Sodium + fluid balance

Question 1

Intracellular vs. extracellular

Answer 1

Intracellular K+
Extracellular Na+
(Cell = potassium package in a salty sea)

Question 2

Osmolality

Answer 2

Total particles in solution
mmol/kg
Measured by osmometer
‘Lifelike’

Question 3

Osmolarity

Answer 3

Defined particles in solution
mmol/L
Calculated by 2(Na + K) + Urea + Glucose
Effectively measured 'pre-set' particles that should be there
'Romanticised'

Question 4

Normal osmolarity

Answer 4

275-295

Question 5

Osmolar gap

Answer 5

Osmolality should = osmolarity
(Should just have pre-set particles)
If osmolality > osmolarity there is an osmolar gap
(Particles that shouldn’t be there)

Question 6

Things causing an osmolar gap

Answer 6

Glucose
Alcohol
Lactate
Mannitol

Question 7

Normal sodium

Answer 7

135-145

Question 8

Symptoms of hyponatraemia

Answer 8

N+V, confusion, seizures, coma

Question 9

Approach to hyponatraemia

Answer 9

1. Is it true (low osmolality)
2. Hypovolaemic / hypervolaemic / euvolaemic
3. Urinary sodium

Question 10

Na <135

High serum osmolality

Answer 10

False hyponatraemia

Glucose, mannitol infusion

Question 11

Na <135

Normal serum osmolality

Answer 11

False hyponatraemia

Drip arm, paraprotein

Question 12

High urinary sodium in hyponatraemia

Answer 12

> 20
Indicates problem with kidneys
In hyponatraemia urinary sodium should be low as the body should be conserving sodium

Question 13

Hypovolaemic hyponatraemia

Causes, Mx

Answer 13

6 Causes: (SaD) Salt losing enteropathy, diuretics
(TED) - Third space losses (ascites, burns), excess sweating, D+V
Mx: Fluid replacement 0.9% saline

Question 14

Treatment hypovolaemic hyponatraemia

Answer 14

Volume replace with 0.9% saline

Question 15

Euvolaemic hyponatraemia

Causes, Ix, Mx

Answer 15

3 ENDOCRINE causes - SIADH*, adrenal failure, hypothyroidism
Ix: Plasma:urine osmolality, short synACTHen test, TFTs
Mx: Restrict + treat underlying cause
Endocrine = Euvolaemic

Question 16

Hypervolaemic hyponatraemia

Causes, Mx

Answer 16

3 FAILURES causes - Heart failure, CKD, Liver failure (+ Too much fluid post-surgery)
Mx: Restrict + treat underlying cause

Question 17

How should Na be replaced and why?

Answer 17

SLOWLY to prevent central pontine myelinolysis (= pseudobulbar palsy, paraparesis, locked-in syndrome)

Question 18

Na <135, serum osmolality <135, urinary Na >20

Low plasma:urine osmolality

Answer 18

SIADH

Question 19

SIADH

Causes, Ix, Mx

Answer 19

Excess ADH secretion increases water reabsorption from kidney
Causes: Malignancy (SCC, prostate, pancreas, lymphoma), CNS pathology / trauma, chest (TB, pneumonia, SCLC), drugs (opiates, SSRIs, PPIs, carbamazepine)
Low plasma:urine osmolality
Mx: Fluid restrict + treat underlying. Demeclocycline, Tolvaptan

Question 20

Is hyponatraemia or hypernatraemia more common?

Answer 20

Hyponatraemia

Question 21

What causes

a) Hyponatraemia
b) Hypernatraemia

Answer 21

a) Too much water
b) Not enough water
(Relative to Na+)

Question 22

Causes of hypernatraemia

Answer 22

May be reduced water intake (elderly, children) but mostly excess water loss: DI, Osmotic diuresis (DM), GI losses (D+V), Conn’s (excess aldosterone), excessive sweating, hypercalcaemia

Question 23

Tx hypernatraemia

Answer 23

5% dextrose to lower Na + treat underlying cause

Question 24

DI

Causes, Ix, Mx

Answer 24

Cranial or nephrogenic
Fluid deprivation test - measure urine osmolality after deprivation / DDAVP
5% dextrose for hypernatraemia + treat underlying cause (DDAVP if cranial)

Question 25

Osmolarity

Answer 25

2(Na + K) + Urea + Glucose

Question 26

Cause of pseudohyponatraemia

Answer 26

Hyperlipidaemia - excess lipids dilute blood resulting in hyponatraemia but normal serum osmolality