ChemPath: Sodium and Fluid Balance Flashcards
What is the underlying pathogenesis of hyponatraemia?
Excess water - concentration of sodium is lower
Which hormone controls water balance?
ADH (vasopressin)
Describe how ADH controls water balance.
ADH is released from the posterior pituitary gland. It acts on V2 receptors on collecting ducts causing insertion of aquaporin-2 water channels. This causes increased water reabsorption.
What receptors may ADH (Vasopressin) work on?
V1 receptors:
- On vascular smooth muscle
- Causes vasoconstriction
- This occurs at higher concentrations
V2 receptors:
- On kidneys
- Insertion of aquaporin-2 channels on collecting ducts
What are the two main stimuli for ADH secretion?
- Serum osmolality - mediated by hypothalamic osmoreceptors
- Blood volume/pressure - mediate by baroreceptors in carotids, atria and aorta
What is the effect of increased ADH secretion on serum sodium?
Hyponatrium
(More water = Less sodium)
What is the first step in the clinical assessment of a patient with hyponatraemia?
- Clinical assessment of volume status
- Look at hands
- Head and neck
- Peripheries
What are clinical signs of hypovolaemia?
- Dry mucous membranes
- Reduced JVP
- Reduces tissue turgor
- Tachycardia
- Postural hypotension
- Confusion/drowsiness
- Reduced urine output
- Low urine Na+ (<20)
If you are hypovolaemic, you need to hold onto sodium so urine sodium will be low → always remember to send off this test
What are clinical signs of hypervolaemia?
- Raised JVP
- Peripheral oedema
- Bibasal crackles (on chest examination)
What makes urine sodium uninterpretable?
Diuretics - these alter the kidney’s ability to retain salt. Must stop it and check 48 hours after.
What are causes of hypovolaemia?
- Diarrhoea
- Vomiting
- Diuretics
- Salt losing nephropathy
What are causes of euvolaemic hyponatraemia?
- Hypothyroidism
- Adrenal insufficiency
- SIADH
What are causes of hypervolaemic hyponatraemia?
- Heart failure
- Cirrhosis
- Nephrotic syndrome
What are causes of hyponatraemia in a hypovolaemic patient?
- Renal: diuretics
- Extra-renal: diarrhoea, vomiting
What the causes of hyponatraemia in a hypervolaemic patient?
- Cardiac failure
- Cirrhosis
- Renal failure
What are causes of hyponatraemia in a euvolaemic patient?
- Hypothyroidism - due to reduction in CO detected by baroreceptors leading to ADH secretion
- Adrenal insufficiency - cortisol needed for water excretion, aldosterone needed for sodium and water retention.
- SIADH
What are the causes of SIADH?
- CNS pathology
- Lung pathology
- DRUGS (SSRI, TCA, opiates, PPIs, carbamazepine)
- Tumours
- Surgery
What investigations would you order in a patient wih euvolaemic hyponatraemia?
- Hypothyroidism: Thryoid function tests
- Adrenal insufficiency: Short synacthen test
- SIADH: Plasma and urine osmolality (low plasma & high urine osmolality)
Will osmolality of plasma and urine be high or low in SIADH?
Plasma osmolality - LOW
Urine osmolality - HIGH (>100)
What does a diagnosis of SIADH require?
- No hypovolaemia
- No hypothyroidism
- No adrenal insufficiency
- Reduced plasma osmolality
- Increased urine osmolality (>100)
How would you manage a hypovolaemic patient with hyponatraemia?
Volume replacement with 0.9% saline - this removes the stimulus for ADH secretion
How would you manage a hypervolaemic patient with hyponatraemia?
Fluid restriction and treat the underlying cause.
Do NOT give saline as the patient will just hold onto the water and exacerbate the hyponatraemia → giving saline won’t address the issue causing ADH secretion if it’s a tumour etc.
How would you manage a euvolaemic patient with hyponatraemia?
Fluid restriction and treat the underlying cause (same as hypervolaemic patient with hyponatraemia)
What are clinical symptoms of severe hyponatraemia?
- Reduced GCS
- Seizures
Seek expert help (treat with hypertonic 3% saline)
How is severe hyponatraemia managed?
Can give boluses of hypertonic 3% saline but only if patient has low GCS or fitting.
DO NOT GIVE 3% saline if alert and orientated.
What is an important point to remember while correcting hyponatraemia?
- Serum Na must NOT be correct >8-10 mmol/L in the first 24 hours
- Risk of osmotic demyelination (central pontine myelinolysis)
- Presents a few days later with quadriplegia, dysarthria, dysphagia, seizures, coma, death
What drugs are used to treat SIADH?
If water restriction is insufficient:
- Demeclocycline
- Tolvaptan
How does Demeclocycline work?
- Reduces responsiveness of collecting tubule cells to ADH
- Need to monitor U&Es (risk of nephrotoxicity)
How does Tolvaptan work?
- V2 receptor antagonist
- Very expensive
What is the treatment for SIADH?
Water restriction
PLUS (but both used rarely)
- Demeclocycline (reduces responsiveness of collecting tubule cells to ADH - but caution because nephrotoxic) OR
- Tolvaptan (V2 receptor antagonist) - use cautiously as they work rapidly
What are the main causes of hypernatraemia?
- Unreplaced water loss
- Gastrointestinal losses, sweat losses
- Renal losses: osmotic diuresis, reduced ADH release/action (Diabetes insipidus)
- Patient cannot control water intake e.g. children, elderly
- Diabetes insipidus
- Not enough ADH
- Resistant to ADH
- Osmotic diuresis in uncontrolled diabetes mellitus
- Loss of water leads to HHS
What investigations would you order in a patient with suspected diabetes insipidus?
- Serum glucose (exclude diabetes mellitus)
- Serum potassium (exclue hypokalaemia)
- Serum calcium (exclude hypercalcaemia)
- Plasma and urine osmolality
- Water deprivation test - urine osmolality will fail to increase
What do patients with diabetes insipidus present with?
Polyuria and polydipsia
How would you treat hypernatraemia?
- Fluid replacement
- Treat the underlying cause
What is the management of hypernatraemia?
- Correct water deficit
- 5% DEXTROSE (free water)
- May need to correct extracellular fluid volume depletion (especially if they’ve been vomiting)
- 0.9% saline
- Serial Na+ measurement
- Every 4-6 hours
What are the effects of diabetes mellitus on serum sodium?
Variable:
- Hyperglycaemia draws water out of the cells leading to hyponatraemia
- Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia
This varies from person to person - based on which factor is pre-dominating
What is the definition of hyponatraemia?
Sodium concentration <135 mmol/L
