Clinical chemistry of disordered fluid and electrolyte balance Flashcards
When is ADH released
IN response to high osmolality and low volume
Role of ADH
increased permeability of kidney distal tubule
Role of aldosterone
Stimulate Na+ reabsorption in exchange for either K+ or H+ ions
Result: Na+ retention and loss of K+ and H+
What is hydrostatic pressure
Pushes fluid form vessel to interstitial fluid
What is oncotic pressure
Pushes fluid from interstitial space to vessel
What is oncotic pressure determined by
ALbumin
Why do patients with nephrotic syndrome have oedema
They lose albumin in urine so have albuminuria
This causes low oncotic pressure so hydrostatic pressure moves fluid into interstitial space
How does liver cirrhosis cause ascites
Increased hydrostatic pressure because of portal hypertension
Low oncotic pressure because liver is not producing albumin
How does heart failure cause oedema
High hydrostatic pressure
Causes of hyponatremia
Sodium depletion
- Renal loss (osmotic diuresis, mineralocorticoid excess)
- GI loss (vomiting diarrhoea)
- Skin loss (excessive sweating)
Excess body water
- Psychogenic polydipsia
- Oedema
- SIADH
Renal causes of sodium depletion
Osmotic diuresis
Mineralocorticoid deficiency
Dilutional causes of excess body water
Cirrhosis
Cardiac Failure
Nephrotic syndrome
SIADH
How do diagnose SIADH
High urine osmolality
High urine sodium (because of inhibition of RAAS)
When should SIADH not be diagnosed
When patient has oedema
No renal, cardiac, thyroid or adrenal problems
What does SIADH respond to
Water restriction
Causes of SIADH
Malignancy
Cerebral (e.g. infection)
Pulmonary (e.g. pulmonary embolus, pneumonia)
Drugs
Which drugs can cause hyponatraemia
Diuretics
ADH potentiation
Stimulate ADH release e.g. nicotine
What are the two causes of artifactual hyponatraemia
Drip arm sample
Pseudohyponatremia
How to treat hyponatremia when the cause is fluid and sodium depletion
Why should you be careful
IV saline
Slow correction to minimise risk of central pontine myelinosis
How to treat acute dilution hyponatremia (severe symptoms and acute onset)
Hypertonic saline to correct serum Na and reduce risk of cerebral oedema
Causes of hypernatremia
Inadequate fluid intake
Water depletion
Excessive sodium intake
Reduced sodium excretion
Causes of water depletion which may cause hypernatremia
Renal- polyuria e.g. diabetes insipidus
Non renal- vomiting, diarrhoea
What can cause reduced sodium excretion which may cause hypernatremia
Mineralocorticoid excess
Artifactual causes of hypernatremia
Contamination from preservative
Drip arm
Symptoms of hypernatremia
Dehydration- thirst, oliguria, confusion, weakness, convulsion and death
investigations in hypernatremia
Clinical history
Polyuria: 24h urine volume, urine osmolality, serum glucose, calcium, potassium. Water deprivation test
Mineralocorticoid excess: spot urine K+, renin and aldosterone
If the patient has mineralocirtocoid excess, what are the first line tests that you do
Spot urine K
Renin and aldosterone
