Chempath 12: Sodium And Fluid Balance Flashcards
What is the most common pathogenesis of hyponatraemia ?
Increased extracellular water
How does ADH increase water reabsorption ?
ADH binds to V2 receptors and causes increased Aquaporin 2 insertion into the collecting duct
Which receptors are found in smooth muscle and cause vasoconstriction ?
A) V1 receptors
B) V2 receptors
A) V1 receptors
They bind Vasopressin and cause vasoconstriction
V2 are found in collecting ducts and respond to ADH
What are the 2 main stimuli for ADH secretion ?
Osmoreceptors- detects high osmolality
Baroreceptors- detect low blood volume/pressure
Where are Osmoreceptors found in the body ?
Hypothalamus
List 3 locations where Baroreceptors are found in the body ?
Carotid
Aorta
atria
What is the most reliable clinical sign of hypovolaemia ?
Low urine Na+
Doesn’t work if on diuretics
List 4 clinical signs of hypovolaemia?
Tachycardia Postural hypotension Reduced skin turgor Dry mucous membranes Sunken eyes
List 4 causes of Hypovolaemic hyponatraemia ?
Diarrhoea
Vomiting
Diuretics
Salt losing nephropathy
List 3 causes of euvolaemic hyponatraemia?
Hypothyroidism
Adrenal insufficiency
SIADH
List 3 causes of hypervolaemic Hyponatraemia ?
Cardiac failure
Nephrotic syndrome
Cirrhosis
How does liver cirrhosis cause hypervolaemic hyponatraemia ?
Cirrhosis causes release of Nitric oxide which causes vasodilation
This causes reduced blood pressure > sensed by baroreceptors
ADH release is increased.
More water is reabsorbed
How does hypothyroidism cause a Euvolaemic hyponatraemia ?
Hypothyroidism causes reduced cardiac contractility
This causes reduced blood pressure
This causes increased ADH release
More water is reabsorbed than Na+
List 3 causes of SIADH ?
CNS tumour
Small cell lung cancer
Meningitis/ subarachnoid haemorrhage
Drugs (psych drugs: SSRIs, Carbamazapine, Amitryptiline)
How do you treat hypovolaemic hyponatraemia?
Volume replacement with 0.9% saline
If volume is returned to normal, the stimulus for ADH release is stopped so allows recovery.
How do you treat hypervolaemic and euvolaemic hyponatraemia?
Fluid restriction
Treat underlying cause
Name the complication that can occur as a result of increasing serum sodium faster than 8-10 mol/L in the first 24 hours ?
Central pontine myelinolysis
Name 2 drugs used to treat SIADH ?
Tolvaptan - ADH receptor antagonist
Demeclocycline - reduces responsiveness of collecting duct cells to ADH
What level of serum sodium defines hypernatraemia ?
> 145 mmol/L
What level of serum sodium defines hyponatraemia ?
<135 mmol/L
Give 3 causes of Hypernatraemia ?
Diarrhoea and vomiting
Diabetes mellitus
Diabetes insipidus
3Ds of hypernatraemia
Name 2 tests used to diagnose diabetes insipidus ?
Serum and urine osmolality
Water deprivation test
Describe the possible outcomes of the water deprivation test ?
Normal: urine becomes very concentrated (sodium >800)
Cranial Diabetes insipidus: unable to concentrate urine (sodium <300)
Nephrogenic Diabetes insipidus : unable to concentrate urine (sodium <300)
Polydipsia: slightly able to concentrate urine, but physiological function has been damaged overtime (sodium <500)
What is the main treatment for hypernatraemia ?
5% Dextrose
How do you treat Hypovolaemic hypernatraemia ?
First correct the extravascular fluid volume with 0.9% saline
Then give 5% dextrose
Explain the 2 methods by which diabetes mellitus affects Sodium ?
1- Hyperglycaemia causes water to leave cells and enter the extravascular compartment causing dilution of sodium (hyponatraemia) (check this one)
2- In diabetes you get an osmotic diuresis which causes increased water loss
List 2 causes of pseudo hyponatraemia ?
Hyperlipidaemia
Hyperproteinaemia
Dilution by excess molecules of lipid or protein in the circulation. Sodium conc. is normal
What is meant by true hyponatraemia ?
When the Na+ conc. is low + Serum osmolality is low
Sodium low
Urine Osmolality > 20mmol/L
Urea elevated
Creatinine elevated
Most likely diagnosis ?
volume Type of hyponatraemia ?
CKD
Hypervolaemic hyponatraemia
CKD causes increased urine protein loss -> Oedema -> low circulating volume -> RAS activated -> Increased ADH release -> Increased water reabsorption
A tanned looking lady
Na+ low
K+ high
BM (glucose) low
Most likely diagnosis ?
Addisons
Lack of Aldosterone -> Increased Na+ reabsorption + Reduced K+ Exretion
High ACTH -> hyperpigmentation
low cortisol -> less gluconeogenesis -> Hypoglycaemia
Hyponatraemia with a higher urine osmolality compared to Serum osmolality ?
Most likely diagnosis?
SIADH
How do you calculate Osmolarity ?
2(Na + K) + Urea + Glucose
How do you calculate Anion gap ?
Na + K - CL - HCO3
How is osmolality determined ?
Measured using an osmometer
What is the normal range for Sodium ?
135-145mM/l
Which atypical pneumonia can cause hyponatraemia ?
Legionella Pneumophilia
hotel air conditioners
