Sodium and Fluid Balance Flashcards
What blood level defines hyponatraemia?
<135 mmol/L
What is the underlying pathogenesis of hyponatraemia?
Increase extracellular water
Which hormone controls water balance?
Anti-diuretic hormone
How does ADH promote water retention?
Inserting aquaporin-2 channels into the cells of the collecting duct
Where and on which receptors does ADH act?
V2 receptors in the collecting duct AND V1 receptors found on vascular smooth muscle
What are the two main stimuli for ADH secretion and where are these detected??
Increased osmolality (hypothalamic osmoreceptors) and decreased blood pressure (baroreceptors in the carotids, atria, and aorta)
Into which 3 categories do we clinically assess hyponatraemic patients?
Hypovolaemic
Euvolaemic
Hypervolaemic
What are clinical features of hypovolaemia?
Tachycardia Postural hypotension Dry mucous membranes Reduced skin turgor Reduced urine output Confusion/drowsiness Low urine sodium
What are clinical features of hypervolaemia?
Raised JVP
Bibasal crackles
Peripheral oedema
What are 4 causes of hypovolaemic hyponatraemia?
Diarrhoea
Vomiting
Diuretics
Salt-losing nephropathy
What are 3 causes of euvolaemic hyponatraemia?
Hypothyroidism
Adrenal insufficiency
SIADH
What are 3 causes of hypervolaemic hyponatraemia?
Cardiac failure
Cirrhosis
Nephrotic syndrome
List 5 main causes of SIADH.
CNS pathology Lung pathology Drugs Tumours Surgery
List drugs that cause SIADH.
SSRIs, TCAs, opiates, PPIs, carbamazepine
What will plasma and urine osmolality be in SIADH?
Reduced plasma osmolality
Increased urine osmolality
How does one treat hypovolaemic hyponatraemia?
Volume replacement with 0.9% saline
How does one treat hypervolaemic hypernatraemia?
Fluid restriction
Treat underlying cause
How does one treat euvolaemic hypernatraemia?
Fluid restriction
Treat underlying cause
What complication can be caused by correcting serum sodium too quickly?
Osmotic demyelination (central pontine myelinolysis)
If fluid restriction is not enough, what else can be done to manage SIADH?
Give frusemide and salt tablets
What blood level defines hyponatraemia?
> 145 mmol/L
What is the underlying pathogenesis for hypernatraemia?
Unreplaced water losses
What are the causes of hypernatraemia?
GI loss (D&V) Sweat loss Renal loss (osmotic diuresis, DI)
What is diabetes insipidus?
DI is the inability to produce a concentrated urine due to:
- a deficiency of antidiuretic hormone (ADH) (cranial diabetes insipidus)
- renal resistance to ADH (renal diabetes insipidus)
What investigations should one do for DI?
- Serum glucose (exclude DM)
- Serum potassium (exclude hypokalaemia - it can induce renal DI)
- Serum calcium (exclude hypercalcaemia)
- Plasma and urine osmolality
- Water deprivation test
How does one treat hypernatraemia?
Fluid replacement - use dextrose
Treat underlying cause
If someone is hypovolaemic, initially give 0.9% saline to treat the hypovolaemia, then give dextrose for the hypernatraemia.
How does diabetes mellitus interact with serum sodium levels?
Hyperglycaemia will draw water out of cells leading to hyponatraemia.
Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia.
