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
