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YEAR 5: Pathology 2 FW > Chemical Pathology 11 - Sodium and fluid balance > Flashcards

Chemical Pathology 11 - Sodium and fluid balance Flashcards

1
Q

What is the definition of hyponatraemia (commonest electrolyte abnormality)?

What is the underlying pathogenesis of hyponatraemia?

A

Serum sodium <135

Increased EXTRACELLULAR water

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2
Q

Upon which receptors does ADH act?

A

V2 (collecting duct - leads to insertion of aquaporins-2) and V1 (on vascular SMCs - leads to vasoconstriction at higher concentrations)

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3
Q

What are the 2 stimuli for ADH secretion?

A

Increased serum osmolality (detected by hypothalamic osmoreceptors)

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

Vomiting - loss of water AND salt - causes hyponatraemia because reduced blood volume -> ADH secretion -> ONLY water reabsorption into the blood

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4
Q

What are the clinical signs of hypovolaemia?

A 
Hypovolaemia:
Tachycardia
Postual hypotension
Dry mucous membranes
Reduce skin turgor
Confusion/drowsiness
Reduced urine output

MOST RELIABLE INDICATOR: LOW URINE Na+ (<20) - if you are hypovolaemic, you need to hold onto sodium so urine sodium will be low
Always remember to send off this test

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5
Q

Recall 4 causes of hypovolaemic hyponatraemia

A

Diarrhoea
Vomiting
Diuretics
Salt losing nephropathy

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6
Q

Recall 3 causes of euvolaemic hyponatraemia

A

Hypothyroidism
Adrenal insufficiency
SIADH
(Euvolaemic = Endocrine - 2 ‘E’s)

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7
Q

Recall 3 causes of hypervolaemic hyponatraemia

A

Cardiac failure
Cirrhosis
Nephrotic syndrome

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8
Q

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

A

Patients on diuretics - because they will be peeing out salts due to the medication

Urine sodium can also be low in very hypervolaemic patients -> high urine sodium points more towards euvolaemic hyponatraemia

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9
Q

How does hypothyroidism cause euvolaemic hyponatraemia?

A

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

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10
Q

How does adrenal insufficiency cause euvolaemic hyponatraemia?

A

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

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11
Q

What are the 5 main causes of SIADH?

A 
CNS pathology
Lung pathology
Drugs (SSRI, PPI, opiates)
Tumours
Surgery
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12
Q

What 3 tests should be done in euvolaemic hyponatraemia?

A

TFTs for hypothyroidism

Short SynACTHen test for adrenal insufficiency

Plasma and urine osmolality for SIADH

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13
Q

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

A

Hyperaldosteronism –> retention of sodium

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14
Q

What will be the plasma and urine osmolality in SIADH

A

Plasma = low (because it’s hyponatraemia!)

Urine = high (>100)

Diagnosis of SIADH can only be made if there is no hypovolaemia, no hypothyroidism and no adrenal insufficiency

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15
Q

What will urine sodium be in cardiac failure?

A

low

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16
Q

Why do you get hyperaldosteronism in cardiac failure?

A

Activation of RAAS

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17
Q

How do you manage a patient with hypovolaemic hyponatraemia?

A

Fluid replacement with 0.9% saline - you’re giving fluid to shut down the issue that’s causing ADH secretion

Be careful - shouldn’t be given if they are NOT hypovolaemic as they will hang onto the water and exacerbate the issue (i.e. an ADH-secreting tumour will still continue to secrete ADH)

18
Q

How do you manage a patient with hypervolaemic hyponatraemia?

A

Fluid restriction

Treat the underlying cause

19
Q

How do you manage a patient with euvolaemic hyponatraemia?

A

Fluid restriction
Treat the underlying cause
If you give fluids, this will exacerbate the hyponatraemia

20
Q

What are the symptoms of SEVERE hyponatraemia?

A

Reduced GCS

Seizures

21
Q

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

A

No more that 8-10mmol/L in 1st 24 hours

Risk of osmotic demyelination (central pontine myelinolysis - happens a few days later with quadraplegia, dysarthria, coma, death)

If sodium is corrected too quickly, reverse it with dextrose and desmopressin

22
Q

How is SIADH treated?

A

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) - use cautiously as they work rapidly
23
Q

Why does SIADH cause euvolaemia?

A

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)

24
Q

What are the main causes of hypernatraemia?

A

Unreplaced water loss
Due to GI losses or renal losses (eg diabetes insipidus)

Diarrhoea and vomiting usually causes hyponatraemia but can sometimes cause hypernatraemia

25
Q

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

A 
Serum glucose (exclude DM)
Serum K+ (exclude hypokalaemia - cause of nephrogenic DI)
Serum Ca (exclude hypercalcaemia)
Plasma and urine osmolality
Water deprivation test
26
Q

How would you manage hypernatraemia?

A

Fluid replacement with 5% DEXTROSE - free water
(NOT saline)
Treat underlying cause

Sometimes, saline might be needed ALONGSIDE dextrose if the patient has been vomiting salts

27
Q

How does diabetes mellitus affect serum Na?

A

Hyperglycaemia –> water drawn out of cells –> hyponatraemia

Osmotic diuresis (polyuria) –> loss of water –> hypernatraemia

28
Q

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

A

Clinically assess volume status

Clinical assessment before measuring plasma and urine osmolality and sodium

29
Q

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

A

Seek expert help and use 2.7% hypertonic saline

30
Q

Does ADH cause water loss or retention?

A

Retention (ADH ADds H2o)

31
Q

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

A

<20mmol/L

32
Q

What is the expected urine sodium in euvolaemic hyponatraemia?

A

High

33
Q

What are the clinical signs of hypervolaemia?

A

Peripheral oedema
Bibasal crackles
Raised JVP

34
Q

What is the expected urine sodium in hypervolaemic hyponatraemia?

A

Low

35
Q

What is the definition of hypernatraemia?

A

Na > 145 mmol/L

36
Q

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

A

Insufficient water

37
Q

What are the units of osmolality vs osmolarity?

A

Osmolality = mmol/kg

Osmolarity = mmol/L

38
Q

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

A

<125mmol/L and symptomatic

39
Q

How does TURP syndrome cause hyponatraemia?

A

Hyponatraemia from irrigation absorbed through damaged prostate

40
Q

How can hyponatraemia and pseudohyponatraemia be differentiated?

A

Pseudohyponatraemia has an increased protein/ lipid volume

Can be differentiated using serum osmolality - in true hyponatraemia, the serum osmolality is LOW

41
Q

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

A

If urine sodium >20 = renal

If urine sodium <20 = non-renal

YEAR 5: Pathology 2 FW (87 decks)

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