Salt and Water balance Flashcards
What endocrine system has the predominant control over sodium
RAAS
Which endocrine system has the predominant control over water
ADH
How does hyponatraemia often present
- Initially: anorexia, nausea, malaise
- Then: Headache, irritability, confusion, weakness, decrease GCS, seizures
- increased risk of falls in elderly
- cardiac failure and oedema can indicate the cause
Symptoms of salt loss – Diarrhoea & vomiting, excessive sweating
describe how the RAAS system works
- angiotensinogen is produced from the liver
- renin from the kidney acts on angiotensingoen and converts it to angiotensin I
- ACE from the lung converts angiotensin I into angiotensin II
- Angiotensin II causes an increase in sympathetic activity, causes aldosterone secretion which causes sodium retention, causes arteriolar vasoconstriction, increases ADH secretion
How does ADH control water
- Osmoreceptors detect an increase in osmotic pressure
- baroreceptors detect a decrease in blood pressure
- this signals to the posterior pituitary
- the posterior pituitary releases ADH
- ADH causes blood vessels to vasoconstrict and causes an increased water reabsorption in the kidney
- this leads to increased blood volume and increased blood pressure
What causes hyponatreaemia
- Dilutional hyponatremia
- salt deficiency or loss
- Pseudohyponatraemia
Name what can cause dilutional hyponatreamia
Too much water ingested
- psychogenic polydipsia - psychiatric problem whereby a person drinks too much water
Too much water reabsorbed (often osmotic)
- cirrhosis
- Nephrotic syndrome
- Congestive cardiac failure
Reduced excretion of water
- syndrome of inappropriate ADH (SIADH)
- hypoadreanlism
How does salt deficiency or loss cause hyponatreaemia
Loss from
- kidneys
- gut
- skin
Hypoadrenalism
How does pseudohyponatraemia cause hyponatraemia
Lab problem Arterfactual low Na due to analytical problems - high lipids - high glucose - paraproteins
How do you determine the cause of hyponatraemia
History
Drugs
Water / Alcohol ingestion
Renal / Liver problems
Examination
Are they euvolaemic, hypovolaemic or hypervolaemic?
Investigations
U&Es, plasma and urine osmolality, glucose, TFT, cortisol
Urinary sodium
what drugs can cause hyponatraemia
- Proton pump inhibitors
- Antidepressants such as citalopram
- thiazides
- loop diuretics
How do you assess volume status
important in patients with hyponatremia
check:
- pulse
- postural blood pressure - look for a drop of 10mmhg
- skin turgor
- JVP - look at the neck carefully to see
- mucus membrnae
- listen to breast sound - crackles due to fluid overloaded in heart failure
- weight chart
- fluid chart
What does it suggest if potassium is high
- suggest that you have glucocorticoid deficiency
what is characterstic of addisons disease
low sodium/high potassium
What does a high urea and creatine suggest
- high suggests AKI/dehydration
What does a low cortisol suggest
- very low suggests glucorticoid deficiency
what does it suggest if urinary sodium is low
- Hypovolaemia - kidneys are trying to preserve salt and water
What does it suggest if urinary sodium is high
- Suggests inappropriate loss of sodium e.g. SIADH
What is SIADH
- this is a condition that is characterised by excess production of ADH that is inappropriate
What can SIADH be due to
- malignancy
- CNS disorders
- Lung disease
- endocrine disease
- Drugs
describe the signs of SIADH
- leads to a dilutional hyponatraemia
- patient is euvolaemic
- low plasma osmolality, inappropriately high urine osmolality - (too much concentrated urine and too much dilute plasma)
Define euvolaemic
Having a normal amount of body fluids
What is the treatment of SIADH
- Treat cause of restrict fluid
- consider salt +/- loop diuretics if severe
- demeclocycline (ADH antagonists) - rarely used for SIADH
- ADH receptor antagonists (Vaptans, e.g. tolvaptan)
what is the treatment of SIADH if the person is confused or severe CNS symptoms
- Hypertonic saline
Tolvaptan
- V2 receptor antagonists - decreases water permeability of collecting duct
- antagonises effects of ADH