ChemPath - General

Question 1

What is the 60-40-20 rule

Answer 1

60% total volume is water
40% volume is intracellular (28L)
20% volume is extracellular

Question 2

What is osmolality vs osmolarity

Answer 2

Osmolality = Measured with osmometer - total no. of particles in a solution (mmol/kg)
Osmolarity = calculated osmolarity using electrolyte levels (mmol/L)

Question 3

How is osmolarity calculated

Answer 3

2(Na + K) + glucose + urea

Question 4

What is the osmolar gap

Answer 4

Osmolality - osmolarity
If raised → suggests extra ions are present

Question 5

Normal sodium range

Answer 5

135-145

Question 6

Hypovolaemia hyponatraemia causes

Answer 6

Urine sodium <20: diarrhoea, vomiting, sweating, third space loss (burns, ascites)

Urine sodium >20: renal failure, salt-losing nephropathy, diuretics, Addison’s

Question 7

Hypovolaemia hyponatraemia management

Answer 7

0.9% saline fluid replacement

Question 8

Normovolaemic hyponatraemia causes

Answer 8

Osmolality >100: SIADH, Hypothyroidism, Hypocortisol (Addison’s)
<100: Water loading, psychogenic polydipsia, tea/toast diet

Question 9

Management for normovolaemia hyponatraemia

Answer 9

Fluid restrict
treat underlying cause

Question 10

Management for SIADH

Answer 10

Fluid restrict
Demococyline → Nephrogenic DI
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