Chemical Pathology- Part 1 (p72-85- Fluid balance, electrolytes, acid-base, LFTs, porphyrias, pituitary + thyroid) Flashcards
What % of body is water?
60
What is ratio of intracellular:extracellular fluid?
2:1
What does extracellular fluid include (three different things)?
Intravascular
Interstitial (between cells- largest component)
Transcellular- epithelial lined spaces e.g. CSF, joint fluid etc
Define osmolality
Total number of particles in solution- measured with an osmometer (units- mmol/kg)
Define osmolarity
The concentration of a solution- calculated (units mmol/l)
What physiological determinants of osmolality and osmolarity are there in serum/plasma?
Na+ K+ Cl- HCO3- Urea Glucose
What pathological determinants of osmolality and osmolarity are there in serum/plasma?
Endogenous (e.g. glucose) or exogenous (e.g. ethanol)
Osmolarity calculation?
2(Na+ + K+) + urea + glucose
What is the osmolar gap?
The difference between osmolality and osmolarity
Why is the osmolar gap useful in metabolic acidosis?
If osmolarity is lower than osmolality we can assume there are extra unmeasured solutes that are dissolved
Normal range for serum osmolality?
275-295 mmol/kg
Normal range for Na+?
135-145mmol/l
What maintains Na+ as mainly an extracellular cation?
Na+/K+ ATPase
What would you see in symptomatic hyponatraemia?
N+V (<136mmol/l)
Confusion (<131mmol/l)
Seizures, pulmonary oedema (<125mmol/l)
Coma (<117mmol/l)
How do you determine whether it is a true hyponatraemia?
Using serum osmolality:
High-
Glucose/mannitol infusion
Normal-
Spurious
Drip arm sample
Pseudohyponatraemia
Low-
True hyponatraemia
What causes of pseudohyponatraemia are there?
Hyperlipidaemia
Paraproteinaemia
What would you be considering aetiology and treatment wise in a hypervolaemic hyponatraemic patient?
Three failures- heart, renal and liver
Treatment:
Fluid restrict and treat cause
What would you be considering aetiology and investigation wise in a euvolaemic hyponatraemic patient?
Hypothyroidism
Glucocorticoid insufficiency
SIADH
investigations:
TFTs
Short synacthen test
Paired urine and serum osmolality
What would you be considering diagnosis and treatment wise in a hypovolaemic hyponatraemic patient?
Diarrhoea
Vomiting
Diuretics
Salt losing nephropathy
Treatment
Fluid restoration with 5% dextrose
How does liver failure cause hyponatraemia?
There is poor breakdown of vasodilators like nitric oxide, these cause a low BP and subsequent ADH release causes water retention, which dilutes down sodium
What is the risk when treating hyponatraemia?
Rapid correction can lead to central pontine myelinolysis (locked in syndrome) therefore increase Na+ by 1mmol/l/hr
Why is hyponatraemia post surgery common?
Over hydration with hypotonic IV fluids
Transient increase in ADH due to stress of surgery
What is the lab criteria for SIADH?
True hyponatraemia
Clinically euvolaemic
Inappropriate high urine osmolality and increased renal sodium excretion (>20mmol/l)
Normal 9am cortisol and TFTs (diagnosis of exclusion)
Causes of SIADH?
Malignancy- small cell lung cancer (most common), pancreas, prostate, lymphoma)
CNS disorders- meningoencephalitis, haemorrhage, abscess
Chest- TB, pneumonia, abscess
Drugs- opiates, SSRIs, carbamazepine, PPIs