Acute Treatment of Hyponatraemia

Sodium concentration less than 135meq/L

ICCU treats those with much lower levels, or very acute drops (as they are symptomatic)

Background

• Remember the value is a concentration

• Hyponatraemia is usually XS water

• Water balance is mediated via ADH

• In hyponatraemia there’s usually either an ADH problem (too much), or it has been overwhelmed (primary polydipsia)

Pseudohyponatraemia

• Serum is 93% water and electrolytes, 7% fat and protein.

• If fat increased (e.g. Propofol infusion syndrome) or protein increased (e.g. myeloma) the water component proportion is reduced.

• The Na conc of the water component is unchanged.

• The Na conc of the sample is reduced.

Osmolarity

• 2(Na+K)+Urea+Glc

• Urea is an ineffective solute as crosses cell membrane.

• The effects of Glucose usually small as rarely high.

• Na main determinant

• As water moves freely, this is the osmolarity of total body water.

Osm is usually reduced in hyponatraemia

• As expected (osm = 2(Na+K)+Urea+Glc)

• Unless the issue is:

– Advanced renal failure• Can’t pass dilute urine so water can’t be excreted, which

would decrease osm BUT urea increases.

– The addition of osmolar active molecules drawing water into the ECF.

• Mannitol, Glucose, Alcohol – Gap between measured and calculated

High ADH

• Volume depletion leading to ADH & RAAS activation.

– True (D&V, Bleeding, Thiazide diuretics)

– Effective • Vasodilation in liver fauilure• Pump failure (cardiac)

– Nb in these conditions chronic Na conc <130 = end stage

• SIADH

Endocrine Causes

• Hypothyroid

• Addisons

Mechanisms unclear

Others

• Exercise & Ecstasy– XS fluid intake combined with RAAS

activation

• Primary Polydipsia– Oral water intake so high that ADH

overwhelmed.

History & Examination

• Volume Status

• Signs Ht, renal, liver failure

• Hypothyroidism & Addisons

• Causes of volume depletion

• Lung Ca or neurological disorders

Check Serum Osm

• Expect it to be low.

• If high consider osmolar active molecules & advanced renal failure

Check Urine Osm

• This is to exclude Primary Polydipsia & to track treatment response.

• If primary polydipsia maximally dilute urine (osm<100)

• Urine osm will decrease if a cause of increased ADH is treated (in reality this is only true hypovolaemia)

• It will be fixed in SIADH – see later

Urine Sodium Conc

• To help dx hypovolaemia.

• If present urine Na conc <25 – the body is retaining salt and water (more water than salt).

Treatment

• If volume deplete give volume

• If overloaded fluid restrict

• If SIADH fluid restrict

• Treat underlying disease

• Treat more quickly if acute (<1 day)

• Treat more quickly if seizures or unconscious

Demyelination

• A complication of over-enthusiastic correction.

• Not a risk if <1 day as the CNS protective measures are not yet in place, and it is these measures that cause the issue.

• Raise conc by <10/24hrs and <1/hour

• A rise of 5-6 is said to be enough to correct seixures / decreased LOC

Fluid restriction

• Aim is to decrease water

• Commonly 800ml/day

• Must be <UOP or isn’t restriction!

• If UOP <800 other measures required.

If volume depleted

• Isotonic saline (although it won’t be isotonic as osm decreased).– Is a salt load (fluid conc > serum conc)– Switches off ADH – Treats the underlying disorder

• TBW (L) = 60% weight (1L=1kg)

• A 50kg woman has a Na conc of 110meq/L

• Her TBW is therefore 30L (0.6x50)

• TBW (L) = 60% weight (1L=1kg)

• A 50kg woman has a Na conc of 110meq/L

• Her TBW is therefore 30L (0.6x50)

• Her total body sodium amount is 110x30 = 3300meq

• TBW (L) = 60% weight (1L=1kg)

• A 50kg woman has a Na conc of 110meq/L

• Her TBW is therefore 30L (0.6x50)

• Her total body sodium amount is 110x30 = 3300meq

• ECV = 1/3 of TBW = 10L

• Her extracellular Na amount is 110X10 = 1100meq

• TBW (L) = 60% weight (1L=1kg)

• A 50kg woman has a Na conc of 110meq/L

• Her TBW is therefore 30L (0.6x50)

• Her total body sodium amount is 110x30 = 3300meq

• ECV = 1/3 of TBW = 10L

• Her extracellular Na amount is 110X10 = 1100meq

• She is given 1000ml 0.9%NaCl, containing 154meq Na

• Extracelular Na amount is now 1254 (1100+154)

• TBW is now 31L

• TBW (L) = 60% weight (1L=1kg)

• A 50kg woman has a Na conc of 110meq/L

• Her TBW is therefore 30L (0.6x50)

• Her total body sodium amount is 110x30 = 3300meq

• ECV = 1/3 of TBW = 10L

• Her extracellular Na amount is 110X10 = 1100meq

• She is given 1000ml 0.9%NaCl, containing 154meq Na

• Extracelular Na is now 1254 (1100+154)

• TBW is now 31L

• Her new Na conc is 3454/31 = 111meq/L

• TBW (L) = 60% weight (1L=1kg)

• A 50kg woman has a Na conc of 110meq/L

• Her TBW is therefore 30L (0.6x50)

• Her total body sodium amount is 110x30 = 3300meq

• ECV = 1/3 of TBW = 10L

• Her extracellular Na amount is 110X10 = 1100meq

• She is given 1000ml 0.9%NaCl, containing 154meq Na

• Extracelular Na is now 1254 (1100+154)

• TBW is now 31L

• Her new Na conc is 3454/31 = 111meq/L

• Her ECV is now 1254/110 = 11.4L– (It has increase by >1L as the sodium has

drawn water out of the cells)

• TBW (L) = 60% weight (1L=1kg)

• A 50kg woman has a Na conc of 110meq/L

• Her TBW is therefore 30L (0.6x50)

• Her total body sodium amount is 110x30 = 3300meq

• ECV = 1/3 of TBW = 10L

• Her extracellular Na amount is 110X10 = 1100meq

• She is given 1000ml 0.9%NaCl, containing 154meq Na

• Extracelular Na is now 1254 (1100+154)

• TBW is now 31L

• Her new Na conc is 3454/31 = 111meq/L

• Her ECV is now 1254/110 = 11.4L

• So in hypovolaemia 1L saline increases Na conc by about 1meq/L and increases ECV by just over a litre.

SIADH• Increased ADH whatever the water status is

(inappropriate)

• Sodium handling is intact (regulated by Aldosterone)

• Pathogenesis– Increased ADH leads to increased TBW and decreased Na

conc.– The hypervolaemia triggers increased Na and water loss– The patient is now euvolaemic but with decreased sodium

amount– Potassium is also excreted to reduce cellular swelling

Causes

• Medication

• Surgery

• Malignancy

• Infection

• CNS disease

It’s therefore not a diagnosis on it’s own!

Treatment

• Fluid restriction if possible

• Not possible if:– Seizures or unconcious– Associated with SAH (risk of vasospasm)

• Nb differentiate from CSW by volume status

• In these situations give fluid

Fluid Tx in SIADH

• The solute conc in the administered fluid MUST be > that of the urine.

• In SIADH the urine Osm is fixed (as ADH is unchanged) so if increased solute increased urine output & vice versa.

• The aim is to make the patient loose water not gain Na

• If urine osm is 616meq/L and serum Na conc is 120meq/L

• 1000ml 0.9%NaCl given (osm = 308)

– Initially Na conc will rise (154>120)

– But the increased Na will be excreted (not hypovolaemic and RAAS system unaffected)

– To excrete 308 osm with a fixed urine osm of 616, 500ml urine will be passed.

– 1000ml in, 500ml out so water increased, Na conc decreases further.

So in SIADH….

• Fluid restrict if you can

• Give hypertonic saline if you can’t