4. Sodium fluid balance

Question 1

What is the usual reference range for serum sodium?

Answer 1

135-145mmol/L

Question 2

What is the serum sodium range for hyponatraemia?

Answer 2

<135mmol/L

Question 3

What is the most common electrolyte abnormality in hospitalised patients?

Answer 3

Hyponatraemia

Question 4

What is the underlying pathogenesis of hyponatraemia?

Answer 4

Increased cellular water. Water balance is controlled by vasopressin (ADH) released from posterior pituitary and acts on the V2 receptors in collecting duct cells to increase water retention by increasing AQP-2 channel insertion.

Question 5

Where are V1 receptors found and what does it do?

Answer 5

Found in vascular smooth muscle and causes vasoconstriction at higher concentrations.

Question 6

What are the 2 main stimuli for secretion of ADH?

Answer 6

Serum osmolality mediated by hypothalamic osmoreceptors. Blood volume or pressure mediated by baroreceptors in carotids, atria and aorta.

Question 7

How does increased ADH secretion affect serum sodium

Answer 7

Decreases

Question 8

First assessment of a patient with hyponatraemia is to clinically assess volume status. What would you notice on examination?

Answer 8

O/E: tachycardia, low BP, dry mucous membranes, reduced tissue turgor, confusion, drowsiness, reduced urne sodium output (<20 - reliable way of telling that this is hypovolaemia as the kidneys are trying to retain fluid). NOTE: If the patient is on diuretics they will have high urine sodium regardless.

Question 9

Clinical features of hypervolaemia

Answer 9

Raised JVP, bibasal crackles, peripheral oedema

Question 10

Causes of hyponatraemia

Answer 10

Common cause of low sodium is hypervolaemia.

Renal cause: diuretics.

Extra-renal causes: diarrhoea, vomiting.

Other causes: cardiac failure (low cardiac output, low BP), cirrhosis, renal failure

Question 11

What are causes of hypovolaemic hyponatraemia?

Answer 11

Diarrhoea, vomiting, diuretics, salt losing nephropathy

Question 12

What are causes of euvolaemic hyponatraemia?

Answer 12

Hypothyroidism, adrenal insufficiency, SIADH

Question 13

What are causes of hypervolaemic hyponatraemia?

Answer 13

Cardiac failure, cirrhosis, nephrotic syndrome

Question 14

How does diarrhoea and vomiting cause hypovolaemic hyponatraemia?

Answer 14

Hypovolaemic patients still have excess water. D+V leads to a loss of salt and water. This leads to low perfusion pressure and consequently increased ADH release. The patient will then reabsorb more water than salt leading to hyponatraemia.

Question 15

Why is there hyponatraemia in cirrhosis?

Answer 15

It leads to the release of various vasodilators like nitric oxide which lower blood perfusion pressure.

Question 16

What investigations to do for a patient with hyponatraemia who is euvolaemic?

Answer 16

Hypothyroidism -> thyroid function tests; adrenal insufficiency -> short synacthen test; syndrome of inappropriate ADH (SIADH) -> low plasma osmolality, high urine osmolality

Question 17

What are causes of SIADH?

Answer 17

CNS pathology, lung pathology (e.g. lung tumour secreting ADH), drugs (e.g. SSRIs, TCA, opiates, PPIs, carbamazepine), tumours, surgery

Question 18

A diagnosis of SIADH is only made if:

Answer 18

No hypovolaemia, no hypothyroidism, no adrenal insufficiency, reduced plasma osmolality AND increased urine osmolality (>100)

Question 19

How does euvolaemic hyponatraemia work?

Answer 19

The initial increase in ADH release will cause water retention. Then atrial expansion caused by increased circulating volume will lead to atrial natriuretic peptide (ANP) release causing sodium (+water) loss.

Question 20

Management of hypovolaemic hyponatraemia

Answer 20

Volume replacement with 0.9% saline to replenish circulating fluid volume to normal levels and switch off the stimulus for ADH release.

However, you need to be sure that the patient is in fact hypovolaemic because giving 0.9% saline to euvolaemic patient can worse hyponatraemia.

Question 21

Management of hypervolaemic hyponatraemia

Answer 21

Fluid restriction and treat underlying cause

Question 22

Management of euvolaemic hyponatraemia

Answer 22

Fluid restriction and treat underlying cause

Question 23

How does severe hyponatraemia present?

Answer 23

Reduced GCS, seizures

Question 24

How to manage severe hyponatraemia?

Answer 24

Requires expert help, treat with hypertonic 3% saline. When treating hyponatraemia, serum sodium should NOT be corrected too rapidly as it can cause osmotic demyelination (central pontine myelinosis).

Question 25

What does osmotic demyelination (central pontine myelinosis) lead to?

Answer 25

Quadriplegia, dysarthria, dysphagia, seizures, coma and death. These Sx only manifest a few days after sodium has been corrected too quickly, by which point it is too late and cerebral damage has been done.

Question 26

How to treat SIADH?

Answer 26

If water restriction is insufficient, give demeclocycline, which reduces responsiveness of collecting tubule cells to ADH. Monitor U+Es for in case there is a risk of nephrotoxicity.

Could give tolvaptan,
V2 receptor antagonist – causes aquaresis, loss of water without salt

Question 27

What is the serum sodium range for hypernatraemia?

Answer 27

> 145mmol/L

Question 28

What are the main causes of hypernatraemia?

Answer 28

Unreplaced water loss. This includes GI losses (common), sweat losses. Renal losses e.g. osmotic diuresis, reduced ADH release or action (diabetes insipidus). NOTE: tends to happen in patients who don’t drink when they are dehydrated e.g. elderly, children

Question 29

Investigations for diabetes inspidus

Answer 29

Serum glucose 🡪 exclude diabetes mellitus

Serum potassium 🡪 exclude hypokalaemia.

NOTE hypokalaemia can induce a nephrogenic diabetes insipidus

Serum calcium 🡪 exclude hypercalcaemia

Plasma and urine osmolality

Water deprivation test

Question 30

What is Tx for hypernatraemia?

Answer 30

Fluid replacement -> use dextrose as this replaces fluid without giving excess salt. Treat underlying cause. Take serial Na+ easurements, every 4-6 hours.

Question 31

What to do if someone is hypovolaemic and hypernatraemic?

Answer 31

You may initially give 0.9% saline to treat hypovolaemia followed by dextrose for the hypernatraemia. Take serial Na+ easurements, every 4-6 hours.

Question 32

How can diabetes mellitus affect serum sodium?

Answer 32

Variable effects on serum sodium depending on dominating mechanisms. Hyperglycaemia will draw water out of cells leading to hyponatraemia
However, osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia.