Sodium Balance Flashcards
What is the most common electrolyte imbalance in hospital?
Hyponatraemia
What is the boundary for hyponatraemia?
<135mmol/L
What is the underlying pathogenesis of hyponatraemia?
Increased extracellular water
Which hormone controls water balance?
ADH (vasopressin) water retention via AQ2
What do V1 and V2 receptors do?
V1: AQ2 in CD
V2: Vasc. smooth muscle, vasoconstriction
What are the two main stimuli for ADH secretion?
High Serum osmolality (hypothalamic osmoreceptors)
Low Blood volume/ pressure (Mediated by baroceptors in carotids/ atria/ aorta)
What is the effect of increased ADH secretion on serum sodium?
Hyponatraemia
What does increased water reabsorption cause?
Reduced sodium concentration in the blood
What is the first step in the clinical assessment of a patient with hyponatraemia?
Assess volume status: Hypo/ Eu/ Hypervolaemic
What are the clinical signs of hypovolaemia?
Tachycardia Postural hypotension Dry mucous membranes Reduced skin turgor Confusion/drowsiness Reduced urine output Low urine Na+ (<20) (most reliable)
What are the clinical signs hypervolaemia?
Raised JVP
Bibasal crackles (on chest examination)
Peripheral oedema
What are the causes of hypovolaemic hyponatraemia?
GI/renal loss: Diarrhoea Vomiting Diuretics Salt losing nephropathy
What are the causes of euvolaemic hyponatraemia?
Endocrine:
Hypothyroidism
Adrenal insufficiency
SIADH
What are the causes of hypervolaemic hyponatraemia?
Failures:
Cardiac Failure
Cirrhosis
Nephrotic syndrome
What are the causes of SIADH?
CNS pathology
Lung pathology
Drugs (SSRI, TCA, opiates, PPIs, carbamazepine)
Tumours
Surgery
How might you investigate the euvolaemic causes of hyponatraemia?
Thyroid: TFTs
Renal insufficiency: Short Synacthen test
SIADH: Plasma & urine osmolality (low plasma & high urine osmolality)
How do you diagnosis 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 500ml-1hr/24 hr
Treat the underlying cause
How would you manage a euvolaemic patient with hyponatraemia?
Fluid restriction 500ml-1hr/24 hr
Treat the underlying cause
What happens/ how do you manage in severe hyponatraemia?
Reduced GCS
Seizures
Seek expert help (Treat with Hypertonic 2.75-3% 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 myelinolysis)
quadriplegia, dysarthria, dysphagia, seizures, coma, death
How do you treat SIADH (With drugs)?
Demeclocycline
Tolvaptan
How does Demeclocycline work?
Reduces responsiveness of collecting tubule cells to ADH
Monitor U&Es (risk of nephrotoxicity)
How does Tolvaptan work?
V2 receptor antagonist
What is the boundary for hypernatraemia?
> 145 mmol/L
What are the main causes of hypernatraemia?
Unreplaced water loss (GI loss/ renal loss, sweat, DI)
Cannot control water intake (child/ elderly)
What investigations would you order in a patient with suspected diabetes insipidus?
Serum glucose (exclude diabetes mellitus)
Serum potassium (exclude hypokalaemia) - causes ADH resistance
Serum calcium (exclude hypercalcaemia) - causes ADH resistance
Plasma high & urine low osmolality
Water deprivation test
How would you treat hypernatraemia?
Fluid replacement (dextrose) Treat the underlying cause
How do you manage hypernatraemia in hypovolaemic patients?
5% dextrose (correct water deficit)
0.9% saline (correct EC fluid volume depletion)
Measure Na every 4-6 hours
What are the effects of diabetes mellitus on serum sodium?
Variable
What may happen in DM which changes serum sodium?
Hyperglycaemia draws water out of the cells leading to hyponatraemia
Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia
What does increased osmolality cause other than a release of ADH?
Thirst
Why does vomiting cause hyponatraemia?
Vomiting causes blood volume drop which stimulates water retention which dilutes sodium in the blood
What is the most reliable clinical sign of hypovolaemia?
Low urine sodium (<20)- needs to be assessed ASAP
Is urine sodium low in hypervolaemia?
Yes- hyperaldosteronism caused by HF etc. causes sodium retention
Why does hypothyroidism cause hyponatraemia?
Reduced BP (contractility/ HR) causes more ADH
How long does it take for symptoms to arise in central myelinolysis?
A few days- if they look fine after a day and the sodium has jumped up by 20 mmol/L in one day- THEY ARE NOT FINE and you MUST bring it back down
When does central myelinolysis happen?
When sodium jumps up more than 8-10 mmol/L
How often should sodium be checked?
Every 2-4 hours
How does SIADH have high sodium in urine?
SIADH causes expansion of atria which releases natriuretic peptides
This causes natriuresis (loss of sodium into the urine)
What is water?
‘5% dextrose is water’
????
What is pseudohyponatraemia?
Low Na, normal osmolality and caused by change in proteins/ lipids )e.g. paraproteinaemia)
