W5 - Sodium and fluid balance Flashcards
What is the commonest electrolyte abnormality in hospitalized patients?
hyponatraemia
Hyponatraemia serum Na+ range?
< 135 mmol/L
What is the underlying pathogenesis of hyponatraemia?
Increased extracellular water aka fluid overload
Which hormone controls water balance? How does it exert its effect?
ADH (vasopressin)
- acts on V2 receptors in collecting duct = insertion of aquaporin-2 channels = water reabsorption
- acts on V1 receptors in vascular SM = vasoconstriction = bring BP up
What are the two main stimuli for ADH secretion? How are they sensed?
- Raised serum osmolality = hypothalamic osmoreceptors
- reduced blood volume/pressure = baroreceptors in carotids/atria/aorta
What is the effect of increased ADH secretion on serum sodium?
Hyponatraemia
What is the first step in the clinical assessment of a patient with hyponatraemia?
clinical assessment of water status
What are the clinical signs (7) of hypovolaemia? What is the MOST reliable?
- Tachycardia
- Postural hypotension
- Dry mucous membranes
- Reduced skin turgor
- Confusion/drowsiness
- Reduced urine output
- Low urine Na+ (<20) ** MOST RELIABLE
After clinical assessment of hyponatraemic patient, what do you need to establish?
Whether patient is euvolaemic, hypovolaemic, or hypervolaemic.
Why is low urine Na+ indicative of hypovolaemic hyponatraemia?
B/c of low volume => kidneys hold onto salts to try to bring BP up (water follows salt) => low urine sodium!
What are the clinical signs (3) of hypervolaemia?
- Raised JVP
- Bibasal crackles (on chest examination)
- Peripheral oedema
What is a contra-indications of interpreting urine sodium?
If patient is on diuretics, you cannot use urine sodium for interpretation
Causes of hypovolaemic hyponatraemia?
- Diarrhoea
- Vomiting
- Diuretics
- Salt-losing nephropathy (uncommon)
Causes of euvolaemic hyponatraemia?
- Hypothyroidism
- Adrenal insufficiency
- SIADH
Causes of hypervolaemic hyponatraemia?
- Cardiac failure
- Cirrhosis
- Nephrotic syndrome (from renal failure)
What are the causes of SIADH?
CNS pathology
lung pathology
drugs (SSRI, TCA, opiates, PPI, carbamazepine)
=> brain, lungs, and drugs!
less common: tumours, surgery
What investigations would you order in a patient with euvolaemic hyponatraemia?
- hypothyroidism? => TFTs
- adrenal insufficiency? => short synACTHen test
- SIADH => only after above 2 are disproven & NO hypovolaemia => low plasma osmolality, high urine osmolality
What are the right exclusions to make before diagnosing SIADH?
No Hypovolaemia
No Hypothyroidism
No Adrenal insufficiency
*Reduced plasma osmolality AND
Increased urine osmolality (>100)*
How would you manage a hypovolaemic patient with hyponatraemia?
Volume replacement with 0.9% saline
How would you manage a hypervolaemic patient with hyponatraemia?
- Fluid restriction (0.5-1L/day)
- Treat the underlying cause such as HF
How would you manage a euvolaemic patient with hyponatraemia?
- Fluid restriction (0.5-1L/day)
- Treat the underlying cause
What is severe hypontraemia? How would you treat it?
- Reduced GCS
- Seizures
SEEK EXPERT HELP => treat with 3% hypertonic saline
What is the most important point to remember while correcting hyponatraemia?
Serum Na must NOT be corrected > 8-10 mmol/L in the first 24 hours => risk of osmotic demyelination (central pontine myelionlysis)
What are the signs of central pontine myelinolysis? MOA?
How long after quick correction of hyponatraemia does it happen?
Can it be corrected?
quadriplegia, dysarthria, dysphgia, seizures, coma, death. MOA: a rapid shift of sodium leads to disruption of the BBB
a few days after quick sodium correction
Yes - explain to the patient and lower Na+ again!
