Chem path 3: Sodium and fluid balance

Question 1

What is the commonest abnormality in hospitalised patients?

Answer 1

Hyponatraemia

Question 2

What is the underlying pathogenesis in hyponatraemia?

Answer 2

Excess extracellular water

Question 3

What hormone is responsible for water balance and how does it act on the kidney?

Answer 3

ADH, acts on V2 receptors in collecting duct cells by inserting AQ2 channels

Question 4

Where are V1 receptors found and how does ADH act on them?

Answer 4

Vascular smooth muscle, ADH –> vasoconstriction at higher concentrations

Question 5

What are the two main stimuli for ADH secretion?

Answer 5

Increased osmolality and reduced blood volume/pressire

Question 6

What are serum osmolality and blood pressure/volume mediated by, respsectively?

Answer 6

Osmoreceptors and baroreceptors

Question 7

Where are osmoreceptors and baroreceptors found, respectively?

Answer 7

Osmoreceptors - hypothalamus

Baroreceptors - Carotids, aorta, atria

Question 8

What is the first step in the clinical assessment of someone with hyponatraemia?

Answer 8

Assess their volume status

Question 9

What is the most reliable clinical sign of hypovolaemia?

Answer 9

Low urine Na+ (<20)

Question 10

In which cohort of patients is urine Na+ not a reliable clinical marker of hypovolaemia?

Answer 10

patients on diuretics as they will have a high urine sodium regardless

Question 11

What are the clinical features of hypervolaemia?

Answer 11

Raised JVP
Bibasal crackles
Peripheral oedema

Question 12

Causes of hypovolaemic hyponatraemia

Answer 12

Diarrhoea/vomiting
Diuretics
Salt losing nephropathy

Question 13

Euvolaemic hypoantraemia

Answer 13

Hypothyroidism
SIADH
Adrenal insufficiency

Question 14

Hypervolaemic hyponatraemia

Answer 14

Cirrhosis
Nephrotic syndrome/renal failure
Heart failure

Question 15

What does a urine sodium <20 indicate and what does one >20 indicate?

Answer 15

if <20, suggests the hyponatraemia is not due to renal causes >20 suggests due to renal causes

Question 16

How does cardiac failure cause hypervolaemic hyponatraemia?

Answer 16

Low pressure is detected by the baroreceptors –> ADH release

Question 17

How does cirrhosis cause hypervolaemic hyponatraemia?

Answer 17

Release of vasodilators e.g. NO which results in low pressure which is detected by baroreceptors –> ADH release

Question 18

How does renal failure cause hypervolaemic hyponatraemia?

Answer 18

Not excreting enough water. CKD results in urinary protein loss and hence oedema. This lower circuilating volume activates RAAS which results in sodium retention. This in turn causes ADH release –> water retention and hypervolaemic hyponatraemia

Question 19

How does hypothyroidism cause euvolaemic hyponatraemia?

Answer 19

Reduced cardiac contractility –> reduced BP –> ADH release

Question 20

How does SIADH cause hyponatraemia?

Answer 20

Excess AQ2 insertion in collecting duct –> water retention and increased blood volume –> suppresses RAAS –> less aldosterone –> less Na+ reabsorption

Question 21

Causes of SIADH

Answer 21

Lung cancer, pneumonia (legionella), CNS pathology, stroke, drugs (SSRIs, TCA, PPI, carbamazepine), surgery

Question 22

Why does SIADH cause a euvolaemic hyponatraemia and not hypervolaemic?

Answer 22

Excess ADH = excess water = volume expansion → secretion for BNP → naturesis –> euvolaemic

Question 23

What are two causes of pseudohyponatraemia?

Answer 23

Hyperlipidaemia and hyperproteinaemia

Question 24

What happens to the urinary sodium in patients with heart failure?

Answer 24

Urinary sodium low as there will be secondary hyperaldosteronism causing sodium retention

Question 25

How do you diagnose SIADH?

Answer 25

No hypovolaemia, no hypothyroidism, no adrenal insufficiency, reduced plasma osmolality and increased urine osmolality (>100!!!!1)

Question 26

Management of pt with hypovolaemic hyponatraemia

Answer 26

VOlume replacement with 0.9% saline

Question 27

Management of pt with euvolaemic hyponatraemia

Answer 27

Fluid restrict + treat underlying cause

Question 28

Management of pt with hypervolaemic hyponatraemia

Answer 28

Fluid restrict + treat underlying cause

Question 29

What are two main signs of severe hyponatraemia and how is this treated?

Answer 29

Seizures and reduced GCS. Mx by seeking expert advice + 3% hypertonic saline

Question 30

At what rate must serum sodium be correcteD?

Answer 30

8-10mmol/L in the first 24 hrs

Question 31

If sodium serum is corrected too quickly what can this lead to?

Answer 31

Central pontine myelinolysis

Question 32

What drugs are used to treat SIADH if fluid restriction is inefficient?

Answer 32

Demeclocycline or tolvaptan

Question 33

What is demeclocyline’s MOA?

Answer 33

Reduces responsiveness of collecting tubule cells to ADH, induces a nephrogenic diabetes insipidus

Question 34

What do you need to monitor in pts taking demeclocycline?

Answer 34

U&Es as can cause nephrotoxicity

Question 35

Tolvaptan MOA

Answer 35

V2 receptor antagonist

Question 36

What is hypernatraemia caused by?

Answer 36

Unreplaced water loss

Question 37

What are the causes of hypernatraemia?

Answer 37

Loss of water: renal losses e.g. osmotic diuresis, diabetes insipidus. non-renal losses e.g. GI, sweat loss. Increase in sodium: Medical, dietary, Conn’s, BAH, RAS, Cushing’s

Question 38

Causes of nephrogenic DI

Answer 38

Hypercalcaemia, hypokalaemia, lithium, SCA

Question 39

What investigations are done for nephrogenic DI?

Answer 39

Serum glucose –> diabetes melltius –> osmotic diuresis
Serum potassium –> hypokalaemia –’> nephrogenic DI
Serum calcium
Plasma and urine osmolality (Exclude hyperaldosteronism)
Water deprivation test

Question 40

How do you treat hypernatraemia?

Answer 40

GIVE 5% DEXTROSE AS WILL REPLACE FLUID WITHOUT GIVING ADDED SALT

Question 41

E.G. 70-year old man, 3-day history of diarrhoea, altered mental status, dry mucous membranes. Serum Na is 168mmol/L

Answer 41

• Correct water deficit → 5% dextrose
• Correct ECF volume depletion → 0.9% saline
• Serial Na+ measurements → every 4-6 hours

Question 42

What is the relationship between diabetes mellitus and sodium?

Answer 42

It is variable as hyperglycaemia will draw water out of cells leading to hyponatraemia however osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia