Chempath - Sodium and Fluid Balance

Question 1

% of body that is water

Answer 1

60%

Question 2

Intracellular:extracellular fluid ratio

Answer 2

2:1

Question 3

Components of ECF

Answer 3

Intravascular, interstitial (largest components), transcellular (within epithelial lined spaces e.g. CSF, joint fluid, urine etc)

Question 4

ECF solute components in comparison to ICF

Answer 4

More Na+ and Cl-
Less K+

Think of cells as primitive organisms that used to be in the sea –> need salty water to survive

Question 5

Osmolality vs osmolarity

Answer 5

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

Question 6

Osmolarity calculation

Answer 6

2(Na+ + K+) + urea + glucose

Question 7

Osmolar gap

Answer 7

Osmolality - osmolarity

Question 8

Normal range for serum osmolality

Answer 8

275-295 mmol/kg

Question 9

Sodium normal range

Answer 9

135-145 mol/l

Question 10

Hyponatraemia =

Answer 10

excess extracellular water relative to Na+

Question 11

Hyponatraemia - Rx

Answer 11

Treat underlying cause, not the hyponatraemia (unless severe (<125mmol/l)) AND symptomatic (chronic hyponatraemias may be asymptomatic). More dangerous to correct too quickly –> central pontine myelinolysis

Question 12

Hyponatraemia - Symptoms

Answer 12

Nausea and vomiting (<134)
Confusion (<131)
Seizures, non-cariogenic pulmonary oedema (<125)
Coma (<117) and eventual death

Question 13

Hormone that controls water balance

Answer 13

ADH (vasopressin) - acts on V2 receptors to insert aquaporin 2 into collecting ducts to increase water reabsorption

Also - V1 receptors:
Vascular smooth muscle
Vasoconstriction (higher concentrations)
Alternative name ‘vasopressin’

Question 14

2 main stimuli for ADH secretion

Answer 14

Serum osmolality (mediated by hypothalamic osmoreceptors). Increase in osmolality –> increased thirst and ADH secretion.

Blood volume/pressure (mediated by baroreceptors in carotids, atria, aorta). Decrease in BP –> increase in ADH

Increased ADH –> increased water reabsorption –> relative hyponatraemia.

Question 15

1st step in assessing hyponatraemic patient

Answer 15

Assessment of fluid status

Question 16

Clinical signs of hypovolaemia

Answer 16

Tachycardia
Postural hypotension
Dry mucous membranes
Reduced skin turgor
Confusion/drowsiness
Reduced urine output
Low urine Na+ (<20) --> kidneys try to hold onto the sodium to hols onto the water

Question 17

Clinical signs of hypervolaemia

Answer 17

Raised JVP
Bibasal crackles (on chest examination)
Peripheral oedema

Question 18

Causes of hypovolaemia

Answer 18

Gastro and renal

Diarrhoea
Vomiting
Diuretics - especially thiazides
Salt losing nephropathy

Question 19

Causes of euvolaemia

Answer 19

Endo

Hypothyroidism
Adrenal insufficiency
SIADH

Question 20

Causes of hypervolaemia

Answer 20

“Three Failures”

Heart failure –> reduced CO –> reduced BP –> increased ADH secretion
Renal failure
Liver failure (cirrhosis) –> NO –> vasodilation –> reduced BP –> increased ADH secretion

Question 21

Difference between hypovolaemia and dehydration

Answer 21

Hypovolaemia - loss of water AND salt

Dehydration - loss of water

Question 22

Causes of SIADH

Answer 22

CNS pathology
Lung pathology
Drugs (SSRIs, TCAs, opiates, PPIs, carbamazepine)
Tumours - (small cell lung cancer, pancreatic, prostate, lymphoma)
Surgery

Question 23

Pseudohyponatraemia

Answer 23

Excess protein/lipid may be sensed by the machine as water - SODIUM WILL APPEAR DILUTED BUT OSMOLALITY WILL BE NORMAL. e.g. in multiple myeloma

Question 24

Euvolaemic hyponatraemia Ix

Answer 24

TFTs
Short syntACTHen test
Plasma & urine osmolality (SIADH will have low plasma & high urine osmolality)

Question 25

Diagnosis of SIADH

Answer 25

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

Question 26

Hypovolaemic Hyponatraemia - Rx

Answer 26

Volume replacement with 0.9% saline

Give 250 ml –> if sodium goes up then it was hypovolaemia, if it goes down then it was euvolaemia (can use diagnostically).

Question 27

Hypervolaemic/Euvolaemic Hyponatraemia - Rx

Answer 27

Fluid restriction

Treat the underlying cause

Question 28

Severe hyponatraemia

Answer 28

Reduced GCS
Seizures
Seek expert help (Treat with HYPERTONIC 3% SALINE)

Serum Na must NOT be corrected > 8-10 mmol/L in the first 24 hours
Risk of osmotic demyelination (central pontine myelionlysis)
quadriplegia, dysarthria, dysphgia, seizures, coma, death

Question 29

SIADH - Rx

Answer 29

Demeclocycline- Reduces responsiveness of collecting tubule cells to ADH. Monitor U&Es (risk of nephrotoxicity).
Tolvaptan - V2 receptor antagonist

Question 30

Hypernatraemia - Main Causes

Answer 30

Unreplaced water loss
Gastrointestinal losses, sweat losses
Renal losses: osmotic diuresis, reduced ADH release/action (Diabetes insipidus)

Patient cannot control water intake e.g. children, elderly

Question 31

Suspected Diabetes Insipidus - Ix

Answer 31

Serum glucose (exclude diabetes mellitus)
Serum potassium (exclude hypokalaemia)
Serum calcium (exclude hypercalcaemia)
Plasma & urine osmolality
Water deprivation test +/- desmopressin

Question 32

Hypernatraemia - Rx

Answer 32

Fluid replacement
Treat the underlying cause

Correct water deficit - 5% DEXTROSE
Correct extracellular fluid volume depletion- 0.9% SALINE
Serial Na+ measurements - Every 4-6 hours

Question 33

Effects of diabetes mellitus on serum sodium

Answer 33

Variable
Hyperglycaemia draws water out of the cells leading to hyponatraemia
Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia

Question 34

Types of DI

Answer 34

Cranial:
Lack of ADH
Causes: surgery, trauma, tumours
Rx - desmopressin

Nephrogenic:
Receptor defect - insensitivity to ADH
Causes: inherited channelopathies, lithium, demeclocycline
Rx - thiazide diuretics

Question 35

Hypernataraemia - Clinical Features

Answer 35

Lethargy, thirst, irritability, confusion, coma, fits