Sodium Flashcards
- What is the definition of hyponatraemia?
Sodium concentration < 135 mmol/L
- What is the underlying pathogenesis of hyponatraemia?
Increased extracellular water
- Describe the action of ADH.
Acts on V2 receptors in the collecting duct
Leads to insertion of AQP2 molecules and an increase in the reabsorption of water
Acts on V1 receptors on vascular smooth muscle leading to vasoconstriction
- What are the two main stimuli for ADH release?
Increased serum osmolality (via hypothalamic osmoreceptors)
Blood volume/pressure (via baroreceptors)
- What is the first step in the management of hyponatraemia?
Assess their volume status
- List some clinical features of hypovolaemia.
Tachycardia Postural hypotension Dry mucous membranes Reduced skin turgor Confusion Reduced urine output
- What is the most reliable clinical sign of hypovolaemia?
Low urine sodium (suggests that you are trying to retain fluid)
NOTE: this may be high in patients on diuretics
- List some clinical features of hypervolaemia.
Raised JVP
Bibasal crackles
Peripheral oedema
- List some causes of hyponatraemia:
a. hypovolaemic
b. euvolaemic
c. hypervolaemic
a. Hypovolaemic Diarrhoea Vomiting Diuretics Salt-losing nephropathy b. Euvolaemic Adrenal insufficiency Hypothyroidism SIADH c. Hypervolaemic Cirrhosis Cardiac failure Nephrotic syndrome
- Explain how patients with hypovolaemic hyponatraemia have too much water.
Diarrhoea and vomiting leads to loss of water and salt
This leads to increased ADH release which causes reabsorption of more water than salt leading to hyponatraemia
- How does cirrhosis lead to hyponatraemia?
Causes the release of various mediators that cause a drop in perfusion pressure
- List some causes of SIADH.
CNS pathology Lung pathology Drugs (SSRIs, TCAs, opiates, PPIs, carbamazepine) Tumours Surgery
- List the main investigative feature of SIADH.
Low plasma osmolality
High urine osmolality
- Which tests would you do for euvolaemic hyponatraemia?
TFTs
Short synacthen test
Plasma and urine osmolality
- Outline the treatment of:
a. hypovolaemic
b. euvolaemic
c. hypervolaemic
a. Hypovolaemic hyponatraemia Volume replacement with 0.9% saline This replenishes the circulating fluid volume and switches off the stimulus for ADH release b. Euvolaemic hyponatraemia Fluid restriction Treat underlying cause c. Hypervolaemic hyponatraemia Fluid restriction Treat underlying cause
- What are some clinical features of severe hyponatraemia?
Reduced GCS
Seizures
- What is the maximum rate of correction of hyponatraemia?
8-10 mmol/L per 24 hours
- What is the main danger of rapidly correcting hyponatraemia?
Can cause central pontine myelinolysis (osmotic demyelination)
This can lead to quadriplegia, dysarthria, dysphagia, seizures, coma and death
- Name and describe the mechanism of action of two drugs used to treat SIADH if fluid restriction is insufficient.
Demeclocycline – reduces the responsiveness of collecting duct cells to ADH
• NOTE: monitor U&E because it can be nephrotoxic
Tolvaptan – V2 receptor antagonist
Alternative: fluid restriction + salt tablets + diuretics
- Define hypernatraemia.
Serum sodium > 145 mmol/L
- List some causes of hypernatraemia.
GI losses
Sweat losses
Renal losses (e.g. osmotic diuresis, DI)
- List some investigations that are used in suspected diabetes insipidus.
Plasma glucose (rule out DM) Plasma K+ (rule out hypokalaemia) Plasma Ca2+ (rule out hypercalcaemia) Plasma and urine osmolality Water deprivation test
- How is hypernatraemia treated?
Fluid replacement – use dextrose because this will replace the fluid without adding to the salt
NOTE: if someone is hypovolaemic with hypernatraemia, they may initially be given 0.9% saline to treat the hypovolaemia before switching to dextrose to treat the hypernatraemia
- How often should serial Na+ measurements be taken in someone being treated for hypernatraemia?
4-6 hours
- How can diabetes mellitus affect serum sodium?
Hyperglycaemia will draw water out of cells (i.e. into the ECF) thereby leading to hyponatraemia
However, high plasma glucose can also lead to an osmotic diuresis (renal losses) which can lead to hypernatraemia
