Sodium and Fluid Balance

Question 1

What blood level defines hyponatraemia?

Answer 1

<135 mmol/L

Question 2

What is the underlying pathogenesis of hyponatraemia?

Answer 2

Increase extracellular water

Question 3

Which hormone controls water balance?

Answer 3

Anti-diuretic hormone

Question 4

How does ADH promote water retention?

Answer 4

Inserting aquaporin-2 channels into the cells of the collecting duct

Question 5

Where and on which receptors does ADH act?

Answer 5

V2 receptors in the collecting duct AND V1 receptors found on vascular smooth muscle

Question 6

What are the two main stimuli for ADH secretion and where are these detected??

Answer 6

Increased osmolality (hypothalamic osmoreceptors) and decreased blood pressure (baroreceptors in the carotids, atria, and aorta)

Question 7

Into which 3 categories do we clinically assess hyponatraemic patients?

Answer 7

Hypovolaemic
Euvolaemic
Hypervolaemic

Question 8

What are clinical features of hypovolaemia?

Answer 8

Tachycardia
Postural hypotension
Dry mucous membranes
Reduced skin turgor
Reduced urine output
Confusion/drowsiness
Low urine sodium

Question 9

What are clinical features of hypervolaemia?

Answer 9

Raised JVP
Bibasal crackles
Peripheral oedema

Question 10

What are 4 causes of hypovolaemic hyponatraemia?

Answer 10

Diarrhoea
Vomiting
Diuretics
Salt-losing nephropathy

Question 11

What are 3 causes of euvolaemic hyponatraemia?

Answer 11

Hypothyroidism
Adrenal insufficiency
SIADH

Question 12

What are 3 causes of hypervolaemic hyponatraemia?

Answer 12

Cardiac failure
Cirrhosis
Nephrotic syndrome

Question 13

List 5 main causes of SIADH.

Answer 13

CNS pathology
Lung pathology
Drugs
Tumours
Surgery

Question 14

List drugs that cause SIADH.

Answer 14

SSRIs, TCAs, opiates, PPIs, carbamazepine

Question 15

What will plasma and urine osmolality be in SIADH?

Answer 15

Reduced plasma osmolality

Increased urine osmolality

Question 16

How does one treat hypovolaemic hyponatraemia?

Answer 16

Volume replacement with 0.9% saline

Question 17

How does one treat hypervolaemic hypernatraemia?

Answer 17

Fluid restriction

Treat underlying cause

Question 18

How does one treat euvolaemic hypernatraemia?

Answer 18

Fluid restriction

Treat underlying cause

Question 19

What complication can be caused by correcting serum sodium too quickly?

Answer 19

Osmotic demyelination (central pontine myelinolysis)

Question 20

If fluid restriction is not enough, what else can be done to manage SIADH?

Answer 20

Give frusemide and salt tablets

Question 21

What blood level defines hyponatraemia?

Answer 21

> 145 mmol/L

Question 22

What is the underlying pathogenesis for hypernatraemia?

Answer 22

Unreplaced water losses

Question 23

What are the causes of hypernatraemia?

Answer 23

GI loss (D&V)
Sweat loss
Renal loss (osmotic diuresis, DI)

Question 24

What is diabetes insipidus?

Answer 24

DI is the inability to produce a concentrated urine due to:

• a deficiency of antidiuretic hormone (ADH) (cranial diabetes insipidus)
• renal resistance to ADH (renal diabetes insipidus)

Question 25

What investigations should one do for DI?

Answer 25

• Serum glucose (exclude DM)
• Serum potassium (exclude hypokalaemia - it can induce renal DI)
• Serum calcium (exclude hypercalcaemia)
• Plasma and urine osmolality
• Water deprivation test

Question 26

How does one treat hypernatraemia?

Answer 26

Fluid replacement - use dextrose
Treat underlying cause
If someone is hypovolaemic, initially give 0.9% saline to treat the hypovolaemia, then give dextrose for the hypernatraemia.

Question 27

How does diabetes mellitus interact with serum sodium levels?

Answer 27

Hyperglycaemia will draw water out of cells leading to hyponatraemia.
Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia.