WEM1
Lab diagnostics MK, JSC
2015/2016
Sodium deficit
Postoperatively a 70-kg patient has a serum
sodium value of 120 mEq/L (120 mmol/L)
to increase this value to 140 mEq/L would require
the administration of how much sodium in mEq?
what is the amount of fluid (0.9% NaCl, ml/hour)?
what is the amount of fluid (3% NaCl, ml/hour)?
rate of replacement 12 mEq/L in the first 24
hours (osmotic melinolysis!!)
rate of replacement 6 to 8 mEq/L in the first 24
hours
𝑈𝑂𝑆𝑀 can vary from about 50 mOsm/kg water
to about 1200 mOsm/kg (plasma osmolality is
280 to 290 mOsm/kg )
urine volume (flow actually!) can vary from
about 500 ml/day to about 20 L/day.
Example
Patient 60 yo, male who presented with
hyponatraemia secondary to SIADH.
On initial presentation,
plasma [Na+] was 125 mmol/L
urinary osmolality 𝑈𝑂𝑆𝑀 was 300 mOsm/kg
urinary Na+ excretion was 80 mmol/day
urinary K+ excretion was 20 mmol/day
daily solute excretion was 600 mOsm/day
Example
• What is the treatment od SIADH?
• How to achieve a negative free water
balance?
• 𝑤𝑎𝑡𝑒𝑟 𝑖𝑛𝑡𝑎𝑘𝑒 < 𝑢𝑟𝑖𝑛𝑎𝑟𝑦 𝑣𝑜𝑙𝑢𝑚𝑒?
Free water clearance (FWC)
Urinary volume (flow) is viewed as
having two components
urinary solutes in a solution that is
isosmotic to plasma (osmolal clearance)
𝐶𝑜𝑠𝑚
a solute free water (free water clearance)
𝐶𝐻2𝑜
𝐶𝑜𝑠𝑚
𝐶𝑜𝑠𝑚
𝐶𝑜𝑠𝑚
𝐶𝐻2𝑜
𝑈𝑂𝑆𝑀 = 𝑃𝑂𝑆𝑀
𝐶𝐻2𝑜
𝐶𝑜𝑠𝑚
𝐶𝐻2𝑜
𝑈𝑂𝑆𝑀 < 𝑃𝑂𝑆𝑀
𝑈𝑂𝑆𝑀 > 𝑃𝑂𝑆𝑀
Correcting hyponatraemia
 in hypovolemic hyponatraemia the goal is to
replace Na, K, and water deficit
 in dilutional (euvolaemic, hypoervolaemic)
hyponatraemia the goal is to induce negative free
water intake
 electrolyte-free water intake must be less than
urinary electrolyte-free water excretion
(assuming insensible loss and fecal water loss
approximate ingested water and metabolic
water!)
Electrolyte-free water clearance
(EFWC)
Urinary volume (flow) is viewed as
having two components
containing a concentration of Na+K that is
isonatric to the plasma
Na(isonatric electrolyte clearance)
does not contain Na and K salts and is
termed electrolyte-free water (electrolytefree water clearance)
Electrolyte-free water clearance
(EFWC)
Example
Patient 60 yo, male who presented with
hyponatraemia secondary to SIADH.
On initial presentation,
plasma [Na+] was 125 mmol/L
urinary osmolality 𝑈𝑂𝑆𝑀 was 300 mOsm/kg
urinary Na+ excretion was 80 mmol/day
urinary K+ excretion was 20 mmol/day
daily solute excretion was 600 mOsm/day
Example
urinary volume (flow)?
urinary [Na+] concentration?
Urinary [K+] concentration?
EFWC, MEFWC?
Example
If a patient was placed on 1 L free water
restriction, a negative balance would be
-0.3 L
Example 2
Patient 55 yo, presented with hyponatremia
secondary to the SIADH.
On initial presentation,
plasma [Na+] was 120 mmol/l,
urinary [Na+K] was 100 mmol/l,
urinary volume (flow) was 1.5 L/day
Example 2
As the urinary volume was 1.5L/day, the
patient was placed on 1 L free water
restriction.
However, this degree of free water restriction
resulted in a decrement in plasma Na.
Why was free water restriction ineffective in
this patient?
Example 2
• Difficult to adhere!
• Not a therapeutic option!
Example 3
• Patient 62 yo, who presented with
hyponatraemia secondary to SIADH.
• On initial presentation,
– Plasma Na 120 mmol/l
– Urinary somolality was 800 mOsm/kg
– Urinary Na excretion was 100 mmol/day
– Urinary K excretion was 40 mmol/day
– Daily solute excretion was 600 mOsm/day
Example 3
• Negative value!
• ADH leads to reabsorption of electrolyte-free
water rather than excretion!
• Any amount of electrolyte-free water intake
will lead to hyponatraemia!
• In such cases increasing urinary electrolytefree water excretion in excess of electrolytefree water intake
Example 3
• Patient was treated with vasopresin-2
receptor antagonist (VRA, vaptans,
demeclocycline, lithium)
• Urinary osmolality decreased to 100 mOsm/kg
• Volume 600/100=6 liters
• Urine Na =100/6=17 mmol/l
• Urine K 40/6=7 mmol/l
• MEFWC=0.62 (increased by 0.8!)