Clinical Chemistry 1 - Sodium and Water Flashcards
The renin-angiotensin-aldosterone system is responsible for the homeostasis of what?
Sodium, water and potassium
What is released from the juxtaglomerular apparatus of the kidney in response to low renal blood flow or raised sympathetic tone?
Renin
What is the role of renin?
Catalyses the conversion of angiotensin to angiotensin I
Angiotensin I is converted to angiotensin II via what?
Angiotensin converting enzyme (ACE)
What is the role of angiotensin II at the level of the glomerulus?
Vasoconstriction of the efferent arteriole
What effect does angiotensin II have on the peripheral circulation?
Vasoconstriction
Angiotensin II promotes the release of what hormone?
Aldosterone
Where does aldosterone act?
On the sodium/potassium pumps of the distal tubule of the nephron
What is the action of aldosterone?
Sodium and water reabsorption, potassium and hydrogen excretion
Sodium concentration in the body is mainly controlled via the action of what?
Aldosterone
Which group of hormones are responsible for reducing sodium reabsorption at the distal tubule and inhibiting the action of renin, in order to decrease sodium levels in the body?
Natriuretic hormones (ANP, BNP, CNP)
Raised plasma osmolarity causes thirst via the hypothalamic thirst centre and the release of what hormone?
ADH
Where does ADH exert its effect?
Collecting ducts of the nephrons
Low plasma osmolarity has what effect on ADH secretion?
Inhibition
Inappropriately high ADH levels cause excess water reabsorption by the kidney- this leads to what clinical condition?
SIADH
What are some examples of causes of SIADH?
Post-operative stress, small cell lung cancers
How can SIADH be treated non-pharmacologically?
Restriction of water intake
What drug class can be used to pharmacologically treat SIADH?
V2 vasopressin receptor antagonists (e.g. tolvaptan)
Inadequate vasopressin action leads to what clinical syndrome?
Diabetes insipidus
What are some causes of cranial (central) diabetes insipidus?
Brain tumours or head trauma
What is the pathology behind cranial (central) diabetes insipidus?
The pituitary gland does not release enough ADH
What is the pathology behind nephrogenic diabetes insipidus?
The kidney fails to respond to ADH
What are some electrolyte abnormalities which can lead to nephrogenic diabetes insipidus?
Hypokalaemia and hypercalcaemia
What are some drugs which can lead to nephrogenic diabetes insipidus?
Lithium and gentamicin
How do patients with diabetes insipidus present clinically?
Polyuria and polydipsia
What happens to plasma osmolarity and plasma sodium levels in diabetes insipidus?
High
What happens to urine osmolarity and urine sodium levels in diabetes insipidus?
Low
What is the clinical test to diagnose diabetes insipidus?
Water deprivation test
In which type of diabetes insipidus will synthetic vasopressin cause a rise in urine osmolarity?
Cranial (central)
ADH levels are high in which type of diabetes insipidus?
Nephrogenic
ADH levels are low in which type of diabetes insipidus?
Cranial (central)
How is cranial (central) diabetes insipidus treated?
Intranasal desmopressin
What effect does Addison’s disease have on sodium, water and potassium levels?
Hyponatraemia, hypovolaemia, hyperkalaemia
What effect do diuretics have on sodium levels?
Hyponatraemia
What effect does excess aldosterone levels (for whatever reason) have on sodium, water and potassium levels?
Hypernatraemia, hypervolaemia, hypokalaemia
What effect does renal failure have on sodium levels?
Hypernatraemia
Which electrolyte abnormality always causes hyperosmolarity?
Hypernatraemia
Most hypernatraemia arises from what?
Unreplaced water loss
The body volume in hypernatraemia is usually what?
Low
What is the body’s main defence against hypernatraemia?
Thirst
What are some potential causes of hypernatraemia?
Fluid losses (D&V, burns), diabetes insipidus, primary hyperaldosteronism
What is a potential iatrogenic cause of hypernatraemia?
Excessive saline as IV fluid replacement
The early clinical features of hypernatraemia are caused by what?
Increased excitability of neurons
What are some examples of early neurological features of hypernatraemia?
Irritability, muscle twitches, brisk reflexes, spasticity
What are some examples of non-neurological features that may be experienced by someone with hypernatraemia?
Thirst, lethargy and weakness
What can hypernatraemia lead to if left untreated?
Seizures and coma
How is hypernatraemia treated if the patient is clinically well?
Oral water replacement
Generally, how is hypernatraemia managed?
Water replacement and treatment of the underlying cause
How is hypernatraemia treated if the patient is clinically unwell?
IV 5% dextrose solution (1 litre every 6 hours)
If a patient with hypernatraemia is hypovolaemic, what should be used for fluid replacement?
IV 0.9% saline
Which type of fluids should always be avoided in patients with hypernatraemia?
Hypertonic solutions
It is important not to correct sodium levels too quickly. You should aim for a change of no more than how many mmol/hour?
0.5mmol/hour
It is important not to correct sodium levels too quickly. You should aim for a change of no more than how many mmol/day?
12mmol/day
If a patient with hyponatraemia is dehydrated, is the cause too little sodium or too much water?
Too little sodium
If a patient with hyponatraemia is not dehydrated, is the cause too little sodium or too much water?
Too much water
If you have established that a person is hyponatraemic due to too little sodium, what is the next most important test to check to establish the diagnosis?
Urinary sodium levels
If a patient is hyponatraemic, dehydrated and has a urinary sodium level of > 20mmol/l, this suggests sodium is being lost from the kidneys. What are some potential causes of this?
Addison’s disease, CKD, diuretic overuse
If a patient is hyponatraemic, dehydrated and has a urinary sodium level of < 20mmol/l, this suggests sodium is being lost from somewhere other than the kidneys. What are some potential causes of this?
D&V, burns, small bowel obstruction, fistulae
When a patient is hyponatraemic, what is the first thing that you want to establish?
Are they dehydrated or not
When a patient is hyponatraemic and not dehydrated, you know that the cause is too much water. In these cases, what is the first question you should ask yourself?
Is the patient oedematous
If a patient is hyponatraemic, not dehydrated and is oedematous, what is the likely underlying cause?
An oedema syndrome (e.g. cardiac, renal or hepatic failure)
If a patient is hyponatraemic, not dehydrated and not oedematous- what is the next test that you should look at to establish the diagnosis?
Urine osmolality
If a patient is hyponatraemic, not dehydrated or oedematous and has a urine osmolality of > 100mg/kg, what is the likely underlying cause?
SIADH
If a patient is hyponatraemic, not dehydrated or oedematous and has a urine osmolality < 100mg/kg, what is the likely underlying cause?
Fluid overload
Which patients are most vulnerable to the neurological effects of hyponatraemia?
Extremes of age, menstruating women and those with underling neurological or metabolic disorders
What effect does hyponatraemia have on the nervous system?
Depressed function
What are some neurological symptoms of hyponatraemia?
Confusion, muscle cramps, reduced reflexes
What are some non-neurological symptoms of hyponatraemia?
Lethargy, nausea
There is a risk of seizures and coma with hyponatraemia, particularly when sodium levels fall below what?
120mmol/l
What is the risk of treating hyponatraemia too quickly?
Central pontine myelinosis
How is asymptomatic, chronic hyponatraemia treated?
Fluid restriction and treatment of the underlying cause
How is acute or symptomatic hyponatraemia treated?
Cautious rehydration with 0.9% saline
If a patient with hyponatraemia is having a seizure or is in a coma- urgent help is needed. What can be considered as treatment?
Hypertonic saline +/- furosemide
What drug can be useful in the treatment of hypervolaemic or euvolaemic hyponatraemia?
Tolvaptan
