ChemPath 12: Sodium and Fluid Balance Flashcards
What is the definition of hyponatraemia?
Sodium concentration < 135 mmol/L
What is the underlying pathogenesis of hyponatraemia?
Increased extracellular water
Describe the action of ADH.
Acts on V2 receptors in the collecting duct
Leads to insertion of AQP2 molecules and an increase in the reabsorption of water
Acts on V1 receptors on vascular smooth muscle leading to vasoconstriction
What are the two main stimuli for ADH release?
Increased serum osmolality (via hypothalamic osmoreceptors)
Blood volume/pressure (via baroreceptors)
What is the first step in the management of hyponatraemia?
Assess their volume status
List some clinical features of hypovolaemia.
Tachycardia
Postural hypotension
Dry mucous membranes
Reduced skin turgor
Confusion
Reduced urine output
What is the most reliable clinical test of hypovolaemia? what is this like in hypervolaemia?
Low urine sodium (suggests that you are trying to retain fluid)
<20 - hypovolaemia
>20 - hypervolaemia
NOTE: this may be high in patients on diuretics
List some clinical features of hypervolaemia
Raised JVP
Bibasal crackles
Peripheral oedema
List some causes of hyponatraemia - Hypovolaemic?
Extra-renal:
- Diarrhoea + Vomiting
Renal:
- Diuretics
- Salt-losing nephropathy
List some causes of hyponatraemia - Euvolaemic?
Adrenal insufficiency (addisons)
Hypothyroidism
SIADH
List some causes of hyponatraemia - Hypervolaemic?
Cirrhosis
Cardiac failure
Renal failure
Nephrotic syndrome
Explain how patients with hypovolaemic hyponatraemia have too much water
Diarrhoea and vomiting leads to loss of water and salt
This leads to increased ADH release which causes reabsorption of more water than salt leading to hyponatraemia
How does cirrhosis lead to hyponatraemia?
Causes the release of various mediators that cause a drop in perfusion pressure
List some causes of SIADH.
CNS pathology
Lung pathology
Tumours
Drugs (SSRIs, TCAs, opiates, PPIs, carbamazepine)
Surgery
List the main investigative feature of SIADH
Low plasma osmolality
High urine osmolality
Which tests would you do for euvolaemic hyponatraemia to diagnose as SIADH
TFTs
Short synacthen test
Plasma and urine osmolality
Outline the treatment of:
- Hypovolaemic hyponatraemia
- Euvolaemic hyponatraemia
- Hypervolaemic hyponatraemia
Hypovolaemic hyponatraemia:
- Volume replacement with 0.9% saline
- This replenishes the circulating fluid volume and switches off the stimulus for ADH release
Euvolaemic hyponatraemia:
- Fluid restriction
- Treat underlying cause
Hypervolaemic hyponatraemia:
- Fluid restriction
- Treat underlying cause
What are some clinical features of severe hyponatraemia?
Reduced GCS
Seizures
What is the maximum rate of correction of hyponatraemia?
8-10 mmol/L per 24 hours
What is the main danger of rapidly correcting hyponatraemia?
Can cause central pontine myelinolysis (osmotic demyelination)
This can lead to quadriplegia, dysarthria, dysphagia, seizures, coma and death
Name and describe the mechanism of action of two drugs used to treat SIADH if fluid restriction is insufficient
Demeclocycline – reduces the responsiveness of collecting duct cells to ADH
NOTE: monitor U&E because it can be nephrotoxic
Tolvaptan – V2 receptor antagonist
Alternative: fluid restriction + salt tablets + diuretics
Define hypernatraemia
Serum sodium > 145 mmol/L - decreased extracellular water
List some causes of hypernatraemia
GI losses
Sweat losses
Renal losses (e.g. osmotic diuresis, DI)
Normaly can just drink enough water to replace - hence hypernatraemia only really affects really young / old people
List some investigations that are used in suspected diabetes insipidus
Rule out causes of osmotic diuresis:
Plasma glucose (rule out DM)
Plasma K+ (rule out hypokalaemia)
Plasma Ca2+ (rule out hypercalcaemia)
Plasma and urine osmolality
Water deprivation test
How is hypernatraemia treated + special case? what happens if too quickly replaced
Fluid replacement – use 5% dextrose because this will replace the fluid without adding to the salt + serial Na measurements
NOTE: if someone is hypovolaemic with hypernatraemia, they may initially be given 0.9% saline to treat the hypovolaemia before switching to dextrose to treat the hypernatraemia
Rapid correction can lead to cerebral oedema!
How often should serial Na+ measurements be taken in someone being treated for hypernatraemia?
4-6 hours
How can diabetes mellitus affect serum sodium?
Hyperglycaemia will draw water out of cells (i.e. into the ECF) thereby leading to hyponatraemia
However, high plasma glucose can also lead to an osmotic diuresis (renal losses) which can lead to hypernatraemia
You are bleeped to review John Doe. A 68-year-old gentlemen on the cardiology ward admitted with Heart Failure. John had contracted Norovirus whilst on the ward is suffering from severe D&V.
On examination, he has dry mucous membranes and reduced skin turgor
Which of the following is the most accurate descriptor of John’s current condition?
- Hypovolaemia Hyponatraemia
- Hypochloraemic Hypokalaemic Metabolic Alkalosis
- Euvolaemic Hyponatraemia
Hypernatraemia - Hypervolaemic Hyponatraemia
Hypovolaemia Hyponatraemia
Why do you end up with Excess Water in hypovolaemic hyponatraemia?
Low Blood Volume -> Baroreceptors -> ADH Release
Why do you end up with excess water in the different causes of hypervolaemic hyponatraemia?
Cardiac Failure – Low BP -> Baroreceptors -> ADH Release
Liver Cirrhosis – Releases vasodilators -> Low BP -> Baroreceptors -> ADH Release
Renal Failure – Reduced Water Excretion
How do you get excess water w/ euvolaemic hyponatraemia?
All of the above (somehow?) cause a spur of ADH to be released
The reabsorption of water increases the circulating volume and therefore increases BP.
The increase in BP is detected in the Atria which release ANP release – Natriuresis.
What can cause SIADH?
- > CNS Pathology
- > Lung Pathology
- > Drugs (SSRI, TCA, Opiates, PPI, Carbamazepine)
- > Tumours
- > Surgery
How to treat euvolaemic hyponatraemia?
- > If fluid restriction isn’t enough
- > Demeclocycline (Reduce cell responsiveness to ADH)
- > Tolvaptan (Antagonist to V2 receptor)
You are bleeped to review John Doe. A 46-year-old gentlemen on the respiratory ward with a diagnosis of Lung Cancer,
On Examination, He appears cachectic ‘but of normal fluid status’,
Low Na+, everything else normal
What is the most appropriate management plan?
Fluid restrict + treat underlying cause - Euvolaemic hyponatraemia
You are bleeped to review Jane Doe. A 46-year-old lady in A&E with a significant background of alcohol abuse who has presented feeling ‘confused’.
On Examination, she has a raised JVP, splenomegaly and shifting dullness.
Low Na+, everything else normal
Which of the following is the most likely cause of her ‘Hypervolaemic Hyponatraemia’?
Renal Failure Diuretic Use Hypothyroidism Liver Cirrhosis Heart Failure
Liver Cirrhosis – Releases vasodilators -> Low BP -> Baroreceptors -> ADH Release
You are bleeped to review Jane Doe. A 98-year-old lady in A&E with poorly controlled Diabetes Mellitus – ‘who lives alone and her water jug is typically too far for her to reach’.
On examination, she has dry mucous membranes and reduced skin turgor.
Raised Na+, everything else normal
What is the most appropriate management plan? what else is needed along side this?
IV 0.9% Saline IV 0.9% Saline + 5% Dextrose 5% Dextrose Fluid Restrict No Management Required
IV 0.9% Saline + 5% Dextrose + Serial Na+ Measurements (4-6h)
