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Pathology > Chemical Pathology - Sodium and Fluid Balance > Flashcards

Chemical Pathology - Sodium and Fluid Balance Flashcards

1
Q

define hyponatraemia

A

serum sodium < 135 mmol/L

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2
Q

what is the underlying pathogenesis of hyponatraemia?

A

increased extracellular water

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3
Q

what is water balance controlled by?

A

ADH

promotes water retention by inserting aquaporin-2 channels into collecting duct cells

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4
Q

where does ADH act?

A
  • V2 receptors on collecting duct (insertion of aquaporin-2)

- V1 on VSMC (causes vasopressin, ADH known as vasopressin here)

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5
Q

what are the 2 main stimuli for ADH secretion?

A
  • inc in serum osmolality (mediated by hypothalamic osmoreceptors)
  • decrease blood volume/ pressure (mediated by baroreceptors in carotids, atria, aorta)
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6
Q

what is the first step in the management of a patient with hyponatraemia?

A
  • assess volume status
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7
Q

what are the clinical features of hypovolaemia?

A
  • tachycardia
  • postural hypotension
  • dry mucous membrane
  • reduced skun turgor
  • confusion/drowsiness
  • dec urine outpur
  • low urine Na (<20)
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8
Q

what is the most reliable clinical sign of hypovolaemia?

A
  • low urine Na

if pt on diuretics, they will have high urine Na

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9
Q

what are the clinical features of hypervolaemia?

A
  • raised JVP
  • bibasal crackles
  • peripheral oedema
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10
Q

causes of hypovolaemic hyponatraemia?

A
  • diarrhoea
  • vomiting
  • diuretics
  • salt losing nephropathy
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11
Q

causes of euvolaemic hyponatraemia?

A
  • hypothyroidism
  • adrenal insufficiency
  • SIADH
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12
Q

causes of hypervolaemic hyponatraemia?

A
  • cardiac failure
  • cirrhosis
  • nephrotic syndrome
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13
Q

explain how hypovolaemic hyponatraemia works

A
  • hypovolaemic pt still have excess water
  • if you have D&V, you will lose a lot of salt and water
  • get a low perfusion pressure and inc in ADH release
  • pt then reabsorb more water than salt
    = hyponatraemia
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14
Q

how does cirrhosis cause hyponatraemia?

A

leads to release of various vasodilators

leads to a drop in perfusion pressure

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15
Q

causes of SIADH

A
  • CNS pathology
  • Lung pathology
  • Drugs (SSRI, TCA, opiates, PPIs, carbamazepine)
  • Tumours
  • Surgery
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16
Q

how do you make a diagnosis of SIADH?

A
  • no hypovolaemia
  • no hypothyroidism
  • no adrenal insufficiency
  • reduced plasma osmolality
  • increased urine osmolality (>100)
17
Q

how should you investigate hypovolaemic hyponatraemia?

A
  • clinically hypovolaemic?
18
Q

how should you investigate euvolaemic hyponatraemia?

A
  • TFTs
  • short synacthen test
  • plasma and urine osmolality
19
Q

how should you investigate hypervolaemic hyponatraemia?

A
  • ? Fluid overload
20
Q

Tx of hypovolaemic hyponatraemia

A
  • volume replacement with 0.9% saline

will replenish circulating fluid volume to normal levels, switch off stimulus for ADH release

21
Q

Tx of hypervolaemic hyponatraemia

A
  • fluid restriction

- treat underlying cause

22
Q

Tx of euvolaemic hyponatraemia

A
  • fluid restriction

- treat underlying cause

23
Q

symptoms of severe hyponatraemia

A
  • reduced GCS
  • seizures
  • seek expert help (treat with 3% hypertonic saline)
24
Q

what is an important consideration when treating hyponatraemia?

A
  • serum sodium must NOT be corrected faster than 8-10mmol/L in first 24 hours
  • if corrected too rapidly = osmotic demyelination (central pontine myelinolysis)
25
Q

how does central pontine myelinolysis present?

A
  • quadraplegia
  • dysarthria
  • dysphagia
  • seizures
  • coma
  • death
26
Q

when are drugs used to treat SIADH?

A

if fluid restriction is insufficient

27
Q

which drugs can be used?

A
  • Demeoclocycline (reduces responsiveness of collecting tubule cells to ADH, monitor U&Es as risk of nephrotoxicity)
  • Tolvaptan (V2 receptor antagonist)
28
Q

define hypernatraemia

A

serum Na concentration >145 mmol/L

29
Q

what is hypernatraemia caused by?

A

unreplaced water loss

  • GI losses
  • sweat losses
  • renal losses (osmotic diuresis, diabetes insipidus)
30
Q

who mostly gets hypernatraemia?

A

patients who tend not to drink when they are dehydrated (e.g. elderly and children)

31
Q

what are the investigations for diabetes insipidus?

A
  • serum glucose (exclude DM)
  • serum potassium (exclude hypokalaemia as can induce nephrogenic DI)
  • serum Ca (exclude hypercalcaemia)
  • plasma and urine osmolality
  • water deprivation test
32
Q

how do you treat hypernatraemia?

A
  • fluid replacement (give dextrose because this will replace fluid without giving excess salt)
  • treat underlying cause
33
Q

how would you treat someone who was hypovolaemic and hypernatraemic?

A
  • initially give 0.9% saline to treat hypovolaemia

- followed by dextrose for hypernatraemia

34
Q

how does diabetes mellitus affect serum sodium>

A
  • hyperglycaemia will draw water out of cells = hyponatraemia
  • osmotic diuresis in uncontrolled diabetes leads to loss of water = hypernatraemia

Pathology (21 decks)

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