ChemPath 12: Sodium and Fluid Balance

Question 1

What is the definition of hyponatraemia?

Answer 1

Sodium concentration < 135 mmol/L

Question 2

What is the underlying pathogenesis of hyponatraemia?

Answer 2

Increased extracellular water

Question 3

Describe the action of ADH.

Answer 3

Acts on V2 receptors in the collecting duct
Leads to insertion of AQP2 molecules and an increase in the reabsorption of water

Acts on V1 receptors on vascular smooth muscle leading to vasoconstriction

Question 4

What are the two main stimuli for ADH release?

Answer 4

Increased serum osmolality (via hypothalamic osmoreceptors)

Blood volume/pressure (via baroreceptors)

Question 5

What is the first step in the management of hyponatraemia?

Answer 5

Assess their volume status

Question 6

List some clinical features of hypovolaemia.

Answer 6

Tachycardia
Postural hypotension

Dry mucous membranes
Reduced skin turgor

Confusion
Reduced urine output

Question 7

What is the most reliable clinical test of hypovolaemia? what is this like in hypervolaemia?

Answer 7

Low urine sodium (suggests that you are trying to retain fluid)

<20 - hypovolaemia
>20 - hypervolaemia

NOTE: this may be high in patients on diuretics

Question 8

List some clinical features of hypervolaemia

Answer 8

Raised JVP

Bibasal crackles

Peripheral oedema

Question 9

List some causes of hyponatraemia - Hypovolaemic?

Answer 9

Extra-renal:
- Diarrhoea + Vomiting

Renal:

• Diuretics
• Salt-losing nephropathy

Question 10

List some causes of hyponatraemia - Euvolaemic?

Answer 10

Adrenal insufficiency (addisons)

Hypothyroidism

SIADH

Question 11

List some causes of hyponatraemia - Hypervolaemic?

Answer 11

Cirrhosis

Cardiac failure

Renal failure
Nephrotic syndrome

Question 12

Explain how patients with hypovolaemic hyponatraemia have too much water

Answer 12

Diarrhoea and vomiting leads to loss of water and salt

This leads to increased ADH release which causes reabsorption of more water than salt leading to hyponatraemia

Question 13

How does cirrhosis lead to hyponatraemia?

Answer 13

Causes the release of various mediators that cause a drop in perfusion pressure

Question 14

List some causes of SIADH.

Answer 14

CNS pathology
Lung pathology
Tumours

Drugs (SSRIs, TCAs, opiates, PPIs, carbamazepine)
Surgery

Question 15

List the main investigative feature of SIADH

Answer 15

Low plasma osmolality

High urine osmolality

Question 16

Which tests would you do for euvolaemic hyponatraemia to diagnose as SIADH

Answer 16

TFTs

Short synacthen test

Plasma and urine osmolality

Question 17

Outline the treatment of:

• Hypovolaemic hyponatraemia
• Euvolaemic hyponatraemia
• Hypervolaemic hyponatraemia

Answer 17

Hypovolaemic hyponatraemia:

• Volume replacement with 0.9% saline
• This replenishes the circulating fluid volume and switches off the stimulus for ADH release

Euvolaemic hyponatraemia:

• Fluid restriction
• Treat underlying cause

Hypervolaemic hyponatraemia:

• Fluid restriction
• Treat underlying cause

Question 18

What are some clinical features of severe hyponatraemia?

Answer 18

Reduced GCS

Seizures

Question 19

What is the maximum rate of correction of hyponatraemia?

Answer 19

8-10 mmol/L per 24 hours

Question 20

What is the main danger of rapidly correcting hyponatraemia?

Answer 20

Can cause central pontine myelinolysis (osmotic demyelination)

This can lead to quadriplegia, dysarthria, dysphagia, seizures, coma and death

Question 21

Name and describe the mechanism of action of two drugs used to treat SIADH if fluid restriction is insufficient

Answer 21

Demeclocycline – reduces the responsiveness of collecting duct cells to ADH
NOTE: monitor U&E because it can be nephrotoxic

Tolvaptan – V2 receptor antagonist

Alternative: fluid restriction + salt tablets + diuretics

Question 22

Define hypernatraemia

Answer 22

Serum sodium > 145 mmol/L - decreased extracellular water

Question 23

List some causes of hypernatraemia

Answer 23

GI losses
Sweat losses
Renal losses (e.g. osmotic diuresis, DI)

Normaly can just drink enough water to replace - hence hypernatraemia only really affects really young / old people

Question 24

List some investigations that are used in suspected diabetes insipidus

Answer 24

Rule out causes of osmotic diuresis:
Plasma glucose (rule out DM)
Plasma K+ (rule out hypokalaemia)
Plasma Ca2+ (rule out hypercalcaemia)

Plasma and urine osmolality
Water deprivation test

Question 25

How is hypernatraemia treated + special case? what happens if too quickly replaced

Answer 25

Fluid replacement – use 5% dextrose because this will replace the fluid without adding to the salt + serial Na measurements NOTE: if someone is hypovolaemic with hypernatraemia, they may initially be given 0.9% saline to treat the hypovolaemia before switching to dextrose to treat the hypernatraemia Rapid correction can lead to cerebral oedema!

Question 26

How often should serial Na+ measurements be taken in someone being treated for hypernatraemia?

Answer 26

4-6 hours

Question 27

How can diabetes mellitus affect serum sodium?

Answer 27

Hyperglycaemia will draw water out of cells (i.e. into the ECF) thereby leading to hyponatraemia However, high plasma glucose can also lead to an osmotic diuresis (renal losses) which can lead to hypernatraemia

Question 28

You are bleeped to review John Doe. A 68-year-old gentlemen on the cardiology ward admitted with Heart Failure. John had contracted Norovirus whilst on the ward is suffering from severe D&V. On examination, he has dry mucous membranes and reduced skin turgor Which of the following is the most accurate descriptor of John’s current condition? - Hypovolaemia Hyponatraemia - Hypochloraemic Hypokalaemic Metabolic Alkalosis - Euvolaemic Hyponatraemia Hypernatraemia - Hypervolaemic Hyponatraemia

Answer 28

Hypovolaemia Hyponatraemia

Question 29

Why do you end up with Excess Water in hypovolaemic hyponatraemia?

Answer 29

Low Blood Volume -> Baroreceptors -> ADH Release

Question 30

Why do you end up with excess water in the different causes of hypervolaemic hyponatraemia?

Answer 30

Cardiac Failure – Low BP -> Baroreceptors -> ADH Release Liver Cirrhosis – Releases vasodilators -> Low BP -> Baroreceptors -> ADH Release Renal Failure – Reduced Water Excretion

Question 31

How do you get excess water w/ euvolaemic hyponatraemia?

Answer 31

All of the above (somehow?) cause a spur of ADH to be released The reabsorption of water increases the circulating volume and therefore increases BP. The increase in BP is detected in the Atria which release ANP release – Natriuresis.

Question 32

What can cause SIADH?

Answer 32

- > CNS Pathology - > Lung Pathology - > Drugs (SSRI, TCA, Opiates, PPI, Carbamazepine) - > Tumours - > Surgery

Question 33

How to treat euvolaemic hyponatraemia?

Answer 33

- > If fluid restriction isn’t enough - > Demeclocycline (Reduce cell responsiveness to ADH) - > Tolvaptan (Antagonist to V2 receptor)

Question 34

You are bleeped to review John Doe. A 46-year-old gentlemen on the respiratory ward with a diagnosis of Lung Cancer, On Examination, He appears cachectic ‘but of normal fluid status’, Low Na+, everything else normal What is the most appropriate management plan?

Answer 34

Fluid restrict + treat underlying cause - Euvolaemic hyponatraemia

Question 35

You are bleeped to review Jane Doe. A 46-year-old lady in A&E with a significant background of alcohol abuse who has presented feeling ‘confused’. On Examination, she has a raised JVP, splenomegaly and shifting dullness. Low Na+, everything else normal Which of the following is the most likely cause of her ‘Hypervolaemic Hyponatraemia’? ``` Renal Failure Diuretic Use Hypothyroidism Liver Cirrhosis Heart Failure ```

Answer 35

Liver Cirrhosis – Releases vasodilators -> Low BP -> Baroreceptors -> ADH Release

Question 36

You are bleeped to review Jane Doe. A 98-year-old lady in A&E with poorly controlled Diabetes Mellitus – ‘who lives alone and her water jug is typically too far for her to reach’. On examination, she has dry mucous membranes and reduced skin turgor. Raised Na+, everything else normal What is the most appropriate management plan? what else is needed along side this? ``` IV 0.9% Saline IV 0.9% Saline + 5% Dextrose 5% Dextrose Fluid Restrict No Management Required ```

Answer 36

IV 0.9% Saline + 5% Dextrose + Serial Na+ Measurements (4-6h)