Sodium and fluid balance

Question 1

Cause of hyponatraemia

Answer 1

increased extracellular water

Question 2

Hormone that controls water balance

Answer 2

ADH (vasopressin)

Question 3

How does ADH control water balance

Answer 3

synthesised in the hypothalamus, ADH is released from the posterior pituitary gland. It acts on V2 receptors on collecting ducts causing insertion of aquaporin-2 water channels. This causes increased water reabsorption.

Question 4

ADH acts on which receptors?

Answer 4

V1 receptors: On vascular smooth muscle, causes vasoconstriction at higher concentrations; V2 receptors: On kidneys, insertion of aquaporin-2 channels on collecting ducts.

Question 5

Stimuli for ADH secretion

Answer 5

Serum osmolality is high - mediated by hypothalamic osmoreceptors; Blood volume/pressure (hypotension) - mediated by baroreceptors in carotids, atria and aorta.

Question 6

Increased ADH effect on serum sodium

Answer 6

Hyponatraemia More water = Less sodium)

Question 7

First step in assessing hyponatraemia

Answer 7

Clinical assessment of volume status: Look at hands, head and neck, peripheries.

Question 8

Signs of hypovolaemia

Answer 8

Dry mucous membranes, reduced JVP, reduced tissue turgor, tachycardia, postural hypotension, confusion/drowsiness, reduced urine output, low urine Na+ (<20).

Question 9

Signs of hypervolaemia

Answer 9

Raised JVP, peripheral oedema, bibasal crackles (on chest examination).

Question 10

What makes urine sodium uniterpretable?

Answer 10

Diuretics - these alter the kidney’s ability to retain salt. Must stop it and check 48 hours after.

Question 11

Causes of hyponatraemia in hypovolaemic patient

Answer 11

Renal: diuretics; Extra-renal: diarrhoea, vomiting.

Question 12

Causes of hyponatraemia in hypervolaemic patient

Answer 12

Cardiac failure, cirrhosis, renal failure.

Question 13

Causes of hyponatraemia in euvolaemic patient

Answer 13

Hypothyroidism - due to reduction in CO detected by baroreceptors leading to ADH secretion; Adrenal insufficiency - cortisol needed for water excretion, aldosterone needed for sodium and water retention; SIADH.

Question 14

Causes of SIADH

Answer 14

CNS pathology, lung pathology, drugs (SSRI, TCA, opiates, PPIs, carbamazepine), tumours, surgery.

Question 15

Investigations for euvolaemic hyponatraemia

Answer 15

Hypothyroidism: Thyroid function tests; Adrenal insufficiency: Short synacthen test; SIADH: Plasma and urine osmolality (low plasma & high urine osmolality).

Question 16

Osmolality in SIADH

Answer 16

Plasma osmolality - LOW; Urine osmolality - HIGH (>100).

Question 17

Diagnosis of SIADH requirements

Answer 17

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

Question 18

Management of hypovolaemic patient with hyponatraemia

Answer 18

Volume replacement with 0.9% saline

Question 19

Management of hypervolaemic patient with hyponatraemia

Answer 19

Fluid restriction and treat the underlying cause.

Question 20

Management of euvolaemic patient with hyponatraemia

Answer 20

Fluid restriction and treat the underlying cause (same as hypervolaemic patient with hyponatraemia).

Question 21

Clinical symptoms of severe hyponatraemia

Answer 21

Reduced GCS, Seizures

Question 22

Management of severe hyponatraemia

Answer 22

Can give boluses of hypertonic 3% saline but only if patient has low GCS or fitting.

Question 23

Important point while correcting hyponatraemia

Answer 23

Serum Na must NOT be corrected >8-10 mmol/L in the first 24 hours. Risk of osmotic demyelination (central pontine myelinolysis). Presents a few days later with quadriplegia, dysarthria, dysphagia, seizures, coma, death.

Question 24

Drugs used to treat SIADH

Answer 24

Demeclocycline, Tolvaptan.

Question 25

How Demeclocycline works

Answer 25

Reduces responsiveness of collecting tubule cells to ADH. Need to monitor U&Es (risk of nephrotoxicity).

Question 26

How Tolvaptan works

Answer 26

V2 receptor antagonist. Very expensive.

Question 27

Treatment for SIADH

Answer 27

Water restriction PLUS (but both used rarely) Demeclocycline (reduces responsiveness of collecting tubule cells to ADH - but caution because nephrotoxic) OR Tolvaptan (V2 receptor antagonist) - use cautiously as they work rapidly.

Question 28

Main causes of hypernatraemia

Answer 28

Unreplaced water loss, Gastrointestinal losses, sweat losses, Renal losses: osmotic diuresis, reduced ADH release/action (Vasopressin deficiency/resistance), Patient cannot control water intake e.g. children, elderly.

Question 29

Investigations for suspected VP deficiency

Answer 29

Serum glucose (exclude diabetes mellitus), Serum potassium (exclude hypokalaemia), Serum calcium (exclude hypercalcaemia), Plasma and urine osmolality, Water deprivation test - urine osmolality will fail to increase.

Question 30

Presentation of VP deficiency

Answer 30

Polyuria and polydipsia.

Question 31

Treatment for hypernatraemia

Answer 31

Fluid replacement and treat the underlying cause.

Question 32

Management of hypernatraemia

Answer 32

Correct water deficit, 5% DEXTROSE (free water), May need to correct extracellular fluid volume depletion (especially if they've been vomiting), 0.9% saline, Serial Na+ measurement every 4-6 hours.

Question 33

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. This varies from person to person - based on which factor is pre-dominating.

Question 34

Definition of hyponatraemia

Answer 34

Sodium concentration <135 mmol/L.

Question 35

severe hyponatraemia sodium concentration

Answer 35

<120 mmol/L

Question 36

Investigations you need to do before you can make a diagnosis of SIADH

Answer 36

CXR, CT head, abdo, pelvis

Question 37

hypernatraemia sodium concentration.

Answer 37

>145 mmol/L

Question 38

Why does cirrhosis cause hyponatraemia?

Answer 38

Excess ADH secreted due to NO, this causes vasodilation - lower blood pressure which is detected by baroreceptors