Sodium and Fluid Balance

Question 1

What is the commonest electrolyte abnormality in hospitalised patients

Answer 1

Hyponatraemia

Question 2

What is hyponatraemia

Answer 2

Serum sodium < 135mmol/L

Question 3

What is the underlying pathogenesis of hyponatraemia

Answer 3

Increased extracellular water

Question 4

What hormone controls water levels in the body

Answer 4

ADH (vasopressin)

Retains water through the action on water channels (aquaporin 2)

Question 5

What is the MOA of ADH

Answer 5

Acts on V2 receptors in the collecting duct
Acts via inserting aquaporin 2 channels

Acts on V1 recptors on vascular smooth muscle as a vasoconstrictor at higher concentrations

Question 6

Where are V1 receptors found

Answer 6

Vascular smooth muscle

Question 7

What are the two main stimuli for ADH secretion

Answer 7

Serum osmolality (mediated by hypothalamic osmoreceptors)

Blood volume/pressure (mediated by baroreceptors in carotids, atria, aorta)

Question 8

What is the effect of increased ADH secretion on serum sodium

Answer 8

Hyponataraemia

Increased water reabsorption leads to dilution of serum sodium

Question 9

What is the first step in the clinical assessment of a patient with hyponatraemia

Answer 9

Clinical assessment of volume status

Question 10

What are the three outcomes of a volume assessment

Answer 10

Hypovolaemic
Euvolaemic
Hypervolaemic

Question 11

What are the clinical signs of hyponatraemic hypovolvaemia

Answer 11

Tachycardia 
Postural hypotension 
Dry mucous membranes 
Reduced skin turgor 
Confusion/drowsiness 
Reduced urine output 
Low urine Na (<20)

Question 12

What are the clinical signs of hyponatraemic hypervolaemia

Answer 12

Raised JVP
Bibasal crackles on chest auscultation
Peripheral oedema

Question 13

Causes of hyponatraemic hypovolaemia

Answer 13

Extra-renal: Diarrhoea, Vomiting

Renal: Diuretics, Salt losing nephropathy

Question 14

Causes of hyponatraemic euvolmaenia

Answer 14

Hypothyroidism
Adrenal insufficiency
SIADH

Question 15

Causes of hyponatraemic hypervolaemia

Answer 15

Cardiac failure
Nephrotic syndrome, renal failure
Cirrhosis

Question 16

What are the causes of SIADH

Answer 16

CNS pathology 
Lung pathology 
Drugs (SSRI, TCA, opiates, PPIs, carbamazepine) 
Tumours 
Surgery

Question 17

Tests in a hyponatraemia hypovolaemic patient

Answer 17

Clinical assessment

Question 18

Tests in a hyponataemia euvolaemic patient

Answer 18

TFTs
Short synacthen test
Plasma and urine osmolality (low plasma and high urine osmolality)

Question 19

Tests in a hyponatraemic hypervolaemic patient

Answer 19

Fluid overload?

Question 20

Diagnosis of SIADH

Answer 20

No hypovolaemia
No hypothyroidism
No adrenal insufficiency
Reduced plasma osmolality AND increased urine osmolality (>100)

Question 21

Management of a hypovolaemic patient with hyponatraemia

Answer 21

Volume replacement with 0.9% saline

Question 22

Management of a hypervolaemic patient with hyponatraemia

Answer 22

Fluid restriction

Treat the underlying cause

Question 23

Management of a euvolaemic patient with hyponatraemia

Answer 23

Fluid restriction

Treat the underlying cause

Question 24

Signs of severe hyponatraemia

Answer 24

Reduced GCS
Seizures
Seek expert help! (treat with hypertonic 3% saline)

Question 25

What is the most important point to remember while correcting hyponatraemia

Answer 25

Serum sodium NOT be corrected >8-10mmol/L in the first 24 hours as there is risk of osmotic demyelination (central pontine myelinolysis)

Question 26

Signs of osmotic demyelination due to rapid correction of hyponatraemia

Answer 26

``` QUadriplegia Dysarthria Dysphagia Seizures Coma Death ```

Question 27

Drugs used to treat SIADH

Answer 27

If water restriction is insufficient Demeclocycline - reduces responsiveness of collecting tubule cells to ADH, monitor U&Es (risk of nephrotoxicity) Tolvaptan - V2 receptor antagonist

Question 28

Hypernatraemia

Answer 28

Serum sodium >145mmol/L

Question 29

Main causes of hypernatraemia

Answer 29

Unreplaced water loss: GI losses, sweat losses, renal losses (osmotic diuresis, reduced ADH release/action (Diabetes insipidus) Patient cannot control water intake (e.g. very young, very old)

Question 30

Investigations in a patient with suspected diabetes insipidus

Answer 30

``` Serum glucose (exclude diabetes mellitus) Serum potassium (exclude hypokalaemia) Serum calcium (exclude hypercalcaemia) Plasma and urine osmolality Water deprivation test ```

Question 31

Treatment of hypernatraemia

Answer 31

Fluid replacement | Treat the underlying cause

Question 32

A 70 yr-old man 3-day history of diarrhoea Altered mental status Dry mucous membranes Serum Na+ is 168 mmol/L Management?

Answer 32

Correct water deficit - 5% dextrose Correct extracellular fluid volume depletion - 0.9% saline Serial sodium measurements - every 4-6 hours

Question 33

What are the effects of diabetes mellitus on serum sodium

Answer 33

Variable Hyperglycaemia draws water out of the cells leading to hyponatraemia Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia