ChemPath 3: Sodium and Fluid Balance Flashcards
Define Hyponatraemia
- Serum sodium <135 mmol/L
Which is the commonest electrolyte abnormality in hospitalised patients?
Hyponatraemia
Which hormone regulates water balance?
ADH
Outline how ADH works
- Synthesised in hypothalamus
- Secreted from posterior pituitary, acts on CD in the kidney
- Water retention through insertion of aquaporin 2 (AQA2)
Briefly explain the pathogenesis of hyponatraemia
Which receptors does ADH act on?
What happens once it has bound to these receptors?
V2 receptors in collecting duct –> insertion of AQA2
Where are V1 receptors found?
- Vascular smooth muscle
- Vasoconstriction – higher concentrations
- “Vasopressin”
What are the 2 main stimuli for ADH secretion?
- Serum osmolality (high) – mediated by hypothalamic osmoreceptors
- Blood volume/pressure (low) – mediated by baroreceptors in carotids, atria, aorta
What is the effect of increased ADH secretion on serum sodium?
- ADH only resorbs water, not any sodium
- –> Hyponatraemia
What is the first step in clinical assessment of a patient with hyponatraemia?
clinical assessment of volume status (Hypovolaemia, euvolemia, hypervolaemia)
What are the clinical signs of hypovolaemia?
- Tachycardia
- Postural hypotension
- Dry mucous membranes
- Reduced skin turgor
- Confusion or drowsiness
- Reduced urine output
- Low urine Na+ (<20)
What is the normal range of urine Na?
- Normal range = 40-220 mEq/L [<20 non-renal loss; >20 in renal loss)
What is the issue with using urine Na as a tool for assessing ?hyponatraemia?
- Patients on diuretics may have urine Na that is not reliable (hypovolemic, but no hyponatraemia)
What are the clinical Signs of hypervolemia?
- Raised JVP
- Bi-basal crackles
- Peripheral oedema
What does hyponatraemia in a hypovolaemic patient require?
requires more sodium than water loss
What are the causes of hyponatraemia in a hypovolaemic patient?
- Diuretics
- Diarrhoea and vomiting
What is the difference between hypovolemic Hyponatraemia and dehydration?
- Hypovolemic Hyponatraemia = loss of water and sodium (as opposed to dehydration = loss of just water)
What is the pathophysiology of hypovolemic hyponatraemia?
- Start euvolaemia and then a hypovolemia develops (quickly)
- –> release of ADH à just water retention
- –> dilutes [Na+] but not to same volume status as they were when euvolaemic
- –> hyponatraemia + hypovolaemic
What are the causes of hyponatraemia in a hypervolemic patient?
- excess water,
- excess ADH
- Cardiac failure → low pressure → detected by baroreceptors → ADH release
- Cirrhosis → vasodilated due to excess NO → low BP → baroreceptors → ADH release
- Renal failure → not excreting enough water
What are the causes of hyponatraemia in euvolemic patient?
- Hypothyroidism à reduced contractility à reduced BP à ADH release
- Adrenal insufficiency à less aldosterone à less Na+ reabsorption
-
SIADH –> AQA2 water retention –> inc. volume –> suppress RAAS –> less aldosterone –> less Na+ reabsorption
- CNS pathology – stroke, haemorrhage, tumour
- Lung pathology – pneumonia (Legionella), pneumothorax
- Drugs – SSRI, TCA, PPI, carbamazepine, opiates
- Tumours
- Surgery
- What investigations would you order in a patient with euvolemic hyponatraemia?
- Hypothyroidism → thyroid function tests
- Adrenal insufficiency → short SynACTHen test
- SIADH → plasma and urine osmolality → low plasma and high urine osmolality
- Excess ADH = excess water = volume expansion → secretion for BNP → naturesis à euvolaemic
- If sodium is high**, it’s a **pseudohyponatraemia (i.e. hyperlipidaemia, hyperproteinaemia)
What is required for the diagnosis of SIADH?
- No hypovolaemia (euvolaemia)
- No hypothyroidism
- No adrenal insufficiency
- Reduced plasma osmolality (resorbing lots of water) AND
- Increased urine osmolality (>100) (concentrating the urine) – need to know this ref range
What is the management of hypovolemic patient with hyponatraemia
volume replacement with 0.9% saline
What is the Management of euvolaemic patient with hyponatraemia
fluid restrict (<750ml/day + ABx infusions) + treat underlying cause
What is the Management of hypervolemic patient with hyponatraemia
fluid restrict (<750ml/day + ABx infusions) + treat underlying cause
What are the signs of severe hyponatraemia?
- Reduced GCS
- Seizures
What is the Tx for severe hyponatraemia?
- Seek expert help – treat with hypertonic 3% saline
What is the complication that arises from too rapid a correction of hyponatraemia?
Central pontine myelinolysis
- a neurological condition involving severe damage to the myelin sheath of nerve cells in the pons
What are the signs and symptoms of Central Pontine Myelinolysis?
- Quadriplegia, dysarthria, dysphagia, seizures, coma, death
What rate should hyponatraemia be corrected at?
not greater than 8-10mmol/L in the first 24 hours
Which drug is used to Tx SIADH if fluid restriction is not enough?
- Demeclocycline –> induce nephrogenic diabetes insipidus
- Tolvaptan – V2 receptor antagonist
How does Demeclocycline work?
- Reduces responsiveness of collecting tubule cells to ADH
- Monitor U&Es as risk of nephrotoxicity
Define hypernatraemia
- Serum [Na+] > 145mmol/L
What are the main causes of hypernatremia?
-
1st –> increase sodium intake AND/OR have a loss of water:
-
Increase in sodium:
- Medical high intake (hypertonic saline, sodium bicarbonate)
- Dietary high intake (salty infant formula, high dietary salt)
- Conn’s syndrome (high aldosterone: renin ratio), BAH (high aldosterone: renin ratio)
- Renal artery stenosis (low GFR from RAS à low BP at JGA à high renin à high aldosterone)
- Cushing’s syndrome (overactivation of MR by cortisol à aldosterone-like effect)
-
Loss of water:
-
Renal losses:
- Osmotic diuresis
-
Diabetes insipidus (less ADH action / release)
- Insensitivity/lack to/of ADH à solitary water losses à hypovolaemia
- Body compensates by resorbing more Na+ to reduce the water loss
- Water loss persists and so you get a hypovolaemic hypernatraemia
-
Non-renal losses:
- GI loss
- sweat loss
-
Renal losses:
-
Increase in sodium:
- 2nd –> maintain the low water intake:
- Child / elderly / dementia
- Fasting for surgery
- Simply cannot keep up with losses
- Busy nightshift and forget to drink
What are the investigations for patient with suspected diabetes insipidus?
- Serum glucose – exclude diabetes mellitus –> osmotic diuresis
- Serum potassium – exclude hypokalaemia –> nephrogenic DI
- Serum calcium – exclude hypercalcaemia –> nephrogenic DI
- Plasma and urine osmolality –> exclude hyperaldosteronism (high plasma osmolality, low urine osmolality)
- Water deprivation test (normal = concentrated urine, no ADH = carry on passing water – dilute urine)
What is the management of hypernatraemia?
- Fluid replacement → dextrose (if the patient is also hypovolemic, then 0.9% saline and 5% dextrose water)
- Treat underlying cause
i.e.
- Correct water deficit → 5% dextrose
- Correct ECF volume depletion → 0.9% saline
- Serial Na+ measurements → every 4-6 hours
What are the effects of diabetes mellitus on serum sodium?
- Variable
- Hyperglycaemia draws water out of cells leading to hyponatraemia
- Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatremia
