P
osm
é
ë
ê
ù
û
ú
C
H
2O
=800´1-
800
270
é
ë
ê
ù
û
ú
C
H
2
O
=-1570ml
 
C
EFW=V´1-
U
Na+U
K
P
Na
é
ë
ê
ù
û
ú
C
EFW=800´1-
5+40
125
é
ë
ê
ù
û
ú
C
EFW
=512
 
C
EFW=V´1-
U
Na+U
K
P
Na
é
ë
ê
ù
û
ú
C
EFW=800´1-
125+40
125
é
ë
ê
ù
û
ú
C
EFW
=-256

18. Etiology of hyponatremia
Hyponatremia occurs when water intake
exceeds water excretion.
Hyponatremia occurs when water intake
exceeds electrolyte free water clearance.
Ingestion > EFW clearance