HYPONATRAEMIA - the hardest one! Flashcards
IMO this is the most difficult conceptual topic in the ACEM exam and the hardest to retain - best resource I found was from anaestheasier.com cos ya know anaesthetists etc.
It is explained SO badly by so many resources - it’s probably explained least well by Dunn’s - I would highly recommend never reading that section -> one of the many YUCK aspects of that “book” (I stopped using it after a week) - If you want times new roman bullet point lists of information just ask meta-AI
Write it out a bunch of times and hopefully SOME key takeaways below
What is the most common electrolyte abnormality?!
You guessed it!
HYPONATRAEMIA
Which is one of the least well answered topics in critical care exams?
HYPONATRAEMIA!
Most importantly… for acute severe hyponatraemia with seizures what are you going to do?
IV 1-2mls of 3% NaCl 0.9% over 20 minutes
(standard dose is 100-150mls in adults)
Repeat until they stop seizing - if you give two boluses of this and they haven’t stopped seizing definitely worth reconsidering the diagnosis!
How much do you need to correct the sodium in severe symptomatic hypoNa and why?
Only 2-5mmol/L increase
The pt is seizing because of cerebral oedema - Monro-Kellie doctrine enclosed space
A small shift in intravascular sodium will cause a significant shift in extracellular cereberal oedema
What are the most important figures you need to know to work out the cause of hyponatraemia?
Normal sodium = 135-145
Normal serum osmolality = 280-295mOsm/kg
Urinary sodium <20mmol/L or >20 mmol/L
Urinary osmolality >100mosm or <100mosm (only in fluid overloaded patients with low urinary sodium)
Serum sodium
Serum osmolality
Urinary sodium and osmolality
That is all
What are the main symptoms hyponatraemia causes?
Acute:
No time for brian to adapt to change in osmolarity shift so…
SEIZURES (increased likelkhood)
Chronic:
Progressive
Altered mental status
Dizziness
Ataxia
If the pt is not seizing what are the two ultimate groups and treatments?
Fluid overloaded = fluid restrict + diuretics OR Rx of underlying cause -> hypothyroid/SIADH
Hypovolaemic = replace salt and water
fluid overloaded here includes SIADH which is “relative” overload
Don’t worry more detail to follow
Try to keep it simple
What is the MAXIMUM you want to correct sodium and WHY?
6-8mmol/L in 24 hours
to prevent Central pontine demyelinolysis
This is a good algorithm from anaestheasier.com
These cards are going to go through this in a little more detail
What are the two main groups of hyponatraemia and what differentiates them?
Pseudohyponatraemia (high or normal osmolality)
AND
Actual hyponatraemia
(low osmolality)
If you’ve lost sodium you’ve lost osmolality
What are the main causes of pseudohyponatraemia?
High osmolality = Glucose/mannitol
Normal osmolality = high fat (hyperlipidaemia) OR high protein (myeloma)
How do you calculate osmolality?
2Na + Urea + glu
Patients with LOW urinary sodium who are DEHYDRATED
NORMAL RESPONSE
Lose a bunch of salt in diarrhoea/bleed = hypotonic
Then ADH retains sodium and water but slightly more water than sodium so hypovolaemic but also hyponatraemic
Causes:
Haemorrhage
Burns
Third spacing
Urinary sodium is LOW because kidneys are working correctly to retain Na to also reabsorb H20
How can patient’s with low urinary sodium who are fluid overloaded be divided?
Poor urine output
OR
Diuresis with low URINARY osmolality (<100mosm)
Describe what is occurring in fluid overloaded patients in whom have low urinary sodium and poor urine output and what is their treatment??
NOT a normal response
CCF/Liver failure cause renal hypoperfusion which inappropriately activates RAAS and post pit to release ADH causing sodium and H20 retention
This causes dilutional hyponatraemia and thus hypotonicity with fluid overload
NEPHROTIC syndrome also causes it due to low intravascular protein causing third spacing therefore intravascularly deplete but with fluid overload
These patient’s need diuretics: Furosemide +/- spironolactone
In liver failure the liver also canot breakdown ADH so longer half life
Complete aside
Why doesn’t nephrotic syndrome cause non-cardiogenic pulmonary oedema?
ALthough albumin is low oncotic pressure is maintained by other proteins e.g. globulins
Describe what is occurring in fluid overloaded patients in whom there is diuresis with low urine osmolality?
THIS IS A NORMAL RESPONSE
Can have a “normal” amount of intravascular sodium but it’s diluted
Happens in:
Beer potomania
Water intoxication (can only happen with pre-existing renal disease)
Beer is water and carbs - if LOTS is drunk the carbs are metabolised which suppresses protein breakdown thus make minimal urea SO get maximally diluted urine and fluid retention with dilutional hyponatraemia
