Chempath 12: Sodium And Fluid Balance

Question 1

What is the most common pathogenesis of hyponatraemia ?

Answer 1

Increased extracellular water

Question 2

How does ADH increase water reabsorption ?

Answer 2

ADH binds to V2 receptors and causes increased Aquaporin 2 insertion into the collecting duct

Question 3

Which receptors are found in smooth muscle and cause vasoconstriction ?

A) V1 receptors
B) V2 receptors

Answer 3

A) V1 receptors

They bind Vasopressin and cause vasoconstriction

V2 are found in collecting ducts and respond to ADH

Question 4

What are the 2 main stimuli for ADH secretion ?

Answer 4

Osmoreceptors- detects high osmolality

Baroreceptors- detect low blood volume/pressure

Question 5

Where are Osmoreceptors found in the body ?

Answer 5

Hypothalamus

Question 6

List 3 locations where Baroreceptors are found in the body ?

Answer 6

Carotid
Aorta
atria

Question 7

What is the most reliable clinical sign of hypovolaemia ?

Answer 7

Low urine Na+

Doesn’t work if on diuretics

Question 8

List 4 clinical signs of hypovolaemia?

Answer 8

Tachycardia
Postural hypotension
Reduced skin turgor
Dry mucous membranes 
Sunken eyes

Question 9

List 4 causes of Hypovolaemic hyponatraemia ?

Answer 9

Diarrhoea
Vomiting
Diuretics
Salt losing nephropathy

Question 10

List 3 causes of euvolaemic hyponatraemia ?

Answer 10

Hypothyroidism
Adrenal insufficiency
SIADH

Question 11

List 3 causes of hypervolaemic Hyponatraemia ?

Answer 11

Cardiac failure
Nephrotic syndrome
Cirrhosis

Question 12

How does liver cirrhosis cause hypervolaemic hyponatraemia ?

Answer 12

Cirrhosis causes release of Nitric oxide which causes vasodilation
This causes reduced blood pressure > sensed by baroreceptors
ADH release is increased.
More water is reabsorbed

Question 13

How does hypothyroidism cause a Euvolaemic hyponatraemia ?

Answer 13

Hypothyroidism causes reduced cardiac contractility
This causes reduced blood pressure
This causes increased ADH release
More water is reabsorbed than Na+

Question 14

List 3 causes of SIADH ?

Answer 14

CNS tumour
Small cell lung cancer
Meningitis/ subarachnoid haemorrhage
Drugs (psych drugs: SSRIs, Carbamazapine, Amitryptiline)

Question 15

How do you treat hypovolaemic hyponatraemia ?

Answer 15

Volume replacement with 0.9% saline

If volume is returned to normal, the stimulus for ADH release is stopped so allows recovery.

Question 16

How do you treat hypervolaemic and euvolaemic hyponatraemia ?

Answer 16

Fluid restriction

Treat underlying cause

Question 17

Name the complication that can occur as a result of increasing serum sodium faster than 8-10 mol/L in the first 24 hours ?

Answer 17

Central pontine myelinolysis

Question 18

Name 2 drugs used to treat SIADH ?

Answer 18

Tolvaptan - ADH receptor antagonist

Demeclocycline - reduces responsiveness of collecting duct cells to ADH

Question 19

What level of serum sodium defines hypernatraemia ?

Answer 19

> 145 mmol/L

Question 20

What level of serum sodium defines hyponatraemia ?

Answer 20

<135 mmol/L

Question 21

Give 3 causes of Hypernatraemia ?

Answer 21

Diarrhoea and vomiting
Diabetes mellitus
Diabetes insipidus

3Ds of hypernatraemia

Question 22

Name 2 tests used to diagnose diabetes insipidus ?

Answer 22

Serum and urine osmolality

Water deprivation test

Question 23

Describe the possible outcomes of the water deprivation test ?

Answer 23

Normal: urine becomes very concentrated (sodium >800)
Cranial Diabetes insipidus: unable to concentrate urine (sodium <300)
Nephrogenic Diabetes insipidus : unable to concentrate urine (sodium <300)
Polydipsia: slightly able to concentrate urine, but physiological function has been damaged overtime (sodium <500)

Question 24

What is the main treatment for hypernatraemia ?

Answer 24

5% Dextrose

Question 25

How do you treat Hypovolaemic hypernatraemia ?

Answer 25

First correct the extravascular fluid volume with 0.9% saline
Then give 5% dextrose

Question 26

Explain the 2 methods by which diabetes mellitus affects Sodium ?

Answer 26

1- Hyperglycaemia causes water to leave cells and enter the extravascular compartment causing dilution of sodium (hyponatraemia) (check this one)

2- In diabetes you get an osmotic diuresis which causes increased water loss

Question 27

List 2 causes of pseudo hyponatraemia ?

Answer 27

Hyperlipidaemia
Hyperproteinaemia

Dilution by excess molecules of lipid or protein in the circulation. Sodium conc. is normal

Question 28

What is meant by true hyponatraemia ?

Answer 28

When the Na+ conc. is low + Serum osmolality is low

Question 29

Sodium low
Urine Osmolality > 20mmol/L
Urea elevated
Creatinine elevated

Most likely diagnosis ?
volume Type of hyponatraemia ?

Answer 29

CKD
Hypervolaemic hyponatraemia

CKD causes increased urine protein loss -> Oedema -> low circulating volume -> RAS activated -> Increased ADH release -> Increased water reabsorption

Question 30

A tanned looking lady

Na+ low
K+ high
BM (glucose) low

Most likely diagnosis ?

Answer 30

Addisons

Lack of Aldosterone -> Increased Na+ reabsorption + Reduced K+ Exretion

High ACTH -> hyperpigmentation

low cortisol -> less gluconeogenesis -> Hypoglycaemia

Question 31

Hyponatraemia with a higher urine osmolality compared to Serum osmolality ?

Most likely diagnosis?

Answer 31

SIADH

Question 32

How do you calculate Osmolarity ?

Answer 32

2(Na + K) + Urea + Glucose

Question 33

How do you calculate Anion gap ?

Answer 33

Na + K - CL - HCO3

Question 34

How is osmolality determined ?

Answer 34

Measured using an osmometer

Question 35

What is the normal range for Sodium ?

Answer 35

135-145mM/l

Question 36

Which atypical pneumonia can cause hyponatraemia ?

Answer 36

Legionella Pneumophilia

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