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

hotel air conditioners