Chemical Pathology - Sodium and Fluid Balance

Question 1

define hyponatraemia

Answer 1

serum sodium < 135 mmol/L

Question 2

what is the underlying pathogenesis of hyponatraemia?

Answer 2

increased extracellular water

Question 3

what is water balance controlled by?

Answer 3

ADH

promotes water retention by inserting aquaporin-2 channels into collecting duct cells

Question 4

where does ADH act?

Answer 4

• V2 receptors on collecting duct (insertion of aquaporin-2)

- V1 on VSMC (causes vasopressin, ADH known as vasopressin here)

Question 5

what are the 2 main stimuli for ADH secretion?

Answer 5

• inc in serum osmolality (mediated by hypothalamic osmoreceptors)
• decrease blood volume/ pressure (mediated by baroreceptors in carotids, atria, aorta)

Question 6

what is the first step in the management of a patient with hyponatraemia?

Answer 6

• assess volume status

Question 7

what are the clinical features of hypovolaemia?

Answer 7

• tachycardia
• postural hypotension
• dry mucous membrane
• reduced skun turgor
• confusion/drowsiness
• dec urine outpur
• low urine Na (<20)

Question 8

what is the most reliable clinical sign of hypovolaemia?

Answer 8

• low urine Na

if pt on diuretics, they will have high urine Na

Question 9

what are the clinical features of hypervolaemia?

Answer 9

• raised JVP
• bibasal crackles
• peripheral oedema

Question 10

causes of hypovolaemic hyponatraemia?

Answer 10

• diarrhoea
• vomiting
• diuretics
• salt losing nephropathy

Question 11

causes of euvolaemic hyponatraemia?

Answer 11

• hypothyroidism
• adrenal insufficiency
• SIADH

Question 12

causes of hypervolaemic hyponatraemia?

Answer 12

• cardiac failure
• cirrhosis
• nephrotic syndrome

Question 13

explain how hypovolaemic hyponatraemia works

Answer 13

• hypovolaemic pt still have excess water
• if you have D&V, you will lose a lot of salt and water
• get a low perfusion pressure and inc in ADH release
• pt then reabsorb more water than salt
  = hyponatraemia

Question 14

how does cirrhosis cause hyponatraemia?

Answer 14

leads to release of various vasodilators

leads to a drop in perfusion pressure

Question 15

causes of SIADH

Answer 15

• CNS pathology
• Lung pathology
• Drugs (SSRI, TCA, opiates, PPIs, carbamazepine)
• Tumours
• Surgery

Question 16

how do you make a diagnosis of SIADH?

Answer 16

• no hypovolaemia
• no hypothyroidism
• no adrenal insufficiency
• reduced plasma osmolality
• increased urine osmolality (>100)

Question 17

how should you investigate hypovolaemic hyponatraemia?

Answer 17

• clinically hypovolaemic?

Question 18

how should you investigate euvolaemic hyponatraemia?

Answer 18

• TFTs
• short synacthen test
• plasma and urine osmolality

Question 19

how should you investigate hypervolaemic hyponatraemia?

Answer 19

• ? Fluid overload

Question 20

Tx of hypovolaemic hyponatraemia

Answer 20

• volume replacement with 0.9% saline

will replenish circulating fluid volume to normal levels, switch off stimulus for ADH release

Question 21

Tx of hypervolaemic hyponatraemia

Answer 21

• fluid restriction

- treat underlying cause

Question 22

Tx of euvolaemic hyponatraemia

Answer 22

• fluid restriction

- treat underlying cause

Question 23

symptoms of severe hyponatraemia

Answer 23

• reduced GCS
• seizures
• seek expert help (treat with 3% hypertonic saline)

Question 24

what is an important consideration when treating hyponatraemia?

Answer 24

• serum sodium must NOT be corrected faster than 8-10mmol/L in first 24 hours
• if corrected too rapidly = osmotic demyelination (central pontine myelinolysis)

Question 25

how does central pontine myelinolysis present?

Answer 25

- quadraplegia - dysarthria - dysphagia - seizures - coma - death

Question 26

when are drugs used to treat SIADH?

Answer 26

if fluid restriction is insufficient

Question 27

which drugs can be used?

Answer 27

- Demeoclocycline (reduces responsiveness of collecting tubule cells to ADH, monitor U&Es as risk of nephrotoxicity) - Tolvaptan (V2 receptor antagonist)

Question 28

define hypernatraemia

Answer 28

serum Na concentration >145 mmol/L

Question 29

what is hypernatraemia caused by?

Answer 29

unreplaced water loss - GI losses - sweat losses - renal losses (osmotic diuresis, diabetes insipidus)

Question 30

who mostly gets hypernatraemia?

Answer 30

patients who tend not to drink when they are dehydrated (e.g. elderly and children)

Question 31

what are the investigations for diabetes insipidus?

Answer 31

- serum glucose (exclude DM) - serum potassium (exclude hypokalaemia as can induce nephrogenic DI) - serum Ca (exclude hypercalcaemia) - plasma and urine osmolality - water deprivation test

Question 32

how do you treat hypernatraemia?

Answer 32

- fluid replacement (give dextrose because this will replace fluid without giving excess salt) - treat underlying cause

Question 33

how would you treat someone who was hypovolaemic and hypernatraemic?

Answer 33

- initially give 0.9% saline to treat hypovolaemia | - followed by dextrose for hypernatraemia

Question 34

how does diabetes mellitus affect serum sodium>

Answer 34

- hyperglycaemia will draw water out of cells = hyponatraemia - osmotic diuresis in uncontrolled diabetes leads to loss of water = hypernatraemia