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Clinical Chemistry > Salt & Water > Flashcards

Salt & Water Flashcards

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1
Q

In which compartment is [Na+] highest?

A

Extracellular (140mmol/L)

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2
Q

What channel moves Na+ across membranes?

A

Na+/K+ ATPase

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3
Q

Which ions determine plasma osmolality?

A

2[Na+] + [K+] + glucose + urea

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4
Q

Which system regulates [Na+]?

A

RAAS

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5
Q

How does ADH regulate plasma water volume?

A

Dehydration = high plasma osmolality
- sensed by osmoreceptors in hypothalamus

SIMULATES: thirst, fluid intake, ADH (renal water reabsorption)

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6
Q

How does RAAS regulate plasma water volume?

A
  1. Arterial baroreceptors sense low BP
  2. SNS: vasoconstriction
  3. # Juxtaglomerular apparatus sense reduced renal arterial perfusion
  4. Increased RENIN -> ATII
  5. # Increased ALDOSTERONE
  6. Na+ retention (distal nephron)
  7. Water retention
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7
Q

What can cause hypernatraemia?

A

Hypoosmolar
- Dehydration (activates RAAS but not enough water)
- Conn’s syndrome
- Cushing’s disease

Hyperosmolar
- DI + dehydration
- diarrhoea, vomiting
- burns, haemorrhage

Artefactual:
- contamination of sample

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8
Q

What are the two types of DI?

A

Central DI - pituitary cannot produce ADH
Nephrogenic DI - DT kidney cannot respond to ADH

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9
Q

How does DI present and how is it managed?

A
  • polyuria, polydipsia
    (Na+ normal if well-hydrated but fluid restriction = HYPERNATRAEMIA)
  • Water deprivation test
  • serial Na+ and osmolality
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10
Q

What are the causes of hyponatraemia?

A

WATER RETENTION
Hypervolaemic (relative hyponatraemia, absolute eu/hyper)
- oedematous state (nephrotic syndrome, HF, RF, LF)
- SIADH
- psychogenic polydipsia

SALT LOSS
Hyper/euvolaemic (relative & absolute hyponatraemia)
- renal: osmotic diuresis (DKA, HHS), diuretics, ESRF (diuretic stage), Addison’s
- extrarenal: diarrhoea & vomiting, burns, fistulae (esp if hypotonic fluids)

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11
Q

What is the commonest cause of hyponatraemia in the community?

A

Thiazide diuretics

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12
Q

A patient presents with hyponatraemia, postural hypotension and no polyuria. What diagnosis must be considered?

A

Addison’s disease

(Low plasma osmolality, high urine osmolality & Na)

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13
Q

What is SIADH?

A

ADH secretion (encourages water retention) inappropriate for plasma osmolality/volume status

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14
Q

What causes SIADH? How is it diagnosed?

A

CNS - infection/malignancy
Pulm - infection/malignancy (ectopic)
Porphyria (rare, hereditary)
Drugs

(Diagnosis of exclusion, must be euvolaemic)

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15
Q

How to distinguish between causes of renal Na+ loss using biochemistry?

A

(Very high urine Na+ = Na+ loss)

  • good clinical hx, DHx
  • plasma glucose
  • plasma cortisol
  • Synacthen test
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16
Q

Why is hyponatraemia causing reduced GCS worrying?

A

Acutely low Na+ (<120) = low brain volume

CNS dysfunction: malaise, confusion, seizures, coma

17
Q

What is the management of SIADH?

A

Fluid restriction (& TULC)

18
Q

How is [Na+] <120 mmol/L managed?

A

IV saline SLOWLY (not more than 12mmol/L/24h)

(Consider: loop diuretics, vasopressin antagonists (Tolvaptan, demeclocycline))

19
Q

Why can rapid correction of hyponatraemia be fatal?

A

Central pontine myelinolysis (CPM)

20
Q

Which IV fluids are isotonic?

A

0.9% saline (154)
Hartmann’s (131 with K+, HCO3-)

21
Q

Which IV fluids are hypotonic?

A

0.45% saline (77)
Dextrose saline (30,40)
5% dextrose (0, 50)

  • dextrose metabolised, equivalent to giving water
22
Q

What is the standard daily requirement of IV fluids? (Per kg)

A

Water - 25-30ml
Ions - 1mmol
Glucose - 50-100kg (total)

23
Q

What are maintenance fluids?

A

All hypotonic fluids (0.45% saline or dextrose solutions)
K+ - 20mmol in 500ml diluent x1-2

24
Q

When is it appropriate to use 0.9% saline or Hartmann’s?

A

Fluid loss
- resus
- replace
- routine maintenance?
- must reassess for inappropriate redistribution

25
Q

What is the ‘tea and toast diet’?

A

Malnutrition seen in elderly (inability/no desire to prepare proper meals)

= Euvolaemic, true hyponatraemia with low urine osmolality (not psychogenic polydipsia)
- also low urine sodium

Clinical Chemistry (7 decks)

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