Sodium and Fluid Balance Flashcards
What is the commonest electrolyte abnormality in hospitalised patients?
Hyponatraemia
Serum sodium <135mmol/L
What is the underlying pathogenesis of hyponatraemia?
Excessive drinking or fluid is the most common cause of hyponatraemia. Excess water, yet the same amount of salt causes the amount of salt in the plasma to be diluted, causing hyponatraemia. So the cause is increased extracellular water
in GI losses e.g. D&V the loss of both salt and water is equal but excess can lead to hyponatraemia when a lot is lost, but also hypovolaemia when a lot of water is lost
Which hormone controls water balance?
ADH/Vasopressin
- What receptors do ADH act on?
2. What are the actions on each of these receptors?
- V1 and V2
- V1 receptors
- Vascular smooth muscle
- Vasoconstriction (higher concentrations)
V2 receptors:
- found in the collecting duct
- Insertion of Aquaporin-2 in order to reabsorb water
What are the two main stimuli for ADH secretion, what are they mediated by and why?
- Serum osmolality (mediated by hypothalamic osmoreceptors) - to reduce serum osmolality
- Blood volume/pressure (mediated by baroreceptors in carotids, atria and aorta) - to increase BP
So a decrease in blood pressure and increased osmolality (thirst) stimulates ADH release
- What is the effect of increased ADH secretion on serum sodium?
- What is the pathogenesis?
- Hyponatraemia
- ADH increases water retention, but unlike aldosterone doesn’t affect sodium reabsorption, so the same amount of sodium is in a larger volume of water, so is diluted and causing hyponatraemia
What is the first step in clinical assessment of a patient with hyponatraemia?
Clinical assessment of volume status
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 useful sign
What are the clinical signs of hypervolaemia?
- Raised JVP
- Bibasal crackles
- Peripheral oedema
What are the causes of hyponatraemia in a hypovolaemic patient?
Renal: diuretics
Extra-renal: D&V
What is the pathogenesis of hypovolaemic hyponatraemia?
Loss of salt and water occurs in D&V, in response ADH is released to deal with the hypovolaemia causing water retention. Sodium levels stay the same but with increased water retention the remaining sodium becomes diluted causing hyponatraemia.
What are the causes of hyponatraemia in a hypovolaemic patient?
Cardiac failure - failure of valves and reduced pressure causing decreased blood pressure, and increased in ADH which increases water retention and diluting sodium levels causing hypontaraemia
Cirrhosis - vasodilation in cirrhosis due to increased nitric oxide, causing reduced blood pressure, causing an increase in ADH and increased water retention, diluting sodium and causing hyponatraemia
Renal failure
What are the causes of hyponatraemia in a euvolaemic patient?
- Hypothyroidism
- Adrenal insufficiency (cortisol is needed for water clearance)
- Syndrome of inappropriate ADH (SIADH)
What are the causes of SIADH?
CNS pathology Lung pathology Drugs (SSRI, TCA, opiates, PPIs and carbamazepine) Tumours Surgery
What investigations would you order in a patient with euvolaemic hypontraemia?
- TFTs - check for hypothyroidism
- Short synacthen test - check for adrenal insufficiency
- Plasma and urine osmolality - check for SIDH
What will the plasma and urine osmolality be in a patient with SIADH?
Low plasma and high urine osmolality
Lack of water in urine and to much water in blood due to excess ADH causing severe water retention
How would you manage a hypervolaemic patient with hyponatraemia?
- Fluid restriction
- Treat the underlying cause
How would you manage a euvolaemic patient with hyponatraemia?
- Fluid restriction
- Treat the underlying cause
What are the clinical signs of severe hyponatraemia?
- Reduced GCS
- Seizures
- Seek expert help (treat with hypertonic 2.7% saline)
What is the most important point to remember while correcting hyponatraemia?
- Serum Na must not be corrected any more than 8-10mmol/L in the first 24 hours.
Risk of osmotic demyelination (central pontine myelinolysis/osmotic demyelination)
Can lead to quadriplegia, dyasarthia, dysphagia, seizures, coma and death
When Na is corrected too fast, and increased sodium into brain cells as water moves out. Increased uptake of sodium into oligodendrocytes which can lead to DNA fragmentation and affecting myelination of nerves. This happens a few days later
What drugs can be used to treat SIADH when water restriction is insufficient?
Demeclocycline
- Reduced responsiveness of collecting tubule cells to ADH. Important to monitor U&Es due to risk of nephrotoxicity
Tolvaptan -V2 receptor antagonist
Hyponatraemia is mostly due to:
a) reduced body sodium
b) Increased extracellular water
B) increased extracellular water
What are the two main stimuli for ADH secretion?
a) Reduced blood volume and reduced serum osmolality
b) Increased blood volume and increased serum osmolality
c) Increased blood volume and reduced serum osmolality
d) Reduced blood volume and increased serum osmolality
d) reduced blood volume and increased serum osmolality
What is the first step in the clinical assessment of a patient with hyponatraemia?
a) measure plasma and urine osmolality
b) measure plasma and urine sodium
c) clinical assessment of volume status
c) clinical assessment of volume status
Which of the following is consistent with SIADH?
a) reduced plasma osmolality and urine osmolality
b) reduced plasma osmolality and increased urine osmolality
b) reduced plasma osmolality and increased urine osmolality
What value is hypernatraemia?
Serum Na > 145mmol/L
What are the main causes of hypernatraemia?
- Unreplaced water loss e.g. GI loss, sweat loss, renal losses (osmotic diuresis, reduced ADH release and action (diabetes insipidus)
- Patient cannot control water intake e.g. children and elderly
What investigations would you order in a patient with suspected diabetes insipdius?
- Serum glucose (exclude T2DM)
- Serum potassium (exclude hypokalaemia)
- Serum calcium (exclude hypercalcaemia)
- Plasma and urine osmolality
- Water deprivation test
How would you treat hypernatraemia with hypovolaemia?
Fluid replacement
Treat the underlying cause
Hypovolaemia and hypernatraemia - 0.9% saline and dextrose
correct water deficit - 5% dextrose
Correct extracellular fluid volume depletion - 0.9% saline
serial Na measurements every 4-6 hours
What are the effects of diabetes mellitus on serum sodium?
Variable:
- Hyperglycemia draws water out of the cells leading to hyponatraemia
- Osmotic diuresis in uncontrolled diabates leads to loss of water and hypernatraemia
