ChemPath MedEd

Question 1

Normal Range for Sodium

Answer 1

135-145 mmol/L

Question 2

What kind of ion is Na+ in the body and how are its levels maintained?

Answer 2

Predominantly an extracellular cation

largely maintained by active pumping from ICF -> ECF
by Na+/K+ ATPase

Question 3

What % of Na+ in the body is freely exchangeable? What is going on with the rest?

Answer 3

70% freely exchangeable, the rest complexed in bone

Question 4

mild hyponatraemia sodium levels

Answer 4

130-135 mmol/L

red common in hospital

Question 5

severe hyponatraemia sodium levels

Answer 5

<125 mmol/L

Question 6

How do we treat hyponatraemia?

Answer 6

treat the underlying cause unless severe (<125 mmol/L) and symptomatic

Hyponatraemia that is compensated (usually chronic) is rarely an emergency to treat: even with
sodium in the 110-120 range that are asymptomatic, it is more dangerous to correct them too fast
than to leave the patient at that level.

Question 7

Sx of symptomatic hyponatraemia

Answer 7

• Nausea + vomiting (<134 mmol/L)
• Confusion (<131 mmol/L)
• Seizures, non-cardiogenic pulmonary oedema (<125 mmol/L)
• Coma (<117 mmol/L) and eventual death

Reduced GCS, Seizures -> Seek expert help (Treat with Hypertonic 3% saline)

Question 8

Is Symptomatic hyponatraemia a medical emergency?

Answer 8

Yes

Question 9

What is the commonest electrolyte abnormality seen in hospitalised patients?

Answer 9

hyponatraemia

Na <135 mmol/L

Question 10

What is the underlying pathogenesis of hyponatraeamia?

Answer 10

increased EC water

Question 11

How does ADH mediated water retention occur?

Answer 11

ADH acts on V2 receptors in the collecting ducts

Insertion of aquaporin-2
channels

-> more water reabsorbed (??)
see if want to add more details here

Question 12

What are the effects of VP?

Answer 12

Acts on V2 receptors (collecting duct) -> Insertion of aquaporin-2

V1 receptors (on vascular smooth muscle)
- vasoconstriction (higher concentrations)
-> alternative name ‘vasopressin’

Question 13

What controls ADH secretion?

Answer 13

• high osmolality measured by hypothalamic osmoreceptors stimulates release
• low blood volume/pressure detected by baroreceptors in carotids, atria and aorta

Question 14

What is the effect of increased ADH secretion on serum sodium concentration?

Answer 14

lowers sodium concentration

HypOnatraemia

increased water reabsorption -> decreased sodium concentration

Question 15

TURP syndrome

Answer 15

• Transurethral resection of the prostate (TURP) syndrome is fluid overload and iso-osmolar hyponatraemia during TURP from large volumes of irrigation fluid being absorbed through venous sinuses
• Irrigation fluid is required to maintain visibility despite bleeding tissue beds
• TUPR syndrome can also occur in other procedures requiring large volumes of irrigation, such as hysteroscopy
• True TURP syndrome is now rare, particularly as glycine-based irrigation fluids are less commonly used

Question 16

60-40-20 rule

Answer 16

The 60-40-20 rule:

• 60% total body weight = water
• 40% of body weight = intracellular
• 20% of body weight = extracellular

Question 17

Osmolality vs Osmolarity

Answer 17

Osmolality = total number of particles in solution - measured with an osmometer, units = mmol/kg.

Osmolarity = calculated, units = mmol/l

Osmolality and osmolarity should roughly equate

Question 18

Osmolarity formula

Answer 18

2(Na + K) + urea + glucose

Question 19

normal range for serum osmolality

Answer 19

275 – 295 mmol/kg

Question 20

Difference between osmolarity and osmolality term

Answer 20

osmolar gap

can be useful in metabolic acidosis cases
This is because if the osmolarity is lower than the osmolality, we can
assume there are extra (unmeasured) solutes that are dissolved in the serum

Question 21

What impacts the osmolality?

Answer 21

Physiological
Na+
K+
Cl-
HCO3-
urea
glucose

Pathological
Endogenous (i.e. glucose), Exogenous (ethanol, mannitol)

Question 22

osmolality in true and in pseudohyponatraemia

Answer 22

true: low

pseudo: normal/high

Question 23

clinical signs of hypovolaemia

Answer 23

Tachycardia
Postural hypotension
Dry mucous membranes
Reduced skin turgor
Confusion/drowsiness
Reduced urine output
Low urine Na+ (<20)

Question 24

clinical signs of hypervolaemia

Answer 24

Raised JVP
Bibasal crackles (on chest examination)
Peripheral oedema

Question 25

Pseudohyponatraemia causes according to osmolality:

Answer 25

normal:
- Spurious
- Drip arm sample
- Pseudohyponatraemia (hyperlipidaemia/ paraproteinaemia)

high:
- Glucose (e.g. HHS)/mannitol infusion
- toxic alcohols

Hyponatraemia with elevated plasma osmolality is due to an excess of osmotically active solutes in the plasma. Often this is glucose (in HHS) but can also be mannitol. This draws water from cells into the plasma, which dilutes down the sodium. This is technically a true hyponatraemia however it is due to another chemical in the blood.

Question 26

What are the causes of hyponatraemia in a hypovolaemic patient?

Answer 26

Renal: diuretics
Extra-renal: diarrhoea, vomiting

-> think: fluid is being lost somewhere

Question 27

What are the causes of hyponatraemia in a hypervolaemic patient?

Answer 27

Cardiac failure
Cirrhosis
Renal failure

-> think failure

Question 28

What are the causes of hyponatraemia in a euvolaemic patient?

Answer 28

Hypothyroidism
Adrenal insufficiency
Syndrome of inappropriate ADH (SIADH)

-> think endocrine

Question 29

Causes of SIADH

Answer 29

CNS pathology

Lung pathology

Drugs (SSRI, TCA, opiates, PPIs, carbamazepine)

Tumours

Surgery

Question 30

What investigations would you order in a patient with euvolaemic hyponatraemia?

Answer 30

? Hypothyroidism: Thyroid function tests

? Adrenal insufficiency: Short Synacthen test

? SIADH: Plasma & urine osmolality (low plasma & high urine osmolality)

Question 31

Findings consistent with SIADH dx

Answer 31

No Hypovolaemia
No Hypothyroidism
No Adrenal insufficiency
Reduced plasma osmolality AND
Increased urine osmolality (>100)

Question 32

How would you manage a hypovolaemic patient with hyponatraemia?

Answer 32

Volume replacement with 0.9% saline

Question 33

How would you manage a hypervolaemic patient with hyponatraemia?

Answer 33

Fluid restriction
Treat the underlying cause (e.g. HF, cirrhosis, primary polydipsia, nephrotic syndrome; renal failure (urine Na >20))

Question 34

Mx of severe hyponatraemia (<125)

Answer 34

i.e. Reduced GCS, Seizures

Seek expert help (Treat with Hypertonic 3% saline)

Question 35

Central pontine myelionlysis

Answer 35

damage to the myelin sheath of the white matter in the CNS caused by a sudden rise in serum osmolality

-> quadriplegia, dysarthria, dysphgia, seizures, coma, death

therefore do not correct serum sodium >8-10 mmol/L in the first 24h

Question 36

What is the most important point to remember while correcting hyponatraemia?

Answer 36

Serum Na must NOT be corrected > 8-10 mmol/L in the first 24h

Risk of osmotic demyelination (central pontine myelionlysis)

quadriplegia, dysarthria, dysphgia, seizures, coma, death