High Yield ChemPath

Question 1

Difference between osmolality and osmolarity?

Answer 1

Osmolality = mOsm/kg of solvent
More accurate
Measured by automated lab machine

OsmolaRity = mOsm/litRe of solvent
More practical
Calculated from blood tests

Question 2

How do you calculate osmolality?

Answer 2

Calculated osmolality = 2 (Na + K) + glucose + urea

Question 3

What is the normal range for osmolality?

Answer 3

275 – 295 mOsmol/kg

Question 4

Why is osmolality more accurate?

Answer 4

E.g. volume of solutions are dependent on temperature but mass will stay constant
Measured by machine in lab

Question 5

What is the osmolar gap?

Answer 5

Measured osmolality – calculated osmolality

Question 6

If the osmolar gap is more that > 10mOsmol/kg what do you need to consider?

Answer 6

Other substances that are not part of the equation:
Alcohol: methanol, ethanol
Sugars: mannitol, sorbitol
Lipids: hypertriglyceridaemia
Proteins: hypergammaglobulinaemia

Question 7

Which electrolyte is the largest contributor to plasma osmolality?

Answer 7

Na

Question 8

Rank the expected calculated osmolality in patients with each of the following outcomes, with 1 being the highest osmolality and 5 being the lowest.
Diabetes insipidus
Diabetic ketoacidosis
Hyperosmolar hyperglycaemic state
Pneumonia
SIADH

Answer 8

HHS
DKA
Diabetes Insipidus
Pneumonia (some cases can cause SIADH)
SIADH

Question 9

How does increasing blood volume affect soduim?

Answer 9

Increased blood volume –> atrial stretch –> increased release of atrial natriuretic peptide (ANP)

Decreasing release of:
Aldosterone (adrenal cortex)
ADH (hypothalamus)
Renin (kidney)

Hence decreasing sodium concentration and blood volume

Question 10

What happens when there is a high blood osmolality?

Answer 10

High osmolality –> thirst + ADH release –> decrease sodium concentration

Question 11

What happens when there is a low blood osmolality?

Answer 11

Low osmolality –> ADH suppression –> increase sodium concentration

Question 12

How do we assess volume status?

Answer 12

BP, HR, CRT
Leg oedema
Pulmonary oedema

Question 12

What is true hyponatraemia?

Answer 12

Low sodium with low plasma osmolality

Question 13

Describe ADH levels in hypovolaemic hyponatraemia?

Answer 13

Appropriately high (want to reabsorb water)

Question 14

How do you ascertain cause of hypovolaemic hyponatraemia?

Answer 14

Check urinary sodium
If < 20 mmol/L = extra-renal loss (vomiting, diarrhoea, burns)
If > 20 mmol/L = renal loss (renal disease, diuretics, cerebral salt wasting)

Question 15

What is the management for Hypovolaemic hyponatraemia?

Answer 15

Treat underlying cause, IV 0.9% NaCl or slow IV hypertonic 3% NaCl

Question 16

When might urine sodium be unreliable?

Answer 16

Pts on diuretics

Question 17

What is the normal range for urine sodium?

Answer 17

Normal range = 40-220 mEq/L [<20 non-renal loss; >20 in renal loss)

Question 18

What are clinical signs of hypervolaemia?

Answer 18

Raised JVP
Bi-basal crackles
Peripheral oedema

Question 19

What are causes of hyponatraemia in a hypervolaemic patient?

Answer 19

Cardiac failure → low pressure → detected by baroreceptors → ADH release

Cirrhosis → vasodilated due to excess NO → low BP → baroreceptors → ADH release

Renal failure → not excreting enough water

Question 20

Describe blood volume in hypervolaemic hyponatraemia?

Answer 20

High total body water but low ‘effective arterial blood volume’

Question 21

How can urine sodium indicate cause of hypervolaemic hyponatraemia?

Answer 21

If < 20 mmol/L = CCF, cirrhosis, nephrotic syndrome
If > 20 mmol/L = CKD

Question 22

What is the management of hypervolaemic hyponatraemia?

Answer 22

Treat underlying cause
Fluid restrict

Question 23

What is the mechanism of euvolaemic hyponatraemia?

Answer 23

Increased total body water relative to sodium

Question 24

How can urine sodium reveal cause of euvolaemic hyponatraemia?

Answer 24

If < 20 mmol/L = psychogenic polydipsia, tea and toast diet
If > 20 mmol/L = hypothyroidism, adrenal insufficiency, SIADH

Question 25

What is the management for euvolaemic hyponatraemia?

Answer 25

Treat underlying cause, fluid restrict, demeclocycline or tolvaptan for resistant SIADH

Question 26

What is the diagnostic criteria for SIADH?

Answer 26

Low plasma sodium (< 135)
Low plasma osmolality (< 270)
High urinary sodium (> 20)
High urinary osmolality (> 100)
No adrenal/thyroid/renal dysfunction
DIAGNOSIS OF EXCLUSION

Question 27

What drugs can induce SIADH?

Answer 27

PPIs
SSRIs
Carbamazepine
Opiates
TCAs

Question 28

What lung conditions can cause SIADH?

Answer 28

pneumonia (legionella), small cell lung cancer (paraneoplastic)

Question 29

What CNS disease can cause SIADH?

Answer 29

stroke, tumour, abscess

Question 30

How does hypothyroidism cause hyponatraemia in a euvolaemic patient?

Answer 30

Hypothyroidism –> reduced contractility –> reduced BP –> ADH release

Question 30

How does adrenal insufficiency lead to hyponatraemia in a euvolaemic patient>

Answer 30

Adrenal insufficiency –> less aldosterone –> less Na+ reabsorption

Question 31

What are the causes of hypernatraemia at different volume status’?

Answer 31

Hypovolaemia – osmotic diuresis, diarrhoea, burns
Hypervolaemia – hypertonic 3% NaCl, hyperaldosteronism
Euvolaemia – diabetes insipidus

Question 32

What is osmotic diuresis?

Answer 32

Urinate glucose (in diabetes)
Drags water out of the body

Question 33

What is the management of hypernatraemia?

Answer 33

Oral intake of water, slow IV 5% dextrose (1L/6hr) guided by urine output and plasma sodium

Question 34

A 65 year old gentleman who is a long-term smoker presented with a 2-month history of cough, shortness of breath and weight loss.
His examination is unremarkable.
His investigation results are as follows: Na 128, K 4.0, adjusted Ca 2.4, normal TSH and cortisol level.
His CXR report is pending.

What is the next best step in investigation?

Answer 34

Assess fluid/volume status
Paired serum and urine osmolalities