Y Lecture 4: Sodium and Fluid Balance

Question 1

What is the definition of hyponatraemia?

Answer 1

Serum sodium <135

Question 2

Upon which receptors does ADH act?

Answer 2

V1 (collecting duct) and V2 (on VSMCs)

Question 3

What are the 2 stimuli for ADH secretion?

Answer 3

Serum osmolality (detected by hypothalamic osmoreceptors)

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

Question 4

What are the clinical signs of hypovolaemia?

Answer 4

Tachycardia
Postural hypotension
Dry mucous membranes
Reduce skin turgor
Confusion/drowsiness
Reduced urine output
KEY: LOW URINE Na+ (<120)

Question 5

Recall 4 causes of hypovolaemic hyponatraemia

Answer 5

Diarrhoea
Vomiting
Diuretics
Salt losing nephropathy

Question 6

Recall 3 causes of euvolaemic hyponatraemia

Answer 6

Hypothyroidism
Adrenal insufficiency
SIADH

(Euvolaemic = Endocrine - 2 ‘E’s)

Question 7

Recall 3 causes of hypervolaemic hyponatraemia

Answer 7

Cardiac failure
Cirrhosis
Nephrotic syndrome

Question 8

In which patients can you not use urine sodium as a reliable test result?

Answer 8

Patients on diuretics

Question 9

How does hypothyroidism cause euvolaemic hyponatraemia?

Answer 9

Hypothyroidism –> Reduced cardiac contractility –> detected by baroreceptors –> more ADH –> increased water resorption –> low plasma Osm secondary to dilution –> less water excreted in urine –> high urinary Osm

Question 10

How does adrenal insufficiency cause euvolaemic hyponatraemia?

Answer 10

Adrenal insufficiency –> low aldosterone and cortisol
Aldosterone is necessary for sodium and water resorption, cortisol is necessary for water clearance, therefore you get excess ADH

Question 11

What are the 5 main causes of SIADH?

Answer 11

CNS pathology
Lung pathology
Drugs (SSRI, PPI, opiates)
Tumours
Surgery

Question 12

What 3 tests should be done in euvolaemic hyponatraemia?

Answer 12

TFTs for hypothyroidism Short SynACTHen test for adrenal insufficiency Plasma and urine osmolality for SIADH

Question 13

Why is urine sodium low in cardiac failure (hypervolaemic hyponatraemia)

Answer 13

Hyperaldosteronism –> retention of sodium

Question 14

What will be the urine and plasma and urine osmolality in SIADH

Answer 14

Plasma = low (because it’s hyponatraemia!) Urine = high (>100)

Question 15

What will urine sodium be in cardiac failure?

Answer 15

low

Question 16

Why do you get hyperaldosteronism in cardiac failure?

Answer 16

Activation of RAAS

Question 17

How do you manage a patient with hypovolaemic hyponatraemia?

Answer 17

Fluid replacement with 0.9% saline

Question 18

How do you manage a patient with hypervolaemic hyponatraemia?

Answer 18

Fluid restriction

Treat the underlying cause

Question 19

How do you manage a patient with euvolaemic hyponatraemia?

Answer 19

Fluid restriction
Treat the underlying cause

If you give fluids, this will exacerbate the hyponatraemia

Question 20

What are the symptoms of SEVERE hyponatraemia?

Answer 20

Reduced GCS

Seizures

Question 21

What is the max rate of serum Na+ correction in hyponatraemia and why?

Answer 21

No more that 8-10mmol/L in 1st 24 hoursRisk of osmotic demyelination (central pontine myelinolysis)

Question 22

How is SIADH treated?

Answer 22

Water restriction PLUS (but both used rarely)

1. Demeclocycline (reduces responsiveness of collecting tubule cells to ADH - but caution because nephrotoxic) OR
2. Tolvaptan (V2 receptor agonist)

Question 23

Why does SIADH cause euvolaemia?

Answer 23

SIADH –> hypervolaemia (due to water retention) –> natiuretic peptide released from heart –> increased sodium excretion to try and pull water into urine –> euvolaemia (as you have now lost the water, but have also lost sodium in order to do so)

Question 24

What are the main causes of hypernatraemia?

Answer 24

Unreplaced water loss

Due to GI losses or renal losses (eg diabetes insipidus)

Question 25

What investigations would you order in a pt with suspected Diabetes insipidus?

Answer 25

Serum glucose (exclude DM) Serum K+ (exclude hypokalaemia) Serum Ca (exclude hypercalcaemia) Plasma and urine osmolality Water deprivation test

Question 26

How would you manage hypernatraemia?

Answer 26

Fluid replacement with 5% DEXTROSE (NOT saline) | Treat underlying cause

Question 27

How does diabetes mellitus affect serum Na?

Answer 27

``` Hyperglycaemia --> water drawn out of cells --> hyponatraemia Osmotic diuresis (polyuria) --> loss of water --> hypernatraemia ```

Question 28

What is the first investigation to do in suspected hyponatraemia/hypernatraemia?

Answer 28

Clinically asses volume status

Question 29

How should severe hyponatraemia (<125mmol/L AND symptomatic) be treated?

Answer 29

Seek expert help and use 2.7% hypertonic saline

Question 30

Does ADH cause water loss or retention?

Answer 30

Retention (ADH ADds H2o)

Question 31

Is hyponatraemia primarily due to excess water or insufficient salt?

Answer 31

Excess water

Question 32

What is the expected urine sodium in a hypovolaemic hyponatraemic patient?

Answer 32

<20mmol/L

Question 33

What is the expected urine sodium in euvolaemic hyponatraemia?

Answer 33

High

Question 34

What are the clinical signs of hypervolaemia?

Answer 34

Peripheral oedema Bibasal crackles Raised JVP

Question 35

What is the expected urine sodium in hypervolaemic hyponatraemia?

Answer 35

Low

Question 36

Does hypervolaeic hyponatraemia cause nephrotic or nephritic syndrome?

Answer 36

Nephrotic

Question 37

What is the definition of hypernatraemia?

Answer 37

Na > 145 mmol/L

Question 38

Is hypernatraemia primarily a problem of excess salt or insuffucuent water?

Answer 38

Insufficient water

Question 39

What are the units of osmolality vs osmolarity?

Answer 39

Osmolality = mmol/kg Osmolarity = mmol/L

Question 40

At what concentration of sodium should the sodium be treated (rather than just treating the cause)?

Answer 40

<125mmol/L and symptomatic

Question 41

How does TURP syndrome cause hyponatraemia?

Answer 41

Hyponatraemia from irrigation absorbed through damaged prostate

Question 42

How can hyponatraemia and pseudohyponatraemia be differentiated?

Answer 42

Pseudohyponatraemia has an increased protein/ lipid volume | Can be differentiated using serum osmolality - in true hyponatraemia, the serum osmolality is LOW

Question 43

How can renal vs non-renal causes of hyponatraemia be differentiated in both hypovolaemic and hypervolaemic patients?

Answer 43

If urine sodium >20 = renal | If urine sodium <20 = non-renal